Geostatistical assessment of groundwater arsenic contamination in the Padana Plain

Arsenic (As) in groundwater from natural and anthropogenic sources is one of the most common pollutants worldwide affecting people and ecosystems. A large dataset from >3600 wells is employed to spatially simulate the depth-averaged As concentration in phreatic and confined aquifers of the Padana Plain (Northern Italy). Results of in-depth geostatistical analysis via PCA and simulations within a Monte Carlo framework allow the understanding of the variability of As concentrations within the aquifers. The most probable As contaminated zones are located along the piedmont areas in the confined aquifers and in the lowland territories in the phreatic aquifers. The distribution of the As contaminated zones has been coupled with hydrogeological, geological, and geochemical information to unravel the sources and mechanisms of As release in groundwater. The reductive dissolution of Fe oxyhydroxides and organic matter mineralization under anoxic conditions resulted to be the major drivers of As release in groundwater. This phenomenon is less evident in phreatic aquifers, due to mixed oxic and reducing conditions. This large-scale study provides a probabilistic perspective on As contamination, e.g. quantifying the spatial probability of exceeding national regulatory limits, and to outline As major sources and drivers.

Salinity origin in the coastal aquifer of the Southern Venice lowland

Groundwater salinization can be natural and anthropogenic in origin, although it often results from a combination of both, especially in low-lying coastal regions that are hydraulically controlled. This study proposes a method to assess the origin of salinity using environmental tracers in porewater, like Cl- and Br-, combined with depositional facies associations detected in sediment cores. Such integrated approach was tested in a target area south of the Venice Lagoon (Italy), where groundwater salinization is triggered by multiple mechanisms due to the complexity of the hydro-geomorphological environment. Batch tests were performed on sediment core samples from boreholes to quantify major anions and total inorganic N. Cl- and Br- porewater concentrations coupled with sedimentary facies association provided insights into the origin of groundwater salinity from a variety of sources, including past and present seawater intrusion, agricultural leaching, and evaporites. The strengths and limitations of the integrated approach are discussed to provide a pathway for improving water resource management and planning measures to prevent groundwater salinization in coastal areas.

Modelling biogeochemical reactions triggered by graphene's addition in a fertilized calcareous sandy soil

Graphene production has dramatically increased in the last years and new ways to recycle this engineered material need to be investigated. To this purpose, a reactive model network was developed using PHREEQC-3 code to quantify the relevant biogeochemical reactions induced by graphene scraps' incorporation in a calcareous sandy soil. The numerical model was calibrated versus a complete dataset of column experiments in water saturated conditions using two different fertilizers, a synthetic NPK fertilizer and fertigation water produced in a wastewater treatment plant. Column experiments consisted of 50 cm columns filled with a mixture of graphene scraps (0.015 % dry weight) and soil in the first 10 cm, while the remaining 40 cm had only soil. The model performance was tested using classical statistical indices (R2, Modelling Efficiency, and Index of Agreement), resulting to be satisfactory. Besides, a simple sensitivity analysis via the perturbation of relevant parameters showed a low degree of uncertainty. The main outcome of this study was the quantification of the increased denitrification rate triggered by graphene incorporation into the soil. Moreover, graphene incorporation substantially increased soil CEC and DOC sorption capacity, demonstrating a good adsorption capacity for ammonium and organic compounds, thus decreasing nutrients leaching that represents a major concern related to agricultural practice. Indeed, Graphene incorporation increased by 40 % the CEC in the first 10 cm of the CSG_NPK column (2.50e-02 mol/L) respect to the CS_NPK column (1.75e-02 mol/L) and increased it by 150 % in the first 10 cm of the CSG_FW column (2.50e-02 mol/L) in comparison with the CS_FW column 1.00e-02 (mol/L). pH fluctuations were most likely due to the precipitation of Ca5(PO4)3OH, indeed the consumption of H+ ions could have triggered the pH lowering during the experiment. These results could be relevant for future graphene applications as a soil improver or as suitable material to enhance soil bioremediation in order to include graphene in a circular economy loop.

Modeling stochastic saline groundwater occurrence in coastal aquifers

The issue of freshwater salinization in coastal areas has grown in importance with the increase of the demand of groundwater supply and the more frequent droughts. However, the spatial patterns of salinity contamination are not easy to be understood, as well as their numerical modeling is subject to various kinds of uncertainty. This paper offers a robust, flexible, and reliable geostatistical methodology to provide a stochastic assessment of salinity distribution in alluvial coastal areas. The methodology is applied to a coastal aquifer in Campania (Italy), where 83 monitoring wells provided depth-averaged salinity data. A Monte Carlo (MC) framework was implemented to simulate depth-averaged groundwater salinity fields. Both MC stochastic fields and the mean across MC simulations enabled to the delineation of which areas are subject to high salinity. Then, a probabilistic approach was developed setting up salinity thresholds for agricultural use to delineate the areas with unsuitable groundwater for irrigation purposes. Furthermore, steady spatial patterns of saline wedge lengths were unveiled through uncertainty estimates of seawater ingression at the Volturno River mouth. The results were compared versus a calibrated numerical model with remarkable model fit (R²=0.96) and versus an analytical solution, obtaining similar wedge lengths. The results pointed out that the high groundwater salinities found inland (more than 2 km from the coastline) could be ascribed to trapped paleo-seawater rather than to actual seawater intrusion. In fact, the inland high salinities were in correspondence of thick peaty layers, which can store trapped saline waters because of their high porosity and low permeability. Furthermore, these results are consistent with the recognition of depositional environments and the position of ancient lagoon alluvial sediments, located in the same areas where are the highest (simulated) salinity fields. This robust probabilistic approach could be applied to similar alluvial coastal areas to understand spatial patterns of present salinization, to disentangle actual from paleo-seawater intrusion, and more in general to delineate zones with unsuitable salinity for irrigation purposes.

