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Well clogging was studied at an aquifer storage transfer and recovery (ASTR) site used to secure freshwater supply for a flower bulb farm. Tile drainage water (TDW) was collected from a 10-ha parcel, stored in a sandy brackish coastal aquifer via well injection in wet periods, and reused during dry periods. This ASTR application has been susceptible to clogging, as the TDW composition largely exceeded most clogging mitigation guidelines. TDW pretreatment by sand filtration did not cause substantial clogging at a smaller ASR site (2 ha) at the same farm. In the current (10 ha) system, sand filtration was substituted by 40-μm disc filters to lower costs (by 10,000–30,000 Euro) and reduce space (by 50–100 m2). This measure treated TDW insufficiently and injection wells rapidly clogged. Chemical, biological, and physical clogging occurred, as observed from elemental, organic carbon, 16S rRNA, and grain-size distribution analyses of the clogging material. Physical clogging by particles was the main cause, based on the strong relation between injected turbidity load and normalized well injectivity. Periodical backflushing of injection wells improved operation, although the disc filters clogged when the turbidity increased (up to 165 NTU) during a severe rainfall event (44 mm in 3 days). Automated periodical backflushing, together with regulating the maximum turbidity (

A field injection experiment was performed in an anoxic sandy aquifer over 6 days to assess sorption characteristics of 7 commonly applied pesticides in agriculture and 2 frequently detected metabolites. Pesticide use changed considerably in the last decades, and there is insufficient knowledge of the fate of currently used pesticides in aquifers. Injected water arrival was monitored at 6 depth intervals of 1 m ranging from 11.4 to 32.2 m-below surface level with varying organic carbon contents (0.057–0.91%d.w.) to examine intra-aquifer variations in sorption. Observed pesticide concentrations were fit using a non-linear least squares routine to an advection-dispersion equation, from which retardation factors (R) were obtained. Pesticide degradation did not significantly influence the simulated R during the experiment. We observed that bentazon and cycloxydim were most mobile with R < 1.1 at all depths. Desphenyl chloridazon, methyl desphenyl chloridazon, and imidacloprid were, on average, less mobile, with maximum R of 1.5. Boscalid, chloridazon, fluopyram, and flutolanil showed a larger range of R, and R > 2.0 were observed in the shallowest part of the aquifer. Largest R were observed at the top of the aquifer and decreased with depth. Koc values varied similarly, which indicates that sorption is not only influenced by sedimentary organic matter (SOM) content but also by its sorption reactivity. Obtained sorption parameters were substantially lower than reported in a widely used pesticide sorption database, which suggests that sorption parameters are influenced by methodological differences and variations in the sorption reactivity of SOM. The large intra-aquifer variations in pesticide sorption highlights that aquifer heterogeneity should be considered in groundwater risk assessments.

Managed Aquifer Recharge (MAR) has been applied as Aquifer Storage, Transfer, and Recovery (ASTR) to provide fresh drinking water for local communities at 99 locations in southwest Bangladesh since 2009. Aerobic freshwater from ponds is filtered and subsequently infiltrated into anaerobic shallow brackish aquifers. At approximately 45% of these sites, relatively higher levels of Fe and As were observed in recovered water, which requires a better understanding of the hydrogeochemical processes that govern the Fe, Mn, and As levels in these MAR systems. Therefore, two representative sites with As above (74 ± 11 μg/L at site GMF11) and below (19 ± 6 μg/L at site JJS91), the Bangladesh drinking water standard of 50 μg/L were weekly monitored on hydrochemical changes from Dec 2017 to Dec 2018. Hydrogeochemical processes occurring during storage were quantified with inverse and forward geochemical mass balance models developed with PHREEQC. The following processes explained the changes in water quality: 1) mixing of infiltration water with native groundwater (∼90%:∼10%); 2) consumption of O2 by a) dissolved Fe2+ that subsequently precipitated as Fe(OH)3 at GMF11 and by b) dissolved and sedimentary organic matter (OM) at site JJS91; 3) reduction of SO4 coupled to the oxidation of OM at both sites; and 4) mixing corrosion and freshening induced cation-exchange (Ca sorption; Na desorption) triggering calcite and siderite dissolution at GMF11. Dissolution of these carbonate minerals occurred to a lesser extent at JJS91, while cation exchange (Na sorption; Ca desorption) suggested that the freshwater was displaced by brackish groundwater because of inadequate infiltration at JJS91. Distinct pH values in recovered water reflected the dominance of Fe2+ versus OM oxidation. Siderite dissolution led to 4.3 ± 3.1 and 1.0 ± 0.5 mg/L Fe in recovered water at GMF11 and JJS91, respectively. Elevated As and Mn levels in recovered water were caused for max. 20% by mixing with native groundwater and for min. 80% by mobilization processes, mainly by desorption of As from Fe-oxides and by the dissolution of Mn-bearing siderite. Recommendations are provided to improve recovered water quality.

Freshwater resources in coastal areas are under intense pressure from excessive groundwater extraction, which amplifies saltwater intrusion (SWI) into coastal freshwater aquifers, such as in the Mekong Delta. Studies that combine socioenvironmental data and households' psychological factors next to salinity measurement data to design groundwater conservation strategies are rare. In this study, these aspects are combined to explore their influence on the public willingness to conserve groundwater using a Bayesian belief network model. We analyzed 313 household survey data spread over three districts in the coastal province of Tra Vinh, located in the Vietnamese Mekong Delta. The level of salinity is significantly correlated with the willingness to conserve groundwater. The top three socioenvironmental characteristics that influence willingness are the level of salinity, type of employment - i.e., being a farmer - and frequency of being exposed to groundwater or SWI promotional activities. Social norm, i.e., perceived social pressure, is the most influential psychological factor that determines willingness. This study reveals an urgency for the local government to intervene and create social pressure regarding the issue.

An aquifer storage transfer and recovery (ASTR) system was studied in which tile drainage water (TDW) was injected with relatively high NO3 (about 14 mg/L) concentrations originating from fertilizers. Here we present the evolution of denitrification kinetics at 6 different depths in the aquifer before, and during ASTR operation. First-order denitrification rate constants increased over time before and during the first days of ASTR operation, likely due to microbial adaptation of the native bacterial community and/or bioaugmentation of the aquifer by denitrifying bacteria present in injected TDW. Push-pull tests were performed in the native aquifer before ASTR operation. Obtained first-order denitrification rate constants were negligible (0.00–0.03 d−1) at the start, but increased to 0.17–0.83 d−1 after a lag-phase of about 6 days. During the first days of ASTR operation in autumn 2019, the arrival of injected TDW was studied at 2.5 m distance from the injection well. First-order denitrification rate constants increased again over time (maximum >1 d−1). Three storage periods without injection were monitored in winter 2019, fall 2020, and spring 2021 during ASTR operation. First-order rate constants ranged between 0.12 and 0.61 d−1. Denitrification coupled to pyrite oxidation occurred at all depths, but other oxidation processes were indicated as well. NO3 concentration trends resembled Monod kinetics but were fitted also to a first-order decay rate model to facilitate comparison. Rate constants during the storage periods were substantially lower than during injection, probably due to a reduction in the exchange rate between aquifer solid phases and injected water during the stagnant conditions. Denitrification rate constants deviated maximally a factor 5 over time and depth for all in-situ measurement approaches after the lag-phase. The combination of these in-situ approaches enabled to obtain more detailed insights in the evolution of denitrification kinetics during AS(T)R.

