Pollutant sources in an arsenic-affected multilayer aquifer in the Po Plain of Italy: Implications for drinking-water supply

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.

Predicting natural arsenic enrichment in peat-bearing, alluvial and coastal depositional systems: A generalized model based on sequence stratigraphy

Hazardously high concentrations of arsenic exceeding the threshold limits for soils and drinking waters have been widely reported from Quaternary sedimentary successions and shallow aquifers of alluvial and coastal lowlands worldwide, raising public health concerns due to potential human exposure to arsenic. A combined sedimentological and geochemical analysis of subsurface deposits, 2.5-50 m deep, from the SE Po Plain (Italy) documents a systematic tendency for naturally-occurring arsenic to accumulate in peat-rich layers, with concentrations invariably greater than maximum permissible levels. A total of 366 bulk sediment samples from 40 cores that penetrated peat-bearing deposits were analysed by X-ray fluorescence. Arsenic concentrations associated with 7 peat-free lithofacies associations (fluvial-channel, levee/crevasse, floodplain, swamp, lagoon/bay, beach-barrier, and offshore/prodelta) exhibit background values invariably below threshold levels (<20 mg/kg). In contrast, total arsenic contents from peaty clay and peat showed 2-6 times larger As accumulation. A total of 204 near-surface (0-2.5 m) samples from modern alluvial and coastal depositional environments exhibit the same trends as their deeper counterparts, total arsenic peaking at peat horizons above the threshold values for contaminated soils. The arsenic-bearing, peat-rich Quaternary successions of the Po Plain accumulated under persisting reducing conditions in wetlands of backstepping estuarine and prograding deltaic depositional environments during the Early-Middle Holocene sea-level rise and subsequent stillstand. Contamination of the Holocene and underlying Pleistocene aquifer systems likely occurred through the release of As by microbially-mediated reductive dissolution. Using high-resolution sequence-stratigraphic concepts, we document that the Late Pleistocene-Holocene lithofacies architecture dictates the subsurface distribution of As. The "wetland trajectory", i.e. the path taken by the landward/seaward shift of peat-rich depositional environments during the Holocene, may help predict spatial patterns of natural As distribution, delineating the highest As-hazard zones and providing a realistic view of aquifer contamination even in unknown areas.

Chemical and mineralogical variability of sediment in a Quaternary aquifer from Huaihe River Basin, China: Implications for groundwater arsenic source and its mobilization

Arsenic (As) is a conspicuous contaminant, and exposure to this element through contaminated drinking groundwater poses a significant challenge to public health. Geogenic groundwater arsenic is associated with sedimentary setting. This work concentrates on the investigation of lithology, elemental abundance and mineralogical compositions about the arsenic profile and its effect to the groundwater from Huaihe River Basin, China. There are 90 sediment samples from the borehole at the field monitoring sites were collected and analyzed. The results reveal that sedimentary concentrations of As, Fe, Mn, S, Al, N, organic carbon and mineralogical compositions vary across the Quaternary aquifer. Arsenic abundance of sediments is 10.63 ± 0.56 mg/kg, and peak As concentrations occur between 59.0 m and 64.8 m in fine particle sediments. The specific higher As concentrations in sedimentary aquifer are concordant with arsenic-rich groundwater around the investigated borehole. Fe, Mn, and Al depth profiles follow similar tendency to those of As. Sedimentary As concentrations are significantly correlated to Fe, Al, and Mn concentrations, but are not correlated to organic carbon and S concentrations. Arsenic probably exists in the form of non-crystalline colloids, and Fe, Al minerals are potential host minerals for arsenic. Under alkaline conditions, groundwater arsenic is released and enriched within the Quaternary aquifer by reductive dissolution of As-hosting Fe and Al minerals.

