Data-Worth Assessment for a Three-Dimensional Optimal Design in Nonlinear Groundwater Systems

Groundwater model predictions are often uncertain due to inherent uncertainties in model input data. Monitored field data are commonly used to assess the performance of a model and reduce its prediction uncertainty. Given the high cost of data collection, it is imperative to identify the minimum number of required observation wells and to define the optimal locations of sampling points in space and depth. This study proposes a design methodology to optimize the number and location of additional observation wells that will effectively measure multiple hydrogeological parameters at different depths. For this purpose, we incorporated Bayesian model averaging and genetic algorithms into a linear data-worth analysis in order to conduct a three-dimensional location search for new sampling locations. We evaluated the methodology by applying it along a heterogeneous coastal aquifer with limited hydrogeological data that is experiencing salt water intrusion (SWI). The aim of the model was to identify the best locations for sampling head and salinity data, while reducing uncertainty when predicting multiple variables of SWI. The resulting optimal locations for new observation wells varied with the defined design constraints. The optimal design (OD) depended on the ratio of the start-up cost of the monitoring program and the installation cost of the first observation well. The proposed methodology can contribute toward reducing the uncertainties associated with predicting multiple variables in a groundwater system.

On the problem of the spatial distribution delineation of the groundwater quality indicators via multivariate statistical and geostatistical approaches

This paper highlights the advantages of multivariate statistical and geostatistical methods to compile the hydro-geochemical properties of groundwater. A total of 123 samples were collected from wells located in Saveh aquifer, in 2015. Seven parameters including total dissolved solids (TDS), sodium adsorption ratio( SAR), electrical conductivity (EC), sodium (Na⁺), total hardness (TH), chloride (Cl^-), and sulfate (SO₄^2-) were analyzed, compiled, and interpreted statistically and geostatistically. At first, factor analysis gave rise to produce a factor representing 94% of the variability. Also, variography was calculated and compiled to define spatial regression and experimental variograms were plotted by GS⁺ software, then, the best theoretical models were fitted on the variograms and an estimation map was prepared based on geostatistical relationship presented in the paper. Smoothing effect is one of the main drawbacks of forward geostatistical methods, on the contrary, inversed methods are subjected to no smoothing effect. Results showed that geostatistical inversed methods could reveal more reliable results than forward methods. Eventually, the map of the estimated factor, as well as error maps, was compiled. According to the evaluation of fractal dimensions, the estimated factor explained the variability of all hydrogeochemical parameters and groundwater quality was categorized as the safe, normal, and anomalous class, ranged from - 1.10 to 1.10, 1.11 to 3.1, and more than 3.1, respectively.

A spatial study of bladder cancer mortality and incidence in the contiguous US: 2000-2014

Bladder cancer is a significant health issue across the United States of America (USA). Evidence of unequal distribution of a disease or condition's incidence and mortality would suggest that important geographically-defined variables may play a role. In this study, a spatial cluster analysis of bladder cancer mortality identified significant hot spots in some parts of the USA. Regression analysis modelling estimated the effects of selected covariates or risk factors for bladder cancer mortality and also incidence. Spatial heat maps and cluster identification were done for mortality and incidence. The main result was the significant association between bladder cancer mortality and arsenic intake from well water. A similar result was also obtained for cancer incidence and arsenic. Additionally, there are certain geographic areas that appear to have bladder cancer mortality rates beyond the simple association with the studied covariates. These geographic areas warrant further investigation to better understand why cancer mortality is unusually high in such geographic areas and to potentially identify additional local concerns or needs to further address bladder cancer mortality in those specific sites.

Hydrogeochemical controls on arsenic mobility in an arid inland basin, Southeast of Iran: The role of alkaline conditions and salt water intrusion