Degradation of 7 common pesticides (bentazon, boscalid, chloridazon, fluopyram, flutolanil, imidacloprid, and methoxyfenozide) and 2 metabolites of chloridazon (desphenyl-chloridazon, and methyl-desphenyl-chloridazon) was studied in an anoxic and brackish sandy aquifer before and during Aquifer Storage Transfer and Recovery (ASTR) operation. Fresh tile drainage water was injected and stored for later re-use as irrigation water. We hypothesized that electron acceptors (O2, NO3), dissolved organic carbon (∼24.7 mg/L), nutrients (NO3: ∼14.1 mg/L, NH4: ∼0.13 mg/L, PO4: ∼5.2 mg/L), and biodegrading bacteria in tile drainage water could stimulate degradation of the pesticides and metabolites (ranging between 0.013 and 10.8 μg/L) introduced in the aquifer. Pesticide degradation was studied at 6 depths in the aquifer using push-pull tests lasting ±18 days before the onset of ASTR operation. Degradation was too limited to quantify and/or could not be assessed because of the potential occurrence of pesticide retardation. Utilizing push-pull tests to obtain degradation constants should only be considered in future studies for non-retarding pesticides with relative low half-lives (here

Urban areas in coastal lowlands host a significant part of the world's population. In these areas, cities have often expanded to unfavorable locations that have to be drained or where excess rain water and groundwater need to be pumped away in order to maintain dry feet for its citizens. As a result, groundwater seepage influences surface water quality in many of such urban lowland catchments. This study aims at identifying the flow routes and mixing processes that control surface water quality in the groundwater-influenced urban catchment Polder Geuzenveld, which is part of the city of Amsterdam. Geuzenveld is a highly paved urban area with a subsurface rain water collection system, a groundwater drainage system, and a main surface water system that receive runoff from pavement and roofs, shallow groundwater and direct groundwater seepage, respectively. We conducted a field survey and systematic monitoring to identify the spatial and temporal variations in water quality in runoff, ditch water, drain water, and shallow and deep groundwater. We found that Geuzenveld receives a substantial inflow of deep, O 2 -depleted groundwater, which is enriched in ammonium and phosphorus due to the subsurface mineralization of organic matter under sulfate-reducing conditions. This groundwater is mixed in the ditches during wet periods with O 2 -rich runoff, and iron- and phosphate-rich drain water. Unlike natural catchments, the newly created, separated urban flow routes lead to mixing of water in the main surface water itself, shortcutting much of the soil and shallow subsurface. This leads to low O 2 and high ammonia concentrations in dry periods, which might be mitigated by water level management or artificially increasing O 2 levels by water inlet or artificially aeration of the main water canals. Further research is necessary how to optimize artificial urban systems to deliver a better ecological and chemical status of the surface water.

The Amsterdam area, a highly manipulated delta area formed by polders and reclaimed lakes, struggles with high nutrient levels in its surface water system. The polders receive spatially and temporally variable amounts of water and nutrients via surface runoff, groundwater seepage, sewer leakage, and via water inlets from upstream polders. Diffuse anthropogenic sources, such as manure and fertiliser use and atmospheric deposition, add to the water quality problems in the polders. The major nutrient sources and pathways have not yet been clarified due to the complex hydrological system in lowland catchments with both urban and agricultural areas. In this study, the spatial variability of the groundwater seepage impact was identified by exploiting the dense groundwater and surface water monitoring networks in Amsterdam and its surrounding polders. A total of 25 variables (concentrations of total nitrogen (TN), total phosphorus (TP), NH4, NO3, HCO3, SO4, Ca, and Cl in surface water and groundwater, N and P agricultural inputs, seepage rate, elevation, land-use, and soil type) for 144 polders were analysed statistically and interpreted in relation to sources, transport mechanisms, and pathways. The results imply that groundwater is a large source of nutrients in the greater Amsterdam mixed urban–agricultural catchments. The groundwater nutrient concentrations exceeded the surface water environmental quality standards (EQSs) in 93 % of the polders for TP and in 91 % for TN. Groundwater outflow into the polders thus adds to nutrient levels in the surface water. High correlations (R2 up to 0.88) between solutes in groundwater and surface water, together with the close similarities in their spatial patterns, confirmed the large impact of groundwater on surface water chemistry, especially in the polders that have high seepage rates. Our analysis indicates that the elevated nutrient and bicarbonate concentrations in the groundwater seepage originate from the decomposition of organic matter in subsurface sediments coupled to sulfate reduction and possibly methanogenesis. The large loads of nutrient-rich groundwater seepage into the deepest polders indirectly affect surface water quality in the surrounding area, because excess water from the deep polders is pumped out and used to supply water to the surrounding infiltrating polders in dry periods. The study shows the importance of the connection between groundwater and surface water nutrient chemistry in the greater Amsterdam area. We expect that taking account of groundwater–surface water interaction is also important in other subsiding and urbanising deltas around the world, where water is managed intensively in order to enable agricultural productivity and achieve water-sustainable cities.

This study demonstrates groundwater quality differences between a limestone and a dolomitic limestone, (sub)oxic coastal aquifer in the Eastern Mediterranean (Lebanon), with and without ongoing moderate salinization since the last decades. For this purpose, 8 major and 50 trace elements (TEs) were analyzed in 80 water and 65 rock samples, and interpreted with a quad-fold approach utilizing: (1) nonparametric statistical tests, (2) concentration deviations from ideal conservative freshwater–seawater mixing lines, (3) a new parameter called Mixing Enrichment Factor to assess the mobility of chemical constituents under salinizing conditions, and (4) 1-D dual porosity flow path modeling with PHREEQC. Dissolution/precipitation of CaxMgySrzCO3 and cation exchange were the main disclosed hydrogeochemical processes besides minor signs of organic matter oxidation. In the dolomitic limestone aquifer, less carbonate dissolved as compared to the limestone aquifer, partly because of lower pCO2 in addition to seawater inflow triggering Mg-calcite precipitation by cation exchange. Saltwater intrusion led to mobilization of As, Ba, Cu, Ni, Rb, Sr and U in both aquifers, sometimes likely by cation exchange (e.g. Ba and Sr). Some of these TEs (notably Cu and Ni) recorded higher concentrations in the dolomitic limestone regardless of salinization. Other elements like Al, Be, Ce, Cr, Nb, Pb, V, Y and Zr revealed no or a low mobilization tendency. The concentration of all TEs in groundwater remained below drinking water limits notwithstanding moderate salinization. This classifies carbonate rocks as a weak geogenic source of TEs, whereas encroaching seawater appears to be a more important source.