Identification of arsenic spatial distribution by hydrogeochemical processes represented by different ion ratios in the Hohhot Basin, China

The Hohhot Basin, a typical inland basin of the Yellow River Basin in China, has high concentrations of arsenic (As) in its shallow groundwater, while the factors dominating the distribution of high arsenic levels remain to be further identified. An analysis of the ratio of hydrogeochemical compositions can help to reveal the spatial characteristics of the shallow groundwater environmental conditions and the distribution of high-arsenic water (As >10 μg/L). In this study, a total of 170 samples of shallow groundwater in the Hohhot Basin were collected and water samples with As >10 μg/L accounted for 29.4% of the total. Based on the slope changes of the cumulative frequency curves of (HCO₃^- + CO₃^2-)/SO₄^2-, Ca²⁺/(HCO₃^- + CO₃^2-), Ca²⁺/Mg²⁺, and Na⁺/Ca²⁺, the groundwater in the study area can be categorized into six different zones according to the environmental characteristics including redox condition, water recharge intensity, and cation exchange level. The result shows that the groundwater in the front of the piedmont alluvial plain and platform is in a weak reducing condition with high lateral recharge intensity, fast runoff, and weak cation exchange. In the Dahei River alluvial plain, which serves as the groundwater discharge zone, the groundwater runoff is sluggish with poor lateral recharge, sufficient exchange between cations in the groundwater and the aquifer matrix, and enhanced reducibility. The degree of oxidation increased in the groundwater near the Hasuhai Lake and the drainage canal, which adverse to the arsenic enrichment. High-arsenic groundwater is mainly distributed in aquifers of (HCO₃^- + CO₃^2-)/SO₄² > 10, Na⁺/Ca²⁺ > 13, and Ca²⁺/(HCO₃^- + CO₃^2-) < 0.1, which represent the strong reducing condition, low surface water recharge intensity, and strong cation exchange condition. Reductive dissolution of iron oxide, strong evaporation and concentration process, and competition from phosphate in aquifers jointly lead to the release of arsenic into groundwater.

Hydrogeochemical Processes and Potential Exposure Risk of Arsenic-Rich Groundwater from Huaihe River Plain, China

Arsenic poses a danger to environmental health, and arsenic-rich groundwater is a key exposure risk for humans. The distribution, migration, and enrichment of arsenic in groundwater is an important environmental and public health problem. Currently, the Huaihe River Basin is identified as a region of arsenic-rich groundwater in China. This study aims to assess arsenic-rich groundwater potential pollution risk, analyze the hydrogeochemical processes, and trace the ion source based on an analysis of groundwater hydrogeochemical data. The results show that arsenic is the main inorganic chemical substances affecting the water quality in the study area, which presents a high exposure risk for public health. The arsenic concentration of groundwater was f 5.75 ± 5.42 μg/L, and 23% of the considered samples exceeded the drinking water standards of the World Health Organization. The groundwater in the study area underwent evaporation, halite dissolution, and ion exchange processes. The total alkalinity (HCO3−) of the arsenic-rich groundwater mainly ranged between 400–700 mg/L, and the chemical type was mainly of HCO3-Na. In an alkaline environment, the oxidative dissolution and reductive dissolution of arsenic bearing minerals might be the formation mechanism of arsenic-rich groundwater.

Origin of nitrate and sulfate sources in volcano-sedimentary aquifers of the East Africa Rift System: An example of the Ali-Sabieh groundwater (Republic of Djibouti)