Performance of graphene and traditional soil improvers in limiting nutrients and heavy metals leaching from a sandy Calcisol

Given the large amount of Graphene produced in the last years, there is the need to introduce this new material into a green and circular economy loop. In this study, for the first time, the fate of nutrients and heavy metals in a sandy Calcisol amended with Graphene was monitored and compared to other traditional improvers such as Compost, Zeolites, and Biochar. This was performed via saturated and unsaturated columns' experiments with two different fertilization regimes: one with NPK fertilizer and one with an innovative fertigation water (FW) produced from a pilot wastewater treatment plant. The breakthrough curves of each nutrient and heavy metal were analysed to understand the main processes occurring in saturated and unsaturated conditions, comparing the columns amended with the improvers versus the unamended Controls. Mass balances for each nutrient and heavy metal were developed to infer whether the different soil improvers were effective in minimizing leaching. Graphene, for most cases, behaved as the Control in nutrients' leaching for all the saturated and unsaturated experiments, both with NPK and FW. Biochar increased EC, K⁺, and pH of the leaching water, which could be an issue for the growth of some plants. Compost increased NO₃^- leaching in all the experiments. Zeolites showed the best N compounds retention, but great PO₄^3- leaching in saturated conditions. Heavy metals leachates were analysed only for unsaturated columns (as more representative of field conditions) and found at concentrations well below the limits suggested by the U.S. Environmental Protection Agency. Overall, Graphene performed well in minimizing nutrients and heavy metals leaching, respect to classical improvers. This study is a starting point for field studies that will be critical to have a clear understanding of how Graphene behaves in the environment.

Assessment of intrinsic aquifer vulnerability at continental scale through a critical application of the drastic framework: The case of South America

An assessment of the intrinsic aquifer vulnerability of South America is presented. The outcomes represent the potential sensitivity of natural aquifers to leaching of dissolved compounds from the land surface. The study, developed at continental scale but retaining regionally a high resolution, is based on a critical application of the DRASTIC method. The biggest challenge in performing such a study in South America was the scattered and irregular nature of environmental datasets. Accordingly, the most updated information on soil, land use, geology, hydrogeology, and climate at continental, national, and regional scale were selected from international and local databases. To avoid spatial discrepancy and inconsistency, data were integrated, harmonized, and accurately cross-checked, using local professional knowledge where information was missing. The method was applied in a GIS environment to allow spatial analysis of raw data along with the overlaying and rating of maps. The application of the DRASTIC method allows to classify South America into five vulnerability classes, from very low to very high, and shows an overall medium to low vulnerability at continental scale. The Amazon region, coastal aquifers, colluvial Andean valleys, and alluvial aquifers of main rivers were the areas classified as highly vulnerable. Moreover, countries with the largest areas with high aquifer vulnerability were those characterized by extended regions of rainforest. In addition, a single parameter sensitivity analysis showed depth to water table to be the most significant factor, while a cross-validation using existing vulnerability assessments and observed concentrations of compounds in groundwater confirmed the reliability of the proposed assessment, even at regional scale. Overall, although additional field surveys and detailed works at local level are needed to develop effective water management plans, the present DRASTIC map represents an essential common ground towards a more sustainable land-use and water management in the whole territory of South America.

In situ arsenic immobilisation for coastal aquifers using stimulated iron cycling: Lab-based viability assessment

Arsenic (As) is one of the most harmful and widespread groundwater contaminants globally. Besides the occurrence of geogenic As pollution, there is also a large number of sites that have been polluted by anthropogenic activities, with many of those requiring active remediation to reduce their environmental impact. Cost-effective remedial strategies are however still sorely needed. At the laboratory-scale in situ formation of magnetite through the joint addition of nitrate and Fe(II) has shown to be a promising new technique. However, its applicability under a wider range of environmental conditions still needs to be assessed. Here we use sediment and groundwater from a severely polluted coastal aquifer and explore the efficiency of nitrate-Fe(II) treatments in mitigating dissolved As concentrations. In selected experiments >99% of dissolved As was removed, compared to unamended controls, and maintained upon addition of lactate, a labile organic carbon source. Pre- and post experimental characterisation of iron (Fe) mineral phases suggested a >90% loss of amorphous Fe oxides in favour of increased crystalline, recalcitrant oxide and sulfide phases. Magnetite formation did not occur via the nitrate-dependent oxidation of the amended Fe(II) as originally expected. Instead, magnetite is thought to have formed by the Fe(II)-catalysed transformation of pre-existing amorphous and crystalline Fe oxides. The extent of amorphous and crystalline Fe oxide transformation was then limited by the exhaustion of dissolved Fe(II). Elevated phosphate concentrations lowered the treatment efficacy indicating joint removal of phosphate is necessary for maximum impact. The remedial efficiency was not impacted by varying salinities, thus rendering the tested approach a viable remediation method for coastal aquifers.

