QMRA of Ralstonia solanacearum in potato cultivation: Risks associated with irrigation water recycled through managed aquifer recharge

Agricultural aquifer storage recovery and transfer (ASTR) stores excess fresh water for later reuse in irrigation. Moreover, water quality improves because chemical pollutants and pathogens will be removed by degradation and attachment to the aquifer material. The source water may contain the bacterial plant pathogen Ralstonia solanacearum which causes plant infections and high yield losses. We used quantitative microbial risk assessment (QMRA) to investigate the removal of R. solanacearum during ASTR to predict infection risks of potato plants after irrigation with the recovered water. Laboratory experiments analyzed the ASTR treatment by investigating the bacterial die-off in the water phase and the removal by attachment to the aquifer sediment. Die-off in the water phase depends on the residence time and ranged between 1.3 and 2.7 log10 after 10 or 60 days water storage, respectively. A subpopulation of the bacteria persisted for a prolonged time at low concentrations which may pose a risk if the water is recovered too early. However, the natural aquifer sand filtration proofed to be highly effective in removing R. solanacearum by attachment which depends on the distance between injection and abstraction well. The high removal by attachment alone (18 log10 after 1 m) would reduce bacterial concentrations to negligible numbers. Upscaling to longer soil passages is discussed in the paper. Infection risks of potato plants were calculated using a dose-response model and ASTR treatment resulted in negligible infection risks of a single plant, but also when simulating the irrigation of a 5 ha potato field. This is the first QMRA that analyzed an agricultural ASTR and the fate of a plant pathogen focusing on plant health. QMRA is a useful (water) management tool to evaluate the treatment steps of water reclamation technologies with the aim to provide safe irrigation water and reduce risks disseminating plant diseases.

Quantitative Microbial Risk Assessment of Contracting COVID-19 Derived from Measured and Simulated Aerosol Particle Transmission in Aircraft Cabins

BACKGROUND

SARS-CoV-2 can be effectively transmitted between individuals located in close proximity to each other for extended durations. Aircraft provide such conditions. Although high attack rates during flights were reported, little was known about the risk levels of aerosol transmission of SARS-CoV-2 in aircraft cabins.

OBJECTIVES

The major objective was to estimate the risk of contracting COVID-19 from transmission of aerosol particles in aircraft cabins.

METHODS

In two single-aisle and one twin-aisle aircraft, dispersion of generated aerosol particles over a seven-row economy class cabin section was measured under cruise and taxi conditions and simulated with a computational fluid dynamic model under cruise conditions. Using the aerosol particle dispersion data, a quantitative microbial risk assessment was conducted for scenarios with an asymptomatic infectious person expelling aerosol particles by breathing and speaking. Effects of flight conditions were evaluated using generalized additive mixed models.

RESULTS

Aerosol particle concentration decreased with increasing distance from the infectious person, and this decrease varied with direction. On a typical flight with an average shedder, estimated mean risk of contracting COVID-19 ranged from 1.3 × 10 - 3 to 9.0 × 10 - 2 . Risk increased to 7.7 × 10 - 2 with a super shedder (< 3 % of cases) on a long flight. Risks increased with increasing flight duration: 2-23 cruise flights of typical duration and 2-10 flights of longer duration resulted in at least 1 case of COVID-19 due to onboard aerosol transmission by one average shedder, and in the case of one super shedder, at least 1 case in 1-3 flights of typical duration cruise and 1 flight of longer duration.

DISCUSSION

Our findings indicate that the risk of contracting COVID-19 by aerosol transmission in an aircraft cabin is low, but it will not be zero. Testing before boarding may help reduce the chance of a (super)shedder boarding an aircraft and mask use further reduces aerosol transmission in the aircraft cabin. https://doi.org/10.1289/EHP11495.

Risk assessment of banknotes as a fomite of SARS-CoV-2 in cash payment transactions

The COVID 19 pandemic has triggered concerns and assumptions globally about transmission of the SARS-CoV-2 virus via cash transactions. This paper assesses the risk of contracting COVID-19 through exposure to SARS-CoV-2 via cash acting as a fomite in payment transactions. A quantitative microbial risk assessment was conducted for a scenario assuming an infectious person at the onset of symptoms, when virion concentrations in coughed droplets are at their highest. This person then contaminates a banknote by coughing on it and immediately hands it over to another person, who might then be infected by transferring the virions with a finger from the contaminated banknote to a facial mucous membrane. The scenario considered transfer efficiency of virions on the banknote to fingertips when droplets were still wet and after having dried up and subsequently being touched by finger printing or rubbing the object. Accounting for the likelihood of the scenario to occur by considering (1) a local prevalence of 100 COVID-19 cases/100,000 persons, (2) a maximum of about one-fifth of infected persons transmit high virus loads, and (3) the numbers of cash transactions/person/day, the risk of contracting COVID-19 via person-to-person cash transactions was estimated to be much lower than once per 39,000 days (107 years) for a single person. In the general populace, there will be a maximum of 2.6 expected cases/100,000 persons/day. The risk for a cashier at an average point of sale was estimated to be much less than once per 430 working days (21 months). The depicted scenario is a rare event, therefore, for a single person, the risk of contracting COVID-19 via person-to-person cash transactions is very low. At a point of sale, the risk to the cashier proportionally increases but it is still low.

Dose-response relationship of Ralstonia solanacearum and potato in greenhouse and in vitro experiments

Ralstonia solanacearum is the causative agent of bacterial wilt of potato and other vegetable crops. Contaminated irrigation water contributes to the dissemination of this pathogen but the exact concentration or biological threshold to cause an infection is unknown. In two greenhouse experiments, potted potato plants (Solanum tuberosum) were exposed to a single irrigation with 50 mL water (non-invasive soil-soak inoculation) containing no or 10² - 10⁸ CFU/mL R. solanacearum. The disease response of two cultivars, Kondor and HB, were compared. Disease development was monitored over a three-month period after which stems, roots and tubers of asymptomatic plants were analyzed for latent infections. First wilting symptoms were observed 15 days post inoculation in a plant inoculated with 5x10⁹ CFU and a mean disease index was used to monitor disease development over time. An inoculum of 5x10⁵ CFU per pot (1.3x10² CFU/g soil) was the minimum dose required to cause wilting symptoms, while one latent infection was detected at the lowest dose of 5x10² CFU per pot (0.13 CFU/g). In a second set of experiments, stem-inoculated potato plants grown in vitro were used to investigate the dose-response relationship under optimal conditions for pathogen growth and disease development. Plants were inoculated with doses between 0.5 and 5x10⁵ CFU/plant which resulted in visible symptoms at all doses. The results led to a dose-response model describing the relationship between R. solanacearum exposure and probability of infection or illness of potato plants. Cultivar Kondor was more susceptible to brown-rot infections than HB in greenhouse experiments while there was no significant difference between the dose-response models of both cultivars in in vitro experiments. The ED₅₀ for infection of cv Kondor was 1.1x10⁷ CFU. Results can be used in management strategies aimed to reduce or eliminate the risk of bacterial wilt infection when using treated water in irrigation.

Removal of bacterial plant pathogens in columns filled with quartz and natural sediments under anoxic and oxygenated conditions

Irrigation with surface water carrying plant pathogens poses a risk for agriculture. Managed aquifer recharge enhances fresh water availability while simultaneously it may reduce the risk of plant diseases by removal of pathogens during aquifer passage. We compared the transport of three plant pathogenic bacteria with Escherichia coli WR1 as reference strain in saturated laboratory column experiments filled with quartz sand, or sandy aquifer sediments. E. coli showed the highest removal, followed by Pectobacterium carotovorum, Dickeya solani and Ralstonia solanacearum. Bacterial and non-reactive tracer breakthrough curves were fitted with Hydrus-1D and compared with colloid filtration theory (CFT). Bacterial attachment to fine and medium aquifer sand under anoxic conditions was highest with attachment rates of max. k_att1 = 765 day^-1 and 355 day^-1, respectively. Attachment was the least to quartz sand under oxic conditions (k_att1 = 61 day^-1). In CFT, sticking efficiencies were higher in aquifer than in quartz sand but there was no differentiation between fine and medium aquifer sand. Overall removal ranged between < 6.8 log₁₀ m^-1 in quartz and up to 40 log₁₀ m^-1 in fine aquifer sand. Oxygenation of the anoxic aquifer sediments for two weeks with oxic influent water decreased the removal. The results highlight the potential of natural sand filtration to sufficiently remove plant pathogenic bacteria during aquifer storage.