Elevated inorganic arsenic concentrations in groundwater has become a major public and environmental health concern in different parts of the world. Currently, As-contaminated groundwater issue in many countries and regions is a major topic for publications at global level. However, there are many regions worldwide where the problem has still not been resolved or fully understood due to inadequate hydrogeochemical investigations. Hence, this study evaluates for the first time the hydrogeochemical behavior of the arid and previously unexplored inland basin of Sirjan Plain, south east (SE) Iran, in order to assess the controlling factors which influence arsenic (As) mobility and its distribution through groundwater resources. Total inorganic arsenic concentration was measured using inductive-coupled plasma optical emission spectrometry (ICP-OES). Arsenic content in groundwater of this region ranged between 2.4 and 545.8 μg/L (mean value: 86.6 μg/L) and 50% of the samples exceeded the World Health Organization (WHO) guideline value of 10 μg/L in drinking water. Groundwater was mainly of Na-Cl type and alkaline due to silicate weathering, ion exchange and evaporation in arid conditions. Elevated As concentrations were generally observed under weakly alkaline to alkaline conditions (pH > 7.4). Multivariate statistical analysis including cluster analysis and bi-plot grouped As with pH and HCO₃ and demonstrated that the secondary minerals including oxyhydroxides of Fe are the main source of As in groundwater in this region. The desorption of As from these mineral phases occurs under alkaline conditions in oxidizing arid environments thereby leading to high levels of As in groundwater. Moreover, evaporation, ion exchange and saltwater intrusion were the secondary processes accelerating As release and its mobility in groundwater. Based on the results of this study, desorption of As from metal oxy-hydroxides surfaces under alkaline conditions, evaporation and intrusion of As-rich saline water are considered to be the major factors causing As enrichment in arid inland basins such as those in southeast Iran. This study proposes the regular monitoring and proper groundwater management practices to mitigate high levels of arsenic in groundwater and related drinking water wells of Sirjan Plain.

Metal-containing nanoparticles derived from concealed metal deposits: An important source of toxic nanoparticles in aquatic environments

The potential environmental risks of engineered nanoparticles in aquatic environment have attracted considerable attention, but naturally produced nanoparticles have relatively been ignored, such as ore-related nanoparticles. To obtain more information about the natural ore-related nanoparticles, deep groundwater and well water samples were respectively collected in or around four major metal deposits in Inner Mongolia, China. These water samples were tested with high resolution transmission electron microscopy (TEM) and abundant metal-containing nanoparticles were found. Major ore-forming elements of corresponding metal deposits, such as Fe, Pb, Zn and Cu, and even associated elements, such as As, Sb, Sn and Cr, significantly contributed to the chemical compositions of these detected nanoparticles. Through comparison analyses, these metal-containing nanoparticles were shown to be originally from deep concealed metal deposits. They were the products of faulting and oxidation of ore minerals, and were transported long distances by water flow. Notably, these ore-related nanoparticles happened to have similar components with those nanoparticles of high environmental risks. Coupled with the analytical results of Atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS), it is recommended that the concentration limits of metal-containing nanoparticles should be considered in the safety assessment of drinking water. This is the first time, so far as we know, that naturally produced ore-related nanoparticles in the aquatic environment were listed as a kind of material with environmental risks. Considering the wide distribution of concealed metal deposits, more attention on related studies was urgently required for establishing specialized risk assessment and monitoring system.

On the influence of groundwater table fluctuations on oil thickness in a well related to an LNAPL contaminated aquifer

This paper presents a new modelling approach to describe and explain the temporal variation of oil thickness in well due to groundwater table fluctuations. This new model, which intends to be simple and easy to implement, was compared to field data obtained by continuous measurements of vertical LNAPL position in wells. Two scenarios have been studied: a pumping well where the oil layer is unconfined, and one where the oil layer is present in a confined porous media. This study shows that the time-depend fluctuation of the oil thickness observed in the wells could not be reproduced only with the differences between the residual oil saturations (S_orw and S_ora) as suggested by Kemblowski and Chiang (1990). It should consider the transient mass exchange between the well and the porous media. Also, the proposed model shows that making the assumption of equilibrium conditions as suggested by Lenhard et al. (2017) for calculating the volume exchanges between the wells and its surrounding introduced errors. Considering transient transfers of oil better reflects the field observations. This observation is a key outcome for improving field data interpretation (e.g.: bail-down test data) and the remedial approach at site polluted by mineral oils.

Dynamics and sources of colloids in shallow groundwater in lowland wells and fracture flow in sloping farmland

