Field Investigation of Vapor‐Phase‐Based Groundwater Monitoring

The use of in‐field analysis of vapor‐phase samples to provide real‐time volatile organic compound (VOC) concentrations in groundwater has the potential to streamline monitoring by simplifying the sample collection and analysis process. A field validation program was completed to (1) evaluate methods for collection of vapor samples from monitoring wells and (2) evaluate the accuracy and precision of field‐portable instruments for the analysis of vapor‐phase samples. The field program evaluated three vapor‐phase sample collection methods: (1) headspace samples from two locations within the well, (2) passive vapor diffusion (PVD) samplers placed at the screened interval of the well, and (3) field vapor headspace analysis of groundwater samples. Two types of instruments were tested: a field‐portable gas chromatograph (GC) and a photoionization detector (PID). Field GC analysis of PVD samples showed no bias and good correlation to laboratory analysis of groundwater collected by low‐flow sampling (slope = 0.96, R² = 0.85) and laboratory analysis of passive water diffusion bag samples from the well screen (slope = 1.03; R² = 0.96). Field GC analysis of well headspace samples, either from the upper portion of the well or at the water‐vapor interface, resulted in higher variability and much poorer correlation (consistently biased low) relative to laboratory analysis of groundwater samples collected by low‐flow sample or passive diffusion bags (PDBs) (slope = 0.69 to 0.76; R² = 0.60 to 0.64). These results indicate that field analysis of vapor‐phase samples can be used to obtain accurate measurements of VOC concentrations in groundwater. However, vapor samples collected from the well headspace were not in equilibrium with water collected from the well screen. Instead, PVD samplers placed in the screened interval represent the most promising approach for field‐based measurement of groundwater concentrations using vapor monitoring techniques and will be the focus of further field testing.     

Screening Indian Mustard Genotypes for Phytoremediating Arsenic‐Contaminated Soils

Ten Indian mustard (Brassica juncea L.) genotypes were screened for their phytoremediation potential for arsenic (As) contaminated water under laboratory‐controlled conditions. The genotypes were grown in a hydroponic chamber for 20 days in 250‐mL beakers containing As‐contaminated water. During plant development, changes in plant growth, biomass, and total As were evaluated. Of the 10 genotypes (Pusa Agrani, BTO, Pusa Kranti, Pusa Bahar, Pusa Bold, Pusa Basant, Pusa Jai Kisan, Arka Vardhan, Varuna, and Vaibhav) Pusa Jai Kisan was the most effective in phytoremediating As‐contaminated water under hydroponic conditions. This will provide new information for Indian mustard genotypes for phytoremediating As‐contaminated soils.     

Reinterpreting the Importance of Oxygen‐Based Biodegradation in Chloroethene‐Contaminated Groundwater

Chlororespiration is common in shallow aquifer systems under conditions nominally identified as anoxic. Consequently, chlororespiration is a key component of remediation at many chloroethene‐contaminated sites. In some instances, limited accumulation of reductive dechlorination daughter products is interpreted as evidence that natural attenuation is not adequate for site remediation. This conclusion is justified when evidence for parent compound (tetrachloroethene, PCE, or trichloroethene, TCE) degradation is lacking. For many chloroethene‐contaminated shallow aquifer systems, however, nonconservative losses of the parent compounds are clear but the mass balance between parent compound attenuation and accumulation of reductive dechlorination daughter products is incomplete. Incomplete mass balance indicates a failure to account for important contaminant attenuation mechanisms and is consistent with contaminant degradation to nondiagnostic mineralization products like CO₂. While anoxic mineralization of chloroethene compounds has been proposed previously, recent results suggest that oxygen‐based mineralization of chloroethenes also can be significant at dissolved oxygen concentrations below the currently accepted field standard for nominally anoxic conditions. Thus, reassessment of the role and potential importance of low concentrations of oxygen in chloroethene biodegradation are needed, because mischaracterization of operant biodegradation processes can lead to expensive and ineffective remedial actions. A modified interpretive framework is provided for assessing the potential for chloroethene biodegradation under different redox conditions and the probable role of oxygen in chloroethene biodegradation.     

Long‐Term Groundwater Depletion in the United States

The volume of groundwater stored in the subsurface in the United States decreased by almost 1000 km³ during 1900–2008. The aquifer systems with the three largest volumes of storage depletion include the High Plains aquifer, the Mississippi Embayment section of the Gulf Coastal Plain aquifer system, and the Central Valley of California. Depletion rates accelerated during 1945–1960, averaging 13.6 km³/year during the last half of the century, and after 2000 increased again to about 24 km³/year. Depletion intensity is a new parameter, introduced here, to provide a more consistent basis for comparing storage depletion problems among various aquifers by factoring in time and areal extent of the aquifer. During 2001–2008, the Central Valley of California had the largest depletion intensity. Groundwater depletion in the United States can explain 1.4% of observed sea‐level rise during the 108‐year study period and 2.1% during 2001–2008. Groundwater depletion must be confronted on local and regional scales to help reduce demand (primarily in irrigated agriculture) and/or increase supply.     