Predictive modeling of selected trace elements in groundwater using hybrid algorithms of iterative classifier optimizer

Trace element (TE) pollution in groundwater resources is one of the major concerns in both developing and developed countries as it can directly affect human health. Arsenic (As), Barium (Ba), and Rubidium (Rb) can be considered as TEs naturally present in groundwater due to water-rock interactions in Campania Plain (CP) aquifers, in South Italy. Their concentration could be predicted via some readily available input variables using an algorithm like the iterative classifier optimizer (ICO) for regression, and novel hybrid algorithms with additive regression (AR-ICO), attribute selected classifier (ASC-ICO) and bagging (BA-ICO). In this regard, 244 groundwater samples were collected from water wells within the CP and analyzed with respect to the electrical conductivity, pH, major ions and selected TEs. To develop the models, the available dataset was divided randomly into two subsets for model training (70% of the dataset) and evaluation (30% of the dataset), respectively. Based on the correlation coefficient (r), different input variables combinations were constructed to find the most effective one. Each model's performance was evaluated using common statistical and visual metrics. Results indicated that the prediction of As and Ba concentrations strongly depends on HCO₃^-, while Na⁺ is the most effective variable on Rb prediction. Also, the findings showed that the most powerful predictive models were those that used all the available input variables. According to models' performance evaluation metrics, the hybrid ASC-ICO outperformed other hybrid (BA- and AR-ICO) and standalone (ICO) algorithms to predict As and Ba concentrations, while both hybrid ASC- and BA-ICO models had higher accuracy and lower error than other algorithms for Rb prediction.

Soil conditioners effects on hydraulic properties, leaching processes and denitrification on a silty-clay soil

Agricultural landscapes are often affected by groundwater quality issues due to fertilizers leaching. To address this worldwide problem several agricultural best practices have been proposed, like limiting the amount of fertilizers and increasing soil organic matter content. To evaluate if these practices may promote groundwater quality enhancement, vadose zone retention time and complex biogeochemical processes must be known in detail. In this study, sequential undisturbed column experiments were performed to determine the amount of nutrients and heavy metals leached after simulated stormwater events. The column was amended with urea then flushed for two pore volumes, then straw residuals were incorporated and flushed for two pore volumes and finally compost was incorporated and flushed for six pore volumes. Dissolved ions, major gasses and heavy metals were determined in leachate samples. Nitrate and nitrite were leached in the urea treatment producing the highest concentrations, followed by compost and straw residuals. The redox conditions were aerobic in all treatments and pH was circumneutral or slightly basic. Denitrification was low but increased with the addition of straw residuals and compost. Heavy metals were all at very low concentrations except for lead and cadmium, which slightly exceeded threshold limits (10 and 1 μg/L, respectively) in all the treatments. The compost treatment, after three pore volumes, was affected by clay swelling due to sodium dispersion, which in turn provoked a reduction of porosity and hydraulic conductivity.

Evaluating SWAT model performance, considering different soils data input, to quantify actual and future runoff susceptibility in a highly urbanized basin

The Soil and Water Assessment Tool (SWAT) is a physical model designed to predict the hydrological processes that could characterize natural and anthropized watersheds. The model can be forced using input data of climate prediction models, soil characteristics and land use scenarios to forecast their effect on hydrological processes. In this study, the SWAT model has been applied in the Aspio basin, a small watershed, highly anthropized and characterized by a short runoff generation. Three simulations setup, named SL1, SL2 and SL3, were investigated using different soil resolution to identify the best model performance. An increase of space requirement and calibration time has been registered in conjunction with the increasing soil resolution. Among all simulations, SL1 has been chosen as the best one in describing watershed streamflow, despite it was characterized by the lower soil resolution. A map of susceptibility to runoff for the entire basin was so created reclassifying the runoff amount of four years in five classes of susceptibility, from very low to very high. Eleven sub-basins, coinciding with the main urban settlements, were identified as highly susceptible to runoff generation. Considering future climate predictions, a slight increase of runoff has been forecasted during summer and autumn. The map of susceptibility successfully identified as highly prone to runoff those sub-basins where extreme flood events were yet recorded in the past, remarking the reliability of the proposed assessment and suggesting that this methodology could represent a useful tool in flood managing plan.