Die-off of plant pathogenic bacteria in tile drainage and anoxic water from a managed aquifer recharge site

Managed aquifer recharge (MAR) can provide irrigation water and overcome water scarcity in agriculture. Removal of potentially present plant pathogens during MAR is essential to prevent crop diseases. We studied the die-off of three plant pathogenic bacteria in water microcosms with natural or filtered tile drainage water (TDW) at 10 and 25°C and with natural anoxic aquifer water (AW) at 10°C from a MAR site. These bacteria were: Ralstonia solanacearum (bacterial wilt), and the soft rot Pectobacteriaceae (SRP) Dickeya solani and Pectobacterium carotovorum sp. carotovorum (soft rot, blackleg). They are present in surface waters and cause destructive crop diseases worldwide which have been linked to contaminated irrigation water. Nevertheless, little is known about the survival of the SRP in aqueous environments and no study has investigated the persistence of R. solanacearum under natural anoxic conditions. We found that all bacteria were undetectable in 0.1 mL samples within 19 days under oxic conditions in natural TDW at 10°C, using viable cell counting, corresponding to 3-log10 reduction by die-off. The SRP were no longer detected within 6 days at 25°C, whereas R. solanacearum was detectable for 25 days. Whereas in anoxic natural aquifer water at 10°C, the bacterial concentrations declined slower and the detection limit was reached within 56 days. Finally, we modelled the inactivation curves with a modified Weibull model that can simulate different curve shapes such as shoulder phenomena in the beginning and long tails reflecting persistent bacterial populations. The non-linear model was shown to be a reliable tool to predict the die-off of the analysed plant pathogenic bacteria, suggesting its further application to other pathogenic microorganisms in the context of microbial risk assessment.

Evaluation of groundwater bacterial community composition to inform waterborne pathogen vulnerability assessments

Microbial water quality evaluations are essential for determining the vulnerability of subsurface drinking water sources to fecal pathogen intrusion. Rather than directly monitor waterborne pathogens using culture- or enumeration-based techniques, the potential of assessing bacterial community using 16S rRNA gene amplicon sequencing to support these evaluations was investigated. A framework for analyzing 16S rRNA gene amplicon sequencing results featuring negative-binomial generalized linear models is demonstrated, and applied to bacterial taxa sequences in purge water samples collected from a shallow, highly aerobic, unconfined aquifer. Bacterial taxa relevant as indicators of fecal source and surface connectivity were examined using this approach. Observed sequences of Escherichia, a genus suggestive of fecal source, were consistently detected but not confirmed by culture-based methods. On the other hand, episodic appearance of anaerobic taxa sequences in this highly aerobic environment, namely Clostridia and Bacteroides, warrants further investigation as potential indicators of fecal contamination. Betaproteobacteria sequences varied significantly on a seasonal basis, and therefore may be linked to understanding surface-water groundwater interactions at this site. However, sequences that are often encountered in surface water bodies (Cyanobacteria and Flavobacteriia) were notably absent or present at very low levels, suggesting that microbial transport from surface-derived sources may be rather limited. This work demonstrates the utility of 16S rRNA gene amplicon sequencing for contextualizing and complementing conventional microbial techniques, allowing for hypotheses about source and transport processes to be tested and refined.

Evaluation of water quality guidelines for public swimming ponds

Swimming ponds are artificial ecosystems for bathing in which people imitate the conditions of natural waters. Swimming in natural water may pose health risks if the water quality is microbiologically poor. Swimming ponds are small water bodies that may be used by relatively large groups of people, moreover, the water is not disinfected, e.g. by using chlorine. The draft new swimming pool legislation in the Netherlands includes water quality requirements for swimming ponds. This study focused on the examination and evaluation of the new microbiological water quality requirements, including Escherichia coli, intestinal enterococci, Pseudomonas aeruginosa and Staphylococcus aureus, in thirteen public swimming pools. In eight of thirteen swimming ponds the water quality met the requirements for fecal indicators; 93-95% of the samples met the requirement for E. coli (≤100/100 ml) and intestinal enterococci (≤50/100 ml). The requirement for P. aeruginosa (≤10/100 ml) was met in eleven of thirteen swimming ponds (99% of the samples). In 68% of the samples the requirement for S. aureus (<1/100 ml) was met. A linear mixed effect analysis showed that E. coli and intestinal enterococci concentrations were significantly dependent on the log₁₀ of turbidity. P. aeruginosa concentrations were significantly dependent on water temperature. 31-45% of the variation between swimming ponds was explained by considering 'pond' as a random effect in the analysis. The monitoring of microbiological parameters in swimming pond water needs selective analytical methods, such as those used in this study, due to large numbers of background bacteria. The draft new Dutch swimming pool legislation provides proper guidance to ensure the microbiological safety of swimming pond water; it would benefit from inclusion of turbidity as an extra parameter. S. aureus is a relevant parameter for non-fecal shedding, although scientific literature does not provide evidence for a norm value based on a dose-response relation for exposure to S. aureus in water.

Modeling the Transport of Human Rotavirus and Norovirus in Standardized and in Natural Soil Matrix-Water Systems

We modeled Group A Rotavirus (RVA) and Norovirus genogroup II (GII NoV) transport experiments in standardized (crystal quartz sand and deionized water with adjusted pH and ionic strength) and natural soil matrix-water systems (MWS). On the one hand, in the standardized MWS, Rotavirus and Norovirus showed very similar breakthrough curves (BTCs), showing a removal rate of 2 and 1.7 log₁₀, respectively. From the numerical modeling of the experiment, transport parameters of the same order of magnitude were obtained for both viruses. On the other hand, in the natural MWS, the two viruses show very different BTCs. The Norovirus transport model showed significant changes; BTC showed a removal rate of 4 log₁₀, while Rotavirus showed a removal rate of 2.6 log₁₀ similar to the 2 log₁₀ observed on the standardized MWS. One possible explanation for this differential behavior is the difference in the isoelectric point value of these two viruses and the increase of the ionic strength on the natural MWS.

Evaluation of Bacterial Contamination as an Indicator of Viral Contamination in a Sedimentary Aquifer in Uruguay

In Uruguay, groundwater is frequently used for agricultural activities, as well as for human consumption in urban and rural areas. As in many countries worldwide, drinking water microbiological quality is evaluated only according to bacteriological standards and virological analyses are not mentioned in the legislation. In this work, the incidence of human viral (Rotavirus A, Norovirus GII, and human Adenovirus) and bacterial (total and thermotolerant coliform and Pseudomonas aeruginosa) contamination in groundwater in the Salto district, Uruguay, as well as the possible correlation between these groups of microorganisms, was studied. From a total of 134 groundwater samples, 42 (32.1%) were positive for Rotavirus, only 1 (0.7%) for both Rotavirus and Adenovirus, and 96 (72.6%) samples were positive for bacterial indicators. Results also show that Rotavirus presence was not associated with changes in chemical composition of the aquifer water. Bacteriological indicators were not adequate to predict the presence of viruses in individual groundwater samples (well scale), but a deeper spatial-temporal analysis showed that they are promising candidates to assess the viral contamination degree at aquifer scale, since from the number of wells with bacterial contamination the number of wells with viral contamination could be estimated.

Transport of bacteriophage MS2 and PRD1 in saturated dune sand under suboxic conditions

Soil passage of (pretreated) surface water to remove pathogenic microorganisms is a highly efficient process under oxic conditions, reducing microorganism concentrations about 8 log¹⁰ within tens of meters. However, under anoxic conditions, it has been shown that removal of microorganisms can be limited very much. Setback distances for adequate protection of natural groundwater may, therefore, be too short if anoxic conditions apply. Because removal of microorganisms under suboxic conditions is unknown, this research investigated removal of bacteriophage MS2 and PRD1 by soil passage under suboxic conditions at field scale. At the field location (dune area), one injection well and six monitoring wells were installed at different depths along three suboxic flow lines, where oxygen concentrations ranged from 0.4 to 1.7 mg/l and nitrate concentrations ranged from 13 to 16 mg/L. PRD1 and MS2 were injected directly at the corresponding depths and their removal in each flow line was determined. The highest bacteriophage removal was observed in the top layer, with about 9 log removal of MS2, and 7 log removal of PRD1 after 16 meters of aquifer transport. Less removal was observed at 12 m below surface, probably due to a higher groundwater velocity in this coarser grained layer. MS2 was removed more effectively than PRD1 under all conditions. Due to short travel times, inactivation of the phages was limited and the reported log removal was mainly associated with attachment of phages to the aquifer matrix. This study shows that attachment of MS2 and PRD1 is similar for oxic and suboxic sandy aquifers, and, therefore, setback distances used for sandy aquifers under oxic and suboxic conditions provide a similar level of safety. Sticking efficiency and the attachment rate coefficient, as measures for virus attachment, were evaluated as a function of the physico-chemical conditions.

