How Effective Is Thermal Remediation of DNAPL Source Zones in Reducing Groundwater Concentrations?

Dense nonaqueous phase liquid (DNAPL) source areas containing chlorinated volatile organic compounds (cVOCs) such as trichloroethene (TCE) and perchloroethene (PCE) often give rise to significant dissolved plumes in groundwater, leading to the closure of downgradient water supply wells and creating vapor intrusion issues in buildings located above the plume. Hydraulic containment via pump‐and‐treat has often been implemented to limit migration but must continue indefinitely. Removal of the DNAPL source area by means such as in situ thermal remediation (ISTR) offers the potential to diminish or end the need for hydraulic containment if the associated dissolved plume attenuates sufficiently following source removal. A question often raised is whether this occurs or whether the back diffusion of contaminants from secondary sources such as low‐permeability lenses in the dissolved plume precludes it. The authors conducted DNAPL source removal using ISTR at dozens of sites. This paper presents a compilation of cases—10 separate DNAPL source areas at five project sites—where data indicate that the implementation of a thorough ISTR in a DNAPL source area can result in the attenuation of the associated dissolved plume, such that in several cases, long‐standing pump‐and‐treat systems could be turned off. Our findings contrast with recent assertions that aggressive source remediation may not be justifiable because dissolved plume concentrations will not decline sufficiently. We show that the application of ISTR can result in the thorough removal of the DNAPL source, effective diminution of dissolved plume groundwater concentrations, and achievement of drinking water standards.     

Evaluation of hydrologic data obtained from a local groundwater monitoring network in a metropolitan city,Korea

In the late 1980s, dramatic increases in water use caused over‐exploitation of groundwater resources and deterioration of water quality in Seoul metropolitan city. To monitor changes in quantity of groundwater resources and their quality, the metropolitan government established a local groundwater monitoring network in 1997 consisting of 119 monitoring wells. Groundwater resources in the urban area were affected by various human activities, including underground construction such as subways, pumping for public or private water use, leaky sewer systems and pavements. The variation patterns of the groundwater levels were mainly classified into four types, reflecting natural recharge due to rainfall events during the wet season, artificial recharge from leaky sewer or water supply systems, and heavy groundwater pumping for drainage or flood control purposes at underground construction sites. Significantly decreasing trends of groundwater levels in the suburbs of Seoul indicate groundwater use for various agricultural activities. Subway construction lowered the water level by an average of 25 m. Electrical conductivity values showed a wide range, from 100 to 1800 µS/cm (mean 470 µS/cm). Groundwater temperature generally showed a stable pattern, except for some sensitive increases at relatively shallow monitoring wells. Detailed analysis of the monitored groundwater data would provide some helpful implications for optimal and efficient management of groundwater resources in this metropolitan city. Copyright © 2005 John Wiley & Sons, Ltd.     

Management of the Wellfield Contaminated by Dense Chlorinated Hydrocarbons in Vilnius,Lithuania

Drinking water supply in Lithuania is entirely based on groundwater. Wellfields of Vilnius develop intermorainic ca. 50 m deep (in average) aquifer which is locally contaminated by chlorinated hydrocarbons— volatile organic compounds (VOCs). Groundwater abstraction activates VOCs migration from an abandoned factory into the pumping wells of one wellfield named “Vingis.” However monitoring data testify that only traces of VOCs were detected on the territory of this factory. Subsequent studies revealed the “secret”: dense VOCs have migrated from the territory of the polluter and have accumulated in lowermost places of pumped aquifer inside the wellfield. An attempt to ensure low concentration of VOCs in pumped water manipulating by pumping rates of more or less contaminated abstraction wells was not effective. Finally, an acceptable concentration of VOCs in supplied drinking water was ensured by permanent pumping out of the most polluted groundwater from some abstraction wells of the wellfield and diverting this water to the Neris River.     