A novel hybrid method of specific vulnerability to anthropogenic pollution using multivariate statistical and regression analyses

Groundwater resources are the main supply of freshwater for human activities. However, in the last fifty years aquifers have become more susceptible to chemical pollution due to human activities. The concept of groundwater vulnerability constitutes a worldwide accepted tool for water protection and planning. However, the existing methods and modified versions do not account for all the hydrogeochemical processes that drive anthropogenic pollution. The hydrogeochemical processes occurring within an aquifer can be determined using multivariate statistical analysis. In this study a specific vulnerability method named SVAP (Specific Vulnerability to Anthropogenic Pollution) is proposed. The index is based on seven quantitative parameters: depth to groundwater, recharge, nitrate losses, hydraulic resistance of the vadose zone, aquifer thickness, hydraulic conductivity of the aquifer, and slope. Weights of anthropogenic factors were determined by factor analysis and used to validate the SVAP methodology. The parameters' classification was selected according to the highest Pearson's correlation coefficient with factor weights and then grouped via a linear combination. The new index was applied in two watersheds: the Florina basin (Greece) and the Garigliano River basin (Italy), both of which possess complex hydrogeochemical regimes. The main hydrogeochemical processes acting in the study areas were identified via factor analysis, which revealed that the anthropogenic pollution in both sites was due mainly to chemical fertilizers and manure. Verification of the SVAP method produced correlation coefficients with nitrate concentrations of 0.75 and 0.62 in Florina and Garigliano, respectively. The proposed SVAP method is more reliable and flexible than standard vulnerability assessment methods and can be easily adapted for complex aquifers.

Effect of ebullition and groundwater temperature on estimated dinitrogen excess in contrasting agricultural environments

Denitrification is a key microbial-mediated reaction buffering the impact of agriculturally-derived nitrate loads. Groundwater denitrification capacity is often assessed by measuring the magnitude and patterns of dinitrogen excess, although this method can be biased by dissolved gasses exsolution and ebullition. To address this issue, shallow groundwater was sampled in two field sites via nested mini-wells on a monthly basis over an entire hydrological year and analysed for dissolved gasses, nitrate and physical parameters. Both sites are located in lowland areas of the Po River basin (Italy) and are characterized by intensive agriculture. The GUA site, a freshwater paleo-river environment, with a low content of organic matter (SOM) and oxic sub-oxic groundwater. The BAN site, a reclaimed brackish swamp environment, with abundant SOM and sulphidic-methanogenic groundwater. Groundwater samples evidenced a general deficit of dinitrogen and Argon concentrations, because of ebullition induced by a total dissolved gasses pressure exceeding the hydrostatic pressure. Ebullition was recorded only during winter at the reclaimed brackish soil and was triggered by methane exsolution. While in summer both sites were affected by ebullition because of the water table drawdown. Denitrification evaluated using dinitrogen excess via dinitrogen-Argon ratio technique, was not only affected by gas exsolution, but also by groundwater temperature fluctuations. In fact, the latter induced large biases in the calculated N₂ excess even in the freshwater paleo-river environment. For these reasons, dinitrogen excess estimate with standard methods resulted to be unreliable in both lowland environments and a modified method is here presented to overcome this issue.

Contrasting biogeochemical processes revealed by stable isotopes of H2O, N, C and S in shallow aquifers underlying agricultural lowlands

Lowland coastal areas as the Po Delta (Italy) are often intensively cultivated and affected by nitrogen imbalance due to fertilizers leaching to groundwater and export via run-off. To address this issue several agricultural best practices have been proposed, like limiting the amount of fertilizers and increasing soil organic matter content. In this study, groundwater samples were analysed for major ions and stable isotopes of H₂O, C, N and S using multi-level sampler (MLS) from two contrasting depositional environments, one representative of alluvial plain (AP) and the other representative of a reclaimed coastal plain (RCP). In each site, controlled plots with different agriculture practice including fertilizers and tillage and compost amendment and no tillage were considered in the study. Tracer test results highlight that recharge water infiltrated at the start of the controlled study has not yet reached the saturated zone, thus current groundwater concentrations are representative of former agricultural practices. Stable isotopes show a clear distinction between different sources of nitrogen in both sites, from synthetic fertilizers to sedimentary nitrogen pool and atmospheric input. The main source of sulphate in groundwater is pyrite and fertilizers. Denitrification, sulphate reduction and methanogenesis were involved in the C, N and S cycle in the RCP site characterized by low hydraulic conductivity sediments and high SOM. These processes were not relevant in the AP site characterized by oxic condition and low SOM, but some evidence of denitrification was found in one of the AP sites. High resolution monitoring was a key tool to identify the different redox zones responsible for N, C and S cycling in these aquifers. This study shows that a clear understanding of transit times in the vadose zone is a key prerequisite to evaluate the effect of controlled agriculture practice on the quality of shallow groundwater.

Intense rainfalls trigger nitrite leaching in agricultural soils depleted in organic matter

Nitrate and ammonium are common inorganic contaminants of anthropogenic origin in many shallow aquifers around the world, while nitrite is less common, but it is most harmful than nitrate and ammonium due to its high reactivity. This paper presents evidence of nitrite accumulation after intense rainfalls in soil samples collected in an agricultural field characterized by organic matter chronic depletion. Moreover, an intact core from the same site was also collected to perform an unsaturated column experiment (60 cm long and 20 cm outer diameter) mimicking heavy rainfalls (230 mm in 2 days). Results from the field site showed nitrite accumulation (up to 0.45 mmol/kg) at 50-70 cm depth, just below the plough layer. The column experiment showed very high initial concentrations of nitrate and nitrite in the leachate and a progressive decrease of nitrate due to denitrification. The numerical flow model was calibrated versus the observed volumetric water contents and leachate flow rates. The numerical reactive transport model was calibrated versus the leachate concentrations of six dissolved species (ammonium, nitrate, nitrite, dissolved organic carbon, chloride and bromide). The optimized model resulted to be robustly calibrated providing insights on the kinetic rates driving the production, accumulation and leakage of nitrite, showing that incomplete denitrification is the source of nitrite. As far as the authors are aware, this is the first study reporting a clear link between high nitrite leaching rates and extreme rainfall events in lowland agricultural soils depleted in organic matter. The proposed methodology could be applied to quantify nitrite cycling processes in many other agricultural areas of the world affected by extreme rainfall events.