Eutrophication of water bodies has been a problem causing severe degradation of water quality in cities. To gain mechanistic understanding of the temporal dynamics of nitrogen and phosphorus in a groundwater fed low-lying urban polder, we applied high frequency monitoring in Geuzenveld, a polder in the city of Amsterdam. The high frequency monitoring equipment was installed at the pumping station where water leaves the polder. From 2016 March to 2017 June, total phosphorus (TP), ammonium (NH4), turbidity, electrical conductivity (EC), and water temperature were measured at intervals smaller than 20 minutes. This paper discusses the results at three time scales: annual scale, rain event scale, and single pumping event scale. Mixing of upwelling groundwater and runoff was the dominant hydrological process and governed the temporal pattern of the EC, while N and P fluxes from the polder were also significantly regulated by primary production and iron transformations. The mixing of groundwater and runoff water governed water quality through variation of the intensity and duration of the events. For NH4, the dominant form of N in surface water originating from groundwater seepage, we observed low concentrations during the algae growing season, while concentrations were governed by mixing of groundwater and precipitation inputs in the late autumn and winter. The depletion of dissolved NH4 in spring suggests uptake by primary producers, consistent with high chlorophyll-a, O2, and suspended solids during this period. Total P and turbidity were high during winter, due to the release of reduced iron and P from anoxic sediment to the water column. Rapid Fe2+ oxidation in the water column is the major cause of turbidity. In the other seasons, P is retained in the sediment by iron oxides. Nitrogen is exported from the polder to the downstream water bodies throughout the whole year, mostly in the form of NH4, but as organic N in spring. P leaves the polder mainly during winter, primarily associated with Fe(OH)3 colloids and as dissolved P. Based on this new understanding of the dynamics of N and P in this low lying urban catchment, it is possible to formulate management strategies that can effectively control and reduce eutrophication situation in urban polders and receiving downstream waters.

Microbially mediated sulfate reduction is a ubiquitous process in many subsurface systems. Isotopic fractionation is characteristic of this anaerobic process, since sulfate-reducing bacteria (SRB) favor the reduction of the lighter sulfate isotopologue (S32O42−) over the heavier isotopologue (S34O42−). Detection of isotopic shifts has been utilized as a proxy for the onset of sulfate reduction in subsurface systems such as oil reservoirs and aquifers undergoing heavy metal and radionuclide bioremediation. Reactive transport modeling (RTM) of kinetic sulfur isotope fractionation has been applied to field and laboratory studies. We developed a benchmark problem set for the simulation of kinetic sulfur isotope fractionation during microbially mediated sulfate reduction. The benchmark problem set is comprised of three problem levels and is based on a large-scale laboratory column experimental study of organic carbon amended sulfate reduction in soils from a uranium-contaminated aquifer. Pertinent processes impacting sulfur isotopic composition such as microbial sulfate reduction and iron-sulfide reactions are included in the problem set. This benchmark also explores the different mathematical formulations in the representation of kinetic sulfur isotope fractionation as employed in the different RTMs. Participating RTM codes are the following: CrunchTope, TOUGHREACT, PHREEQC, and PHT3D. Across all problem levels, simulation results from all RTMs demonstrate reasonable agreement.

Riparian zones are important buffer zones for streams as they are hotspots of nitrate transformation and removal in agricultural catchments. However, mixing of water from different sources and various transformation processes can complicate the quantification of nitrate turnover in riparian zones. In this study, we analyzed nitrate concentration and isotope data in riparian groundwater along a 2-km stream section in central Germany. We developed a mathematical model combining end-member mixing and isotope modeling to account for mixing of river water and groundwater and quantify nitrate transformation in riparian groundwater. This enabled us to explicitly determine the extent of denitrification (as process leading to permanent nitrate removal from riparian groundwater) and transient nitrate removal by additional processes associated with negligible isotope fractionation (e.g., plant uptake and microbial assimilation) and to perform an extensive uncertainty analysis. Based on the nitrogen isotope data of nitrate, the simulations suggest a mean removal of up to 28% by additional processes and only about 9% by denitrification. Nitrate removal from riparian groundwater by additional processes exceeded denitrification particularly in winter and at larger distance from the river, underlining the role of the river as organic carbon source. This highlights that nitrate consumption by additional processes predominates at the field site, implying that a substantial fraction of agricultural nitrogen input is not permanently removed but rather retained in the riparian zone. Overall, our model represents a useful tool to better compare nitrogen retention to permanent nitrogen removal in riparian zones at various temporal and spatial scales.

Degradation of chlorinated ethenes (CEs) in low conductivity layers of aquifers reduces pollution plume tailing and accelerates natural attenuation timeframes. The degradation pathways involved are often different from those in the higher conductive layers and might go undetected when only highly conductive layers are targeted in site assessments. Reactive transport model simulations (PHT3D in FloPy) were executed to assess the performance of dual carbon and chlorine compound specific stable isotope analysis (CSIA) in degradation pathway identification and quantification in a coupled physical-chemical heterogeneous virtual aquifer. Degradation rate constants were assumed correlated to the hydraulic conductivity: positively for oxidative transformation (higher oxygen availability in coarser sands) and negatively for chemical reduction (higher content of reducing solids in finer sediments). Predicted carbon isotope ratios were highly heterogeneous. They generally increased downgradient of the pollution source but the large variation across depth illustrates that monotonously increasing isotope ratios downgradient, as were associated with the oxidative component, are not necessarily a common situation when degradation is favorable in low conductivity layers. Dual carbon-chlorine CSIA performed well in assessing the occurrence of the spatially separated degradation pathways and the overall degradation, provided appropriate enrichment factors were known and sufficiently different. However, pumping to obtain groundwater samples especially from longer well screens causes a bias towards overestimation of the contribution of oxidative transformation associated with the higher conductive zones. As degradation was less intense in these highly conductive zones under the simulated conditions, overall degradation was underestimated. In contrast, in the usual case of limited CSIA data, dual CSIA plots may rather indicate dominance of chemical reduction, while oxidative transformation could go unnoticed, despite being an equally important degradation pathway.

Push-pull tests (PPTs) were evaluated with 1-D radially axisymmetric multi-component geochemical reactive transport modelling (RTM) to assess aquifer reactivity controlling groundwater quality. Nutrient fate and redox processes were investigated in an Aquifer Storage and Recovery (ASR) system, in which oxic tile drainage water (TDW; nitrate ~14 mg/L; phosphate ~17 mg/L) was stored in an anoxic aquifer for later re-use as irrigation water. During the PPTs, the ASR system did not operate. PPTs were performed in two monitoring wells (MW2, MW3), with 1 m well screens in contrasting geochemical formations at different depths. In these wells, 300 L TDW was injected, and consecutively 720 L was abstracted within 12 days, during which water quality changes were studied. The RTM simulated cation exchange, precipitation of Hydrous Ferric Oxides, Iron (III)-phosphate and Calcium-phosphate minerals, and surface complexation as equilibrium processes. Oxidation of Pyrite, soil organic matter, and dissolved ferrous iron were simulated with kinetic rate expressions. Oxygen (within 2 days) and nitrate (within 4–7 days) were fully reduced during the PPTs. The main reductants were ferrous iron (Monitoring Well (MW) 2: 2%, MW3: 13%), soil organic matter (MW2: 93%, MW3: 6%), and Pyrite (MW2: 5%, MW3: 81%). The intra aquifer differences in dominant reduction pathways are remarkable as higher reduction rates coincided with lower contents of soil organic matter and Pyrite, respectively. Phosphate was mostly re-abstracted (MW2: 73%, MW3: 64%) and partially immobilized due to precipitation of Iron-hydroxyphosphates (MW2: 4.6%, MW3: 35%), Hydroxyapatite (MW2: 23%, MW3: 0%), and to a lesser extent by surface complexation on various minerals (MW2-3

In spite of trying to understand processes in the same spatial domain, the catchment hydrology and water quality scientific communities are relatively disconnected and so are their respective models. This is emphasized by an inadequate representation of transport processes, in both catchment-scale hydrological and water quality models. While many hydrological models at the catchment scale only account for pressure propagation and not for mass transfer, catchment scale water quality models are typically limited by overly simplistic representations of flow processes. With the objective of raising awareness for this issue and outlining potential ways forward we provide a nontechnical overview of (1) the importance of hydrology-controlled transport through catchment systems as the link between hydrology and water quality; (2) the limitations of current generation catchment-scale hydrological and water quality models; (3) the concept of transit times as tools to quantify transport; and (4) the benefits of transit time based formulations of solute transport for catchment-scale hydrological and water quality models. There is emerging evidence that an explicit formulation of transport processes, based on the concept of transit times has the potential to improve the understanding of the integrated system dynamics of catchments and to provide a stronger link between catchment-scale hydrological and water quality models. WIREs Water 2016, 3:629-657. doi: 10.1002/wat2.1155 For further resources related to this article, please visit the .

