The influence of application rate on the bacterial mutagenicity of soil amended with municipal sewage sludge

Evaluation of the potential agricultural use of biostimulated sewage sludge using mammalian cell culture assays

Among the bioremediation processes, biostimulation is an effective methodology for the decontamination of organic waste by the addition of agents that stimulate the indigenous microbiota development. Rice hull is a biostimulating agent that promotes the aeration of edaphic systems and stimulates the aerobiotic activity of soil microorganisms. The present study aimed to evaluate the efficacy of the bioremediation and biostimulation processes in reducing the toxicity of sewage sludge (SS) and to evaluate its possible application in agriculture using cytotoxic and genotoxic assays in human hepatoma cells (HepG2). SS of domestic origin was tested as both the pure product (PSS) and mixed with soil (S) and with a stimulating agent, such as rice hull (RH), in different proportions (SS + S and SS + S + RH); we also examined different remediation periods (3 months - T1 and 6 months - T2). For the PSS sample, a significant induction of micronucleus (MN) in T2 was observed with nuclear buds in all of the periods assessed, and we observed the presence of more than one alteration per cell (MN and nuclear bud) in T1 and T2. The PSS sample caused genotoxic effects in the HepG2 cells even after being bioremediated. For the samples containing soil and/or rice hull, no toxic effects were observed in the test system used. Therefore, the addition of SS to agricultural soils should be conducted with caution, and it is important that the SS undergoes a remediation process, such as bioremediation and biostimulation treatments.

Attesting the efficiency of monitored natural attenuation in the detoxification of sewage sludge by means of genotoxic and mutagenic bioassays

A viable alternative to the use of sewage sludge (SS) would be using it as a reconditioner of agricultural soils, due to its high content of organic matter and nutrients. However, this solution may contaminate the soil, since SS may contain toxic substances. Monitored natural attenuation is a process that can be used in the decontamination of SS before its disposal into the environment. The effectiveness of the natural attenuation of a domestic SS was evaluated over 12 months by assays of Salmonella/microsome and micronucleus (MN) in human hepatoma cells (HepG2). Mutagenic activity was observed for the Salmonella strain TA 100, with S9, for the extracts from periods 0-6 months of natural attenuation. Genotoxic effects were observed in HepG2 cells, for 0 and 2 months, in almost all tested concentrations. Comparing obtained data by MN test to chemical analyses, it is possible to observe a coincidence between the induction of MN and the quantity of the m- and p-cresol, since these compounds were present in the initial SS and after 2 months of natural attenuation, decreasing their concentrations in samples from 6 to 12 months. The positive results obtained with Salmonella/microsome (from 6 months) suggest a combined action of other substances in SS. These results indicated that this SS, in the earlier periods tested, is potentially genotoxic and mutagenic and that its disposal can lead to severe environmental problems. Thus, the use of the studied SS as reconditioner requires pre-processing for over than 6 months of natural attenuation.

Mutagenicity of an aged gasworks soil during bioslurry treatment

This study investigated changes in the mutagenic activity of organic fractions from soil contaminated with polycyclic aromatic hydrocarbons (PAHs) during pilot-scale bioslurry remediation. Slurry samples were previously analyzed for changes in PAH and polycyclic aromatic compound content, and this study examined the correspondence between the chemical and toxicological metrics. Nonpolar neutral and semipolar aromatic fractions of samples obtained on days 0, 3, 7, 24, and 29 of treatment were assayed for mutagenicity using the Salmonella mutation assay. Most samples elicited a significant positive response on Salmonella strains TA98, YG1041, and YG1042 with and without S9 metabolic activation; however, TA100 failed to detect mutagenicity in any sample. Changes in the mutagenic activity of the fractions across treatment time and metabolic activation conditions suggests a pattern of formation and transformation of mutagenic compounds that may include a wide range of PAH derivatives such as aromatic amines, oxygenated PAHs, and S-heterocyclic compounds. The prior chemical analyses documented the formation of oxygenated PAHs during the treatment (e.g., 4-oxapyrene-5-one), and the mutagenicity analyses showed high corresponding activity in the semipolar fraction with and without metabolic activation. However, it could not be verified that these specific compounds were the underlying cause of the observed changes in mutagenic activity. The results highlight the need for concurrent chemical and toxicological profiling of contaminated sites undergoing remediation to ensure elimination of priority contaminants as well as a reduction in toxicological hazard. Moreover, the results imply that remediation efficacy and utility be evaluated using both chemical and toxicological metrics.