GALDIT-SUSI a modified method to account for surface water bodies in the assessment of aquifer vulnerability to seawater intrusion

The salinization of coastal aquifers is one of the major environmental issue worldwide. Overexploitation is the most common reason of salinization, since it generates a piezometric inversion, which in turn leads to groundwater flow from the coast towards inland. This also occurs in water bodies connected to the sea like lagoons, rivers, torrents and wetlands. In this study, a modification of the GALDIT method including "SUperficial Seawater Intrusion (SUSI)" is proposed. Six new parameters were added to the classical ones. The analytical hierarchy process and the sensitivity analysis were performed for weights definition and validation of the proposed GALDIT-SUSI method. Two study areas, with different characteristics were chosen for the application of both methods: the coastal area of Epanomi (Greece) and the Po River lowland (Italy). The application of the standard GALDIT in both sites showed a poor discrimination of the vulnerability to seawater intrusion, confining it only in proximity to the coastline. Conversely, GALDIT-SUSI divided the two sites in five classes of vulnerability ranging from very low to very high, stressing the higher vulnerability of lagoons and wetland for Epanomi and lagoons and rivers for the Po River lowland. GALDIT-SUSI is easy to apply and versatile, since it can be adapted to the specific hydrogeological setting of the area of interest. Moreover, GALDIT-SUSI can be further improved to deal with other salinization mechanisms.

Direct measurement of dissolved dinitrogen to refine reactive modelling of denitrification in agricultural soils

Nitrogen fertilizers used in agriculture often cause nitrate leaching towards shallow groundwater, especially in lowland areas where soil permeability, ploughing, clay content, and the flat topography minimizes surface runoff. The introduction of good agricultural practices to reduce the nitrate amount entering the groundwater system is crucial to ameliorate the kinetic control on nitrate denitrification capacity. With this aim, a series of anaerobic mesocosms, consisting of loamy and clay soils and nitrate rich water, were modelled using acetate and natural organic matter as electron donors. Acetate was chosen because it is the main intermediate in many biodegradation pathways of organic compounds, and hence it is a suitable carbon source for denitrification. To account for the spatial variability of soil parameters, the experiments were performed in triplicates. The geochemical code PHREEQC(3) was used to simulate kinetic denitrification, and equilibrium reactions of gas and mineral phases. The reactive modelling results highlighted a rapid acetate and nitrate degradation rate, a rapid production of dissolved inorganic carbon and dinitrogen, and a steady concentration of dissolved iron and sulphate, suggesting that the main pathway of nitrate attenuation is through denitrification; concomitantly excluding the occurrence of other processes leading to nitrate consumption. In the absence of acetate, the loamy soil, poor of natural organic matter, did not allow to complete the denitrification process. This modelling study investigates in detail the relationship between the denitrification process in natural soils, with excess and in limitation of organic substrates, and the occurrence and fate of dissolved dinitrogen analysed with a high precision membrane inlet mass spectrometer. Results demonstrate that modelling nitrate degradation processes as a whole, using geochemical datasets and codes, will improve the estimates of agricultural landscapes denitrification and support better nitrogen management, especially in lowland environments.

Multivariate statistical analysis to characterize/discriminate between anthropogenic and geogenic trace elements occurrence in the Campania Plain, Southern Italy

Shallow aquifers are the most accessible reservoirs of potable groundwater; nevertheless, they are also prone to various sources of pollution and it is usually difficult to distinguish between human and natural sources at the watershed scale. The area chosen for this study (the Campania Plain) is characterized by high spatial heterogeneities both in geochemical features and in hydraulic properties. Groundwater mineralization is driven by many processes such as, geothermal activity, weathering of volcanic products and intense human activities. In such a landscape, multivariate statistical analysis has been used to differentiate among the main hydrochemical processes occurring in the area, using three different approaches of factor analysis: (i) major elements, (ii) trace elements, (iii) both major and trace elements. The elaboration of the factor analysis approaches has revealed seven distinct hydrogeochemical processes: i) Salinization (Cl^-, Na⁺); ii) Carbonate rocks dissolution; iii) Anthropogenic inputs (NO₃^-, SO₄^2-, U, V); iv) Reducing conditions (Fe²⁺, Mn²⁺); v) Heavy metals contamination (Cr and Ni); vi) Geothermal fluids influence (Li⁺); and vii) Volcanic products contribution (As, Rb). Results from this study highlight the need to separately apply factor analysis when a large data set of trace elements is available. In fact, the impact of geothermal fluids in the shallow aquifer was identified from the application of the factor analysis using only trace elements. This study also reveals that the factor analysis of major and trace elements can differentiate between anthropogenic and geogenic sources of pollution in intensively exploited aquifers.