Climate change impact on infection risks during bathing downstream of sewage emissions from CSOs or WWTPs

Climate change is expected to influence infection risks while bathing downstream of sewage emissions from combined sewage overflows (CSOs) or waste water treatment plants (WWTPs) due to changes in pathogen influx, rising temperatures and changing flow rates of the receiving waters. In this study, climate change impacts on the surface water concentrations of Campylobacter, Cryptosporidium and norovirus originating from sewage were modelled. Quantitative microbial risk assessment (QMRA) was used to assess changes in risks of infection. In general, infection risks downstream of WWTPs are higher than downstream CSOs. Even though model outputs show an increase in CSO influxes, in combination with changes in pathogen survival, dilution within the sewage system and bathing behaviour, the effects on the infection risks are limited. However, a decrease in dilution capacity of surface waters could have significant impact on the infection risks of relatively stable pathogens like Cryptosporidium and norovirus. Overall, average risks are found to be higher downstream WWTPs compared to CSOs. Especially with regard to decreased flow rates, adaptation measures on treatment at WWTPs may be more beneficial for human health than decreasing CSO events.

Long-term inactivation of bacteriophage PRD1 as a function of temperature, pH, sodium and calcium concentration

The two most significant processes controlling virus mobility in the subsurface environment are virus attachment and inactivation. In particular, models that predict subsurface virus transport are highly sensitive to inactivation. Virus inactivation is known to depend on temperature as well as hydrochemical conditions. The aim of the current work was to study the effects of temperature and hydrochemical conditions on the inactivation of bacteriophage PRD1 as a model virus, and to develop a quantitative relation for these effects. Series of batch experiments under controlled temperature were conducted, for a range of conditions: 9.5 °C and 12 °C, pH4 - pH8, sodium concentrations of 1, 10 and 20 mM, and calcium concentrations of 0.5, 1.5, and 3 mM. By multivariate regression analysis, a joint log-square model was developed that describes the inactivation rate of PRD1 as a function of these hydrochemical conditions. This model approximates two rate and Weibull models and accounts for the observed non-linear inactivation at increased pH and salt concentrations. Model predictions are within ±0.4 log10 (0.4-2.5 times) virus concentration reduction. The nature of the log-square model does not allow extrapolation of virus inactivation beyond the experimental conditions. Inactivation rate of PRD1 was found to increase with increasing temperature and increasing sodium and calcium concentrations, and to be lowest between pH 6.5 and pH 7.5. Within the studied conditions, the developed log-square model may be applied at field scale for predicting inactivation during subsurface transport of viruses.

Fate of Extended-Spectrum β-Lactamase-Producing Escherichia coli from Faecal Sources in Surface Water and Probability of Human Exposure through Swimming

The goal of this study was to determine the fate of ESBL-producing Escherichia coli (ESBL-EC) emitted from faecal sources in surface water, and the probability of human exposure through swimming. Concentrations of ESBL-EC were measured in recreational waters and in source waters, being water in ditches surrounding poultry farms and municipal wastewater. Additionally, the potential of ESBL-EC in source waters to reach recreational waters located downstream of these sources was modeled. Modeled ESBL-EC concentrations in recreational waters appeared to be mostly determined by the concentrations in the source waters and by subsequent dilution in surface water, and less by inactivation. The mean (95%) risk of human exposure to ESBL-EC per person per swimming event, as assessed from measured ESBL-EC concentrations in recreational waters, was 0.16 (0.89) for men, 0.13 (0.72) for women and 0.20 (0.95) for children. Similar exposure risks were estimated for hypothetical recreational waters containing 100- or 1000-times diluted source water, located 10 days water travel time downstream of the sources. Human exposure to ESBL-EC through swimming is likely, if recreational waters are located downstream of poultry farms and municipal wastewater discharge points.

Effect of Climate Change on the Concentration and Associated Risks of Vibrio Spp. in Dutch Recreational Waters

Currently, the number of reported cases of recreational- water-related Vibrio illness in the Netherlands is low. However, a notable higher incidence of Vibrio infections has been observed in warm summers. In the future, such warm summers are expected to occur more often, resulting in enhanced water temperatures favoring Vibrio growth. Quantitative information on the increase in concentration of Vibrio spp. in recreational water under climate change scenarios is lacking. In this study, data on occurrence of Vibrio spp. at six different bathing sites in the Netherlands (2009-2012) were used to derive an empirical formula to predict the Vibrio concentration as a function of temperature, salinity, and pH. This formula was used to predict the effects of increased temperatures in climate change scenarios on Vibrio concentrations. For Vibrio parahaemolyticus, changes in illness risks associated with the changed concentrations were calculated as well. For an average temperature increase of 3.7 °C, these illness risks were calculated to be two to three times higher than in the current situation. Current illness risks were, varying per location, on average between 10(-4) and 10(-2) per person for an entire summer. In situations where water temperatures reached maximum values, illness risks are estimated to be up to 10(-2) and 10(-1) . If such extreme situations occur more often during future summers, increased numbers of ill bathers or bathing-water-related illness outbreaks may be expected.

Entero- and parechovirus distributions in surface water and probabilities of exposure to these viruses during water recreation

Numerous studies have reported quantitative data on viruses in surface waters generated using different methodologies. In the current study, the impact of the use of either cell culture-based or molecular-based methods in quantitative microbial risk assessment was assessed. Previously and newly generated data on the presence of infectious human enteroviruses (HEV) and enterovirus and parechovirus RNA were used to estimate distributions of virus concentrations in surface waters. Because techniques for the detection of infectious human parechoviruses (HPeV) in surface waters were not available, a 'Parallelogram Approach' was used to estimate their concentrations based on the ratio infectious HEV/HEV RNA. The obtained virus concentrations were then used to estimate the probability of exposure for children during recreation in such virus contaminated surface waters. Human enterovirus cell culture/PCR ratios ranged from 2.3 × 10(-3) to 0.28. This broad range of ratios indicates that care should be taken in assuming a fixed ratio for assessing the risk with PCR based virus concentrations. The probabilities of exposure to both enteroviruses and parechoviruses were calculated, using our Parallelogram Approach for the calculation of infectious parechoviruses. For both viruses it was observed that the detection method significantly influenced the probability of exposure. Based on the calculated culture data, PCR data, and the ingestion volume, it was estimated that the mean probabilities of exposure, of recreating children, to surface water containing viruses were 0.087 (infectious enteroviruses), 0.71 (enterovirus particles), 0.28 (parechovirus particles) and 0.025 (calculated infectious parechoviruses) per recreation event. The mean probabilities of exposure of children recreating in surface water from which drinking water is produced to infectious enteroviruses were estimated for nine locations and varied between 1.5 × 10(-4) - 0.09 per recreation event. In this study, the use of the rotavirus dose response relationship as a surrogate was avoided. Instead, the probabilities of exposure were derived as a function of the distributions of the calculated doses. Our 'Parallelogram Approach' was used to estimate the unavailable infectious parechovirus concentrations using Monte Carlo simulations, and the exposure assessment carried out showed that virus concentrations present in surface waters could pose a health risk for children and other vulnerable populations.

Quantitative assessment of infection risk from exposure to waterborne pathogens in urban floodwater

Flooding and heavy rainfall have been associated with waterborne infectious disease outbreaks, however, it is unclear to which extent they pose a risk for public health. Here, risks of infection from exposure to urban floodwater were assessed using quantitative microbial risk assessment (QMRA). To that aim, urban floodwaters were sampled in the Netherlands during 23 events in 2011 and 2012. The water contained Campylobacter jejuni (prevalence 61%, range 14- >10(3) MPN/l), Giardia spp. (35%, 0.1-142 cysts/l), Cryptosporidium (30%, 0.1-9.8 oocysts/l), noroviruses (29%, 10(2)-10(4) pdu/l) and enteroviruses (35%, 10(3)-10(4) pdu/l). Exposure data collected by questionnaire, revealed that children swallowed 1.7 ml (mean, 95% Confidence Interval 0-4.6 ml) per exposure event and adults swallowed 0.016 ml (mean, 95% CI 0-0.068 ml) due to hand-mouth contact. The mean risk of infection per event for children, who were exposed to floodwater originating from combined sewers, storm sewers and rainfall generated surface runoff was 33%, 23% and 3.5%, respectively, and for adults it was 3.9%, 0.58% and 0.039%. The annual risk of infection was calculated to compare flooding from different urban drainage systems. An exposure frequency of once every 10 years to flooding originating from combined sewers resulted in an annual risk of infection of 8%, which was equal to the risk of infection of flooding originating from rainfall generated surface runoff 2.3 times per year. However, these annual infection risks will increase with a higher frequency of urban flooding due to heavy rainfall as foreseen in climate change projections.

