Aquifer Contamination Research Articles

Quantifying aquifer contamination risk from casing rupture using support vector machine: a comprehensive assessment

Quantifying aquifer contamination risk from casing rupture using support vector machine: a comprehensive assessment

Estimation of pollution sources and hydraulic conductivity field in a coastal aquifer under tidal effects

In the past decades, most of the efforts have often been invested in the joint identification of contamination and hydraulic conductivity field in inland subsurface aquifers. However, the booming economy and dense population around the coast have brought a new challenge to the forefront: the serious pollution of coastal aquifers. Coastal aquifers are highly vulnerable ecological areas, which are affected by inland pollutants carried by groundwater during its entry into the sea and are also influenced by seawater intrusion. This article addresses the inverse problem of coastal aquifer pollution, which is subject to tidal effects, using the Iterative Local Updating Ensemble Smoother (ILUES) algorithm. The adopted ILUES algorithm is leveraged to efficiently extract pollutant concentration and salinity data for the identification of the source release history and characterization of the hydraulic conductivity field of a site with multiple pollutant sources. This study is based on a variant of the traditional benchmark problem, analyzing the influence of algorithm parameters, tidal effects, and heterogeneity on data assimilation. The article further demonstrates the great potential of ILUES to address new challenges. In addition, the significance of multi-level monitoring wells for the estimation of hydraulic conductivity in coastal multi-layered heterogeneous aquifers is explained.

Use of water quality index and DRASTIC index correlation for better assessment of groundwater vulnerability to pollution: a case study

Abstract DRASTIC method's vulnerability assessment for estimating the potential risk of contamination in shallow groundwater aquifers is the most acceptable technique. The method uses seven parameters, namely, depth to groundwater table, net recharge, aquifer media, soil media, topography, land use and land cover, and hydraulic conductivity to obtain vulnerability index values. These index values are validated with one or a few elements of water characteristics as pollution indicators. In most of the reported studies, the correlation coefficient between the DRASTIC index and the concentration of individual element(s) was low. The present study uses the water quality index (WQI) to correlate with the DRASTIC index value. In this paper, 11 physical and chemical water characteristics data of 31 well locations within the Faridabad District of Haryana, India are utilized to estimate the WQI. The correlation coefficients for single element concentration with DRASTIC index vary between the range of 0.104 to 0.304. The correlation coefficients for the concentration of NO3- and WQI are found to be 0.104 and 0.533, respectively. Thus, the DRASTIC index demonstrates a stronger correlation with the WQI when compared to single element concentration.

Occurrence of and Factors Affecting Groundwater Fluoride in the Western Coastal Area of Hainan Island, South China

Hainan, a well-known center of tropical agricultural production in south China, has received little attention regarding groundwater fluoride contamination. This study investigates the occurrence of fluoride in the western coastal area of Hainan Island and discusses factors affecting groundwater fluoride contamination in various aquifers and areas with different land-use types using hydrochemistry and multivariate statistical analysis. A total of 100 groundwater samples were collected from the western coastal area of Hainan Island. The results show that the groundwater fluoride concentration is as high as 4.18 mg/L and that F−-high (>1 mg/L) groundwater accounts for 9% of total groundwater. The proportion of F−-high fissure water is about two times that of F−-high pore water. Among the different land-use types, the proportion of F−-high groundwater from highest to lowest is as follows: bare land > cultivated land > woodland > construction land > grassland. The main factor affecting fluoride in pore water is the leaching of fluorine/aluminum-containing minerals such as phlogopite and calcite in the vadose zone, which is characterized by the co-enrichment of fluoride and aluminum in pore water. The leading cause of fluoride in fissure water is the leaching of fluorine-containing fertilizers, and continuous irrigation promotes the cation exchange of sodium, strontium, and calcium, which is characterized by the co-enrichment of fluoride with sodium and strontium in fissure water. Consequently, it is advised to minimize the excessive use of fluoride fertilizers and increase groundwater quality monitoring in order to decrease the emergence of F−-high groundwater in the western coastal area of Hainan Island.