Within the East African Rift System (EARS), the complex Ali-Sabieh aquifers system, located in the south of the Republic of Djibouti, was overexploited and subjected to anthropogenic and/or geogenic pollution with high concentrations of dissolved nitrate (up to 181 mg/l) and sulfates (up to 1540 mg/l). This study is the first undertaken on the hydrochemistry of this aquifer system, combining geochemical tools and multi-isotope - δ²H(H₂O), δ¹⁸O(H₂O), δ¹⁸O(SO₄), δ³⁴S(SO₄), δ¹⁵N(NO₃), δ¹⁸O(NO₃), δ¹³C(DIC), and ¹⁴C- was used to decipher the origin and fate of different nitrate and sulfate sources to groundwater. The groundwater samples of the region show a chemical evolution from fresh Ca(Na)-bicarbonate to brackish Na-Cl , mainly due to water-rock interaction. The combined chloride and water isotope data show that evaporation and transpiration are present, with the latter occurring primarily in the shallow alluvial aquifer waters. Inspection of δ¹⁵N(NO₃) vs. δ¹⁸O(NO₃) and NO₃/Cl vs. Cl diagrams show that dissolved nitrates are primarily of anthropogenic origin. In particular, higher nitrate concentrations may be related to animal manure used as organic fertilizers during agricultural activities. Sulfates are from a natural origin related to the interaction of water with gypsum of hydrothermal or sedimentary origin. SO₄/Cl ratio and isotopic composition show that dissolved sulfates in saline and ancient groundwater of the Cretaceous sandstone aquifer (between 7.4 ± 2.2 and 5.8 ± 1.4 k-years before the present) are generated by interaction with gypsum from oxidation of pre-existing (Jurassic?) sulfides. This work highlight that isotopic ratios of the two molecules -δ¹⁸O(SO₄), δ³⁴S(SO₄), δ¹⁵N(NO₃), δ¹⁸O(NO₃)- are not sufficient for tracing the origin of nitrate and sulfates in groundwater, but that a complete hydrogeochemical study is needed. In the absence of this, the relatively high concentration of chloride and sulfates could be wrongly linked to the anthropogenic source of nitrate (manure or sewage).

Arsenic exposure from groundwater: environmental contamination, human health effects, and sustainable solutions

Arsenic (As) occurs naturally in geologic conditions, but groundwater contamination might also be found due to the consequences of mining, agricultural and industrial processes. Human exposure to As after drinking contaminated water is commonly associated with acute toxicity outcomes and chronic effects ranging from skin lesions to cancer. Integrated actions from environmental and health authorities are needed to reduce exposure, monitoring outcomes, and promotion of actions to offer sustainable As-safe water alternatives. Considering recent research trends, the present review summarizes and discusses current issues associated with the process and effects of contamination and decontamination in an environmental health perspective. Recent findings reinforce the harmful effects of the consumption of As-contaminated water and broaden the scope of related diseases including intestinal maladies, type 2 diabetes, cancers of bladder, kidneys, lung, and liver. Among the main strategies to diminish or remove As from water, the following are highlighted (1) ion exchange system and membrane filtration (micro, ultra, and nanofiltration) as physicochemical treatment systems; (2) use of cyanobacteria and algae in bioremediation programs and (3) application of nanotechnology for water treatment.

Overlapping redox zones control arsenic pollution in Pleistocene multi-layer aquifers, the Po Plain (Italy)

Understanding the factors that control As concentrations in groundwater is vital for supplying safe groundwater in regions with As-polluted aquifers. Despite much research, mainly addressing Holocene aquifers hosting young (<100 yrs) groundwater, the source, transport, and fate of As in Pleistocene aquifers with fossil (>12,000 yrs) groundwaters are not yet fully understood and so are assessed here through an evaluation of the redox properties of the system in a type locality, the Po Plain (Italy). Analyses of redox-sensitive species and major ions on 22 groundwater samples from the Pleistocene arsenic-affected aquifer in the Po Plain shows that groundwater concentrations of As are controlled by the simultaneous operation of several terminal electron accepters. Organic matter, present as peat, is abundant in the aquifer, allowing groundwater to reach a quasi-steady-state of highly reducing conditions close to thermodynamic equilibrium. In this system, simultaneous reduction of Fe-oxide and sulfate results in low concentrations of As (median 7 μg/L) whereas As reaches higher concentrations (median of 82 μg/L) during simultaneous methanogenesis and Fe-reduction. The position of well-screens is an additional controlling factor on groundwater As: short screens that overlap confining aquitards generate higher As concentrations than long screens placed away from them. A conceptual model for groundwater As, applicable worldwide in other Pleistocene aquifers with reducible Fe-oxides and abundant organic matter is proposed: As may have two concentration peaks, the first after prolonged Fe-oxide reduction and until sulfate reduction takes place, the second during simultaneous Fe-reduction and methanogenesis.