Community‐Based Groundwater Monitoring Network Using a Citizen‐Science Approach

Water level monitoring provides essential information about the condition of aquifers and their responses to water extraction, land‐use change, and climatic variability. It is important to have a spatially distributed, long‐term monitoring well network for sustainable groundwater resource management. Community‐based monitoring involving citizen scientists provides an approach to complement existing government‐run monitoring programs. This article demonstrates the feasibility of establishing a large‐scale water level monitoring network of private water supply wells using an example from Rocky View County (3900 km²) in Alberta, Canada. In this network, community volunteers measure the water level in their wells, and enter these data through a web‐based data portal, which allows the public to view and download these data. The close collaboration among the university researchers, county staff members, and community volunteers enabled the successful implementation and operation of the network for a 5‐year pilot period, which generated valuable data sets. The monitoring program was accompanied by education and outreach programs, in which the educational materials on groundwater were developed in collaboration with science teachers from local schools. The methodology used in this study can be easily adopted by other municipalities and watershed stewardship groups interested in groundwater monitoring. As governments are starting to rely increasingly on local municipalities and conservation authorities for watershed management and planning, community‐based groundwater monitoring provides an effective and affordable tool for sustainable water resources management.     

Simulating Stable Isotope Ratios in Plumes of Groundwater Pollutants with BIOSCREEN‐AT‐ISO

BIOSCREEN is a well‐known simple tool for evaluating the transport of dissolved contaminants in groundwater, ideal for rapid screening and teaching. This work extends the BIOSCREEN model for the calculation of stable isotope ratios in contaminants. A three‐dimensional exact solution of the reactive transport from a patch source, accounting for fractionation by first‐order decay and/or sorption, is used. The results match those from a previously published isotope model but are much simpler to obtain. Two different isotopes may be computed, and dual isotope plots can be viewed. The dual isotope assessment is a rapidly emerging new approach for identifying process mechanisms in aquifers. Furthermore, deviations of isotope ratios at specific reactive positions with respect to “bulk” ratios in the whole compound can be simulated. This model is named BIOSCREEN‐AT‐ISO and will be downloadable from the journal homepage.     

Using DNA‐Stable Isotope Probing to Identify MTBE‐ and TBA‐Degrading Microorganisms in Contaminated Groundwater

Although the anaerobic biodegradation of methyl tert‐butyl ether (MTBE) and tert‐butyl alcohol (TBA) has been documented in the laboratory and the field, knowledge of the microorganisms and mechanisms involved is still lacking. In this study, DNA‐stable isotope probing (SIP) was used to identify microorganisms involved in anaerobic fuel oxygenate biodegradation in a sulfate‐reducing MTBE and TBA plume. Microorganisms were collected in the field using Bio‐Sep® beads amended with ¹³C₅‐MTBE, ¹³C₁‐MTBE (only methoxy carbon labeled), or ¹³C₄‐TBA. ¹³C‐DNA and ¹²C‐DNA extracted from the Bio‐Sep beads were cloned and 16S rRNA gene sequences were used to identify the indigenous microorganisms involved in degrading the methoxy group of MTBE and the tert‐butyl group of MTBE and TBA. Results indicated that microorganisms were actively degrading ¹³C‐labeled MTBE and TBA in situ and the ¹³C was incorporated into their DNA. Several sequences related to known MTBE‐ and TBA‐degraders in the Burkholderiales and the Sphingomonadales orders were detected in all three ¹³C clone libraries and were likely to be primary degraders at the site. Sequences related to sulfate‐reducing bacteria and iron‐reducers, such as Geobacter and Geothrix, were only detected in the clone libraries where MTBE and TBA were fully labeled with ¹³C, suggesting that they were involved in processing carbon from the tert‐butyl group. Sequences similar to the Pseudomonas genus predominated in the clone library where only the methoxy carbon of MTBE was labeled with ¹³C. It is likely that members of this genus were secondary degraders cross‐feeding on ¹³C‐labeled metabolites such as acetate.     