Mutagens in contaminated soil: a review

The intentional and accidental discharges of toxic pollutants into the lithosphere results in soil contamination. In some cases (e.g., wood preserving wastes, coal-tar, airborne combustion by-products), the contaminated soil constitutes a genotoxic hazard. This work is a comprehensive review of published information on soil mutagenicity. In total, 1312 assessments of genotoxic activity from 118 works were examined. The majority of the assessments (37.6%) employed the Salmonella mutagenicity test with strains TA98 and/or TA100. An additional 37.6% of the assessments employed a variety of plant species (e.g., Tradescantia clone 4430, Vicia faba, Zea mays, Allium cepa) to assess mutagenic activity. The compiled data on Salmonella mutagenicity indicates significant differences (p<0.0001) in mean potency (revertents per gram dry weight) between industrial, urban, and rural/agricultural sites. Additional analyses showed significant empirical relationships between S9-activated TA98 mutagenicity and soil polycyclic aromatic hydrocarbon (PAH) concentration (r2=0.19 to 0.25, p<0.0001), and between direct-acting TA98 mutagenicity and soil dinitropyrene (DNP) concentration (r2=0.87, p<0.0001). The plant assay data revealed excellent response ranges and significant differences between heavily contaminated, industrial, rural/agricultural, and reference sites, for the anaphase aberration in Allium cepa (direct soil contact) and the waxy locus mutation assay in Zea mays (direct soil contact). The Tradescantia assays appeared to be less responsive, particularly for exposures to aqueous soil leachates. Additional data analyses showed empirical relationships between anaphase aberrations in Allium, or mutations in Arabidopsis, and the 137Cs contamination of soils. Induction of micronuclei in Tradescantia is significantly related to the soil concentration of several metals (e.g., Sb, Cu, Cr, As, Pb, Cd, Ni, Zn). Review of published remediation exercises showed effective removal of genotoxic petrochemical wastes within one year. Remediation of more refractory genotoxic material (e.g., explosives, creosote) frequently showed increases in mutagenic hazard that remained for extended periods. Despite substantial contamination and mutagenic hazards, the risk of adverse effect (e.g., mutation, cancer) in humans or terrestrial biota is difficult to quantify.

Evaluation of the genotoxic and teratogenic potential of a municipal sludge and sludge-amended soil using the amphibian Xenopus laevis and the tobacco: Nicotiana tabacum L. var. xanthi Dulieu

The toxic, genotoxic and teratogenic potential of a municipal sewage sludge was assessed using the micronucleus assay on the larvae of the amphibian Xenopus laevis and with the tobacco somatic mutation test using the yellow-green xanthi Dulieu mutant a(1)(+)/a(1) a(2)(+)/a(2). The teratogenic potential was assessed by means of the Frog Embryo Teratogenesis Assay-Xenopus (FETAX). Various doses of the pasty sludge added to a crop soil were tested using the three bioassays. The test systems were performed either directly with sludge or sludge-amended soil samples (plant model) or with aqueous extracts (aquatic animal model). Using the tobacco model, we found no mutagenic impact of the soil amended with the sludge, perhaps because the clay-like nature of the soil, with its high adsorption capacity, may have prevented the contaminants from reaching the target. All leachates of amended soils produced a significant size reduction in Xenopus embryos. Depending on the soil/sludge ratio, some leachates were found to be genotoxic but were never teratogenic. This battery of in vivo test systems enabled us to estimate the global long-term effects under agricultural conditions with various genetic endpoints on ecologically relevant organisms characteristic of the aquatic and terrestrial compartments.

Screening the environmental fate of organic contaminants in sewage sludges applied to agricultural soils: 1. The potential for downward movement to groundwaters

The potential for organic contaminants present in sewage sludge to leach and cause groundwater contamination following sludge application to agricultural land has been assessed. Models used to predict compound mobility in soil on the basis of physico-chemical parameters were applied to a range of contaminants prioritised and/or detected in sludge and a provisional list of potential "leachers' compiled. In addition, theoretical soil water concentrations following sludge application were calculated using mean reported sludge contaminant concentrations and soil/water partition coefficients. These estimated aqueous phase concentrations were compared with Dutch groundwater quality standards in the absence of appropriate UK standards to identify those compounds which could be present in groundwater at levels of concern. The two prioritised lists were used to identify compounds in sludge which could pose a possible threat to groundwater. Appropriate experimental data were not available to qualify model results. However, the screening exercise indicated that under routine operation practice with typical sludge application rates, and the usual range of compound concentrations detected in sludge, groundwater quality standards were unlikely to be exceeded. However, data variability, reliability and scarcity limited the usefulness of this screening approach.