A modified SINTACS method for groundwater vulnerability and pollution risk assessment in highly anthropized regions based on NO3- and SO42- concentrations

Groundwater vulnerability and risk assessment are worldwide tools in supporting groundwater protection and land planning. In this study, we used three of these different methodologies applied to the Campanian Plain located in southern Italy: SINTACS, AVI and LOS. However, their capability to describe the observed chemical pollution of the area has resulted quite poor. For such a reason, a modified SINTACS method has been then implemented in the area in order to get a more reliable view of groundwater vulnerability. NO₃^- and SO₄^2- from more than 400 monitoring wells were used for specific vulnerability assessment. Land use was chosen as key parameter to infer the risk of groundwater pollution in our area. The new methodology seems to show a higher correlation with observed NO₃^- concentrations and a more reliable identification of aquifer's pollution hot spots. The main sources of NO₃^- were found in sub-urban areas, where vulnerability and risk are higher than in other areas. Otherwise due to reducing conditions triggered by the presence of elevated sedimentary organic matter and peat, concentrations below agricultural areas were lower than in sub-urban areas. The SO₄^2- specific vulnerability map showed a positive correlation with observed concentrations, due to geogenic and anthropogenic SO₄^2- sources present in the area. The combination of both NO₃^- and SO₄^2- derived risk maps becomes essential to improve the conceptual model of aquifer pollution in this severely anthropized area. The application of this new and original approach shed light on the strengths and weaknesses of each of the described previous methods and clearly showed how anthropogenic activities have to be taken into account in the assessment process.

Chlorate origin and fate in shallow groundwater below agricultural landscapes

In agricultural lowland landscapes, intensive agricultural is accompanied by a wide use of agrochemical application, like pesticides and fertilizers. The latter often causes serious environmental threats such as N compounds leaching and surface water eutrophication; additionally, since perchlorate can be present as impurities in many fertilizers, the potential presence of perchlorates and their by-products like chlorates and chlorites in shallow groundwater could be a reason of concern. In this light, the present manuscript reports the first temporal and spatial variation of chlorates, chlorites and major anions concentrations in the shallow unconfined aquifer belonging to Ferrara province (in the Po River plain). The study was made in 56 different locations to obtain insight on groundwater chemical composition and its sediment matrix interactions. During the monitoring period from 2010 to 2011, in June 2011 a nonpoint pollution of chlorates was found in the shallow unconfined aquifer belonging to Ferrara province. Detected chlorates concentrations ranged between 0.01 and 38 mg/l with an average value of 2.9 mg/l. Chlorates were found in 49 wells out of 56 and in all types of lithology constituting the shallow aquifer. Chlorates concentrations appeared to be linked to NO₃^-, volatile fatty acids (VFA) and oxygen reduction potential (ORP) variations. Chlorates behaviour was related to the biodegradation of perchlorates, since perchlorates are favourable electron acceptors for the oxidation of labile dissolved organic carbon (DOC) in groundwater. Further studies must take into consideration to monitor ClO₄^- in pore waters and groundwater to better elucidate the mass flux of ClO₄^- in shallow aquifers belonging to agricultural landscapes.

Monitoring nutrients fate after digestate spreading in a short rotation buffer area

One of the main sources of reactive nitrogen pollution is animal manure. The disposal of digestate (material remaining after the anaerobic digestion of a biodegradable feedstock) in agricultural soils could solve both the problems of soil fertilization and waste removal, but the fate of digestate in the environment must be assessed carefully before its massive utilization. To investigate whether digestate could be safely employed as a soil fertilizer, an agricultural field located in Monastier di Treviso (Northern Italy) and characterized by the presence of low hydraulic conductivity clay soils, was selected to be amended with bovine digestate. The experimental site was intensively monitored by a three-dimensional array of probes recording soil water content, temperature, and electrical conductivity, to solve the water and bulk mass fluxes in the unsaturated zone. High-resolution soil coring allowed the characterization of soil water composition over two hydrological years. Chloride, found in high concentrations in the digestate, was used as environmental tracer to track the fate of the percolating water. The study concluded that digestate could be confidently employed in short rotation buffer areas at an average rate of 195 ± 26 kg-N/ha/year in low hydraulic conductivity soils not affected by diffuse fracturing during dry periods.

Natural and anthropogenic variations in the Po river waters (northern Italy): insights from a multi-isotope approach

Po is the main Italian river and the δ(18)O and δ(2)H of its water reveal a similarity between the current meteoric fingerprint and that of the past represented by groundwater. As concerns the hydrochemisty, the Ca-HCO3 facies remained constant over the last 50 year, and only nitrate significantly increased from less than 1 mg/L to more than 10 mg/L in the 1980s, and then attenuated to a value of 9 mg/L. Coherently, δ(13)CDIC and δ(34)SSO4 are compatible with the weathering of the lithologies outcropping in the basin, while extremely variable δ(15)NNO3 indicates contribution from pollutants released by urban, agricultural and zootechnical activities. This suggests that although the origin of the main constituents of the Po river water is geogenic, anthropogenic contributions are locally significant. Noteworthy, the associated aquifers have the same nitrogen isotopic signature of the Po river, but are characterized by significantly higher NO(-) 3 concentration. This implies that aquifers' pollution is not ascribed to inflow of current river water, and that the attenuation of the nitrogen load recorded in the river is not occurring in the aquifers, due to their longer water residence time and delayed recovery from anthropogenic contamination.