Bacteriophage PRD1 batch experiments to study attachment, detachment and inactivation processes

Knowledge of virus removal in subsurface environments is pivotal for assessing the risk of viral contamination of water resources and developing appropriate protection measures. Columns packed with sand are frequently used to quantify attachment, detachment and inactivation rates of viruses. Since column transport experiments are very laborious, a common alternative is to perform batch experiments where usually one or two measurements are done assuming equilibrium is reached. It is also possible to perform kinetic batch experiments. In that case, however, it is necessary to monitor changes in the concentration with time. This means that kinetic batch experiments will be almost as laborious as column experiments. Moreover, attachment and detachment rate coefficients derived from batch experiments may differ from those determined using column experiments. The aim of this study was to determine the utility of kinetic batch experiments and investigate the effects of different designs of the batch experiments on estimated attachment, detachment and inactivation rate coefficients. The experiments involved various combinations of container size, sand-water ratio, and mixing method (i.e., rolling or tumbling by pivoting the tubes around their horizontal or vertical axes, respectively). Batch experiments were conducted with clean quartz sand, water at pH 7 and ionic strength of 20 mM, and using the bacteriophage PRD1 as a model virus. Values of attachment, detachment and inactivation rate coefficients were found by fitting an analytical solution of the kinetic model equations to the data. Attachment rate coefficients were found to be systematically higher under tumbling than under rolling conditions because of better mixing and more efficient contact of phages with the surfaces of the sand grains. In both mixing methods, more sand in the container yielded higher attachment rate coefficients. A linear increase in the detachment rate coefficient was observed with increased solid-water ratio using tumbling method. Given the differences in the attachment rate coefficients, and assuming the same sticking efficiencies since chemical conditions of the batch and column experiments were the same, our results show that collision efficiencies of batch experiments are not the same as those of column experiments. Upscaling of the attachment rate from batch to column experiments hence requires proper understanding of the mixing conditions. Because batch experiments, in which the kinetics are monitored, are as laborious as column experiments, there seems to be no major advantage in performing batch instead of column experiments.

Quantitative risk estimation for a Legionella pneumophila infection due to whirlpool use

Quantitative microbiological risk assessment was used to quantify the risk associated with the exposure to Legionella pneumophila in a whirlpool. Conceptually, air bubbles ascend to the surface, intercepting Legionella from the traversed water. At the surface the bubble bursts into dominantly noninhalable jet drops and inhalable film drops. Assuming that film drops carry half of the intercepted Legionella, a total of four (95% interval: 1-9) and 4.5×10(4) (4.4×10(4) - 4.7×10(4) ) cfu/min were estimated to be aerosolized for concentrations of 1 and 1,000 legionellas per liter, respectively. Using a dose-response model for guinea pigs to represent humans, infection risks for active whirlpool use with 100 cfu/L water for 15 minutes were 0.29 (∼0.11-0.48) for susceptible males and 0.22 (∼0.06-0.42) for susceptible females. A L. pneumophila concentration of ≥1,000 cfu/L water was estimated to nearly always cause an infection (mean: 0.95; 95% interval: 0.9-∼1). Estimated infection risks were time-dependent, ranging from 0.02 (0-0.11) for 1-minute exposures to 0.93 (0.86-0.97) for 2-hour exposures when the L. pneumophila concentration was 100 cfu/L water. Pool water in Dutch bathing establishments should contain <100 cfu Legionella/L water. This study suggests that stricter provisions might be required to assure adequate public health protection.

Reduction of bacteriophage MS2 by filtration and irradiation determined by culture and quantitative real-time RT-PCR

Molecular methods are increasingly applied for virus detection in environmental samples without rendering data on viral infectivity. Infectivity data are important for assessing public health risks from exposure to human pathogenic viruses in the environment. Here, treatment efficiencies of three (drinking) water treatment processes were estimated by quantification of the indicator virus bacteriophage MS2 with culture and real-time reverse transcription polymerase chain reaction (qRT-PCR). We studied the virus reduction by slow sand filtration at a pilot plant. No decay of MS2 RNA was observed, whereas infectious MS2 particles were inactivated at a rate of 0.1 day(-1). Removal of MS2 RNA and infectious MS2 particles was 1.2 and 1.6 log10-units, respectively. Virus reduction by UV and gamma irradiation was determined in laboratory-scale experiments. The reduction of MS2 RNA based on qRT-PCR data was negligible. Reduction of infectious MS2 particles was estimated at 3.0-3.6 log10-units (UV dose up to 400 or 800 J/m(2)) and 4.7-7 log10-units (gamma dose up to 200 Gray). As shown in this study, estimations of viral reduction, both inactivation and removal, obtained by molecular methods should be interpreted carefully when considering treatment options to provide virus-safe drinking water. Combining culture-based methods with molecular methods may provide supplementary information on mechanisms of virus reduction.

A mathematical model for removal of human pathogenic viruses and bacteria by slow sand filtration under variable operational conditions

Slow sand filtration (SSF) in drinking water production removes pathogenic microorganisms, but detection limits and variable operational conditions complicate assessment of removal efficiency. Therefore, a model was developed to predict removal of human pathogenic viruses and bacteria as a function of the operational conditions. Pilot plant experiments were conducted, in which bacteriophage MS2 and Escherichia coli WR1 were seeded as model microorganisms for pathogenic viruses and bacteria onto the filters under various temperatures, flow rates, grain sizes and ages of the Schmutzdecke. Removal of MS2 was 0.082-3.3 log10 and that of E. coli WR1 0.94-4.5 log10 by attachment to the sand grains and additionally by processes in the Schmutzdecke. The contribution of the Schmutzdecke to the removal of MS2 and E. coli WR1 increased with its ageing, with sticking efficiency and temperature, decreased with grain size, and was modelled as a logistic growth function with scale factor f0 and rate coefficient f1. Sticking efficiencies were found to be microorganism and filter specific, but the values of f0 and f1 were independent of microorganism and filter. Cross-validation showed that the model can be used to predict log removal of MS2 and ECWR1 within ±0.6 log. Within the range of operational conditions, the model shows that removal of microorganisms is most sensitive to changes in temperature and age of the Schmutzdecke.

A mathematical model for removal of human pathogenic viruses and bacteria by slow sand filtration under variable operational conditions

Slow sand filtration (SSF) in drinking water production removes pathogenic microorganisms, but detection limits and variable operational conditions complicate assessment of removal efficiency. Therefore, a model was developed to predict removal of human pathogenic viruses and bacteria as a function of the operational conditions. Pilot plant experiments were conducted, in which bacteriophage MS2 and Escherichia coli WR1 were seeded as model microorganisms for pathogenic viruses and bacteria onto the filters under various temperatures, flow rates, grain sizes and ages of the Schmutzdecke. Removal of MS2 was 0.082-3.3 log10 and that of E. coli WR1 0.94-4.5 log10 by attachment to the sand grains and additionally by processes in the Schmutzdecke. The contribution of the Schmutzdecke to the removal of MS2 and E. coli WR1 increased with its ageing, with sticking efficiency and temperature, decreased with grain size, and was modelled as a logistic growth function with scale factor f0 and rate coefficient f1. Sticking efficiencies were found to be microorganism and filter specific, but the values of f0 and f1 were independent of microorganism and filter. Cross-validation showed that the model can be used to predict log removal of MS2 and ECWR1 within ±0.6 log. Within the range of operational conditions, the model shows that removal of microorganisms is most sensitive to changes in temperature and age of the Schmutzdecke.

Effects of fluctuations in river water level on virus removal by bank filtration and aquifer passage--a scenario analysis

Riverbank filtration is an effective process for removing pathogenic viruses from river water. Despite indications that changing hydraulic conditions during floods can affect the efficacy of riverbank filtration to remove viruses, the impact on advection and dispersion of viruses in the riverbank is not well understood. We investigated the effects of fluctuations in river water level on virus transport during riverbank filtration, considering 3-D transient groundwater flow and virus transport. Using constant removal rates from published field experiments with bacteriophages, removal of viruses with distance from the riverbank was simulated for coarse gravel, fine gravel and fine sandy gravel. Our simulations showed that, in comparison with steady flow conditions, fluctuations in river water level cause viruses to be transported further at higher concentrations into the riverbank. A 1-5 m increase in river water levels led to a 2- to 4-log (log10 reduction in concentration relative to the initial concentration in the river) increase in virus concentration and to up to 30% shorter travel times. For particular cases during the receding flood, changing groundwater flow conditions caused that pristine groundwater was carried from further inland and that simulated virus concentrations were more diluted in groundwater. Our study suggests that the adverse effect of water level fluctuations on virus transport should be considered in the simulation of safe setback distances for drinking water supplies.