Optimization of the Sampling Technique for the Determination of Dissolved Hydrogen in Groundwater

In this study a field‐sampling technique for dissolved hydrogen (H₂) in groundwater will be presented which allows the transport of gaseous samples into the laboratory for further analysis. The method consists of transferring the headspace trapped in a gas‐sampling bulb which is continuously purged by groundwater into previously evacuated vials using a gas‐tight syringe. Three transfer steps with preceding evacuation of the vial led to a H₂‐recovery of 100 % in laboratory experiments. The method has been applied to determine H₂ concentrations in an aquifer contaminated with chlorinated solvents. Tests concerning the effect of different pumping techniques on H₂ concentrations revealed that most reliable values were obtained with a bladder pump, while an electrically driven submersible pump generated considerable amounts of hydrogen due to electrochemical interactions with the sampled water. Concentrations of dissolved hydrogen in field and laboratory samples were about two orders of magnitude higher when sampling was performed with the electrically driven submersible pump compared to sampling with the bladder pump and a peristaltic pump. Lab experiments with a Plexiglas reservoir to produce H₂‐enriched water were used to study the effect of two tubing materials (PVC, polyamide) on H₂ losses. PVC tubing turned out to allow transfer of H₂‐enriched water over 25 m without significant losses, while PA‐tubing was not suitable for sampling of H₂.     

Influence of Shallow Geology on Volatile Organic Chemical Attenuation from Groundwater to Deep Soil Gas

Vapor intrusion pathway evaluations commonly begin with a comparison of volatile organic chemical (VOC) concentrations in groundwater to generic, or Tier 1, screening levels. These screening levels are typically quite low reflecting both a desired level of conservatism in a generic risk screening process as well as limitations in understanding of physical and chemical processes that impact vapor migration in the subsurface. To study the latter issue, we have collected detailed soil gas and groundwater vertical concentration profiles and evaluated soil characteristics at seven different sites overlying chlorinated solvent contaminant plumes. The goal of the study was to evaluate soil characteristics and their impacts on VOC attenuation from groundwater to deep soil gas (i.e., soil gas in the unsaturated zone within 2 feet of the water table). The study results suggest that generic screening levels can be adjusted by a factor of 100× at sites with fine‐grained soils above the water table, as identified by visual observations or soil air permeability measurements. For these fine‐grained soil sites, the upward‐adjusted screening levels maintain a level of conservatism while potentially eliminating the need for vapor intrusion investigations at sites that may not meet generic screening criteria.     

The Sensitivity of Four Monitoring Well Sampling Systems to Low Concentrations of Three Volatile Organics

Four state-of-the-art ground water sampling systems were analyzed to determine their reliability in providing representative samples of the volatile chlorinated hydrocarbons trichloroethylene (TCE), perchloroethylene (PCE), and 1,1,1-trichloroethane (TCA) from a simulated monitoring well. The sampling systems studied represent four commonly used devices, including a stainless steel and Teflon® piston pump, a Teflon bailer, a Teflon bladder pump, and a PVC air-lift pump.
Controlled laboratory sampling experiments were conducted in a tank and well test chamber designed to approximate field conditions. A well purging and sampling procedure was used in the test apparatus to determine the accuracy and precision of each device for detecting low concentrations of the compounds in ground water. The compounds selected are some of the most ubiquitous hazardous contaminants found in shallow aquifers near hazardous waste sites throughout the United States.
No significant statistical difference was found among the four sampling systems in detecting the compounds.     

Modeling Dehalococcoides sp. Augmented Bioremediation in a Single Fracture System

A numerical reactive transport model was developed to simulate the bioremediation processes in a perchloroethene (PCE) contaminated single fracture system augmented with Dehalococcoides sp. (DHC). The model describes multispecies bioreactive transport processes that include bacterial growth and detachment dynamics, biodegradation of chlorinated species, competitive inhibition of various reactive species, and the loss of daughter products because of back‐partitioning effects. Two sets of experimental data, available in the study by Schaefer et al. (2010b) , were used to calibrate and test the model. The model was able to simulate both datasets. The simulation results indicated that the yield coefficient and the DHC maximum utilization rate coefficient were the two important process parameters. A detailed sensitivity study was completed to quantify the sensitivity of the model to variations in these two parameter values. The results show that an increase in yield coefficient increases bacterial growth and thus expedites the dechlorination process, whereas an increase in maximum utilization rate coefficient greatly increased dechlorination rates. The proposed model provides a mathematical framework for simulating remediation systems that employ DHC bioaugmentation for restoring chlorinated‐solvent contaminated groundwater aquifers.     