Hydrogeochemical Evolution of Interaction Between Surface Water and Groundwater Affected by Exploitation

Hydrogeochemical evolution of interactions between surface water and groundwater is crucial for guaranteeing water supply quality in a riverside water source area. This study focuses on the seasonal and spatial characteristics of hydrogeochemical evolution affected by groundwater exploitation in the Hulan water source area using hydrochemical analyses and stable isotope tracers. Results show that the concentrations of major ions and total dissolved solids (TDS) increase considerably during the dry season. A bicarbonate water type is primarily produced by the dissolution of calcite, dolomite and gypsum, as well as the cation exchange and human activities. Along the typical infiltration path, the proportions of surface water increase with proximity to the river from 8%-63% during the wet season to 11%-84% during the dry season, which are attributed to an increased hydraulic gradient by exploitation. The typical path is classified into two zones. The first is the intensive mixing zone (within 1 km) with increasing concentrations of major ions and TDS due to mixing effect. The second is the exploitation influence zone (1-3.3 km) with increased concentrations of Ca²⁺, Mg²⁺, SO₄²⁻, and HCO₃⁻ during the dry season due to two reasons of seasonal variations in evaporation, stronger water-rock interactions and mixing effects with increased surface water by exploitation.     

Efficacy of Hollow‐Fiber Ultrafiltration for Microbial Sampling in Groundwater

The goal of this study was to test hollow‐fiber ultrafiltration as a method for concentrating in situ bacteria and viruses in groundwater samples. Water samples from nine wells tapping a shallow sandy aquifer in a densely populated village in Bangladesh were reduced in volume approximately 400‐fold using ultrafiltration. Culture‐based assays for total coliforms and Escherichia coli, as well as molecular‐based assays for E. coli, Bacteroides, and adenovirus, were used as microbial markers before and after ultrafiltration to evaluate performance. Ultrafiltration increased the concentration of the microbial markers in 99% of cases. However, concentration factors (CF = post‐filtration concentration/pre‐filtration concentration) for each marker calculated from geometric means ranged from 52 to 1018 compared to the expected value of 400. The efficiency was difficult to quantify because concentrations of some of the markers, especially E. coli and total coliforms, in the well water (WW) collected before ultrafiltration varied by several orders of magnitude during the period of sampling. The potential influence of colloidal iron oxide precipitates in the groundwater was tested by adding EDTA to the pre‐filtration water in half of the samples to prevent the formation of precipitates. The use of EDTA had no significant effect on the measurement of culturable or molecular markers across the 0.5 to 10 mg/L range of dissolved Fe²⁺ concentrations observed in the groundwater, indicating that colloidal iron did not hinder or enhance recovery or detection of the microbial markers. Ultrafiltration appears to be effective for concentrating microorganisms in environmental water samples, but additional research is needed to quantify losses during filtration.     

Homotopy Perturbation Method‐Based Analytical Solution for Tide‐Induced Groundwater Fluctuations

The groundwater variations in unconfined aquifers are governed by the nonlinear Boussinesq's equation. Analytical solution for groundwater fluctuations in coastal aquifers under tidal forcing can be solved using perturbation methods. However, the perturbation parameters should be properly selected and predefined for traditional perturbation methods. In this study, a new dimensional, higher‐order analytical solution for groundwater fluctuations is proposed by using the homotopy perturbation method with a virtual perturbation parameter. Parameter‐expansion method is used to remove the secular terms generated during the solution process. The solution does not require any predefined perturbation parameter and valid for higher values of amplitude parameter A/D, where A is the amplitude of the tide and D is the aquifer thickness.     

A Small‐Diameter NMR Logging Tool for Groundwater Investigations

A small‐diameter nuclear magnetic resonance (NMR) logging tool has been developed and field tested at various sites in the United States and Australia. A novel design approach has produced relatively inexpensive, small‐diameter probes that can be run in open or PVC‐cased boreholes as small as 2 inches in diameter. The complete system, including surface electronics and various downhole probes, has been successfully tested in small‐diameter monitoring wells in a range of hydrogeological settings. A variant of the probe that can be deployed by a direct‐push machine has also been developed and tested in the field. The new NMR logging tool provides reliable, direct, and high‐resolution information that is of importance for groundwater studies. Specifically, the technology provides direct measurement of total water content (total porosity in the saturated zone or moisture content in the unsaturated zone), and estimates of relative pore‐size distribution (bound vs. mobile water content) and hydraulic conductivity. The NMR measurements show good agreement with ancillary data from lithologic logs, geophysical logs, and hydrogeologic measurements, and provide valuable information for groundwater investigations.     