Field-scale studies of natural colloid mobilization and transport in finely fractured aquifer as well as the source identification of groundwater colloids are of great importance to the safety of shallow groundwater. In this study, the daily monitoring of fracture flow from a sloping farmland plot and the biweekly monitoring of three lowland shallow wells within the same catchment were carried out simultaneously in 2013. The effects of physicochemical perturbations on groundwater colloid dynamics were explored in detail using partial redundancy analysis, structural equation modeling, Pearson correlation and multi-linear regression analyses. The characterization and source identification of groundwater colloids were addressed via multiple parameters. The daily colloid concentration in the fracture flow varied between 0.54 and 31.90 mg/L (1.64 mg/L on average). Unique periods of high colloid concentration (5.59 mg/L on average) occurred during the initially generated flow following the dry season. In comparison, a narrower colloid concentration range of 0.24-11.66 mg/L was observed in the lowland shallow wells, with a smaller temporal variation than that of the fracture flow. A low percentage (2.4-7.0%) of colloids and a high percentage (47.7-92.0%) of coarse particles (2-10 μm) were present in the lowland well water. Hydraulic perturbation by rainwater infiltration in the sloping farmland was the dominant mechanism for colloid mobilization in general; this effect retreated to secondary importance behind chemical perturbations (pH, Mg²⁺ and DOC) at low flow discharges (<1.3 L/min). In contrast, water chemistry (e.g., EC, cations and DOC concentrations) exhibited a major effect on colloid dynamics in the water of the lowland wells, except for the extremely high-salinity water of one well, in which water temperature showed a negative dominant influence on colloid stability. The combined use of multiple parameters (e.g., mineral composition and organic matter, calcium carbonate and δ¹³C contents) traced groundwater colloids to the shallow soil in the upper farmlands. It is strongly advised that in finely fractured aquifers within agricultural catchments, not only the small colloids but also the coarse particles in the size range of 2-10 μm should be monitored in case of colloid-associated contamination from agricultural wastes e.g., N, P, pesticides and/or heavy metals, especially at the early stages of the rainy seasons.

Human health risk assessment for the occurrence of enteric viruses in drinking water from wells: Role of flood runoff injections

We demonstrated that floods can induce severe microbiological contamination of drinking water from wells and suggest strategies to better address water safety plans for groundwater drinking supplies. Since 2002, the Italian Water Research Institute (IRSA) has detected hepatitis A virus, adenovirus, rotavirus, norovirus, and enterovirus in water samples from wells in the Salento peninsula, southern Italy. Perturbations in the ionic strength in water flow can initiate strong virus detachments from terra rossa sediments in karst fractures. This study therefore explored the potential health impacts of prolonged runoff injections in Salento groundwater caused by severe flooding during October 2018. A mathematical model for virus fate and transport in fractures was applied to determine the impact of floodwater injection on groundwater quality by incorporating mechanisms that affect virus attachment/detachment and survival in flowing water at microscale. This model predicted target concentrations of enteric viruses that can occur unexpectedly in wells at considerable distances (5-8 km) from the runoff injection site (sinkhole). Subsequently, the health impact of viruses in drinking water supplied from contaminated wells was estimated during the summer on the Salento coast. Specific unpublished dose-response model coefficients were proposed to determine the infection probabilities for Echo-11 and Polio 1 enteroviruses through ingestion. The median (50%) risk of infection was estimated at 6.3 · 10^-3 with an uncertainty of 23%. The predicted burden of diseases was 4.89 disability adjusted life years per year, i.e., twice the maximum tolerable disease burden. The results highlight the requirement for additional water disinfection treatments in Salento prior to the distribution of drinking water. Moreover, monthly controls of enteric virus occurrence in water from wells should be imposed by a new water framework directive in semiarid regions because of the vulnerability of karst carbonate aquifers to prolonged floodwater injections and enteric virus contamination.

Mapping access to domestic water supplies from incomplete data in developing countries: An illustrative assessment for Kenya

Water point mapping databases, generated through surveys of water sources such as wells and boreholes, are now available in many low and middle income countries, but often suffer from incomplete coverage. To address the partial coverage in such databases and gain insights into spatial patterns of water resource use, this study investigated the use of a maximum entropy (MaxEnt) approach to predict the geospatial distribution of drinking-water sources, using two types of unimproved sources in Kenya as illustration. Geographic locations of unprotected dug wells and surface water sources derived from the Water Point Data Exchange (WPDx) database were used as inputs to the MaxEnt model alongside geological/hydrogeological and socio-economic covariates. Predictive performance of the MaxEnt models was high (all > 0.9) based on Area Under the Receiver Operator Curve (AUC), and the predicted spatial distribution of water point was broadly consistent with household use of these unimproved drinking-water sources reported in household survey and census data. In developing countries where geospatial datasets concerning drinking-water sources often have necessarily limited resolution or incomplete spatial coverage, the modelled surface can provide an initial indication of the geography of unimproved drinking-water sources to target unserved populations and assess water source vulnerability to contamination and hazards.