Nitrate stimulated microbial and viral activity and the subsequent influence on uranium mobility in sedimentary systems

Mobilization of naturally-occurring uranium(U) has been recognized to give rise to geogenic U groundwater contamination in aquifers. In addition to carbonate ligand complexation, nitrate has been demonstrated to play a role in controlling U mobility by altering uranium solubility through redox reactions. Nitrate is a common anthropogenic contaminant often prevalent at high concentrations in alluvial aquifers overlaying managed land. Alluvial deposition processes that form these aquifers create a lithologically heterogeneous subsurface with defined contacts between sands, silts, and clays. This leads to deposition of organic carbon and accumulation of reduced metals/radionuclides, including U(IV), in the finer grained silts and clays. The addition of high nitrate porewater into uranium-bearing alluvial aquifer silt sediments stimulated a nitrate reducing microbial community capable of catalyzing U(IV) oxidation and mobilization of U into porewaters. However, metadata from an aquifer wide study and a subsequent experiment revealed that this result is concentration dependent. Low concentrations of nitrate bearing pore-water added into organic-rich, uranium bearing sediments and resulted in a decrease in dissolved U(VI), consistent with reduction. XANES analysis of sediments supported U(VI) reduction with the precipitation of U(IV). U(VI) reduction activity occurred concurrent with an increase in dissolved organic carbon (DOC) and cell and virus abundance and activity. Metagenome assembled genomes from the microbial community revealed the metabolic potential indicating complex carbon degradation, fermentation, mineralization as well as the potential for anaerobic respiration of nitrate, metal/radionuclides, and sulfate. The virome recovered from the samples indicated a change in viral community in response to nitrate amendments and viral-encoded carbohydrate active enzymes were upregulated indicating a coupled response of both viral and microbial community regulating nitrate stimulated carbon biogeochemical cycling. These data together suggest that the addition of an electron acceptor in to organic carbon reduced sediments stimulates not only microbial but also viral activity leading to upregulation of genes associated with carbon biogeochemical cycling in sedimentary systems. While genes associated with metal oxidation are observed, net reduction of uranium prevails leading to uranium immobilization at low nitrate concentrations. Thus together these data indicate a tipping point whereby the influx of nitrate into the reduced environment can influence uranium mobility in DOC and carbon cycling supporting microbial activity and reducing conditions subsurface systems.

Extreme exposure of fluoride and arsenic contamination in shallow coastal aquifers of the Ganges delta, transboundary of the Indo-Bangladesh region

Globally, shallow aquifer groundwater (GW) has been severely affected in recent decades for both geogenic and anthropogenic reasons. The hydro-geochemical characteristics of the GW change inconsistently with the addition of unwanted inorganic trace elements into the GW aquifer of the Indo-Bangladesh delta region (IBDR), such as arsenic (As) along with fluoride (F−) contamination. Contaminated GW can have a negative impact on drinking water supplies and agricultural output. GW pollution can have serious adverse effects on the environment and human health. Thus, the GW quality of this region is deteriorating progressively, and human health threatening by various life-threatening disorders. Hence, the current study concentrated on the GW quality evaluation and prediction of possible health issues in the IBDR due to elevated contamination of As along with F− within GW aquifers by considering sixteen causative. Field survey-based statistical methods such as entropy quality index (EWQI) combined with health risk index (HRI) was implemented for evaluating the As and F− sensitivity with the help of correlation testing and principal component analysis. The study's outcome explains that a substantial portion of the IBDR has been vastly experiencing inferior GW quality, environmental issues, and health-related problems in dry and wet seasons, correspondingly for As and F− exposure. Piper diagram verified the suitability of water that almost 55% of GW across the study area’s aquifers are unfit for drinking as well as cultivation of crops. Sensitivity analysis and the Monte Carlo simulation method were also applied to assess the contaminant's concentration level and probable health risk appraisal. The present study concludes that the elevated exposure of As and F− pollution has to be monitored regularly and prevent unwanted GW contamination through implementing sustainable approaches and policies to fulfil the sustainable development goal 6 (SDG-6) till 2030, ensuring the most basic human right of clean, safe, and hygienic water.

Enhanced contaminant degradation by FeS under oxic conditions with the coexistence of cobalt and nickel

The process of FeS oxygenation is intrinsically associated with the fate of contaminants in heterogeneous aquifers, whereas the impacts of common heavy metals are poorly understood. In this study, we found that the introduction of Co2+ and Ni2+ (Me2+) resulted in a remarkable 3–10-fold enhancement in the degradation of bisphenol A, which stemmed from the significant increase in the generation of •OH and SO4•−. The improved production of reactive species was primarily attributed to the accelerated regeneration of Fe(II); the introduction of Me2+ manipulated the electronic structure of FeS and consequently facilitated electron transfer from S2- to Fe(III), as revealed by experimental evidence and density functional theory calculations. This study represents the first endeavor to underscore the influence of coexisting heavy metals on contaminant degradation during FeS oxygenation.