Closed‐Form Approximations for Two‐Dimensional Groundwater Age Patterns in a Fresh Water Lens

Simple closed‐form approximations are presented for calculating the steady‐state groundwater age distribution in two‐dimensional vertical cross sections of idealized fresh water lenses overlying salt water, for aquifers that are vertically semi‐infinite and of finite thickness. The approximations are developed on the basis of existing one‐dimensional analytical solutions for travel‐time calculation in fresh water lenses and approximate streamline formulations. The two‐dimensional age distributions based on the closed‐form solutions match convincingly with numerical simulations. As expected, notable deviations from the numerical solution are encountered at the groundwater flow divide and when submarine groundwater discharge occurs. Ratios of recharge over hydraulic conductivities are varied to explore how the magnitude of the deviations changes, and it is found that the approximate closed‐form solutions perform well over a range of conditions found in natural systems.     

Opportunities to Build Groundwater Resilience in the Semi‐Arid Tropics

Agricultural water management (AWM) is the adaptation strategy for increasing agricultural production through enhancing water resources availability while maintaining ecosystem services. This study characterizes groundwater hydrology in the Kothapally agricultural watershed, in hard rock Deccan plateau area in India and assesses the impact of AWM interventions on groundwater recharge using a calibrated and validated hydrological model, SWAT, in combination with observed water table data in 62 geo‐referenced open wells. Kothapally receives, on average, 750 mm rainfall (nearly 90% of annual rainfall) during the monsoon season (June to October). Water balance showed that 72% of total rainfall was converted as evapotranspiration (ET), 16% was stored in aquifer, and 8% exported as runoff from the watershed boundary with AWM interventions. Nearly 60% of the runoff harvested by AWM interventions recharged shallow aquifers and rest of the 40% increased ET. Water harvesting structures (WHS) contributed 2.5 m additional head in open wells, whereas hydraulic head under natural condition was 3.5 m, resulting in total 6 m rise in water table during the monsoon. At the field scale, WHSs recharged open wells at a 200 to 400 m spatial scale.     

A Comparison of Data‐Driven Groundwater Vulnerability Assessment Methods

Increasing availability of geo‐environmental data has promoted the use of statistical methods to assess groundwater vulnerability. Nitrate is a widespread anthropogenic contaminant in groundwater and its occurrence can be used to identify aquifer settings vulnerable to contamination. In this study, multivariate Weights of Evidence (WofE) and Logistic Regression (LR) methods, where the response variable is binary, were used to evaluate the role and importance of a number of explanatory variables associated with nitrate sources and occurrence in groundwater in the Milan District (central part of the Po Plain, Italy). The results of these models have been used to map the spatial variation of groundwater vulnerability to nitrate in the region, and we compare the similarities and differences of their spatial patterns and associated explanatory variables. We modify the standard WofE method used in previous groundwater vulnerability studies to a form analogous to that used in LR; this provides a framework to compare the results of both models and reduces the effect of sampling bias on the results of the standard WofE model. In addition, a nonlinear Generalized Additive Model has been used to extend the LR analysis. Both approaches improved discrimination of the standard WofE and LR models, as measured by the c‐statistic. Groundwater vulnerability probability outputs, based on rank‐order classification of the respective model results, were similar in spatial patterns and identified similar strong explanatory variables associated with nitrate source (population density as a proxy for sewage systems and septic sources) and nitrate occurrence (groundwater depth).     

Pump‐and‐Treat Groundwater Remediation Using Chlorine/Ultraviolet Advanced Oxidation Processes

Comparative studies of the use of chlorine/ultraviolet (Cl₂/UV) and hydrogen peroxide/ultraviolet (H₂O₂/UV) Advanced oxidation processes (AOPs) to remove trichloroethylene (TCE) from groundwater in a pump‐and‐treat application were conducted for the first time at the full‐scale operational level at two water treatment facilities in Northern California. In these studies, aqueous chlorine replaced hydrogen peroxide in the AOP treatment step, where the oxidant is exposed to UV light to produce highly reactive radical species that degrade groundwater contaminants. TCE removal rates as a function of initial chlorine dose and pH were then determined. At the site where the natural pH of the water was 7.1, TCE was removed (to a concentration of less than 0.5 µg/L) for nearly every chlorine dose point tested, and pH adjustment slightly enhanced the treatment process at this facility. The second site had a high natural pH of 7.7, and here, TCE was not completely removed for any chlorine dose up to 5.7 mg/L, although TCE removal did increase when the chlorine dose increased between 0.9 and 3.6 mg/L. Residual TCE remaining in the water post‐Cl₂/UV was readily removed using active carbon filtration, which is part of the overall treatment train at this facility. These studies also verified that Cl₂/UV AOP did not interfere with the photolysis of N‐nitrosodimethylamine or result in an effluent acutely toxic toward Ceriodaphnia dubia. Comparative economic analysis revealed that the chemical costs associated with Cl₂/UV AOP were 25 to 50% of the costs associated with in place H₂O₂/UV AOP treatment.