Fluoride Occurrence and Human Health Risk in Drinking Water Wells from Southern Edge of Chinese Loess Plateau

Fluoride hydrogeochemistry and associated human health risks implications are investigated in several aquifers along the southern edge of the Chinese Loess Plateau. Locally, 64% shallow groundwater samples in loess aquifer exceed the fluoride limit (1.5 mg/L) with the maximum of 3.8 mg/L. Presently, the shallow groundwater is the main source of private wells for domestic use, and this is clearly a potential risk for human health. Hydrogeochemistry and stable isotopes are used to elucidate the diversity of occurrence mechanisms. Enrichment of fluoride in groundwater is largely controlled by the F-containing minerals dissolution. Furthermore, alkaline condition and calcium-removing processes promote water-rock interactions. Stable isotopes of hydrogen and oxygen (δD and δ18O) in study area waters demonstrate that groundwater in loess aquifer is old, which means groundwater remains in the aquifer for a long time. Long residence time induces sufficient water-rock interactions, which play significant roles in the resolution of fluoride minerals. Samples from the shallow loess aquifer show elevated fluoride levels, which may pose human health risk for both adults (60%) and children (94%) via oral intake. To ensure drinking water safety, management measures such as popularizing fluoride-removing techniques and optimizing water supply strategies need to be implemented.

Temporal Scaling Analytical Method to Identify Multi-Fractionality in Groundwater Head Fluctuations

Natural dynamics such as groundwater head fluctuations may exhibit multi-fractionality, likely caused by multi-scale aquifer heterogeneity and other controlling factors, whose statistics requires efficient quantification methods. As a scaling exponent, the Hurst exponent can describe the temporal correlation or multifractal behavior in groundwater level fluctuation processes. However, the scaling behavior may change with time under natural conditions, likely due to the non-stationary evolution of internal and external conditions, which cannot be characterized by traditional methods using a single or several scaling exponents for the complex features of the overall process. This methods note quantifies the multi-fractionality using the timescale local Hurst exponent (TS-LHE) and then proposes a systematic statistical method to analyze groundwater head fluctuations. Time series of daily groundwater level fluctuations from three wells located in the lower Mississippi valley are analyzed, after removing the seasonal cycle, which leads to transient TS-LHE, implying multi-fractionality and multifractal-scaling behavior that changes with time and location. Therefore, the temporal scaling analysis proposed here may provide useful and quantitative information to understand the nature of dynamic hydrologic systems.

Affordable and efficient adsorbent for arsenic removal from rural water supply systems in Newfoundland

The fly ash from the Corner Brook Pulp and Paper (CBPP) mill was used in this study as the raw material for the preparation of a low-cost adsorbent for arsenic removal from the well water of Bell Island. The CBPP fly ash was physically activated in two different ways: (a) activation with pure CO₂ (CAC) with the iodine number and methylene value of 704.53 mg/g and 292.32 mg/g, respectively; and (b) activation with a mixture of CO₂ and steam (CSAC) with the iodine number and methylene value of 1119.98 mg/g and 358.95 mg/g, respectively, at the optimized temperature of 850 °C and the time of 2 h for both activations. The BET surface areas of the CAC and CSAC at the optimized conditions were 847.26 m²/g and 1146.25 m²/g, respectively. The optimized CSAC was used for impregnation with iron (III) chloride (FeCl₃) with different concentrations (0.01 M to 1 M). The study shows that the adsorbent impregnated with 0.1 M FeCl₃ is the most efficient adsorbent for arsenic removal. Isotherm analysis shows that the Langmuir model better describes the equilibrium behavior of the arsenic adsorption from both local well water and synthesized water compared to the other models. The maximum arsenic adsorption capacity was 35.6 μg/g of carbon for local well water and 1428.6 μg/g of carbon for synthesized water. Furthermore, the kinetic behavior of arsenic adsorption from synthesized and local well water was well depicted by the pseudo-second order kinetic model.

Hydrogeochemical characterization and evaluation of groundwater quality in Kangayam taluk, Tirupur district, Tamil Nadu, India, using GIS techniques