Hydrological modelling for assessing spatio-temporal groundwater recharge variations in the water-stressed Amathole Water Supply System,Eastern Cape,South Africa

To increase the resilience of regional water supply systems in South Africa in the face of anticipated climatic changes and a constant increase in water demand, water supply sources require diversification. Many water-stressed metropolitan regions in South Africa depend largely on surface water to cover their water demand. While climatic and river discharge data is widely available in these regions, information on groundwater resources – which could support supply source diversification – is scarce. Groundwater recharge is a key parameter that is used to estimate groundwater amounts that can be sustainably exploited at a sub-watershed level. Therefore, the objective of this study was to develop a reliable hydrological modelling routine that enables the assessment of regional spatio-temporal variations of groundwater recharge to discern the most promising areas for groundwater development. Accordingly, we present a semi-distributed hydrological modelling approach that incorporates water balance routines coupled with baseflow modelling techniques to yield spatio-temporal variations of groundwater recharge on a regional level. The approach is demonstrated for the actively managed catchment areas of the Amathole Water Supply System situated in a semi-arid part of the Eastern Cape of South Africa. In the investigated study area, annual groundwater recharge exhibits a high spatio-temporal heterogeneity and is estimated to vary between ~0.5% and 8% of annual rainfall. Despite some uncertainties induced by limited data availability, calibration and validation of the model were found to be satisfactory and yielded model results similar to (point) data of annual groundwater recharge reported in earlier studies. Our approach is therefore found to derive crucial information for efficiently targeting more detailed groundwater exploration studies and could work as a blueprint for orientating groundwater potential exploration in similar environments.     

Evaluation of Zerovalent Zinc for Treatment of 1,2,3‐Trichloropropane‐Contaminated Groundwater: Laboratory and Field Assessment

The efficacy and feasibility of using zerovalent zinc (ZVZ) to treat 1,2,3‐trichloropropane (TCP)‐contaminated groundwater was assessed in laboratory and field experiments. In the first portion of the study, the reactivity of commercially available granular ZVZ toward TCP was measured in bench‐scale batch‐reactor and column experiments. These results were used to design columns for on‐site pilot‐scale treatment of contaminated groundwater at a site in Southern California. Two of the ZVZ materials tested were found to produce relatively high rates of TCP degradation as well as predictable behavior when scaling from bench‐scale to field testing. In addition, there was little decrease in the rates of TCP degradation over the duration of field testing. Finally, no secondary impacts to water quality were identified. The results suggest that ZVZ may be an effective and feasible material for use in engineered treatment systems, perhaps including permeable reactive barriers.     

Numerical studies on the influences of the South‐to‐North Water Transfer Project on groundwater level changes in the Beijing Plain,China

Groundwater is an important component of the water supply, and overexploitation has triggered many problems in the Beijing Plain. The South‐to‐North Water Transfer Project has been proposed as a promising solution to alleviate these problems. Evaluation of different scenarios of groundwater management after the implementation of the South‐to‐North Water Transfer Project is necessarily required. In this study, a numerical model of groundwater flow was established using FEFLOW software and was well calibrated by parameter optimization and groundwater withdrawal inversion in the Beijing Plain. Sixteen scenarios that considered groundwater exploitation, artificial recharge, and precipitation were designed to simulate the groundwater dynamics after 11 years of the project. The results showed that the groundwater level in the study area would recover to various degrees due to the reductions of groundwater withdrawal and the increments of infiltration; additionally, it was concluded that groundwater was significantly affected by precipitation. Generally, in the designed scenarios, the groundwater‐level increment in the upper streams of the model area was higher than that in the lower streams. The groundwater level would obviously increase from artificial recharge in the immediate and adjacent areas. In addition, modes of reducing exploitation had no significant influence on the change in groundwater level during the 11‐year study period. The developed model offers a reliable and effective way to improve groundwater management.     

Screening of Emerging Volatile Organic Contaminants in Shallow Groundwater in East China

In order to collect baseline information on the environmental occurrence of volatile organic compounds (VOCs) in groundwater in East China, shallow groundwater samples were collected from five alluvial plains in East China in 2008 to 2009. All samples were analyzed for 54 VOCs representing a wide variety of uses and origins. Sampling sites were mainly selected in the areas to be susceptible to contamination from human activities in terms of previous hydrogeological survey. The data of all samples showed a variety of different hydrogeological systems with potential sources of VOCs, with 36 of the 54 VOCs being found. The most frequently detected compounds include naphthalene (56.9%), chloroform (16.9%), 1,2‐dichloroethane (16.2%), 1,2‐dichloropropane (13.1%), and 1,2,3‐trichlorobenzene (12.3%). The concentrations of methylene chloride, 1,2‐dichloroethane, carbon tetrachloride, trichloroethene, 1,2‐dichloropropane, and tetrachloroethene exceeded the relating drinking water standards. Future work will be needed to identify those factors that are most important in determining the occurrence and concentrations of VOCs in groundwater in China.     