Groundwater and surface water quality characterization through positive matrix factorization combined with GIS approach

This study aims at testing the effectiveness of Positive Matrix Factorization in characterizing groundwater and surface water quality, in terms of identifying main hydrochemical features and processes (natural and anthropogenic) that govern them. This method is applied in a hydro-system featured by a strong interrelation between groundwater and surface water and highly impacted by agricultural activities. Therefore, a holistic approach considering groundwater together with the surface water bodies, consisting in lake, several rivers and springs, was used. Multivariate statistical analysis, in particular Factor Analysis, has been proved to be effective in elaborating and interpreting water quality data highlighting the information carried within them, but it presents some limitations: it does not consider data uncertainty and it groups variables which are correlated positively and negatively. Moreover, in some cases the resulting factors are not clearly interpretable, describing each one various overlapping features/processes. Here, Positive Matrix Factorization is applied to groundwater and surface water quality data, and the results are compared to those obtained through a Factor Analysis in terms of both factor profiles and their spatial distribution through a GIS approach. Results of isotopes analysis are used to validate PMF output and support interpretation. Positive Matrix Factorization allows to consider data uncertainty and the solution respects two positivity constraints, based on the concept of chemical mass balance, which leads to a more environmentally interpretable solution. Results show that Positive Matrix Factorization identifies five different factors reflecting main features and natural and anthropogenic processes affecting the study area: 1) surface water used for irrigation, 2) groundwater subjected to reducing processes at advanced stages, 3) groundwater subjected to reducing processes at early stages, 4) groundwater residence time and 5) the effects of the agricultural land use on both groundwater and surface water. Positive Matrix Factorization leads to a more detailed understanding of the studied system as compared to Factor Analysis which identifies only three factors with overlapping information. Based on the results of this study, Positive Matrix Factorization could be a useful technique to perform groundwater and surface water quality characterization and to reach a deeper understanding of the phenomena that govern water chemistry.

Identification of groundwater pollution sources in a landfill site using artificial sweeteners, multivariate analysis and transport modeling

In this study, sources of groundwater pollution in a landfill site were identified, using artificial sweeteners as chemical tracers, multivariate statistical analysis and a quantitative analysis of the groundwater flow system through particle tracking and transport modeling. The study area, located in northern Italy, hosts an older unlined landfill and a newer lined municipal solid waste landfill placed downstream of the former. Groundwater, surface water, treated wastewater, and leachate samples were collected in March 2017 for analysis of the artificial sweeteners saccharin, cyclamate, acesulfame and sucralose together with major cations and anions, inorganic nitrogen compounds, total phosphorus, COD and some further parameters. The interpretation of the results suggests that two main leachate leaks/spills are affecting the study area. The first one concerns leachate probably spilling out of the leachate collection system serving the younger lined landfill, the other one involves leachate from the older unlined landfill that also seems to affect an area downstream of the lined landfill. Direct leachate leaks from the lined landfill seem unlikely, although they cannot be definitively excluded. This work underlines the importance of a multi-methods approach, which integrates here chemical tracers, multivariate analysis and transport modeling, for assessing groundwater pollution sources generated from complex landfill sites, where multiple and different sources may exist. In particular, this work highlights how artificial sweeteners can be used for tracing leachate plumes from landfills. The methodology applied in this study can have a broad applicability also in other polluted landfill sites worldwide.