Effect of mixed physical barrier on seawater intrusion and nitrate accumulation in coastal unconfined aquifers.

Physical barrier has been proven to be one of the most effective measures to prevent and control seawater intrusion (SWI) in coastal areas. Mixed physical barrier (MPB), a new type of physical barrier, has been shown to have higher efficiency in SWI control. As with conventional subsurface dam and cutoff wall, the construction of MPB may lead to the accumulation of nitrate contaminants in coastal aquifers. We investigated the SWI control capacity and nitrate accumulation in the MPB using a numerical model of variable density flow coupling with reactive transport, and performed sensitivity analysis on the subsurface dam height, cutoff wall depth and opening spacing in the MPB. The differences in SWI control and nitrate accumulation between MPB and conventional subsurface dam and cutoff wall were compared to assess the applicability of different physical barrier. The numerical results show that the construction of MPB will increase the nitrate concentration and contaminated area in the aquifer. The prevention and control efficiency of MPB against SWI is positively correlated with the depth of the cutoff wall, reaching the highest efficiency at the minimum effective dam height, and the retreat distance of the saltwater wedge is positively correlated with the opening spacing. We found a non-monotonic relationship between the change in subsurface dam height and the extent of nitrate accumulation, with total nitrate mass and contaminated area increasing and then decreasing as the height of the subsurface dam increased. The degree of nitrate accumulation increased linearly with increasing the height of the cutoff wall and the opening spacing. Under certain conditions, MPB is 46-53% and 16-57% more efficient in preventing and controlling SWI than conventional subsurface dam and cutoff wall, respectively. However, MPB caused 14-27% and 2-12% more nitrate accumulation than subsurface dam and cutoff wall, respectively. The findings of this study are of great value for the protection of coastal groundwater resources and will help decision makers to select appropriate engineering measures and designs to reduce the accumulation of nitrate pollutants while improving the efficiency of SWI control.

Groundwater sulfate in the Pearl River Delta driven by urbanization: Spatial distribution, sources and factors

Groundwater sulfate contamination becomes a big concern in urbanized areas where urbanization expansion continues. This study investigates the spatial distribution of sulfate in shallow groundwater of the Pearl River Delta (PRD) where urbanization has lasted for more than 40 years, and discusses sources and factors for groundwater sulfate contamination in various aquifers and areas with different land-use types by using hydrochemistry and principal component analysis. A total of 330 groundwater samples, 19 river samples, and 9 landfill leachate samples were collected from the PRD. The results show that groundwater SO42− concentration in the PRD is up to 857.57 mg/L, and the SO42−-rich (>150 mg/L) groundwater accounts for 7.27%. SO42−-rich groundwater in the coastal alluvial aquifers is 3.8 times that in the alluvial-proluvial aquifers, while fissured and karst aquifers are absence of SO42−-rich groundwater. In coastal alluvial aquifers, SO42−-rich groundwater in urbanized areas is 8.5 times that in peri-urban areas. By contrast, in alluvial-proluvial aquifers, SO42-rich groundwater just occurs in urbanized areas rather than other areas. Exogenous input driven by urbanization is the primary factor for SO42−-rich groundwater in the PRD. The occurrence of SO42−-rich groundwater in urbanized and peri-urban areas of the PRD is mainly attributed to the industrial wastewater infiltration and acid precipitation. Lateral flow from rivers is another prominent source for SO42-rich groundwater in the PRD. As a consequence, in order to reduce the emergence of SO42−-rich groundwater in areas of accelerated urban expansion such as the PRD, strengthening the treatment and supervision of wastewater from township enterprises and waste gas from industrial enterprises is recommended.