Performance of different assessment methods to evaluate contaminant sources and fate in a coastal aquifer

The present study deals with the application of different monitoring techniques and numerical models to characterize coastal aquifers affected by multiple sources of contamination. Specifically, equivalent freshwater heads in 243 monitoring wells were used to reconstruct the piezometric map of the studied aquifer; flow meter tests were carried out to infer vertical groundwater fluxes at selected wells; deuterium and oxygen isotopes were used to identify the groundwater origin, and tritium was analyzed to estimate the residence time; compound-specific isotope analyses and microbial analyses were employed to track different sources of contamination and their degradation; numerical modelling was used to estimate and verify groundwater flow direction and magnitude throughout the aquifer. The comparison of the information level for each technique allowed determining which of the applied approaches showed the best results to locate the possible sources and better understanding of the fate of the contaminants. This study reports a detailed site characterization process and outcomes for a coastal industrial site, where a comprehensive conceptual model of pollution and seawater intrusion has been built using different assessment methods. Information and results from this study encourages combining different methods for the design and implementation of the monitoring activities in real-life coastal contaminated sites in order to develop an appropriate strategy for control and remediation of the contamination.

Fate of arsenic, phosphate and ammonium plumes in a coastal aquifer affected by saltwater intrusion

A severe groundwater contamination with extensive plumes of arsenic, phosphate and ammonium was found in a coastal aquifer beneath a former fertilizer production plant. The implementation of an active groundwater remediation strategy, based on a comprehensive pump and treat scheme, now prevents the migration of the dissolved contaminants into the marine environment. However, due to the site's proximity to the coastline, a seawater wedge was induced by the pumping scheme. Additionally the groundwater flow and salinity patterns were also strongly affected by leakage from the site's sewer system and from a seawater-fed cooling canal. The objective of this study was to elucidate the fate of arsenic and its co-contaminants over the site's history under the complex, coupled hydrodynamic and geochemical conditions that prevail at the site. A detailed geochemical characterisation of samples from sediment cores and hydrochemical data provided valuable high-resolution information. The obtained data were used to develop various conceptual models and to constrain the development and calibration of a reactive transport model. The reactive transport simulations were performed for a sub-domain (two-dimensional transect) of an earlier developed three-dimensional flow and variable density solute transport model. The results suggest that in the upper sub-oxic zone the influx of oxygenated water promoted As attenuation via co-precipitation with Al and Fe oxides and copper hydroxides. In contrast, in the deeper aquifer zone, iron reduction, associated with the release of adsorbed As and the dissolution of As bearing phases, provided and still provides to date a persistent source for groundwater pollution. The presented monitoring and modelling approach could be broadly applied to coastal polluted sites by complex contaminant mixture containing As.

The Po river water from the Alps to the Adriatic Sea (Italy): new insights from geochemical and isotopic (δ(18)O-δD) data

Although the Po river is the most important fluvial system of Northern Italy, the systematic geochemical and isotopic investigations of its water are rare and were never reported for the whole basin. The present contribution aims to fill this knowledge gap, reporting a comprehensive data set including oxygen and hydrogen stable isotopes as well as major and trace element concentration of dissolved species for 54 Po river water samples, mainly collected in different hydrological conditions (peak discharge in April, drought in August) at increasing distance from the source, i.e., from the upper part of the catchment to the terminal (deltaic) part of the river at the confluence with the Adriatic Sea. The isotopic compositions demonstrate that the predominant part of the runoff derives from the Alpine sector of the catchment through important tributaries such as the Dora Baltea, Ticino, Adda, and Tanaro rivers, whereas the contribution from the Apennines tributaries is less important. The geochemical and isotopic compositions show that the Po river water attains a homogeneous composition at ca. 100 km from the source. The average composition is characterized by δ(18)O -9.8‰, δD -66.2‰, total dissolved solid (TDS) 268 mg/L, and chloride 17 mg/L and by a general Ca-HCO3 hydrochemical facies, which is maintained for most of the river stream, only varying in the terminal part where the river is diverted in a complex deltaic system affected by more significant evaporation and mixing with saline water evidenced by higher TDS and chloride content (up to 8198 and 4197 mg/L, respectively). Geochemical and isotopic maps have been drawn to visualize spatial gradients, which reflect the evolution of the river water composition at progressive distance from the source; more detailed maps were focused on the deltaic part in order to visualize the processes occurring in the transitional zone toward the Adriatic Sea. The data also highlight anthropogenic contributions, mainly represented by significant concentrations of nitrate (average 8 mg/L) and possibly arsenic (average 12 μg/L). These data allow the calculation of geochemical fluxes transferred from the river to the sea, and generally, they contribute to the definition of a "hydro-archive" which is useful to highlight ongoing variations in the related ecosystems.