The main objective of the present study is to evaluate the hydrogeochemical characteristics of groundwater and its suitability for drinking water supply in Kangayam taluk, Tirupur district, Tamil Nadu, India. To achieve this objective, seventy-eight groundwater samples were collected from the wells spread over the study area during December 2016. The collected groundwater samples were tested in the laboratory for various hydrogeochemical parameters such as hydrogen ion concentration (pH), electrical conductivity, total dissolved solids, total hardness, calcium, magnesium, sodium, potassium, chloride, bicarbonate, carbonate, nitrate, sulphate and fluoride. The analytical results were compared with WHO drinking water standards to assess the suitability of groundwater for drinking purposes. To understand the spatial variation of hydrogeochemical parameters over the study area, choropleth (zonation) maps were prepared using geographical information system (GIS). Overall groundwater quality zones were demarcated by overlaying and integrating all the spatial plots using GIS. Three groundwater quality zones such as (1) most desirable, (2) maximum allowable and (3) not permissible were demarcated based on the limits prescribed by the WHO for drinking purposes. This study indicates that 49% of the study area does not possess potable groundwater. About 21% of the area represents "most desirable" category, and the remaining 30% area represents "maximum allowable" category for drinking purposes. The Piper's trilinear diagram indicates that groundwater of this region is Mixed CaMgCl type. As the groundwater quality is poor nearly 49% of the total area, it is necessary to go for treatment before drinking water supply. It is also essential to recharge the aquifer artificially to improve the quantity and quality of groundwater.

Experimental and numerical evaluation of Groundwater Circulation Wells as a remediation technology for persistent, low permeability contaminant source zones

Contaminants removal stoked inside low permeability zones of aquifers is one of the most important challenge of groundwater remediation process today. Low permeability layers can be considered persistent secondary sources of contamination because they release pollutants by molecular diffusion after primary source of contamination is reduced, causing long plum tails (Back-Diffusion). In this study, the Groundwater Circulation Well (GCW) system was investigated as an alternative remediation technology to the low efficient traditional pumping technologies to restore contaminated low permeability layers of aquifers. The GCW system creates vertical groundwater circulation cells by drawing groundwater through a screen of a multi-screen well and discharging it through another screen. The suitability of this technology to remediate contaminated low permeability zones was investigated by laboratory test and numerical simulations. The collected data were used to calibrate a model created to simulate the Back-Diffusion process and to evaluate the effect of different pumping technologies on the depletion time of that process. Results show that the efficiency of the GCW is dependent on the position and on the geometry of the low permeability zones, however the GCW system appears more suitable to restore contaminated low permeability layers of aquifers than the traditional pumping technology.

Groundwater flow velocities in a fractured carbonate aquifer-type: Implications for contaminant transport

Contaminants that are highly soluble in groundwater are rapidly transported via fractures in mechanically resistant sedimentary rock aquifers. Hence, a rigorous methodology is needed to estimate groundwater flow velocities in such fractured aquifers. Here, we propose an approach using borehole hydraulic testing to compute flow velocities in an un-faulted area of a fractured carbonate aquifer by applying the cubic law to a parallel plate model. The Cadeby Formation (Yorkshire, NE England) - a Permian dolostone aquifer present beneath the University of Leeds Farm - is the fractured aquifer selected for this hydraulic experiment. The bedding plane fractures of this dolostone aquifer, which are sub-horizontal, sub-parallel and laterally persistent, largely dominate the flow at shallow (<~40 mBGL) depths. These flowing bedding plane discontinuities are separated by a rock matrix which is relatively impermeable (K_well-test/K_core-plug~10⁴) as is common in fractured carbonate aquifers. In the workflow reported here, the number of flowing fractures - mainly bedding plane fractures - intersecting three open monitoring wells are found from temperature/fluid conductivity and acoustic/optical televiewer logging. Following well installation, average fracture hydraulic apertures for screened intervals are found from analysis of slug tests. For the case study aquifer, this workflow predicts hydraulic apertures ranging from 0.10 up to 0.54 mm. However, groundwater flow velocities range within two order of magnitude from 13 up to 242 m/day. Notably, fracture apertures and flow velocities rapidly reduce with increasing depth below the water table; the upper ~10 m shows relatively high values of hydraulic conductivity (0.30-2.85 m/day) and corresponding flow velocity (33-242 m/day). Permeability development around the water table in carbonate aquifer-types is common, and arises where high pCO₂ recharge water from the soil zone causes calcite/dolomite dissolution. Hence, agricultural contaminants entering the aquifer with recharge water are laterally transported rapidly within this upper part. Computation of groundwater flow velocities allows determination of the Reynolds number. Values of up ~1, indicating the lower limit of the transition from laminar to turbulent flow, are found at the studied site, which is situated away from major fault traces. Hence, turbulent flow is likely to arise in proximity to tectonic structures, such as normal faults, which localize flow and enhance karstification. The occurrence of turbulent flow in correspondence of such tectonic structures should be represented in regional groundwater flow simulations.