Back-diffusion of chlorinated ethenes (CEs) from low-permeability layers (LPLs) causes contaminant persistence long after the primary spill zones have disappeared. Naturally occurring degradation in LPLs lowers remediation time frames, but its assessment through sediment sampling is prohibitive in conventional remediation projects. Scenario simulations were performed with a reactive transport model (PHT3D in FloPy) accounting for isotope effects associated with degradation, sorption, and diffusion, to evaluate the potential of CSIA data from aquifers in assessing degradation in aquitards. The model simulated a trichloroethylene (TCE) DNAPL and its pollution plume within an aquifer-aquitard-aquifer system. Sequential reductive dechlorination to ethene and sorption were uniform in the aquitard and did not occur in the aquifer. After 10 years of loading the aquitard through diffusion from the plume, subsequent source removal triggered release of TCE by back-diffusion. In the upper aquifer, during the loading phase, δ13C-TCE was slightly enriched (up to 2‰) due to diffusion effects stimulated by degradation in the aquitard. In the upper aquifer, during the release phase, (i) source removal triggered a huge δ13C increase especially for higher CEs, (ii) moreover, downstream decreasing isotope ratios (caused by downgradient later onset of the release phase) with temporal increasing isotope ratios reflect aquitard degradation (as opposed to downstream increasing and temporally constant isotope ratios in reactive aquifers), and (iii) the carbon isotope mass balance (CIMB) enriched up to 4‰ as lower CEs (more depleted, less sorbing) have been transported deeper into the aquitard. Thus, enriched CIMB does not indicate oxidative transformation in this system. The CIMB enrichment enhanced with more sorption and lower aquitard thickness. Thin aquitards are quicker flushed from lower CEs leading to faster CIMB enrichment over time. CIMB enrichment is smaller or nearly absent when daughter products accumulate. Aquifer CSIA patterns indicative of aquitard degradation were similar in case of linear decreasing rate constants but contrasted with previous simulations assuming a thin bioactive zone. The Rayleigh equation systematically underestimates the extent of TCE degradation in aquifer samples especially during the loading phase and for conditions leading to long remediation time frames (low groundwater flow velocity, thicker aquitards, strong sorption in the aquitard). The Rayleigh equation provides a good and useful picture on aquitard degradation during the release phase throughout the sensitivity analysis. This modelling study provides a framework on how aquifer CSIA data can inform on the occurrence of aquitard degradation and its pitfalls.

Microbial communities in groundwater ecosystems can develop the capacity to degrade complex mixtures of chemicals resulting from pollution by landfill leachate. Monitoring this natural attenuation requires insight into the metabolic potential and activity of microbial communities. We contrasted the metagenomes and metatranscriptomes from a leachate-polluted aquifer downstream of the Banisveld (the Netherlands) landfill with uncontaminated groundwater, which revealed changes in microbial genomic content and activity. Banisveld landfill leachate contains mono-aromatic hydrocarbons and the assessment of natural attenuation of these compounds in the aquifer had been a focal point of research. In the contaminated groundwater, active microbial functions were the ones involved in degradation of complex carbon substrates and organic pollutants. We found that benzylsuccinate synthase genes-involved in the catabolism of toluene-were highly expressed close to the source of contamination, confirming the ongoing natural attenuation of organic mono-aromatic hydrocarbon pollution in this aquifer. Additionally, metatranscriptomes were indicative of phosphorus limitation that can constrain total microbial activity and agree with the low phosphate concentrations ( < 0.4 μmol/L) in this aquifer. Through the application of metagenomics and metatranscriptomics, we were able to determine functional potential and expression patterns to assess the natural attenuation processes and constraints on microbial communities.

The ability to evaluate stream hydrochemistry is often constrained by the capacity to sample streamwater at an adequate frequency. While technology is no longer a limiting factor, costs and sample management can still be a barrier to high-resolution water quality instrumentation. We propose a new framework for investigating the electrical conductivity (EC) of streamwater, which can be measured continuously through inexpensive sensors. We show that EC embeds information about individual ion content that can be isolated to retrieve solute concentrations at high resolution. The essence of the approach is the decomposition of the EC signal into its "harmonics", i.e., the specific contributions of the major ions that conduct current in water. The ion contribution is used to explore water quality patterns and to develop algorithms that reconstruct solute concentrations starting from EC during periods where solute measurements are not available. The approach is validated on a hydrochemical data set from Plynlimon, Wales, showing that improved estimates of high-frequency solute dynamics can easily be achieved. Our results support the installation of EC probes to complement water quality campaigns and suggest that the potential of EC measurements in rivers is currently far from being fully exploited.

Compound-specific stable isotope analysis (CSIA) has proven beneficial in the characterization of contaminant degradation in groundwater, but it has never been used to assess pesticide transformation on catchment scale. This study presents concentration and carbon CSIA data of the herbicides S-metolachlor and acetochlor from three locations (plot, drain, and catchment outlets) in a 47 ha agricultural catchment (Bas-Rhin, France). Herbicide concentrations at the catchment outlet were highest (62 µg L−1) in response to an intense rainfall event following herbicide application. Increasing δ13C values of S-metolachlor and acetochlor by more than 2 ‰ during the study period indicated herbicide degradation. To assist the interpretation of these data, discharge, concentrations, and δ13C values of S-metolachlor were modelled with a conceptual mathematical model using the transport formulation by travel-time distributions. Testing of different model setups supported the assumption that degradation half-lives (DT50) increase with increasing soil depth, which can be straightforwardly implemented in conceptual models using travel-time distributions. Moreover, model calibration yielded an estimate of a field-integrated isotopic enrichment factor as opposed to laboratory-based assessments of enrichment factors in closed systems. Thirdly, the Rayleigh equation commonly applied in groundwater studies was tested by our model for its potential to quantify degradation on catchment scale. It provided conservative estimates on the extent of degradation as occurred in stream samples. However, largely exceeding the simulated degradation within the entire catchment, these estimates were not representative of overall degradation on catchment scale. The conceptual modelling approach thus enabled us to upscale sample-based CSIA information on degradation to the catchment scale. Overall, this study demonstrates the benefit of combining monitoring and conceptual modelling of concentration and CSIA data and advocates the use of travel-time distributions for assessing pesticide fate and transport on catchment scale.