Effect of dissolved calcium on the removal of bacteriophage PRD1 during soil passage: the role of double-layer interactions

The objective of this work was to investigate and obtain quantitative relations for the effects of Ca(2+) concentration on virus removal in saturated soil and to compare the experimental findings with predictions of the DLVO theory. In order to do so, a systematic study was performed with a range of calcium concentrations corresponding to natural field conditions. Experiments were conducted in a 50-cm column with clean quartz sand under saturated conditions. Inflow solutions were prepared by adding CaCl(2,) NaCl and NaHCO(3) to de-ionized water. Values of pH and ionic strength were fixed at 7 and 10mM, respectively. Bacteriophage PRD1 was used as a conservative model virus for virus removal. The samples were assayed using the plaque forming technique. Attachment, detachment and inactivation rate coefficients were determined from fitting breakthrough curves. Attachment rate coefficients were found to increase with increasing calcium concentration. Results were used to calculate sticking efficiency, for which an empirical formula as a function of Ca(2+) was developed. Numerical solutions of the Poisson-Boltzmann equation were obtained to evaluate the effect of Ca(2+) on the double-layer interactions between quartz and PRD1. Based on these results, the DLVO interaction energies were calculated. It turned out that the experimental findings cannot be explained with the distance profiles of the DLVO interaction. The discrepancy between theory and experiment can be attributed to underestimation of the van der Waals interactions, chemisorption of Ca(2+) onto the surfaces, or by factors affecting the double-layer interactions, which are not included in the Poisson-Boltzmann equation. When abruptly changing from inflow solution containing Ca(2+) to a Ca(2+)-free solution, pronounced mobilization of viruses was observed. This indicates virus removal is not irreversible and that chemical perturbations of the groundwater can cause a burst of released viruses.

Definition and quantification of initial anthropogenic pollutant release in swimming pools

Pollutants, brought into a swimming pool by bathers, will react with chlorine to form disinfection by-products (DBPs). Some of these DBPs are found to be respiratory and ocular irritant and might be associated with asthma, or might even be carcinogenic. As DBPs in swimming pools are formed from bather-shed-pollutants, a reduction of these pollutants will lead to a reduction of DBPs. Until now, however, the release of pollutants by bathers has not been studied in detail. The study described in this paper focuses on the release of these pollutants, further called anthropogenic pollutants. The objective was to define and quantify the initial anthropogenic pollutants, by using a standardised shower cabin and a standardised showering protocol in laboratory time-series experiments and on-site experiments in swimming pools. The time-series experiments resulted in a definition of the initial anthropogenic pollutant release: the amount of pollutants released from a person in a standardised shower cabin during the first 60 s of showering. The data from the time-series experiments were used to create a model of pollutant release. The model can be used to predict the initial anthropogenic pollutant release as well as the effects of showering. On-site experiments were performed at four different swimming pools, including one outdoor pool. Results of these on-site showering experiments correspond with the time-series and model outcomes. Anthropogenic pollutant release (both chemical and microbiological) in swimming pool water can be reduced by pre-swim showering, very likely resulting in decreased DBPs formation and chlorine demand.

The impact of temperature on the inactivation of enteric viruses in food and water: a review

Temperature is considered as the major factor determining virus inactivation in the environment. Food industries, therefore, widely apply temperature as virus inactivating parameter. This review encompasses an overview of viral inactivation and virus genome degradation data from published literature as well as a statistical analysis and the development of empirical formulae to predict virus inactivation. A total of 658 data (time to obtain a first log(10) reduction) were collected from 76 published studies with 563 data on virus infectivity and 95 data on genome degradation. Linear model fitting was applied to analyse the effects of temperature, virus species, detection method (cell culture or molecular methods), matrix (simple or complex) and temperature category (<50 and ≥50°C). As expected, virus inactivation was found to be faster at temperatures ≥50°C than at temperatures <50°C, but there was also a significant temperature-matrix effect. Virus inactivation appeared to occur faster in complex than in simple matrices. In general, bacteriophages PRD1 and PhiX174 appeared to be highly persistent whatever the matrix or the temperature, which makes them useful indicators for virus inactivation studies. The virus genome was shown to be more resistant than infectious virus. Simple empirical formulas were developed that can be used to predict virus inactivation and genome degradation for untested temperatures, time points or even virus strains.

QMRAspot: a tool for Quantitative Microbial Risk Assessment from surface water to potable water

In the Netherlands, a health based target for microbially safe drinking water is set at less than one infection per 10,000 persons per year. For the assessment of the microbial safety of drinking water, Dutch drinking water suppliers must conduct a Quantitative Microbial Risk Assessment (QMRA) at least every three years for the so-called index pathogens enterovirus, Campylobacter, Cryptosporidium and Giardia. In order to collect raw data in the proper format and to automate the process of QMRA, an interactive user-friendly computational tool, QMRAspot, was developed to analyze and conduct QMRA for drinking water produced from surface water. This paper gives a description of the raw data requirements for QMRA as well as a functional description of the tool. No extensive prior knowledge about QMRA modeling is required by the user, because QMRAspot provides guidance to the user on the quantity, type and format of raw data and performs a complete analysis of the raw data to yield a risk outcome for drinking water consumption that can be compared with other production locations, a legislative standard or an acceptable health based target. The uniform approach promotes proper collection and usage of raw data and, warrants quality of the risk assessment as well as enhances efficiency, i.e., less time is required. QMRAspot may facilitate QMRA for drinking water suppliers worldwide. The tool aids policy makers and other involved parties in formulating mitigation strategies, and prioritization and evaluation of effective preventive measures as integral part of water safety plans.

Exposure assessment for swimmers in bathing waters and swimming pools

Bathing water compliant with bathing water legislation may nevertheless contain pathogens to such a level that they pose unacceptable health risks for swimmers. Quantitative Microbiological Risk Assessment (QMRA) can provide a proper basis for protective measures, but the required data on swimmer exposure are currently limited or lacking. The objective of this study was to collect exposure data for swimmers in fresh water, seawater and swimming pools, i.e. volume of water swallowed and frequency and duration of swimming events. The study related to swimming in 2007, but since the summer of 2007 was wet and this might have biased the results regarding surface water exposure, the study was repeated relating to swimming in 2009, which had a dry and sunny summer. Exposure data were collected through questionnaires administered to approximately 19 000 persons representing the general Dutch population. Questionnaires were completed by 8000 adults of whom 1924 additionally answered the questions for their eldest child (< 15 years). The collected data did not differ significantly between 2007 and 2009. The frequency of swimming and the duration of swimming were different for men, women and children and between water types. Differences between men and women were small, but children behaved differently: they swam more often, stayed in the water longer, submerged their heads more often and swallowed more water. Swimming pools were visited most frequently (on average 13-24 times/year) with longest duration of swimming (on average 67-81 min). On average, fresh and seawater sites were visited 6-8 times/year and visits lasted 41-79 min. Dependent on the water type, men swallowed on average 27-34 ml per swimming event, women 18-23 ml, and children 31-51 ml. Data on exposure of swimmers to recreational waters could be obtained by using a questionnaire approach in combination with a test to measure mouthfuls of water for transformation of categorical data to numerical data of swallowed volumes of water. Previous assumptions on swimmer exposure were replaced with estimates of exposure parameters, thus reducing uncertainty in assessing the risk of infection with waterborne pathogens and enabling identification of risk groups. QMRA for Cryptosporidium and Giardia was demonstrated based on data from previous studies on the occurrence of these pathogens in recreational lakes and a swimming pool.

Systematic study of effects of pH and ionic strength on attachment of phage PRD1

Objectives of this work are to investigate effects of pH and ionic strength (IS) on virus transport in saturated soil and to develop a quantitative relationship for these effects. A series of 50-cm column experiments with clean quartz sand under saturated conditions and with pH values of 5, 6, 7, 8, and IS values of 1, 10, and 20 mM were conducted. Bacteriophage PRD1 was used as a model virus. Applying a one-site kinetic model, attachment, detachment, and inactivation rate coefficients were determined from fitting breakthrough curves using the software package Hydrus-1D. Attachment rate coefficients increased with decreasing pH and increasing IS, in agreement with DLVO theory. Sticking efficiencies were calculated from the attachment rate coefficients and used to develop an empirical formula for sticking efficiency as a function of pH and IS. This relationship is applicable under unfavorable conditions for virus attachment. We compared sticking efficiencies predicted by the empirical formula with those from field and column experiments. Within the calibrated range of pH and IS, the predicted and observed sticking efficiencies are in reasonable agreement for bacteriophages PRD1 and MS2. However, the formula significantly overestimates sticking efficiencies for IS higher than 100 mM. In addition, it performs less well for viruses with different surface reactivity than PRD1 and MS2. Effects of pH and IS on detachment and inactivation rate coefficients were also investigated but the experimental results do not allow constraining these parameters with sufficient certainty.

Vulnerability of unconfined aquifers to virus contamination

An empirical formula was developed for determining the vulnerability of unconfined sandy aquifers to virus contamination, expressed as a dimensionless setback distance r(s)(*). The formula can be used to calculate the setback distance required for the protection of drinking water production wells against virus contamination. This empirical formula takes into account the intrinsic properties of the virus and the unconfined sandy aquifer. Virus removal is described by a rate coefficient that accounts for virus inactivation and attachment to sand grains. The formula also includes pumping rate, saturated thickness of the aquifer, depth of the screen of the pumping well, and anisotropy of the aquifer. This means that it accounts also for dilution effects as well as horizontal and vertical virus transport. Because the empirical model includes virus source concentration it can be used as an integral part of a quantitative viral risk assessment.