Pesticides in aquifers from Latin America and the Caribbean

In Latin America and the Caribbean (LAC), agriculture is the primary consumer of water and the biggest user of pesticides worldwide. Given that groundwater is a crucial resource in this region, a systematic review was conducted to evaluate the current state of knowledge on the presence of pesticides in aquifers. The review examined 48 research papers published between 1998 and 2020, and found that only six countries in the region have information on pesticides in groundwater. A total of 70 agrochemicals were detected, encompassing legacy pesticides, herbicides, insecticides, fungicides, and metabolites. Herbicides, including the widely used atrazine and glyphosate, were the most commonly detected current-use pesticides. These herbicides are being gradually banned or restricted due to their potentially harmful effects on the environment. Factors that contribute to the presence of these contaminants in aquifers include preferential flows, seasonal variations in rainfall, aquifer type, unsaturated zone thickness, and land use and management practices. Researchers noted that analysis of these contaminants is often beyond the economic or methodological scope, and analytical capacity in the region is generally limited. Based on the findings of this review, there is a clear need for groundwater pesticide monitoring in the region to reduce health risks to humans and ecosystems.

Demystifying the decadal shift in the extent of groundwater in the coastal aquifers of Gujarat, India: A case of reduced extent but increased magnitude of seawater intrusion

Catastrophic increase in urbanisation and industrialisation along the coastal region leads to increased stress on groundwater reservoirs worldwide. As a growing economy, India faces extreme water crises due to rising water demand and escalating salinisation, specifically in the coastal districts. Therefore, this study shows the implication of a comprehensive modelling approach to assess the spatiotemporal changes in hydrogeochemical processes in the coastal aquifer of the Surat district. Using a multi-model assessment approach, the present study focuses on the decadal evolution in groundwater quality of the coastal aquifers of Surat, Gujarat. Fifty-one groundwater samples were collected for 2008, 2012, and 2018 to assess the spatio-temporal shift in groundwater quality. Piper diagram revealed a shift of hydrogeochemical facies from Mg2+-HCO3− type to Ca2+-Mg2+-Cl− type, indicating the increased salinisation over a decade. The result suggests that rock-water interaction, seawater intrusion mechanism, and anthropogenic activities (intensive agricultural activities and improper waste management) govern the hydrogeochemical processes in the coastal aquifer. A shift of dominance of carbonate weathering to silicate weathering with the dissolution of calcite, dolomite, and gypsum, changing the hydrogeochemistry, was observed over the last decades. This shift leads to the increasing hardness of groundwater. The enrichment of nutrients in groundwater during 2018 (NO3− = 2 to 85 mg. L−1) compared to 2008 (NO3− = 1 to 36 mg.L−1) indicates the increasing imprints of agricultural fertilizer application and human organic waste through sewage contamination on the coastal aquifer. The seawater mixing index model demonstrates that extent of seawater intrusion reduced in 2018 compared to 2012, but the magnitude increased near the coastal talukas (SMI =9.5). The present study helps to understand the increasing anthropogenic activities over a decade leading to increased salinisation and groundwater contamination in the aquifer system. This work can help local stakeholders, water resource managers, and the state government manage the groundwater resources and the future potential threat of aquifer contamination.

A DFN-based framework for probabilistic assessment of groundwater contamination in fractured aquifers

It is challenging to conduct groundwater contamination risk assessment in fractured aquifers containing a large number of complex fractures, especially in a situation where the uncertainty of massive fractures and fluid-rock interactions is inevitable. In this study, a novel probabilistic assessment framework based on discrete fracture network (DFN) modeling is proposed to assess the uncertainty of groundwater contamination in fractured aquifers. The Monte Carlo simulation technique is employed to quantify the uncertainty of fracture geometry, and the environmental and health risks of the contaminated site are probabilistically analyzed in conjunction with the water quality index (WQI) and hazard index (HI). The results show that the contaminant transport behavior in fractured aquifers can be strongly affected by the distribution of the fracture network. The proposed framework of groundwater contamination risk assessment is capable of practically accounting for the uncertainties involved in the mass transport process and effectively assessing the contamination risk of fractured aquifers.

Anthropogenic Contamination in the Free Aquifer of the San Luis Potosí Valley.