Compound-specific stable isotope analysis (CSIA) has proven beneficial in the characterization of contaminant degradation in groundwater, but it has never been used to assess pesticide transformation at catchment scale. This study presents concentration and carbon CSIA data of the herbicides S-metolachlor and acetochlor from three locations (plot, drain, and catchment outlets) in a 47-ha agricultural catchment (Bas-Rhin France). Herbicide concentrations at the catchment outlet were highest (62 μg L−1) in response to an intense rainfall event following herbicide application. Increasing δ13C-values of S-metolachlor and acetochlor by more than 2 ‰ during the study period indicated herbicide degradation. To assist the interpretation of these data, discharge, concentrations and δ13C-values of S-metolachlor were modelled with a conceptual mathematical model using the transport formulation by travel time distributions. Testing of different model setups supported the assumption that degradation half-lives (DT50) increase with increasing soil depth, which can be straightforwardly implemented in conceptual models using travel time distributions. Moreover, model calibration yielded an estimate of a field-integrated isotopic enrichment factor as opposed to laboratory-based assessments of enrichment factors in closed systems. Thirdly, the Rayleigh equation commonly applied in groundwater studies was tested by our model for its potential to quantify degradation at catchment scale. It provided conservative estimates on the extent of degradation as occurred in stream samples. However, largely exceeding the simulated degradation within the entire catchment, these estimates were not representative of overall degradation at catchment scale. The conceptual modelling approach thus enabled us to upscale sample-based CSIA information on degradation to the catchment scale. Overall, this study demonstrates the benefit of combining monitoring and conceptual modelling of concentrations and CSIA data, and advocates the use of travel time distributions for assessing pesticide fate and transport at catchment scale.

Aquifers used for the production of drinking water are increasingly being used for the generation of shallow geothermal energy. This causes temperature perturbations far beyond the natural variations in aquifers and the effects of these temperature variations on groundwater quality, in particular trace elements, have not been investigated. Here, we report the results of column experiments to assess the impacts of temperature variations (5°C, 11°C, 25°C and 60°C) on groundwater quality in anoxic reactive unconsolidated sandy sediments derived from an aquifer system widely used for drinking water production in the Netherlands. Our results showed that at 5°C no effects on water quality were observed compared to the reference of 11°C (in situ temperature). At 25°C, As concentrations were significantly increased and at 60°C, significant increases were observed pH and DOC, P, K, Si, As, Mo, V, B, and F concentrations. These elements should therefore be considered for water quality monitoring programs of shallow geothermal energy projects. No consistent temperature effects were observed on Na, Ca, Mg, Sr, Fe, Mn, Al, Ba, Co, Cu, Ni, Pb, Zn, Eu, Ho, Sb, Sc, Yb, Ga, La, and Th concentrations, all of which were present in the sediment. The temperature-induced chemical effects were probably caused by (incongruent) dissolution of silicate minerals (K and Si), desorption from, and potentially reductive dissolution of, iron oxides (As, B, Mo, V, and possibly P and DOC), and mineralisation of sedimentary organic matter (DOC and P). © 2013 Elsevier Ltd.

Carbon (C), chlorine (Cl), and hydrogen (H) isotope effects were determined during dechlorination of TCE to ethene by a mixed Dehalococcoides (Dhc) culture. The C isotope effects for the dechlorination steps were consistent with data published in the past for reductive dechlorination (RD) by Dhc. The Cl effects (combined with an inverse H effect in TCE) suggested that dechlorination proceeded through nucleophilic reactions with cobalamin rather than by an electron transfer mechanism. Depletions of (37)Cl in daughter compounds, resulting from fractionation at positions away from the dechlorination center (secondary isotope effects), further support the nucleophilic dechlorination mechanism. Determination of C and Cl isotope ratios of the reactants and products in the reductive dechlorination chain offers a potential tool for differentiation of Dhc activity from alternative transformation mechanisms (e.g., aerobic degradation and reductive dechlorination proceeding via outer sphere mechanisms), in studies of in situ attenuation of chlorinated ethenes. Hydrogenation of the reaction products (DCE, VC, and ethene) showed a major preference for the (1)H isotope. Detection of depleted dechlorination products could provide a line of evidence in discrimination between alternative sources of TCE (e.g., evolution from DNAPL sources or from conversion of PCE).

Groundwater systems are increasingly used for seasonal aquifer thermal energy storage (SATES) for periodic heating and cooling of buildings. Its use is hampered in contaminated aquifers because of the potential environmental risks associated with the spreading of contaminated groundwater, but positive side effects, such as enhanced contaminant remediation, might also occur. A first reactive transport study is presented to assess the effect of SATES on the fate of chlorinated solvents by means of scenario modeling, with emphasis on the effects of transient SATES pumping and applicable kinetic degradation regime. Temperature effects on physical, chemical, and biological reactions were excluded as calculations and initial simulations showed that the small temperature range commonly involved (ΔT < 15 C) only caused minor effects. The results show that a significant decrease of the contaminant mass and (eventually) plume volume occurs when degradation is described as sediment-limited with a constant rate in space and time, provided that dense non-aqueous phase liquid (DNAPL) is absent. However, in the presence of DNAPL dissolution, particularly when the dissolved contaminant reaches SATES wells, a considerably larger contaminant plume is created, depending on the balance between DNAPL dissolution and mass removal by degradation. Under conditions where degradation is contaminant-limited and degradation rates depend on contaminant concentrations in the aquifer, a SATES system does not result in enhanced remediation of a contaminant plume. Although field data are lacking and existing regulatory constraints do not yet permit the application of SATES in contaminated aquifers, reactive transport modeling provides a means of assessing the risks of SATES application in contaminated aquifers. The results from this study are considered to be a first step in identifying the subsurface conditions under which SATES can be applied in a safe or even beneficial manner. © 2013 Elsevier B.V.

Aquifer thermal energy storage (ATES) uses groundwater to store energy for heating or cooling purposes in the built environment. This paper presents field and laboratory results aiming to elucidate the effects that ATES operation may have on chemical groundwater quality. Field data from an ATES site in the south of the Netherlands show that ATES results in chemical quality perturbations due to homogenisation of the initially present vertical water quality gradient. We tested this hypothesis by numerical modelling of groundwater flow and coupled SO4 transport during extraction and injection of groundwater by the ATES system. The modelling results confirm that extracting groundwater from an aquifer with a natural quality stratification, mixing this water in the ATES system, and subsequent injection in the second ATES well can adequately describe the observation data. This mixing effect masks any potential temperature effects in typical low temperature ATES systems (

Reactive transport modeling of multi-element, compound-specific isotope analysis (CSIA) data has great potential to quantify sequential microbial reductive dechlorination (SRD) and alternative pathways such as oxidation, in support of remediation of chlorinated solvents in groundwater. As a key step towards this goal, a model was developed that simulates simultaneous carbon, chlorine, and hydrogen isotope fractionation during SRD of trichloroethene, via cis-1,2-dichloroethene (and trans-DCE as minor pathway), and vinyl chloride to ethene, following Monod kinetics. A simple correction term for individual isotope/isotopologue rates avoided multi-element isotopologue modeling. The model was successfully validated with data from a mixed culture Dehalococcoides microcosm. Simulation of Cl-CSIA required incorporation of secondary kinetic isotope effects (SKIEs). Assuming a limited degree of intramolecular heterogeneity of δ37Cl in TCE decreased the magnitudes of SKIEs required at the non-reacting Cl positions, without compromising the goodness of model fit, whereas a good fit of a model involving intramolecular CCl bond competition required an unlikely degree of intramolecular heterogeneity. Simulation of H-CSIA required SKIEs in H atoms originally present in the reacting compounds, especially for TCE, together with imprints of strongly depleted δ2H during protonation in the products. Scenario modeling illustrates the potential of H-CSIA for source apportionment.