Fate and transport of viruses during sewage treatment in a mound system

Studies undertaken to assess the performance of filter materials to remove phosphorus in decentralised sewage systems have not reported on the broader performance of these systems. This study aimed to identify virus fate and transport mechanisms at the laboratory scale for comparison with field experiments on a mound system amended with blast furnace slag. Inactivation was a significant removal mechanism for MS2 bacteriophage, but not for PRD1 bacteriophage. Column studies identified rapid transport of PRD1. Laboratory studies predicted lower removal of PRD1 in a full scale system than was experienced in the field study, highlighting the importance of considering pH and flow rate in pathogen removal estimates. The results highlight the necessity for studying a range of organisms when assessing the potential for pathogen transport.

Transport of Escherichia coli and solutes during waste water infiltration in an urban alluvial aquifer

Recharge of waste water in an unconsolidated poorly sorted alluvial aquifer is a complex process, both physically and hydrochemically. The aim of this paper is to analyse and conceptualise vertical transport mechanisms taking place in an urban area of extensive wastewater infiltration by analysing and combining the water balance, the microbial (Escherichia coli) mass balance, and the mass balance for dissolved solutes. For this, data on sediment characteristics (grain size, organic carbon, reactive iron, and calcite), groundwater levels, and concentrations of E. coli in groundwater and waste water were collected. In the laboratory, data on E. coli decay rate coefficients, and on bacteria retention characteristics of the sediment were collected via column experiments. The results indicated that shallow groundwater, at depths of 50 m below the surface, was contaminated with E. coli concentrations as high as 10(6) CFU/100 mL. In general, E. coli concentrations decreased only 3 log units from the point of infiltration to shallow groundwater. Concentrations were lower at greater depths in the aquifer. In laboratory columns of disturbed sediments, bacteria removal was 2-5 log units/0.5 cm column sediment. Because of the relatively high E. coli concentrations in the shallow aquifer, transport had likely taken place via a connected network of pores with a diameter large enough to allow bacterial transport instead of via the sediment matrix, which was inaccessible for bacteria, as was clear from the column experiments. The decay rate coefficient was determined from laboratory microcosms to be 0.15 d(-1). Assuming that decay in the aquifer was similar to decay in the laboratory, then the pore water flow velocity between the point of infiltration and shallow groundwater, coinciding with a concentration decrease of 3 log units, was 0.38 m/d, and therefore, transport in this connected network of pores was fast. According to the water balance of the alluvial aquifer, determined from transient groundwater modelling, groundwater flow in the aquifer was mainly in vertical downward direction, and therefore, the mass balance for dissolved solutes was simulated using a 1D transport model of a 200 m column of the Quaternary Alluvium aquifer. The model, constructed with PHREEQC, included dual porosity, and was able to adequately simulate removal of E. coli, cation-exchange, and nitrification. The added value of the use of E. coli in this study was the recognition of relatively fast transport velocities occurring in the aquifer, and the necessity to use the dual porosity concept to investigate vertical transport mechanisms. Therefore, in general and if possible, microbial mass balances should be considered more systematically as an integral part of transport studies.

Removal and fate of Cryptosporidium parvum, Clostridium perfringens and small-sized centric diatoms (Stephanodiscus hantzschii) in slow sand filters

The decimal elimination capacity (DEC) of slow sand filtration (SSF) for Cryptosporidium parvum was assessed to enable quantitative microbial risk analysis of a drinking water production plant. A mature pilot plant filter of 2.56m(2) was loaded with C. parvum oocysts and two other persistent organisms as potential surrogates; spores of Clostridium perfringens (SCP) and the small-sized (4-7microm) centric diatom (SSCD) Stephanodiscus hantzschii. Highly persistent micro-organisms that are retained in slow sand filters are expected to accumulate and eventually break through the filter bed. To investigate this phenomenon, a dosing period of 100 days was applied with an extended filtrate monitoring period of 150 days using large-volume sampling. Based on the breakthrough curves the DEC of the filter bed for oocysts was high and calculated to be 4.7log. During the extended filtrate monitoring period the spatial distribution of the retained organisms in the filter bed was determined. These data showed little risk of accumulation of oocysts in mature filters most likely due to predation by zooplankton. The DEC for the two surrogates, SCP and SSCD, was 3.6 and 1.8log, respectively. On basis of differences in transport behaviour, but mainly because of the high persistence compared to the persistence of oocysts, it was concluded that both spores of sulphite-reducing clostridia (incl. SCP) and SSCD are unsuited for use as surrogates for oocyst removal by slow sand filters. Further research is necessary to elucidate the role of predation in Cryptosporidium removal and the fate of consumed oocysts.

Determination of protection zones for Dutch groundwater wells against virus contamination--uncertainty and sensitivity analysis

Protection zones of shallow unconfined aquifers in The Netherlands were calculated that allow protection against virus contamination to the level that the infection risk of 10(-4) per person per year is not exceeded with a 95% certainty. An uncertainty and a sensitivity analysis of the calculated protection zones were included. It was concluded that protection zones of 1 to 2 years travel time (206-418 m) are needed (6 to 12 times the currently applied travel time of 60 days). This will lead to enlargement of protection zones, encompassing 110 unconfined groundwater well systems that produce 3 x 10(8) m3 y(-1) of drinking water (38% of total Dutch production from groundwater). A smaller protection zone is possible if it can be shown that an aquifer has properties that lead to greater reduction of virus contamination, like more attachment. Deeper aquifers beneath aquitards of at least 2 years of vertical travel time are adequately protected because vertical flow in the aquitards is only 0.7 m per year. The most sensitive parameters are virus attachment and inactivation. The next most sensitive parameters are grain size of the sand, abstraction rate of groundwater, virus concentrations in raw sewage and consumption of unboiled drinking water. Research is recommended on additional protection by attachment and under unsaturated conditions.

Effect of goethite coating and humic acid on the transport of bacteriophage PRD1 in columns of saturated sand

The transport of bacteriophage PRD1, a model virus, was studied in columns containing sediment mixtures of quartz sand with goethite-coated sand and using various solutions consisting of monovalent and divalent salts and humic acid (HA). Without HA and in the absence of sand, the inactivation rate of PRD1 was found to be as low as 0.014 day(-1) (at 5+/-3 degrees C), but in the presence of HA it was much lower (0.0009 day(-1)), indicating that HA helps PRD1 to survive. When the fraction of goethite in the sediment was increased, the removal of PRD1 also increased. However, in the presence of HA, C/C0 values of PRD1 increased by as much as 5 log units, thereby almost completely eliminating the effect of addition of goethite. The sticking efficiency was not linearly dependent on the amount of goethite added to the quartz sand; this is apparently due to surface charge heterogeneity of PRD1. Our results imply that, in the presence of dissolved organic matter (DOM), viruses can be transported for long distances thanks to two effects: attachment is poor because DOM has occupied favourable sites for attachment and inactivation of virus may have decreased. This conclusion justifies making conservative assumptions about the attachment of viruses when calculating protection zones for groundwater wells.

Evaluation of data from the literature on the transport and survival of Escherichia coli and thermotolerant coliforms in aquifers under saturated conditions

Escherichia coli and thermotolerant coliforms are of major importance as indicators of fecal contamination of water. Due to its negative surface charge and relatively low die-off or inactivation rate coefficient, E. coli is able to travel long distances underground and is therefore also a useful indicator of fecal contamination of groundwater. In this review, the major processes known to determine the underground transport of E. coli (attachment, straining and inactivation) are evaluated. The single collector contact efficiency (SCCE), eta0, one of two parameters commonly used to assess the importance of attachment, can be quantified for E. coli using classical colloid filtration theory. The sticking efficiency, alpha, the second parameter frequently used in determining attachment, varies widely (from 0.003 to almost 1) and mainly depends on charge differences between the surface of the collector and E. coli. Straining can be quantified from geometrical considerations; it is proposed to employ a so-called straining correction parameter, alpha(str). Sticking efficiencies determined from field experiments were lower than those determined under laboratory conditions. We hypothesize that this is due to preferential flow mechanisms, E. coli population heterogeneity, and/or the presence of organic and inorganic compounds in wastewater possibly affecting bacterial attachment characteristics. Of equal importance is the inactivation or die-off of E. coli that is affected by factors like type of bacterial strain, temperature, predation, antagonism, light, soil type, pH, toxic substances, and dissolved oxygen. Modeling transport of E. coli can be separated into three steps: (1) attachment rate coefficients and straining rate coefficients can be calculated from Darcy flow velocity fields or pore water flow velocity fields, calculated SCCE fields, realistic sticking efficiency values and straining correction parameters, (2) together with the inactivation rate coefficient, total rate coefficient fields can be generated, and (3) used as input for modeling the transport of E. coli in existing contaminant transport codes. Areas of future research are manifold and include the effects of typical wastewater characteristics, including high concentrations of organic compounds, on the transport of E. coli and thermotolerant coliforms, and the upscaling of experiments to represent typical field conditions, possibly including preferential flow mechanisms and the aspect of population heterogeneity of E. coli.