The San Luis Potosí valley is an endorheic basin that contains three aquifers: a shallow unconfined aquifer of alluvial material and two deep aquifers, free and confined. The groundwater contamination documented for the shallow aquifer generates contamination of the deep unconfined type aquifer, from which part of the population's drinking water needs are met. This study records incipient anthropogenic contamination of two types: biogenic and potentially toxic trace elements. The studied contaminants include fecal coliform bacteria, total coliform, nitrate, and potentially toxic elements such as: manganese (Mn), mercury (Hg), arsenic (As), and cadmium (Cd). This contamination in some locations exceeds the permissible limit for human consumption. Some major consequences to health, including severe illness, may be caused by the trace elements. The present results give a first signal about the contamination of the deep unconfined type aquifer due to anthropogenic activity in the valley. This is a priority issue because this aquifer supplies drinking water, and in the short or medium term it will have an effect on public health.

Mapping the impacts of the anthropogenic activities and seawater intrusion on the shallow coastal aquifer of Port Said, Egypt

The quality and quantity of groundwater resources have been continuously deteriorating as a result of anthropogenic activities and their excessive usage. This has intensified seawater intrusion, particularly in the coastal area of Egypt. The management of this issue and preventing ongoing groundwater contamination are crucial responsibilities. Thus, an integrated strategy using remote sensing, geophysical technique, and hydrogeochemical analysis is used in this work to identify the causes of degradation and evaluate their impacts on the groundwater quality in East Port Said, Egypt. The following points were identified: 1) Remote sensing analysis between 1984 and 2015 showed an increase in anthropogenic activities, such as the construction of fish farms and vegetation, which became their areas of 12.5 and 37.8 km2 respectively. 2) Field observations demonstrated that the groundwater resources are being overexploited and it is expected that these human activities could have an impact on the groundwater quality. 3) The results of the resistivity approach indicated that sand and clay constitute the underlying layers and the shallow subsurface strata contain a high concentration of saline water. As a result, the aquifer is vulnerable to seawater intrusion due to its homogeneity. 4) Nineteen samples of groundwater were collected from the shallow Quaternary aquifer and the hydrochemical characteristic of the samples was identified. The hydrochemical analysis showed that the groundwater across the research area is of the Na-Cl water type and is highly saline (from 7,558 to 23,218 mg/L). By integrating the aforementioned techniques, it is evident that the research region is affected by anthropogenic activities as well as seawater intrusion on groundwater quality. These results serve as a solid base for further research on groundwater-surface water interactions and the evaluation of possible sources of contamination in the shallow aquifers under stress from anthropogenic activity in such environments.

Geostatistical estimates of groundwater nitrate-nitrogen concentrations with spatial auxiliary information on DRASTIC-LU-based aquifer contamination vulnerability.

Groundwater nitrate-nitrogen contamination typically involves several natural and anthropogenic factors, including those related to hydrology, hydrogeology, topography, and land use (LU). DRASTIC-LU-based aquifer contamination vulnerability could be used to characterize the pollution potentials of groundwater nitrate-nitrogen and to determine groundwater protection zones. This study used regression kriging (RK) with environmental auxiliary information on DRASTIC-LU-based aquifer contamination vulnerability to investigate groundwater nitrate-nitrogen pollution in the Pingtung Plain of Taiwan. First, the relationship between groundwater nitrate-nitrogen pollution and assessments of aquifer contamination vulnerability was determined using stepwise multivariate linear regression (MLR). Subsequently, the residuals between the nitrate-nitrogen observations and MLR predictions were estimated by kriging techniques. Finally, the groundwater nitrate-nitrogen distributions were spatially analyzed using RK, ordinary kriging (OK), and MLR. The findings indicated that the land used for orchards and the medium- and coarse-sand fractions of vadose zones were associated with groundwater nitrate-nitrogen concentrations. The fertilizer used for orchards was identified as the primary source of groundwater nitrate-nitrogen pollution. The RK estimates could be used to analyze the characteristics of the pollution source for land used for orchards and exhibited high spatial variability and accuracy after residual correction. Moreover, RK had an excellent estimate ability for extreme data compared to MLR and OK. Correctly determining groundwater nitrate-nitrogen distributions using RK was useful for administering environmental resources and preventing public health hazards.