Comparison of Phytoscreening and Direct‐Push‐Based Site Investigation at a Rural Megasite Contaminated with Chlorinated Ethenes

The reliable characterization of subsurface contamination of spatially extended contaminated sites is a challenging task, especially with an unknown history of land use. Conventional technologies often fail due to temporal and financial constraints and thus hinder the redevelopment of abandoned areas in particular. Here we compare two site screening techniques that can be applied quickly at relatively low cost, namely Direct Push (DP)‐based groundwater sampling and tree core sampling. The effectiveness of both methods is compared for a rural megasite contaminated with chlorinated hydrocarbons. Unexpected pollution hot spots could be identified using both of these methods, while tree coring even enabled the delineation of the contaminant plume flowing into an adjacent wetland inaccessible for DP units. Both methods showed a good agreement in revealing the spatial pattern of the contamination. The correlation between groundwater concentrations and equivalent concentrations in wood was linear and highly significant for trichloroethene. Correlation was less obvious for its metabolite cis‐dichloroethene, but still significant. As outcome of our study we recommend tree coring and for initial screening in combination with a DP sampling to retrieve quantitative data on groundwater pollutants in order to assess the contamination situation of a non‐ or only partly investigated site. The subsequent placement of monitoring wells for long‐term monitoring of contamination levels is recommended. A combination of methods would achieve more relevant information at comparable or possibly even lower efforts in comparison to a conventional site investigation.     

Assessment of the hydrological status of Doñana dune ponds: a natural World Heritage Site under threat

ABSTRACT

The hydrological response of shallow ponds to groundwater withdrawal has been of growing concern in the Doñana National Park (southern Spain) in recent decades. This study examines the role of groundwater in maintaining the hydroperiod (i.e. the hydrological regime) in the park’s main dune ponds, by quantifying the groundwater fluxes to/from them. The hydrological characterization was performed by applying different methodologies. Daily hydrological balances registered in the ponds revealed groundwater contributions ranging from 80% of the total water inflows (i.e. groundwater discharge) to a net groundwater recharge from the ponds to the aquifer, and enabled the studied water bodies to be classified as discharge or recharge systems. The recharge systems must have been influenced by the lowering of piezometric levels due to groundwater extraction for urban supply in a nearby coastal resort.     

Effects of Linking a Soil‐Water‐Balance Model with a Groundwater‐Flow Model

A previously published regional groundwater‐flow model in north‐central Nebraska was sequentially linked with the recently developed soil‐water‐balance (SWB) model to analyze effects to groundwater‐flow model parameters and calibration results. The linked models provided a more detailed spatial and temporal distribution of simulated recharge based on hydrologic processes, improvement of simulated groundwater‐level changes and base flows at specific sites in agricultural areas, and a physically based assessment of the relative magnitude of recharge for grassland, nonirrigated cropland, and irrigated cropland areas. Root‐mean‐squared (RMS) differences between the simulated and estimated or measured target values for the previously published model and linked models were relatively similar and did not improve for all types of calibration targets. However, without any adjustment to the SWB‐generated recharge, the RMS difference between simulated and estimated base‐flow target values for the groundwater‐flow model was slightly smaller than for the previously published model, possibly indicating that the volume of recharge simulated by the SWB code was closer to actual hydrogeologic conditions than the previously published model provided. Groundwater‐level and base‐flow hydrographs showed that temporal patterns of simulated groundwater levels and base flows were more accurate for the linked models than for the previously published model at several sites, particularly in agricultural areas.     

Removal of Volatile Organic Compounds in Vertical Flow Filters: Predictions from Reactive Transport Modeling

Vertical flow filters are containers filled with porous medium that are recharged from top and drained at the bottom, and are operated at partly saturated conditions. They have recently been suggested as treatment technology for groundwater containing volatile organic compounds (VOCs). Numerical reactive transport simulations were performed to investigate the relevance of different filter operation modes on biodegradation and/or volatilization of the contaminants and to evaluate the potential limitation of such remediation mean due to volatile emissions. On the basis of the data from a pilot‐scale vertical flow filter intermittently fed with domestic waste water, model predictions on the system’s performance for the treatment of contaminated groundwater were derived. These simulations considered the transport and aerobic degradation of ammonium and two VOCs, benzene and methyl tertiary butyl ether (MTBE). In addition, the advective‐diffusive gas‐phase transport of volatile compounds as well as oxygen was simulated. Model predictions addressed the influence of depth and frequency of the intermittent groundwater injection, degradation rate kinetics, and the composition of the filter material. Simulation results show that for unfavorable operation conditions significant VOC emissions have to be considered and that operation modes limiting VOC emissions may limit aerobic biodegradation. However, a suitable combination of injection depth and composition of the filter material does facilitate high biodegradation rates while only little VOC emissions take place. Using such optimized operation modes would allow using vertical flow filter systems as remediation technology suitable for groundwater contaminated with volatile compounds.     