Clogging of water wells by iron-hydroxide incrustations due to mixing of anoxic and oxic groundwater is a common well-ageing problem. The relation between well operation (on and off), the spatial and temporal variations in hydrochemistry outside and inside a supply well, and the distribution of clogging iron-hydroxides were studied in an artificial recharge well field in the Netherlands. Camera inspection, high-resolution multi-level water sampling outside the well and detailed in-well pH/EC/O2 profiles revealed remarkable patterns. During pumping, the top of the upper well screen abstracted oxic filtrate, although the larger part of the in-well water column was anoxic. The column rapidly turned oxic after shutdown due to a downward short-circuiting of oxic water via the well. Within 15 d it became anoxic due to the slow advance of anoxic lake filtrate created by local changes in flow direction as the neighboring wells continued to pump. Severe clogging occurred where the oxic filtrate entered the well, while half-clogging of the upper well screen occurred due to less inflow of oxic filtrate on the lake side. Transport of iron flocs and bacterial slimes after shutdown seemed to clog the lower part of the well screen. Frequent on/off switching should be avoided in iron-clogged wells.

Enhanced In situ Biodenitrification (EIB) is a capable technology for nitrate removal in subsurface water resources. Optimizing the performance of EIB implies devising an appropriate feeding strategy involving two design parameters: carbon injection frequency and C:N ratio of the organic substrate nitrate mixture. Here we model data on the spatial and temporal evolution of nitrate (up to 1.2 mM), organic carbon (ethanol), and biomass measured during a 342 day-long laboratory column experiment (published in Vidal-Gavilan et al., 2014). Effective porosity was 3% lower and dispersivity had a sevenfold increase at the end of the experiment as compared to those at the beginning. These changes in transport parameters were attributed to the development of a biofilm. A reactive transport model explored the EIB performance in response to daily and weekly feeding strategies. The latter resulted in significant temporal variation in nitrate and ethanol concentrations at the outlet of the column. On the contrary, a daily feeding strategy resulted in quite stable and low concentrations at the outlet and complete denitrification. At intermediate times (six months of experiment), it was possible to reduce the carbon load and consequently the C:N ratio (from 2.5 to 1), partly because biomass decay acted as endogenous carbon to respiration, keeping the denitrification rates, and partly due to the induced dispersivity caused by the well developed biofilm, resulting in enhancement of mixing between the ethanol and nitrate and the corresponding improvement of denitrification rates. The inclusion of a dual-domain model improved the fit at the last days of the experiment as well as in the tracer test performed at day 342, demonstrating a potential transition to anomalous transport that may be caused by the development of biofilm. This modeling work is a step forward to devising optimal injection conditions and substrate rates to enhance EIB performance by minimizing the overall supply of electron donor, and thus the cost of the remediation strategy.

Enhanced in situ biodenitrification (EIB) is a feasible technology to clean nitrate-polluted groundwater and reachdrinking water standards. Aimed at enabling a better monitoring and management of the technology at the fieldscale, we developed a two-dimensional reactive transport model (RTM) of a cross section (26.5 × 4 m) of a fracturedaquifer composed of marls involving both biogeochemical processes and associated isotope fractionation.The RTM was based on the upscaling of a previously developed batch-scale model and on a flow model that wasconstructed and calibrated on in situ pumping and tracer tests. The RTM was validated using the experimentaldata provided by Vidal-Gavilan et al. (2013). The model considers several processes including (i) exogenous andendogenousmicrobial nitrate and sulfate respiration coupled to ethanol oxidation and linked tomicrobial growthand decay, and (ii) geochemical interactions (dissolution/precipitation of calcite), and (iii) isotopic fractionationof the reaction network (15N–NO3, 18O–NO3, 13C–DIC, 13C–ethanol, 13C–biomass, and 13C–calcite). Most of thecalibrated microbiological parameter values at field scale did not change more than one order of magnitudefrom those obtained at batch scale, which indicates that parameters determined at the batch scale can be used as initial estimates to reproduce field observations provided that groundwater flow is well known. In contrast, the calcite precipitation rate constant increased significantly (fifty times)with respect to batch scale. The incorporation of isotope fractionation into the model allowed to confirm the overall consistency of the model and to test the practical usefulness of assessing the efficiency of EIB through the Rayleigh equation approach. The large underestimation of the Rayleigh equation of the extent of EIB (from 10 to 50%) was caused by the high value of hydrodynamic dispersion observed in this fractured aquifer together with the high reaction rates.

The effects of transverse hydrodynamic dispersion on altering transformation-induced compound-specific isotope analysis (CSIA) signals within groundwater pollution plumes have been assessed with reactive transport modeling accommodating diffusion-induced isotope fractionation (DIF) and implementing different parameterizations of local transverse dispersion. The model reproduced previously published field data showing a negative carbon isotope pattern (-2 %) at the fringes of a nondegrading PCE plume. We extended the study to reactive transport scenarios considering vinyl chloride as a model compound and assessing, through a detailed sensitivity analysis, the coupled effects of transverse hydrodynamic dispersion (with and without DIF) and aerobic fringe degradation on the evolution of carbon and chloride isotope ratios. Transformation-induced positive isotope signals were increasingly attenuated with distance from the source and higher degradation rate. The effect of DIF on the overall isotope signal attenuation was greatest near the source and for low values of groundwater flow velocity, transverse dispersion coefficient, molecular weight, rate constant, and isotope fractionation factor, α, of the degradation reaction. Models disregarding DIF underestimate the actual α. The approximately twice larger DIF effect for chlorine than for carbon together with the low α for oxidation resulted in strong chlorine CSIA depletions for VC at the plume fringe. © 2012 American Chemical Society.

There is increasing interest in combined carbon-chlorine compound-specific isotope analysis (CSIA) to differentiate between contaminant sources and to assess transformation processes. However, the significant abundance of polychlorinated molecules with several heavy chlorine isotopes complicates the evaluation of chlorine isotope trends. Therefore, the goal of this study was to develop a conceptual and mathematical framework that describes the expected chlorine isotope fractionation patterns during multistep transformation of chlorinated compounds. Reductive dechlorination of chlorinated ethenes served as an example. The study demonstrates that chlorine isotope trends can be simulated by reproducing the average behavior of light and heavy isotopes or by explicitly simulating molecules with different numbers of heavy isotopes (isotopologues). The calculations reveal that initial chlorine isotope ratios of products equal the isotope ratios of their parent compounds in the absence of secondary isotope effects, while steadily increasing during transformation. The slopes in dual isotope plots are linear for reactant and product during a one-step reaction. They become nonlinear for products that are degraded further but converge to characteristic slopes. Consideration of different scenarios reveals that combined carbon-chlorine isotope analysis bears high potential to differentiate between contaminant sources, to elucidate reaction mechanisms in laboratory studies, and to identify transformation processes in the field. © 2009 American Chemical Society.