Investigating the Potential Impact on Shallow Groundwater Quality of Oily Wastewater Injection in Deep Petroleum Reservoirs: A Multidisciplinary Evaluation at the Val d’Agri Oilfield (Southern Italy)

The increase in oil production from petroleum reservoirs has led to studies examining the effects of these activities on groundwater quality. Oily wastewater associated with oil production is often reinjected through abandoned wells into the unproductive portions of the reservoir to avoid its discharge on the surface. The reinjection process is designed to be environmentally friendly and to exclude direct interactions between injected fluids and the surrounding groundwater; nevertheless, the evaluation of the compatibility between this process and the protection of the surrounding environment is of utmost importance when oilfields are located within sensitive and protected areas. The present work aimed to evaluate the impact of the oily wastewater reinjection into a long-term and high-rate disposal well in the Val d’Agri oilfield (Southern Italy). Previous preliminary investigations carried out at the study site led researchers to hypothesize the possible hydrocarbon contamination of the shallower aquifer caused by reinjection well integrity issues. Our strategy is based on an integrated and multidisciplinary approach involving isotopic (stable isotopes 2H and 18O), chemical, and microbiological (characterization of bacterial and archaeal communities) analyses. After a comprehensive and meticulous examination of the research data, it has been ascertained that significant discrepancies exist between the shallow and reinjection water systems. This allowed us to clarify the area’s complex flow dynamics and exclude hydrocarbon contamination of spring waters caused by the reinjection process.

Three-Dimensional High-Precision Numerical Simulations of Free-Product DNAPL Extraction in Potential Emergency Scenarios: A Test Study in a PCE-Contaminated Alluvial Aquifer (Parma, Northern Italy)

Chlorinated organic compounds are widespread aquifer contaminants. They are known to be dense non-aqueous phase liquids (DNAPLs). Therefore, they are denser than water and immiscible with other fluids. Their migration into the environment in variably saturated zones can cause severe damage. For this reason, optimizing those actions that minimize the negative impact of these compounds in the subsurface is essential. This paper presented a numerical model simulating the free-product DNAPL migration and extraction through a purpose-designed pumping well in a potential emergency scenario. The numerical simulations were performed using CactusHydro, a numerical code that uses a high-resolution shock-capturing flux conservative method to resolve the non-linear coupled partial differential equations of a three-phase immiscible fluid flow recently proposed in the literature, including the contaminant extraction at the base of the aquifer. We investigated the temporal (and spatial) evolution of its migration in the Parma (Northern Italy) porous alluvial aquifer following the saturation contour profiles of the three-phase fluid flow in variably saturated zones. The results indicated that this numerical approach can simulate the contaminant migration in the subsurface and the pumping of the free-product from a well screened at the base of the aquifer system. Moreover, the simulation showed the possibility of recovering about two-thirds of the free-product, in agreement with the scientific literature.

Chromium removal from aqueous solution using bimetallic Bi0/Cu0-based nanocomposite biochar.

Chromium (Cr), due to its greater contamination in aquifers and distinct eco-toxic impacts, is of greater environmental concern. This study aimed to synthesize nanocomposites of almond shells biochar (BC) with zerovalent bismuth and/or copper (Bi0/BC, Cu0/BC, and Bi0-Cu0/BC) for the removal of Cr from aqueous solution. The synthesized nanocomposites were investigated using various characterization techniques such as XRD, FTIR spectroscopy, SEM, and EDX. The Cr removal potential by the nanocomposites was explored under different Cr concentrations (25-100mg/L), adsorbent doses (0.5-2.0g/L), solution pH (2-8), and contact time (10-160min). The above-mentioned advanced techniques verified successful formation of Bi0/Cu0 and their composite with BC. The synthesized nanocomposites were highly effective in the removal of Cr. The Bi0-Cu0/BC nano-biocomposites showed higher Cr removal efficiency (92%) compared to Cu0/BC (85%), Bi0/BC (76%), and BC (67%). The prepared nanocomposites led to effective Cr removal at lower Cr concentrations (25mg/L) and acidic pH (4.0). The Cr solubility changes with pH, resulting in different degrees of Cr removal by Bi0-Cu0/BC, with Cr(VI) being more soluble and easier to adsorb at low pH levels and Cr(III) being less soluble and more difficult to adsorb at high pH levels. The experimental Cr adsorption well fitted with the Freundlich adsorption isotherm model (R2 > 0.99) and pseudo-second-order kinetic model. Among the prepared nanocomposites, the Bi0-Cu0/BC showed greater stability and reusability. It was established that the as-synthesized Bi0-Cu0/BC nano-biocomposite showed excellent adsorption potential for practical Cr removal from contaminated water.