Transport of E. coli in columns of geochemically heterogeneous sediment

To elucidate the parameters determining the transport of Escherichia coli in aquifers, the attachment of E. coli in low concentrations to column sediments was investigated. The sediments comprised 0.18-0.50mm quartz sand, grains coated with goethite, calcite grains or grains of activated carbon (AC), in varying fractions (lambda=0, 0.05, 0.1, 0.2, 0.4, 0.7, 1.0) and all of similar diameter to the quartz sand. The weighted sum of favourable and unfavourable sticking efficiencies (alpha(total)) showed that upon increasing the fraction of favourable mineral grains (lambda) there was an initial rapid increase, which then slowed down. This was most pronounced in the AC experiments, followed by the calcite experiments and then the goethite experiments. We ascribe this non-linear relation to surface charge and hydrophobic heterogeneity of the E. coli population.

Determining straining of Escherichia coli from breakthrough curves

Though coliform bacteria are used world wide as an indication of faecal pollution, the parameters determining the transport of Escherichia coli in aquifers are relatively unknown, especially for the period after the clean bed collision phase brought about by prolonged infiltration of waste water. In this research, the breakthrough curves of E. coli after total flushing of 50-200 pore volumes were studied for various influent concentrations in various sediments at different pore water flow velocities. The results indicated that straining in Dead End Pores (DEPs) was an important process that dominated bacteria breakthrough in fine-grained sediment (0.06-0.2 mm). The filling of the DEP space with bacteria took 5-65 pore volumes and was dependent on concentration. Column breakthrough curves were modelled and from this the DEP volumes were determined. These volumes (0.21-0.35% of total column volume) corresponded well with values calculated with a formula based on purely geometrical considerations and also with values calculated with a pore size density function. For this function the so-called Van Genuchten parameters of the sediments used in the experiments were determined. The results indicate that straining might be a dominant process affecting colloid transport in the natural environment and therefore it is concluded that proper knowledge of the pore size distribution is crucial to an understanding of the retention of bacteria.

Effect of climate changes on waterborne disease in The Netherlands

Effects of climate change in The Netherlands in the 21st century on the microbiological quality of water for drinking water production and recreation were evaluated. The following was concluded: increased temperature leads to significant additional inactivation of enteric pathogens in surface waters with residence times of more than a month, but not in waters with residence times of up to ten days. Increased precipitation, runoff and storm water overflow lead to more peak concentrations of waterborne pathogens in surface water. Peak concentrations strongly determine the infection risk through drinking water consumption. Drought lowers and increased precipitation elevates groundwater tables, but an effect on the risk of groundwater contamination with waterborne pathogens is not clear. Climate effects are not noticeable near a groundwater well, where the groundwater table and flow rate are mainly determined by the pumping rate. Exposure of recreants to waterborne pathogens that can grow in the water is expected to increase due to increased recreation and increased growth opportunities of these pathogens. Due to warmer summers, pathogens, like amoeba, that have not caused problems up to date in The Netherlands, may now emerge in recreational waters.

Release of Cryptosporidium and Giardia from dairy cattle manure: physical factors

Various physical factors affecting the release rate of naturally occurring Cryptosporidium parvum oocysts and Giardia duodenalis cysts from dairy manure disks to sprinkled water were studied. The investigated factors included temperature (5 or 23 degrees C), manure type (calf manure, a 50% calf and 50% cow manure mixture, and a 10% calf and 90% cow manure mixture), and water application method (mist or drip) and flow rate. Effluent concentrations of manure and (oo)cysts were always several orders of magnitude below their initial concentration in the manure, decreased gradually, and exhibited persistent concentration tailing. Release of manure and (oo)cysts were found to be related by a constant factor, the so-called release efficiency of (oo)cysts. A previously developed (oo)cyst release model that included these release efficiencies provided a satisfactory simulation of the observed release. An effect of temperature on the release of manure and (oo)cysts was not apparent. The manure and (oo)cyst release rates from cow manure decreased faster than those from calf manure, and (oo)cyst release efficiencies from cow manure were higher than those from calf manure. In comparison with mist application, dripping water resulted in higher release rates of manure and (oo)cysts and in higher (oo)cyst release efficiencies due to the increased mechanical forces associated with droplet impact. Mist application at a higher flow rate resulted in faster release, but did not affect the (oo)cyst release efficiencies. The data and modeling approach described herein provide insight and an enhanced ability to describe the influence of physical factors on (oo)cyst release.

Elimination of viruses, bacteria and protozoan oocysts by slow sand filtration

The decimal elimination capacity (DEC) of slow sand filters (SSF) for viruses, bacteria and oocysts of Cryptosporidium has been assessed from full-scale data and pilot plant and laboratory experiments. DEC for viruses calculated from experimental data with MS2-bacteriophages in the pilot plant filters was 1.5-2 log10. E. coli and thermotolerant coliforms (Coli44) were removed at full-scale and in the pilot plant with 2-3 log10. At full-scale, Campylobacter bacteria removal was 1 log10 more than removal of Coli44, which indicated that Coli44 was a conservative surrogate for these pathogenic bacteria. Laboratory experiments with sand columns showed 2-3 and >5-6 log10 removal of spiked spores of sulphite-reducing clostridia (SSRC; C. perfringens) and oocysts of Cryptosporidium respectively. Consequently, SSRC was not a good surrogate to quantify oocyst removal by SSF. Removal of indigenous SSRC by full-scale filters was less efficient than observed in the laboratory columns, probably due to continuous loading of these filter beds with spores, accumulation and retarded transport. It remains to be investigated if this also applies to oocyst removal by SSF. The results additionally showed that the schmutzdecke and accumulation of (in)organic charged compounds in the sand increased the elimination of microorganisms. Removal of the schmutzdecke reduced DEC for bacteria by +/-2 log10, but did not affect removal of phages. This clearly indicated that, besides biological activity, both straining and adsorption were important removal mechanisms in the filter bed for microorganisms larger than viruses.

Bacteriophages and Clostridium spores as indicator organisms for removal of pathogens by passage through saturated dune sand

In a field study on the efficiency of dune recharge for drinking water production, bacteriophage MS2 was shown to be removed 8 log(10) by passage through the dune sand. The question of whether pathogenic viruses would be removed as much as MS2 was studied by comparing complete breakthrough curves of MS2 with those of the human viruses Coxsackievirus B4 (CB4) and Poliovirus 1 (PV1) in laboratory columns. The columns were designed to closely simulate the field conditions: same sand, water, porewater velocity and temperature. Employing a two-site kinetic model to simulate breakthrough curves, attachment/detachment to two types of kinetic sites as well as inactivation of free and attached viruses were evaluated. It was found that attachment to only one of the sites is of significance for determining overall removal. At field scale, removal of the less negatively charged PV1 was extrapolated to be about 30 times greater than that of MS2, but removal of CB4 would be only as much as that of MS2. Also, removal of spores of Clostridium perfringens D10, a potential surrogate for Cryptosporidium oocysts, was studied. The attachment rate coefficient of the spores was 7.5 times greater than that of MS2. However, this does not imply that the removal of the spores is 7.5 times greater than that of MS2. Due to negligible inactivation in combination with detachment of previously attached spores, the actual removal rate of the spores depends on the duration of contamination and eventually all spores will break through. Provided no irreversible attachment or physical straining occurs, this may also be the case for other persistent microorganisms, like oocysts of Cryptosporidium.

Column experiments to study nonlinear removal of bacteriophages by passage through saturated dune sand

In a recent field study on dune recharge, bacteriophages MS2 and PRD1 were found to be removed 3 log10 over the first 2.4 m and only 5 log10 over the next 27 m. To understand the causes of this nonlinear removal, column experiments were carried out under conditions similar to the field: same recharge water, temperature (5 +/- 3 degrees C) and pore water velocity (1.5 m day(-1)). Soil samples were taken along a streamline between the recharge canal and the first monitoring well. Bacteriophage phiX174 was included for comparison. The high initial removal in the field was found not to be due to heterogeneity of phage suspensions but to soil heterogeneity. Phage removal rates correlated strongly positively with soil organic carbon content, and relatively strongly positively with silt content and the presence of ferric oxyhydroxides. Soil organic carbon content, silt content and the presence of ferric oxyhydroxides were found to decrease exponentially with travel distance. Removal rates of phiX174 were found to be 3-10 times higher than those of MS2 and PRD1 due to the lower electrostatic repulsion that the less negatively charged phiX174 experiences. It is suggested that the high initial removal in the field is due to the presence of favorable sites for attachment formed by ferric oxyhydroxides that decrease exponentially with travel distance. Similar removal rates may be found at both laboratory and field scale. However, due to local variations at field scale detailed knowledge on soil heterogeneity may be needed to enable a reliable prediction of removal.