Deep tubewell microbial water quality and access in arsenic mitigation programs in rural Bangladesh

The objective of this paper is to determine whether deep tubewells installed through arsenic mitigation efforts in rural Bangladesh provide better drinking water microbial quality compared to shallow tubewells. We conducted a stratified random cross-sectional survey of 484 households to assess microbial contamination of deep tubewell water at source and at point of use (POU) compared to shallow tubewell water using the Compartment Bag Test. In addition, we measured storage time, distance, travel time and ownership status among both sets of users to assess deep tubewell efficacy and under what conditions they offer poorer or better water quality. Differences in tubewell characteristics were compared using non-parametric Mann-Whitney U tests and two-proportion Z-tests. Prevalence ratios of microbial contamination stratified by water quality, storage time and distance to tubewells and ownership were estimated using unadjusted Mantel-Haenszel tests. There was no significant difference in microbial contamination between shallow and deep tubewells at source. The presence of POU water microbial contamination in storage containers in deep tubewell households was 1.11 times the prevalence in shallow tubewell storage containers (95% CI = 0.97-1.27). Deep tubewell users stored water longer and walked significantly farther to obtain water compared to shallow tubewell users. Among deep tubewell households, those residing farther away from the source were 1.24 times as likely to drink contaminated water from storage containers compared to those located nearby (95% CI = 1.04-1.48). Our findings suggest that deep tubewells have comparable water quality to shallow tubewells at source, but increasing distance from the household exacerbates risk of microbial contamination at POU.

Changes in arsenic exposure in Araihazar, Bangladesh from 2001 through 2015 following a blanket well testing and education campaign

BACKGROUND

Concentrations of arsenic (As) are elevated in a large proportion of wells in Bangladesh but are spatially variable even within a village. This heterogeneity can enable exposed households to switch to a nearby well lower in As in response to blanket (area-wide) well As testing.

OBJECTIVES

We document the evolution of As exposure in Araihazar, Bangladesh following a blanket well testing and education campaign, as well as the installation of a considerable number of low As community wells.

METHODS

We use well water and urinary As data collected between 2000 and 2008, along with household interviews extending through 2016, within a 25 km2 area of Araihazar upazila for nearly 12,000 participants enrolled in the Health Effects of Arsenic Longitudinal Study (HEALS). We observe changes in participants' well water and urinary As concentrations following interventions to lower their exposure and use logistic regression to determine the factors associated with participants' decisions to switch primary household wells.

RESULTS

Urinary As for participants drinking from wells with >100 μg/L As at baseline declined from a mean of 226 μg/L at baseline to 173 μg/L two years later, and further declined to 139 μg/L over 8 years. For comparison, urinary As concentrations for participants drinking from wells with ≤10 μg/L As remained close to 50 μg/L throughout. Whereas the interventions only partially reduced exposure, well status with respect to As was predictive of well-switching decisions for at least a decade after the initial testing. Participants with high-As wells were 7 times more likely to switch wells over the first two years and 1.4-1.8 times more likely to switch wells over the ensuing decade.

CONCLUSIONS

Arsenic exposure gradually declined following blanket well testing, an education campaign, and the installation of community wells but remained almost three times higher than for a subgroup of the participants drinking from wells with ≤10 μg/L. In addition, the number of participants with unknown As concentrations in their primary household wells increased substantially over time, indicating the importance of additional well testing as new wells continue to be installed, in addition to other means of reducing As exposure.

An alternative approach to control saltwater intrusion in coastal aquifers using a freshwater surface recharge canal

Aquifers are a major source of freshwater in many parts of the world. Saltwater intrusion from the sea or saline lakes into freshwater aquifers degrades the potable quality of these resources. Various methods have been introduced to mitigate saltwater intrusion, such as recharge wells and physical subsurface barriers. This paper presents an alternative approach to control saltwater intrusion in coastal aquifers using a surface water recharge canal. In this paper, the effectiveness of a recharge canal at mitigating saltwater intrusion is evaluated numerically using SEAWAT. The results indicate that the recharge canal leads to a reduction in the extent of the saltwater intrusion. Under a fixed hydraulic gradient, the extent of this reduction is dependent on the location of the recharge canal relative to the saltwater source. As the hydraulic gradient increases, with the optimum location of the recharge canal approaches the saltwater source location. The results also indicate that more effective saltwater repulsion is achieved when the recharge canal is located near the toe of the saltwater wedge. The results of a field scale case study indicate that a recharge canal with relatively small dimensions could have a significant effect on reduction in the extent of the saltwater intrusion.