Reactive modelling of 1,2-DCA and DOC near the shoreline

1,2-Dichloroethane (1,2-DCA) was found to be the most abundant compound among chlorinated hydrocarbons detected in a petrochemical plant in southern Italy. This site is located near the coastline, and it is set above an unconfined coastal aquifer, where seawater intrusion is present. The presence of organic and inorganic contaminants at this site has required the implementation of remediation strategies, consisting of pumping wells (hydraulic barrier) and a horizontal flow barrier. The purpose of this work was to assess the influence of salt water intrusion on the degradation rate of 1,2-DCA. This was done on a three-dimensional domain relative to a limited portion of a well characterized field site, accounting for density-dependent flow and reactive transport modelling of 1,2-DCA and Dissolved Organic Carbon (DOC). The modelling procedure was performed employing SEAWAT-4.0 and PHT3D, to reproduce the complex three-dimensional flow and transport domain. In order to determine the fate of 1,2-DCA, detailed field investigations provided intensive depth profile information. Different, kinetically controlled degradation rates were simulated to explain the observed, selective degradation of pollutants in groundwater. Calibration of the model was accomplished by comparison with the two different sets of measurements obtained from the MLS devices and from pumping wells. With the calibrated model, it was possible to distinguish between dispersive non-reactive processes and bacterially mediated reactions. In the non-reactive model, 1,2-DCA sorption was simulated using linear sorption coefficient determined with field data and 1,2-DCA degradation was simulated using a first order decay coefficient using literature data as initial guess. Finally, on the reactive transport model, where a two-step approach with partial equilibrium approach was implemented, the effects of neglecting the cation exchange capacity, omitting density-dependent flow, and refining the vertical discretization of the model were investigated. Comparison of results from various scenarios shows that geochemical changes in inorganic constituents can be used to improve the site's conceptual model, and establishes that natural degradation processes can be suitable for 1,2-DCA as a remediation option.

Evaluation of saline tracer performance during electrical conductivity groundwater monitoring

Saline solutions are the most commonly used hydrological tracers, because they can be easily and economically monitored by in situ instrumentation such as electrical conductivity (EC) loggers in wells or by geoelectrical measurements. Unfortunately, these low-cost techniques only provide information on the total concentration of ions in solution, i.e., they cannot resolve the ionic composition of the aqueous solution. This limitation can introduce a bias in the estimation of aquifer parameters where sorption phenomena between saline tracers and sediments become relevant. In general, only selected anions such as Cl(-) and Br(-) are recognised to be transported unretarded and they are referred to as conservative tracers or mobile anions. However, cations within the saline tracer may interact with the soil matrix through a range of processes such as ion exchange, surface complexation and via physical mass-transfer phenomena. Heterogeneous reactions with minerals or mineral surfaces may not be negligible where aquifers are composed of fine alluvial sediments. The focus of the present study was to examine and to quantify the bias between the aquifer parameters estimated during model-based interpretation of experimental data of EC measurements of saline tracer relative to the aquifer parameters found by specific measurements (i.e. via ionic chromatography, IC) of truly conservative species. To accomplish this, column displacement experiments with alluvial aquifer materials collected from the Po lowlands (Italy) were performed under water saturated conditions. The behaviour of six selected, commonly used saline tracers (i.e., LiCl, KCl, and NaCl; LiBr, KBr, and NaBr) was studied and the data analysed by inverse modelling. The results demonstrate that the use of EC as a tracer can lead to an erroneous parameterisation of the investigated porous media, if the reactions between solute and matrix are neglected. In general, errors were significant except for KCl and KBr, which is due to the weak interaction between dissolved K(+) and the sediment material. The study shows that laboratory scale pre-investigations can help with tracer selection and to optimise the concentration range targeted for in situ multilevel monitoring by unspecific geoelectrical instrumentation.

Modelling the fate of styrene in a mixed petroleum hydrocarbon plume

Severe petroleum hydrocarbon contamination (styrene and the BTEX compounds: benzene, toluene, ethylbenzene and the isomers of xylene) from leaking sewers was detected in a Quaternary aquifer below a chemical plant in the Padana Plain, Italy. From 1994, active pump and treat remediation has been employed. The site is bordered by canals which, in combination with variable pumping rates and groundwater flow directions, control groundwater levels. In this study we sought to determine the fate of styrene at the site within a mixed styrene/BTEX plume where the hydraulic boundaries induced strong seasonal variations in flows. In order to determine the fate of styrene, detailed field investigations provided intensive depth profile information. This information was then incorporated into a staged flow and reactive transport modelling. Three sets of measurements were obtained from sampling multilevel samplers (MLSs) under different hydraulic conditions at the site. These included measurements of BTEX, styrene, all major ions, pH and redox potential. A three-dimensional transient flow model was developed and calibrated to simulate an unconfined sandy aquifer with a variable flow field. Subsequently a reactive, multi-component transport model was employed to simulate the fate of dissolved BTEX and styrene along a selected flow line at the site. Each petroleum hydrocarbon compound was transported as independent species. Different, kinetically controlled degradation rates and a toxicity effect were simulated to explain the observed, selective degradation of pollutants in groundwater. Calibration of the model was accomplished by comparison with the three different sets of measurements obtained from the MLS devices. The results from various scenarios show that the detailed simulation of geochemical changes can be very useful to improve the site's conceptual model.