Large-scale arsenic mobilization from legacy sources in anoxic aquifers: Multiple methods and multi-decadal perspectives

While geogenic arsenic (As) contamination of aquifers have been intensively investigated across the world, the mobilization and transport of As from anthropogenic sources have received less scientific attention, despite emerging evidence of poor performance of widely used risk assessment models. In this study we hypothesize that such poor model performance is largely due to insufficient attention to heterogeneous subsurface properties, including the hydraulic conductivity K and the solid-liquid partition (Kd), as well as neglect of laboratory-to-field scaling effects. Our multi-method investigation includes i) inverse transport modelling, ii) in-situ measurements of As concentrations in paired samples of soil and groundwater, and iii) batch equilibrium experiments combined with (iv) geochemical modelling. As case study we use a unique 20-year series of spatially distributed monitoring data, capturing an expanding As plume in a Chromated Copper Arsenate (CCA)-contaminated anoxic aquifer in southern Sweden. The in-situ results showed a high variability in local Kd values of As (1 to 107 L kg−1), implying that over-reliance of data from only one or few locations can lead to interpretations that are inconsistent with field-scale As transport. However, the geometric mean of the local Kd values (14.4 L kg−1) showed high consistency with the independently estimated field-scale “effective Kd” derived from inverse transport modelling (13.6 L kg−1). This provides empirical evidence for the relevance of using geometric averaging when estimating large-scale “effective Kd” values from local measurements within highly heterogenous, isotropic aquifers. Overall, the considered As plume is prolonged by about 0.7 m year−1, now starting to extend beyond the borders of the industrial source area, a problem likely shared with many of the world's As-polluted sites. In this context, geochemical modelling assessments, as presented here, provided a unique understanding of the processes governing As retention, including local variability in, e.g., Fe/Al-(hydr)oxides contents, redox potential and pH.

Coupled experimental assessment and machine learning prediction of mechanical integrity of MICP and cement paste as underground plugging materials

Compromised integrity of cementitious materials can lead to potential geo-hazards such as detrimental fluid flow to the wellbore (borehole), potential leakage of underground stored fluids, contamination of water aquifers, and other issues that could impact environmental sustainability during underground construction operations. The mechanical integrity of wellbore cementitious materials is critical to prevent wellbore failure and leakages, and thus, it is imperative to understand and predict the integrity of oilwell cement (OWC) and microbial-induced calcite precipitation (MICP) to maintain wellbore integrity and ensure zonal isolation at depth. Here, we investigated the mechanical integrity of two cementitious materials (MICP and OWC), and assessed their potential for plugging leakages around the wellbore. Further, we applied Machine Learning (ML) models to upscale and predict near-wellbore mechanical integrity at macro-scale by adopting two ML algorithms, Artificial Neural Network (ANN) and Random Forest (RF), using 100 datasets (containing 100 observations). Fractured portions of rock specimens were treated with MICP and OWC, respectively, and their resultant mechanical integrity (unconfined compressive strength, UCS; fracture toughness, Ks) were evaluated using experimental mechanical tests and ML models. The experimental results showed that although OWC (average UCS = 97 MPa, Ks = 4.3 MPa·√m) has higher mechanical integrity over MICP (average UCS = 86 MPa, Ks = 3.6 MPa·√m), the MICP showed an edge over OWC in sealing microfractures and micro-leakage pathways. Also, the OWC can provide a greater near-wellbore seal than MICP for casing-cement or cement-formation delamination with relatively greater mechanical integrity. The results show that the degree of correlation between the mechanical integrity obtained from lab tests and the ML predictions is high. The best ML algorithm to predict the macro-scale mechanical integrity of a MICP-cemented specimen is the RF model (R2 for UCS = 0.9738 and Ks = 0.9988; MAE for UCS = 1.04 MPa and Ks = 0.02 MPa·√m). Similarly, for OWC-cemented specimen, the best ML algorithm to predict their macro-scale mechanical integrity is the RF model (R2 for UCS = 0.9984 and Ks = 0.9996; MAE for UCS = 0.5 MPa and Ks = 0.01 MPa·√m). This study provides insights into the potential of MICP and OWC as near-wellbore cementitious materials and the applicability of ML model for evaluating and predicting the mechanical integrity of cementitious materials used in near-wellbore to achieve efficient geo-hazard mitigation and environmental protection in engineering and underground operations.