Elemental enrichment of sediments in an unprotected shallow groundwater of Lagos and Ogun States, Nigeria

Sediments quality is a good indicator of pollution in a water body where various elements were concentrated. Limited information is available on sediments from hand-dug wells. The present study evaluates sediment samples collected from groundwater of Lagos and Ogun States, Southwest Nigeria. Twenty sediment samples were collected from shallow groundwater, and ten rock samples were also collected from the vicinity of recently dug wells. Trace elements were determined in both the sediments and the rock samples using inductively coupled plasma-mass spectrometry. The enrichment factor was calculated for different elements using Fe, Ti, Mn and Cu as normalizing elements. The order of sediment contamination with each normalizing elements are Cu > Mn > Ti > Fe. The geo-accumulation (Igeo) indexes for Cr, Pb, Cu and Ni are 1.31, 1.05, 1.94 and 1.85, respectively. The Igeo for Lagos sediments is in the order Cr > Pb > Ni > Cu, while in Ogun sediments the order is Cu > Ni > Cr > Pb. The results were compared with Canadian Council of Ministers of Environment values of threshold effect level and probable effect level, which shows the sediments are not toxic. The pollution load index and ecological risk index values are 2.463 and 0.0014, respectively, which further indicates the sediments are not toxic in nature. The major source of most elements in sediments is the host rocks found in the vicinity of the groundwater while high level of some elements recorded in sediments are from the anthropogenic sources.

Redesigning and monitoring groundwater quality and quantity networks by using the entropy theory

This study aimed at redesigning and monitoring the groundwater network of Naqadeh plain in the southwest of Lake Urmia to examine the number and position of optimal wells for the salinity information transfer (EC) and survey of groundwater level at aquifer. In this regard, groundwater level data (35 wells) and electrical conductivity values (24 wells) were used during a 10-year period (2002-2012). In the first stage, simulation was conducted using the multivariate regression method and quantitative and qualitative values and the interaction of wells was observed. In the next stage, number of different classes was considered for clustering quantitative and quantitative values. The results of studying different classes of data clustering showed that the 12-class cluster had more accurate results based on the root mean square error and coefficient of determination. The root mean square error was improved by about 40, 21, and 15%, respectively, compared to the 3, 5, and 9-classe clusters. Finally, by choosing proper cluster of data, entropy indicators were investigated for quantitative and qualitative values at the aquifer level. The results of entropy indices at the aquifer showed that there was a severe shortage of information in terms of salinity in the Northwest of the aquifer, which necessitates drilling a new well in this area to accurately monitor the EC values. However, since more than 90% of the basin area is in surplus and approximately surplus conditions in terms of transferring information, the studied area has a good dispersion for qualitative monitoring. Information transfer index for the quantitative groundwater network monitoring showed that piezometers near Lake Urmia were faced with a lack of information, which according to piezometers ranking, is ranked last in terms of value of maintaining or keeping the network. Eastern areas of aquifer are also faced with shortage of piezometers accounting for about 3% of the total area. The results of survey of surplus wells in the aquifer showed that nine and six surplus wells are in the aquifer for the qualitative and quantitative network, respectively. There were also wells in which information transfer was not well done and their information could not be assured. Finally, based on the conditions, a new arrangement of wells and a new optimal network were proposed.

Arsenic contamination of rural community wells in Nicaragua: A review of two decades of experience

Several surveys have been conducted in Nicaragua between 1996 and 2015 confirming the presence of high levels of arsenic (>10 μg/L). In this paper, these peer-reviewed (n = 2) and non-peer reviewed sources (n = 14) have been combined to provide an extensive overview of the arsenic contamination of drinking water sources in Nicaragua. So far, arsenic contamination has been detected in over 80 rural communities located in 34 municipalities of the country and arsenic poisoning has been identified in at least six of those communities. The source of arsenic contamination in Nicaragua is probably volcanic in origin, both from volcanic rocks and geothermal fluids which are distributed across the country. Arsenic may have directly entered into the groundwater by geothermally-influenced water bodies, or indirectly by reductive dissolution or alkali desorption, depending on the local geochemical conditions.