Open waste dump systems are still widely used in Indonesia. The Jatibarang landfill receives 650-700 tons of municipal waste per day from the city of Semarang, Central Java. Some of the leachate from the landfill flows via several natural and collection ponds to a nearby river. The objectives of the study were to identify seasonal landfill leachate characteristics in this surface water and to determine the occurrence of natural attenuation, in particular the potential for biodegradation, along the flow path. Monthly measurements of general landfill leachate parameters, organic matter-related factors and redox-related components revealed that leachate composition was influenced by seasonal precipitation. In the dry season, electrical conductivity and concentrations of BOD, COD, N-organic matter, ammonia, sulphate and calcium were significantly higher (1.1-2.3 fold) than during the wet season. Dilution was the major natural attenuation process acting on leachate. Heavy metals had the highest impact on river water quality. Between the landfill and the river, a fivefold dilution occurred during the dry season due to active springwater infiltration, while rainwater led to a twofold dilution in the wet season. Residence time of leachate in the surface leachate collection system was less than 70 days. Field measurements and laboratory experiments showed that during this period hardly any biodegradation of organic matter and ammonia occurred (less than 25%). However, the potential for biodegradation of organic matter and ammonia was clearly revealed during 700 days of incubation of leachate in the laboratory (over 65%). If the residence time of leachate discharge can be increased to allow for biodegradation processes and precipitation reactions, the polluting effects of leachate on the river can be diminished. © 2008 Elsevier Ltd. All rights reserved.

Heterogeneity in eukaryotic and bacteria community structure in surface and subsurface sediment samples downgradient of the Banisveld landfill (The Netherlands) was studied using a culturing-independent molecular approach. Along a transect covering the part of the aquifer most polluted by landfill leachate, sediment was sampled at 1-m depth intervals, until a depth of 5.5 m, at four distances from the landfill. Two drillings were placed in a nearby clean area as a reference. Denaturing gradient gel electrophoresis banding patterns revealed high bacterial and eukaryotic diversity and complex community structures. Bacteria and eukaryotic community profiles in polluted samples grouped different from those in clean samples. Bacteria community profiles in surface samples clustered together and separately from subsurface community profiles. Subsurface bacteria profiles clustered in a location-specific manner. Eukaryotic community structure did not significantly relate to distance from the landfill or depth. No significant spatial autocorrelation of bacteria or eukaryotic communities was observed over 1-m depth intervals per sampling location. Spatial heterogeneity in sediment-associated bacterial communities appears to be much larger than in groundwater. We discuss how on the one hand, spatial heterogeneity may complicate the assessment of microbial community structure and functioning, while on the other it may provide better opportunities for natural attenuation.

Eukaryotes may influence pollutant degradation processes in groundwater ecosystems by activities such as predation on bacteria and recycling of nutrients. Culture-independent community profiling and phylogenetic analysis of 18S rRNA gene fragments, as well as culturing, were employed to obtain insight into the sediment-associated eukaryotic community composition in an anaerobic sandy aquifer polluted with landfill leachate (Banisveld, The Netherlands). The microeukaryotic community at a depth of 1 to 5 m below the surface along a transect downgradient (21 to 68 m) from the landfill and at a clean reference location was diverse. Fungal sequences dominated most clone libraries. The fungal diversity was high, and most sequences were sequences of yeasts of the Basidiomycota . Sequences of green algae (Chlorophyta) were detected in parts of the aquifer close (Heteromita globosa (Cercozoa) was retrieved in enrichments, and its sequences dominated the clone library derived from the polluted aquifer at a depth of 5 m at a location 21 m downgradient from the landfill. The number of culturable eukaryotes ranged from 10 2 to 10 3 cells/g sediment. Culture-independent quantification revealed slightly higher numbers. Groundwater mesofauna was not detected. We concluded that the food chain in this polluted aquifer is short and consists of prokaryotes and fungi as decomposers of organic matter and protists as primary consumers of the prokaryotes.

The hydrochemistry of landfill leachate and groundwater is affected by not only waste degradation processes, but also by external factors such as the geography of the landfilling site. Knowledge on the fate of landfill leachate in tropical countries will be beneficial for monitoring and regulatory purposes. We studied the Keputih landfill close to the sea at Surabaya, Indonesia: (1) to assess leachate and groundwater hydrochemistry with respect to contamination and seawater intrusion, (2) to investigate the seasonal effects on hydrochemical composition; and (3) to determine redox conditions in order to evaluate the potential for natural attenuation through microbe-mediated electron-accepting processes. We document an influence from sea water intrusion on groundwater hydrochemistry on top of the influences from the landfill itself. Leachate had a high electrical conductivity and high COD, and contained high concentrations of NH4+, HCO3-, SO42-, Fe2+ and Cl-. Concentrations were significantly influenced by season, except for COD and SO42-. The groundwater at locations surrounding the landfill was also contaminated by leachate and concentrations of groundwater contaminants were higher than national regulatory standards in Indonesia for drinking water. The abundance of SO42- in groundwater indicates a large potential for anaerobic biodegradation of organic compounds. Based on the relative concentrations of Cl- and SO42- an influence of the sea water on groundwater hydrochemistry was obvious. Landfilling in developing countries often occurs in coastal areas, therefore we emphasize the need to study microbial community structure and functioning in relation to degradation of landfill leachate in tropical coastal areas impacted by seawater infiltration.

Identification of the functional groups of microorganisms that are predominantly in control of fluxes through, and concentrations in, microbial networks would benefit microbial ecology and environmental biotechnology: the properties of those controlling microorganisms could be studied or monitored specifically or their activity could be modulated in attempts to manipulate the behaviour of such networks. Herein we present ecological control analysis (ECA) as a versatile mathematical framework that allows for the quantification of the control of each functional group in a microbial network on its process rates and concentrations of intermediates. In contrast to current views, we show that rates of flow of matter are not always limited by a single functional group; rather flux control can be distributed over several groups. Also, control over intermediate concentrations is always shared. Because of indirect interactions, through other functional groups, the concentration of an intermediate can also be controlled by functional groups not producing or consuming it. Ecological control analysis is illustrated by a case study on the anaerobic degradation of organic matter, using experimental data obtained from the literature. During anaerobic degradation, fermenting microorganisms interact with terminal electron-accepting microorganisms (e.g. halorespirers, methanogens). The analysis indicates that flux control mainly resides with fermenting microorganisms, but can shift to the terminal electron-accepting microorganisms under less favourable redox conditions. Paradoxically, halorespiring microorganisms do not control the rate of perchloroethylene and trichloroethylene degradation even though they catalyse those processes themselves. © 2006 The Authors.

The Rayleigh equation relates the change in isotope ratio of an element in a substrate to the extent of substrate consumption via a single kinetic isotopic fractionation factor (α). Substrate consumption is, however, commonly distributed over several metabolic pathways each potentially having a different α. Therefore, extended Rayleigh-type equations were derived to account for multiple competing degradation pathways. The value of a as expressed in the environment appears a function of the α values and rate constants of the various involved degradation pathways. Remarkably, the environmental or apparent α value changes and shows non-Rayleigh behavior over a large and relevant concentration interval if Monod kinetics applies and the half-saturation constants of the competing pathways differ. Derived equations were applied to previously published data and enabled (i) quantification of the share that two competing degradation pathways had on aerobic 1,2-dichloroethane (1,2-DCA) biodegradation in laboratory batch experiments and (ii) calculation of the extent of methyl tert-butyl ether (MTBE) biodegradation shared over aerobic and anaerobic degradation at a field site by means of an improved solution to two-dimensional (carbon and hydrogen) compound-specific isotope analysis (CSIA). © 2007 American Chemical Society.