The effects of irrigation on groundwater quality and quantity in a human-modified hydro-system: The Oglio River basin, Po Plain, northern Italy

For several hundred years, farming in the Po Plain of Italy (46,000 km², 20 million inhabitants) has been supported by intensive surface irrigation with lake and river water. Despite the longevity of irrigation, its effects on the quality and quantity of groundwater is poorly known and so is investigated here through seasonal measurements of hydraulic heads and water quality in groundwaters, rivers, lake, springs and rainwaters. In the north of the study region, an unconfined coarse-grained alluvial aquifer, infiltration of surface irrigation water, sourced from the Oglio River and low in NO₃, contributes much to aquifer recharge (up to 88%, as evidenced by a δ²H-Cl/Br mixing model) and has positive effects on groundwater quality by diluting high concentrations of NO₃ (decrease by 17% between June and September)_. This recharge also helps to maintain numerous local springs that form important local micro-environments. Any increase in water-use efficiency in irrigation will reduce this recharge, imperil the spring environments, and lessen the dilution of NO₃ leading to increasing NO₃ concentrations in groundwater. These findings can be extended by analogy to the entire Po Plain region and other surface-water-irrigated systems worldwide where inefficient irrigation methods are used and similar hydrogeological features occur.

Evaluation of energy consumption of treating nitrate-contaminated groundwater by bioelectrochemical systems

Nitrate contamination of groundwater is a mounting concern for drinking water production due to its healthy and ecological effects. Bioelectrochemical systems (BES) are a promising method for energy efficient nitrate removal, but its energy consumption has not been well understood. Herein, we conducted a preliminary analysis of energy consumption based on both literature information and multiple assumptions. Four scenarios were created for the purpose of analysis based on two treatment approaches, microbial fuel cells (MFCs) and controlled biocathodic denitrification (CBD), under either in situ or ex situ deployment. The results show a specific energy consumption based on the mass of NO₃^--N removed (SEC_N) of 0.341 and 1.602 kWh kg NO₃^--N^-1 obtained from in situ and ex situ treatments with MFCs, respectively; the main contributor was the extraction of the anolyte (100%) in the former and pumping the groundwater (74.8%) for the latter. In the case of CBD treatment, the energy consumption by power supply outcompeted all the other energy items (over 85% in all cases), and a total SEC_N of 19.028 and 10.003 kWh kg NO₃^--N^-1 were obtained for in situ and ex situ treatments, respectively. The increase in the water table depth (from 10 to 30 m) and the decrease of the nitrate concentration (from 25 to 15 mg NO₃^--N) would lead to a rise in energy consumption in the ex situ treatment. Although some data might be premature due to the lack of sufficient information in available literature, the results could provide an initial picture of energy consumption by BES-based groundwater treatment and encourage further thinking and analysis of energy consumption (and production).

Determination of trigger levels for groundwater quality in landfills located in historically human-impacted areas

Landfills are one of the most recurrent sources of groundwater contamination worldwide. In order to limit their impacts on groundwater resources, current environmental regulations impose the adoption of proper measures for the protection of groundwater quality. For instance, in the EU member countries, the calculation of trigger levels for identifying significant adverse environmental effects on groundwater generated by landfills is required by the Landfill Directive 99/31/EC. Although the derivation of trigger levels could be relatively easy when groundwater quality data prior to the construction of a landfill are available, it becomes challenging when these data are missing and landfills are located in areas that are already impacted by historical contamination. This work presents a methodology for calculating trigger levels for groundwater quality in landfills located in areas where historical contaminations have deteriorated groundwater quality prior to their construction. This method is based on multivariate statistical analysis and involves 4 steps: (a) implementation of the conceptual model, (b) landfill monitoring data collection, (c) hydrochemical data clustering and (d) calculation of the trigger levels. The proposed methodology was applied on a case study in northern Italy, where a currently used lined landfill is located downstream of an old unlined landfill and others old unmapped waste deposits. The developed conceptual model stated that groundwater quality deterioration observed downstream of the lined landfill is due to a degrading leachate plume fed by the upgradient unlined landfill. The methodology led to the determination of two trigger levels for COD and NH₄-N, the former for a zone representing the background hydrochemistry (28 and 9 mg/L for COD and NH₄-N, respectively), the latter for the zone impacted by the degrading leachate plume from the upgradient unlined landfill (89 and 83 mg/L for COD and NH₄-N, respectively).