Modeling groundwater nitrate exposure in private wells of North Carolina for the Agricultural Health Study

Unregulated private wells in the United States are susceptible to many groundwater contaminants. Ingestion of nitrate, the most common anthropogenic private well contaminant in the United States, can lead to the endogenous formation of N-nitroso-compounds, which are known human carcinogens. In this study, we expand upon previous efforts to model private well groundwater nitrate concentration in North Carolina by developing multiple machine learning models and testing against out-of-sample prediction. Our purpose was to develop exposure estimates in unmonitored areas for use in the Agricultural Health Study (AHS) cohort. Using approximately 22,000 private well nitrate measurements in North Carolina, we trained and tested continuous models including a censored maximum likelihood-based linear model, random forest, gradient boosted machine, support vector machine, neural networks, and kriging. Continuous nitrate models had low predictive performance (R2 < 0.33), so multiple random forest classification models were also trained and tested. The final classification approach predicted <1 mg/L, 1-5 mg/L, and ≥5 mg/L using a random forest model with 58 variables and maximizing the Cohen's kappa statistic. The final model had an overall accuracy of 0.75 and high specificity for the higher two categories and high sensitivity for the lowest category. The results will be used for the categorical prediction of private well nitrate for AHS cohort participants that reside in North Carolina.

Modeling Stream-Aquifer Interactions Under Seasonal Groundwater Pumping and Managed Aquifer Recharge

In South Korea, a significant amount of groundwater is used for the heating of water-curtain insulated greenhouses during the winter dry season, which had led to problems of groundwater depletion. A managed aquifer recharge (MAR) project is currently underway with the goal of preventing such groundwater depletion in a typical cultivation area, located on an alluvial aquifer near the Nam River. In the present study, FEFLOW, a three-dimensional finite element model, was used to evaluate different strategies for MAR of the cultivation areas. A conceptual model was developed to simulate the stream-aquifer dynamics under the influence of seasonal groundwater pumping and MAR. The optimal rates and duration of MAR were assessed by analyzing the recovery of the groundwater levels and the change in the groundwater temperature. The simulation results indicate that a MAR rate of 8000 m³ /d effectively restores the groundwater level when the injection wells are located inside the groundwater depletion area. It is also demonstrated that starting the MAR before the beginning of the seasonal pumping is more effective. Riverbank filtration is preferable for securing the injection water owing to plentiful source of induced recharge from the river. Locating the pumping wells adjacent to the river where there are thick permeable layers could be a good strategy for minimizing decreases in the groundwater level and temperature.

Effects of Intraborehole Flow on Purging and Sampling Long-Screened or Open Wells

Hydraulic head differences across the screened or open interval of a well significantly influence the sampled water mixture. Sample bias can occur due to an insufficient pumping rate and/or due to native groundwater displacement by intraborehole flow (IBF). Proper understanding of the sampled water mixture is crucial for accurate interpretation of environmental tracers and groundwater chemistry data, and hence groundwater characterization. This paper uses numerical modeling to quantify sample bias caused by IBF in an un-pumped high-yield well, and the influence of pumping rate and heterogeneity on the volume of pumpage required to purge an IBF plume. The results show that (1) the pumping rate must be at least an order of magnitude greater than the IBF rate to achieve permeability-weighted yield, (2) purge volume was 2.2 to 20.6 times larger than the IBF plume volume, with the ratio depending on plume location relative to hydraulic conductivity and head distributions, and (3) after an example 1000-day un-pumped period, purging required removal of at least three orders of magnitude more water than the common practice of three to five well volumes. These results highlight the importance of knowing the borehole flow regime to identify IBF inflow and outflow zones, estimate IBF rates, and to develop a strategic sampling approach.

Multi-Objective Optimization of Managed Aquifer Recharge

This study demonstrates the utilization of a multi-objective hybrid global/local optimization algorithm for solving managed aquifer recharge (MAR) design problems, in which the decision variables included spatial arrangement of water injection and abstraction wells and time-variant rates of pumping and injection. The objective of the optimization was to maximize the efficiency of the MAR scheme, which includes both quantitative and qualitative aspects. The case study used to demonstrate the capabilities of the proposed approach is based on a published report on designing a real MAR site with defined aquifer properties, chemical groundwater characteristics as well as quality and volumes of injected water. The demonstration problems include steady state and transient scenarios. The steady state scenario demonstrates optimization of spatial arrangement of multiple injection and recovery wells, whereas the transient scenario was developed with the purpose of finding optimal regimes of water injection and recovery at a single location. Both problems were defined as multi-objective problems. The scenarios were simulated by applying coupled numerical groundwater flow and solute transport models: MODFLOW-2005 and MT3D-USGS. The applied optimization method was a combination of global (the non-dominated sorting genetic algorithm [NSGA-2]) and local (the Nelder-Mead downhill simplex search algorithms). The analysis of the resulting Pareto optimal solutions led to the discovery of valuable patterns and dependencies between the decision variables, model properties, and problem objectives. Additionally, the performance of the traditional global and the hybrid optimization schemes were compared.