Quantifying the share of destructive and nondestructive processes to natural attenuation (NA) of groundwater pollution plumes is of high importance to the evaluation and acceptance of NA as remediation strategy. Dilution as consequence of hydrodynamic dispersion may contribute considerably to NA, however, without reducing the mass of pollution. Unfortunately, tracers to quantify dilution are usually lacking. Degradation though of low-molecular-weight organic chemicals such as BTEX, chlorinated ethenes, and MTBE is uniquely associated with increases in isotope ratios for steady-state plumes. Compound-specific isotope analysis (CSIA) data are commonly interpreted by means of the Rayleigh equation, originally developed for closed systems, to calculate the extent of degradation under open system field conditions. For that reason, the validity of this approach has been questioned. The Rayleigh equation was accordingly modified to account for dilution, and showed that dilution contributed several to many times more to NA than biodegradation at a groundwater benzene plume. Derived equations also (i) underlined that field-derived isotopic enrichment factors underestimate actual values operative as a consequence of dilution, and (ii) provided a check on the lower limit of isotopic fractionation, thereby resulting in more reliable predictions on the extent of degradation. © 2007 American Chemical Society.

Managers of landfill sites are faced with enormous challenges when attempting to detect and delineate leachate plumes with a limited number of monitoring wells, assess spatial and temporal trends for hundreds of contaminants, and design long-term monitoring (LTM) strategies. Subsurface microbial ecology is a unique source of data that has been historically underutilized in LTM groundwater designs. This paper provides a methodology for utilizing qualitative and quantitative information (specifically, multiple water quality measurements and genome-based data) from a landfill leachate contaminated aquifer in Banisveld, The Netherlands, to improve the estimation of parameters of concern. We used a principal component analysis (PCA) to reduce nonindependent hydrochemistry data, Bacteria and Archaea community profiles from 16SrDNA denaturing gradient gel electrophoresis (DGGE), into six statistically independent variables, representing the majority of the original dataset variances. The PCA scores grouped samples based on the degree or class of contamination and were similar over considerable horizontal distances. Incorporation of the principal component scores with traditional subsurface information using cokriging improved the understanding of the contaminated area by reducing error variances and increasing detection efficiency. Combining these multiple types of data (e.g., genome-based information, hydrochemistry, borings) may be extremely useful at landfill or other LTM sites for designing cost-effective strategies to detect and monitor contaminants. © 2005 American Chemical Society.

Subsurface removal of arsenic by injection with oxygenated groundwater has been proposed as a viable technology for obtaining 'safe' drinking water in Bangladesh. While the oxidation of ferrous iron to solid ferric iron minerals, to which arsenic adsorbs, is assumed to be driven by abiotic reactions, metal-cycling microorganisms may potentially affect arsenic removal. A cultivation-independent survey covering 24 drinking water wells in several geographical regions in Bangladesh was conducted to obtain information on microbial community structure and diversity in general, and on specific functional groups capable of the oxidation or reduction of arsenic or iron. Each functional group, targeted by either group-specific 16S rRNA or functional gene amplification, occurred in at least 79% of investigated samples. Putative arsenate reducers and iron-oxidizing Gallionellaceae were present at low diversity, while more variation in potentially arsenite-oxidizing microorganisms and iron-reducing Desulfuromonadales was revealed within and between samples. Relations between community composition on the one hand and hydrochemistry on the other hand were in general not evident, apart from an impact of salinity on iron-cycling microorganisms. Our data suggest widespread potential for a positive contribution of arsenite and iron oxidizers to arsenic removal upon injection with oxygenated water, but also indicate a potential risk for arsenic re-mobilization by anaerobic arsenate and iron reducers once injection is halted.

Compound-specific carbon and hydrogen isotope analysis was used to investigate biodegradation of benzene and ethylbenzene in contaminated groundwater at Dow Benelux BV industrial site. delta13C values for dissolved benzene and ethylbenzene in downgradient samples were enriched by up to 2+/-0.5 per thousand, in 13C, compared to the delta13C value of the source area samples. delta2H values for dissolved benzene and ethylbenzene in downgradient samples exhibited larger isotopic enrichments of up to 27+/-5 per thousand for benzene and up to 50+/-5 per thousand for ethylbenzene relative to the source area. The observed carbon and hydrogen isotopic fractionation in downgradient samples provides evidence of biodegradation of both benzene and ethylbenzene within the study area at Dow Benelux BV. The estimated extents of biodegradation of benzene derived from carbon and hydrogen isotopic compositions for each sample are in agreement, supporting the conclusion that biodegradation is the primary control on the observed differences in carbon and hydrogen isotope values. Combined carbon and hydrogen isotope analyses provides the ability to compare biodegradation in the field based on two different parameters, and hence provides a stronger basis for assessment of biodegradation of petroleum hydrocarbon contaminants.

Enhanced in-situ biodenitrification (EIB) is a potential technology for remediating nitrate-polluted groundwater. EIB aims to create optimal biodenitrification conditions through the addition of carbon sources, enabling the autochthonous microbial community to degrade nitrate via different redox pathways. Biogeochemical numerical models are useful tools for predicting and designing such biodenitrification applications. Compound-specific stable isotope analysis (CSIA) is another valuable method for determining the degree of nitrate transformation. Therefore, incorporating isotope fractionation in biogeochemical models combines the two tools and is a key step in the development of reactive transport models of EIB under field conditions. In this work, we developed such an integrated model using the Phreeqc code and calibrated the model with batch scale experimental data using either ethanol or glucose as external carbon sources. The model included the following: microbiological processes —exogenous and endogenous nitrate respiration coupled to microbial growth and decay; geochemical processes —precipitation or dissolution of calcite; and isotopic fractionation —δ15N-NO3−, δ18O-NO3−, and δ13C-DIC, incorporating the full δ13C isotope geochemistry involved in EIB. The modeled results fit well with the hydrochemical and isotopic experimental data. The model also incorporated nitrite accumulation observed during the glucose experiment. The biogeochemical model indicates that, depending on the added carbon source, calcite precipitates (using ethanol) or dissolves (using glucose). In both cases, changes in hydraulic conductivity can be induced for actual and long-term EIB applications. The incorporation of isotope fractionation in the model better enables to account for other natural attenuation processes, such as dilution and dispersion, in EIB applications at field scale. Both calibrated enrichment factors (+ 8‰ for ethanol and + 17‰ for glucose) suggest that an inverse fractionation effect occurred (in which the heavy isotope reacts faster than the light isotope) during their oxidation.

Aquifer thermal energy storage (ATES) systems are increasingly being used to acclimatize buildings and are often constructed in aquifers used for drinking water supply. This raises the question of potential groundwater quality impact. Here, we use laboratory column experiments to develop and calibrate a reactive transport model (PHREEQC) simulating the thermally induced (5-60 °C) water quality changes in anoxic sandy sediments. Temperature-dependent surface complexation, cation-exchange, and kinetic dissolution of K-feldspar were included in the model. Optimization results combined with an extensive literature survey showed surface complexation of (oxy)anions (As, B, and PO4) is consistently exothermic, whereas surface complexation of cations (Ca and Mg) and cationic heavy metals (Cd, Pb, and Zn) is endothermic. The calibrated model was applied to simulate arsenic mobility in an ATES system using a simple yet powerful mirrored axi-symmetrical grid. Results showed that ATES mobilizes arsenic toward the fringe of the warm water bubble and the center of the cold water bubble. This transient redistribution of arsenic causes its aqueous concentrations in the cold and warm groundwater bubbles to become similar through multiple heating cycles, with a final concentration depending on the average injection temperature of the warm and cold ATES wells.