Enhanced adsorption performance of aspartic acid intercalated Mg-Zn-Fe-LDH materials for arsenite

A series of hydrophobic and hydrophilic amino acid (aspartic acid, phenylalanine, glutamic acid, and proline) intercalated LDH materials were synthesized and characterized. The results of batch experiments showed that Mg7Zn1Fe4-Asp-LDH and Mg7Zn1Fe4-Phe-LDH showed good adsorption performances for both arsenate and arsenite in aqueous solutions. The effects of various experimental conditions have been investigated by the batch test, which included the effects of initial pH, arsenic concentration, contact time and coexisting ions. For Mg7Zn1Fe4-Asp-LDH under the optimal experimental conditions, the maximum adsorption capacity of As(iii) and As(v) reached 94.81 mg g-1 and 57.42 mg g-1, respectively. It showed a higher adsorption capacity for As(iii) than that for As(v), which is of great significance to remove the trivalent arsenic species with higher toxicity. When the dosage of Mg7Zn1Fe4-Asp-LDH was 0.8 g L-1, the concentration of As(iii) in the aqueous solution could be reduced from 2 mg L-1 to below 10 μg L-1.When Mg-Zn-Fe-Asp-LDH was applied in practical water samples with a dosage of 0.2 g L-1, the residual concentrations of arsenic in three actual water samples were all lower than 10 μg L-1 after adsorption. The column test showed that 1.0 g of Mg7Zn1Fe4-Asp-LDH could continuously treat 2.6 L of As(iii) aqueous solution (2 mg L-1) and reduced the concentration of As(iii) to below 10 μg L-1 or handle 0.4 L of arsenic-contaminated (10 mg L-1, As(iii) : As(v) = 1 : 1) water, and the effluent concentration was below 10 μg L-1. Compared with the previously reported hydrophobic amino acid intercalated LDHs, aspartic acid (hydrophilic amino acid) intercalated LDH has a good removal efficiency for arsenic. The synthesized Mg7Zn1Fe4-Asp-LDH is considered to be a potentially functional material that can be used to treat arsenic contamination in water.

Sources of groundwater salinity and potential impact on arsenic mobility in the western Hetao Basin, Inner Mongolia

The quality of groundwater used for human consumption and irrigation in the Hetao Basin of Inner Mongolia, China is affected by elevated salinity as well as high arsenic (As) concentrations. However, the origin of high salinity and its potential impact on As mobility in the Basin remain unclear. This study explores both issues using stable isotopic compositions and Cl/Br ratios of groundwater as well as the major ions of both groundwater and leachable salts in aquifer sediments. Limited variations in δ¹⁸O and δ²H (-11.13 to -8.10, -82.23 to -65.67) with the wide range of Total Dissolved Solid (TDS, 351-6734mg/L) suggest less contribution of direct evaporation to major salinity in groundwater. Deuterium excess shows that non-direct evaporation (capillary evaporation, transpiration) and mineral/evaporite dissolution contribute to >60% salinity in groundwater with TDS>1000mg/L. Non-direct evaporation, like capillary evaporation and transpiration, is proposed as important processes contributing to groundwater salinity based on Cl/Br ratio and halite dissolution line. The chemical weathering of Ca, Mg minerals and evaporites (Na₂SO₄ and CaSO₄) input salts into groundwater as well. This is evidenced by the fact that lacustrine environment and the arid climate prevails in Pleistocene period. Dissolution of sulfate salts not only promotes groundwater salinity but affects As mobilization. Due to the dissolution of sulfate salts and non-direct evaporation, groundwater SO₄^2- prevails and its reduction may enhance As enrichment. The higher As concentrations (300-553μg/L) are found at the stronger SO₄^2- reduction stage, indicating that reduction of Fe oxide minerals possibly results from HS^- produced by SO₄^2- reduction. This would have a profound impact on As mobilization since sulfate is abundant in groundwater and sediments. The evolution of groundwater As and salinity in the future should be further studied in order to ensure sustainable utilization of water resource in this water scarce area.