Bayesian Chance‐Constrained Hydraulic Barrier Design under Geological Structure Uncertainty

The groundwater community has widely recognized geological structure uncertainty as a major source of model structure uncertainty. Previous studies in aquifer remediation design, however, rarely discuss the impact of geological structure uncertainty. This study combines chance‐constrained (CC) programming with Bayesian model averaging (BMA) as a BMA‐CC framework to assess the impact of geological structure uncertainty in remediation design. To pursue this goal, the BMA‐CC method is compared with traditional CC programming that only considers model parameter uncertainty. The BMA‐CC method is employed to design a hydraulic barrier to protect public supply wells of the Government St. pump station from salt water intrusion in the “1500‐foot” sand and the “1700‐foot” sand of the Baton Rouge area, southeastern Louisiana. To address geological structure uncertainty, three groundwater models based on three different hydrostratigraphic architectures are developed. The results show that using traditional CC programming overestimates design reliability. The results also show that at least five additional connector wells are needed to achieve more than 90% design reliability level. The total amount of injected water from the connector wells is higher than the total pumpage of the protected public supply wells. While reducing the injection rate can be achieved by reducing the reliability level, the study finds that the hydraulic barrier design to protect the Government St. pump station may not be economically attractive.     

The performance of rainwater tanks for stormwater retention and water supply at the household scale: an empirical study

Urban stormwater run‐off degrades the ecological condition of streams. The use of rainwater tanks to supplement water supply can reduce the frequency and volume of urban stormwater run‐off that is otherwise conveyed directly to streams via conventional stormwater drainage systems. Few studies, however, have examined the use of tanks in the context of managing flow regimes for stream protection, with most focussed uniquely on their water conservation benefits. We used measured tank water level data to assess the performance of 12 domestic rainwater tanks against the dual criteria of their ability to (i) reduce potable mains water usage and (ii) retain run‐off from rainfall events and thus reduce the volume and frequency of stormwater run‐off. We found that five households relied almost entirely on tank water. Three of the tanks achieved stormwater retention performance approaching that of the same area of pre‐developed land, although nine did not – a consequence of limited demand and small tank capacity. Our results suggest that tank water usage can result in substantial reductions in mains water use, if regular and sufficiently large domestic demands are connected to tanks. In many cases, such demands will also result in the best stormwater retention performance. Our results highlight an opportunity to design tank systems to achieve multiple objectives. Application of similar analyses in different locations will help to optimize tanks for simultaneous water supply and stormwater retention purposes. Copyright © 2014 John Wiley & Sons, Ltd.     

Nature and Estimated Human Toxicity of Polar Metabolite Mixtures in Groundwater Quantified as TPHd/DRO at Biodegrading Fuel Release Sites

Polar metabolites resulting from petroleum biodegradation are measured in groundwater samples as TPHd unless a silica gel cleanup (SGC) is used on the sample extract to isolate hydrocarbons. Even though the metabolites can be the vast majority of the dissolved organics present in groundwater, SGC has been inconsistently applied because of regulatory concern about the nature and toxicity of the metabolites. A two‐step approach was used to identify polar compounds that were measured as TPHd in groundwater extracts at five sites with biodegrading fuel sources. First, gas chromatography with mass spectrometry (GC‐MS) was used to identify and quantify 57 individual target polar metabolites. Only one of these compounds—dodecanoic acid, which has low potential human toxicity—was detected. Second, nontargeted analysis was used to identify as many polar metabolites as possible using both GC‐MS and GC×GC‐MS. The nontargeted analysis revealed that the mixture of polar metabolites identified in groundwater source areas at these five sites is composed of approximately equal average percentages of organic acids, alcohols and ketones, with few phenols and aldehydes. The mixture identified in downgradient areas at these five sites is dominated by acids, with fewer alcohols, far fewer ketones, and very few aldehydes and phenols. A ranking system consistent with systems used by USEPA and the United Nations was developed for evaluating the potential chronic oral toxicity to humans of the different classes of identified polar metabolites. The vast majority of the identified polar metabolites have a “Low” toxicity profile, and the mixture of identified polar metabolites present in groundwater extracts at these five sites is unlikely to present a significant risk to human health.