Two-site kinetic modeling of bacteriophages transport through columns of saturated dune sand

Breakthrough curves, on a semi-log scale, from tests in porous media with block-input of viruses, bacteria, protozoa and colloidal particles often exhibit a typical skewness: a rather slowly rising limb and a smooth transition of a declining limb to a very long tail. One-site kinetic models fail to fit the rising and declining limbs together with the tail satisfactorily. Inclusion of an equilibrium adsorption site does not seem to improve simulation results. This was encountered in the simulation of breakthrough curves from a recent field study on the removal of bacteriophages MS2 and PRD1 by passage through dune sand. In the present study, results of laboratory experiments for the study of this issue are presented. Breakthrough curves of salt and bacteriophages MS2, PRDI, and phiX174 in 1 D column experiments have been measured. One- and two-site kinetic models have been applied to fit and predict breakthrough curves from column experiments. The two-site model fitted all breakthrough curves very satisfactorily, accounting for the skewness of the rising limb as well as for the smooth transition of the declining limb to the tail of the breakthrough curve. The one-site model does not follow the curvature of the breakthrough tail, leading to an overestimation of the inactivation rate coefficient for attached viruses. Interaction with kinetic site 1 is characterized by relatively fast attachment and slow detachment, whereas attachment to and detachment from kinetic site 2 is fast. Inactivation of viruses and interaction with kinetic site 2 provide only a minor contribution to removal. Virus removal is mainly determined by the attachment to site 1. Bacteriophage phiX174 attached more than MS2 and PRD1, which can be explained by the greater electrostatic repulsion that MS2 and PRD1 experience compared to the less negatively charged phiX174.

Kinetic modeling of virus transport at the field scale

Bacteriophage removal by soil passage in two field studies was re-analyzed with the goal to investigate differences between one- and two-dimensional modeling approaches, differences between one- and two-site kinetic sorption models, and the role of heterogeneities in the soil properties. The first study involved removal of bacteriophages MS2 and PRDI by dune recharge, while the second study represented removal of MS2 by deep well injection. In both studies, removal was higher during the first meters of soil passage than thereafter. The software packages HYDRUS-ID and HYDRUS-2D, which simulate water flow and solute transport in one- and two-dimensional variably saturated porous media, respectively, were used. The two codes were modified by incorporating reversible adsorption to two types of kinetic sites. Tracer concentrations were used first to calibrate flow and transport parameters of both models before analyzing transport of bacteriophages. The one-dimensional one-site model did not fully describe the tails of the measured breakthrough curves of MS2 and PRD1 from the dune recharge study. While the one-dimensional one-site model predicted a sudden decrease in virus concentrations immediately after the peaks, measured data displayed much smoother decline and tailing. The one-dimensional two-site model simulated the overall behavior of the breakthrough curves very well. The two-dimensional one-site model predicted a more gradual decrease in virus concentrations after the peaks than the one-dimensional one-site model, but not as good as the one-dimensional two-site model. The dimensionality of the problem hence can partly explain the smooth decrease in concentration after peak breakthrough. The two-dimensional two-site model provided the best results. Values for k(att2) and k(det2) could not be determined at the last two of four monitoring wells, thus suggesting that either a second type of kinetic sites is present in the first few meters of dune passage and not beyond the second monitoring well, or that effects of soil heterogeneity and dimensionality of the problem overshadowed this process. Variations between single collector efficiencies were relatively small, whereas collision efficiencies varied greatly. This implies that the nonlinear removal of MS2 and PRD1 is mainly caused by variations in interactions between grain and virus surfaces rather than by physical heterogeneity of the porous medium. Similarly, a two-site model performed better than the one-site model in describing MS2 concentrations for the deep well injection study. However, the concentration data were too sparse in this study to have much confidence in the fitted parameters.

Virus removal by soil passage at field scale and groundwater protection of sandy aquifers

Virus removal from groundwater by soil passage often appears to be much higher during the first few metres due to the presence of more favorable sites for attachment than thereafter. A model is presented which interprets virus removal as a function of collision efficiencies alpha(beta) and alpha(lambda), inactivation rate coefficient mu(t) and rate parameter gamma. Initial high removal is determined by ab, which decreases exponentially at a rate g to a constant base removal rate that is determined by alpha(lambda) and mu(t). A hypothetical worst case was simulated to calculate the travel distance and time required for 9 log10 protection against virus contamination of groundwater wells in anoxic sandy aquifers. Unfavorable conditions for attachment were assumed. Virus was constantly leaking from a sewage pipe lying at the groundwater table. Mixing reduced virus concentration by 3.1 to 4.0 log10. For an additional 5.0 to 5.9 l log10 protection against virus contamination by attachment and inactivation, residence times of about three to seven times longer than the current guideline of 60 days are needed, depending on abstraction rates, aquifer thickness and grain size of the sand.

Modelling the sewage discharge and dispersion of Cryptosporidium and Giardia in surface water

Modelling the discharge of parasitic protozoa into surface water and the dispersion in rivers and streams gives insight into the contribution of the different sources of environmental contamination and in the transmission of these organisms from the point of discharge to drinking water abstraction points and bathing sites. We tested the applicability of emission (PROMISE) and dispersion (WATNAT) models developed for chemical pollutants to describe the environmental behaviour of Cryptosporidium and Giardia in the Netherlands. The annual load of Cryptosporidium and Giardia in domestic wastewater was 3.2 x 10(13) and 3.8 x 10(14) respectively. The majority (85%) of the Cryptosporidium oocysts was discharged with effluent of wastewater treatment plants. while the majority (82%) of the Giardia cysts was discharged with untreated wastewater discharges and sewer overflows. The estimated annual import through the river Rhine and Meuse was 3.2 x 1014 Cryptosporidium oocysts and 2.1 x 10(15) Giardia cysts, of which the river Rhine contributed 87 and 66%, respectively. This outweighed the total load of the discharges of treated and untreated wastewater in the Netherlands. The combination of PROMISE and WATNAT predicted concentrations of Cryptosporidium and Giardia in surface water that were in the same order of magnitude as the concentrations that were observed at 5 of the 6 sites compared. At a site with primarily agricultural contamination, the models predicted concentrations that were 1 10log-unit lower than the observed concentrations. This is a first step in the direction of a quantitative description of the transmission cycle of Cryptosporidum and Giardia through water. The use of these models combines observational occurrence data and experimental data from laboratory survival studies into a single integrated description. The description needs further improvement by incorporation of agricultural run-off and increasing the number and time frame of input monitoring data.

Risk of otitis externa after swimming in recreational fresh water lakes containing Pseudomonas aeruginosa

OBJECTIVE

To determine whether an outbreak of otitis externa was due to bathing in recreational fresh water lakes and to establish whether the outbreak was caused by Pseudomonas aeruginosa in the water.

DESIGN

Matched case-control study.

SETTING

The Achterhoek area, the Netherlands.

SUBJECTS

98 cases with otitis externa and 149 controls matched for age, sex, and place of residence.

MAIN OUTCOME MEASURES

Odds ratios for type of swimming water and frequency of swimming; presence of P aeruginosa in ear swabs and fresh water lakes.

RESULTS

Otitis externa was strongly associated with swimming in recreational fresh water lakes in the previous two weeks (odds ratio 15.5 (95% confidence interval) 4.9 to 49.2) compared with non-swimming). The risk increased with the number of days of swimming, and subjects with recurrent ear disease had a greatly increased risk. The lakes met the Dutch bathing water standards and those set by the European Commission for faecal pollution in the summer of 1994, but P aeruginosa was isolated from all of them, as well as from the ear swabs of 78 (83%) of the cases and 3 (4%) of the controls.

CONCLUSIONS

Even when current bathing water standards are met, swimming can be associated with a substantial risk of otitis externa because of exposure to P aeruginosa. People with recurrent ear disease should take special care when swimming in waters containing P aeruginosa.

A Simple and Widely Applicable Method for Preparing Homogeneous and Stable Quality Control Samples in Water Microbiology

Test strains suspended in skim milk, quickly frozen in dry ice-ethanol, and stored at - 70°C can be used as quality control samples that are immediately available by quickly thawing at 37°C. The samples remain homogeneous and stable for at least 1 year, except for Aeromonas hydrophila , which decreases 20 to 30% in 1 year.