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Lack of water quality data for private drinking water sources prevents robust evaluation of exposure risk for communities co-located with historically contaminated sites and ongoing industrial activity. Areas of the Appalachian region of the United States (i.e., Pennsylvania, Ohio and West Virginia) contain extensive hydraulic fracturing activity, as well as other extractive and industrial technologies, in close proximity to communities reliant on private drinking water sources, creating concern over potential groundwater contamination. In this study, we characterized volatile organic compound (VOC) occurrence at 307 private groundwater well sites within Pennsylvania, Ohio, and West Virginia. The majority (97%) of water samples contained at least one VOC, while the average number of VOCs detected at a given site was 5 ± 3. The majority of individual VOC concentrations fell below applicable U.S. Environmental Protection Agency (EPA) Maximum Contamination Levels (MCLs), except for chloroform (MCL of 80 μg L−1; n = 1 at 98 μg L−1), 1,2-dibromoethane (MCL of 0.05 μg L−1; n = 3 ranging from 0.05 to 0.35 μg L−1), and 1,2-dibromo-3-chloropropane (MCL of 0.2 μg L−1; n = 7 ranging from 0.20 to 0.58 μg L−1). To evaluate well susceptibility to VOCs from industrial activity, distance to hydraulic fracturing site was used to assess correlations with contaminant occurrences. Proximity to closest hydraulic fracturing well-site revealed no statistically significant linear relationships with either individual VOC concentrations, or frequency of VOC detections. Evaluation of other known industrial contamination sites (e.g., US EPA Superfund sites) revealed elevated levels of three VOCs (chloroform, toluene, benzene) in groundwaters within 10 km of those Superfund sites in West Virginia and Ohio, illuminating possible point source influence. Lack of correlation between VOC concentrations and proximity to specific point sources indicates complex geochemical processes governing trace VOC contamination of private drinking water sources. While individual concentrations of VOCs fell well below recommended human health levels, the low dose exposure to multiple VOCs occurring in drinking supplies for Appalachian communities was noted, highlighting the importance of groundwater well monitoring. [ABSTRACT FROM AUTHOR]

Unconventional oil and gas (UOG) development, made possible by horizontal drilling and high‐volume hydraulic fracturing, has been fraught with controversy since the industry's rapid expansion in the early 2000's. Concerns about environmental contamination and public health risks persist in many rural communities that depend on groundwater resources for drinking and other daily needs. Spatial disparities in UOG risks can pose distributive environmental injustice if such risks are disproportionately borne by marginalized communities. In this paper, we analyzed groundwater vulnerability to contamination from UOG as a physically based measure of risk in conjunction with census tract level sociodemographic characteristics describing social vulnerability in the northern Appalachian Basin. We found significant associations between elevated groundwater vulnerability and lower population density, consistent with UOG development occurring in less densely populated rural areas. We also found associations between elevated groundwater vulnerability and lower income, higher proportions of elderly populations, and higher proportion of mobile homes, suggesting a disproportionate risk burden on these socially vulnerable groups. We did not find a statistically significant association between elevated groundwater vulnerability and populations of racial/ethnic minorities in our study region. Household surveys provided empirical support for a relationship between sociodemographic characteristics and capacity to assess and mitigate exposures to potentially contaminated water. Further research is needed to probe if the observed disparities translate to differences in chemical exposure and adverse health outcomes. Plain Language Summary: Hydraulic fracturing ("fracking") and horizontal drilling have paved the way for renewed expansion of fossil fuel extraction across the country in the last two decades. Communities have voiced concerns over the industry's potential risks to the environment and public health, especially in regions that heavily rely on groundwater. We developed a new approach to assess groundwater vulnerability to contamination and examined its overlap with measures of social vulnerability. We found significant overlaps between elevated groundwater vulnerability and several measures of higher social vulnerability, suggesting higher risks of drinking water contamination among certain disadvantaged populations. The implications of the co‐occurrence of these vulnerabilities on the inequitable distribution of risks should be considered in policies that aim to balance socioeconomic benefits from the fracking industry and its environmental burdens on marginalized communities. Key Points: Groundwater vulnerability to contamination from unconventional oil and gas development was assessed using a physically based modeling frameworkElevated groundwater vulnerability was associated with several sociodemographic indicators of high social vulnerabilityCo‐occurrence of groundwater vulnerability and social vulnerability poses distributive environmental injustice [ABSTRACT FROM AUTHOR]

BACKGROUND: Unconventional oil and gas development (UOGD) releases chemicals that have been linked to cancer and childhood leukemia. Studies of UOGD exposure and childhood leukemia are extremely limited. OBJECTIVE: The objective of this study was to evaluate potential associations between residential proximity to UOGD and risk of acute lymphoblastic leukemia (ALL), the most common form of childhood leukemia, in a large regional sample using UOGD-specific metrics, including a novel metric to represent the water pathway. METHODS: We conducted a registry-based case–control study of 405 children ages 2–7 y diagnosed with ALL in Pennsylvania between 2009–2017, and 2,080 controls matched on birth year. We used logistic regression to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for the association between residential proximity to UOGD (including a new water pathway-specific proximity metric) and ALL in two exposure windows: a primary window (3 months preconception to 1 y prior to diagnosis/reference date) and a perinatal window (preconception to birth). RESULTS: Children with at least one UOG well within 2 km of their birth residence during the primary window had 1.98 times the odds of developing ALL in comparison with those with no UOG wells [95% confidence interval (CI): 1.06, 3.69]. Children with at least one vs. no UOG wells within 2 km during the perinatal window had 2.80 times the odds of developing ALL (95% CI: 1.11, 7.05). These relationships were slightly attenuated after adjusting for maternal race and socio-economic status [odds ratio (OR) = 1:74 (95% CI: 0.93, 3.27) and OR = 2:35 (95% CI: 0.93, 5.95)], respectively). The ORs produced by models using the water pathway-specific metric were similar in magnitude to the aggregate metric. DISCUSSION: Our study including a novel UOGD metric found UOGD to be a risk factor for childhood ALL. This work adds to mounting evidence of UOGD’s impacts on children’s health, providing additional support for limiting UOGD near residences [ABSTRACT FROM AUTHOR]

We constructed a numerical model of transient ground water flow and solute transport for a portion of the Biscayne Aquifer in Florida, and calibrated the model with three different combinations of data from a 193-day period: head (h) data alone, data on h and ground water discharge to a canal (q), and data on h, q, and ground water chloride concentration (C). We used each of the three calibrated models to predict h and q during a 182-day test period separate from the calibration period. All three calibrated models predicted h equally well during the test period (r = 0.95, where r = 1 indicates perfect agreement between measured and simulated values), though the model calibrated on h alone had significantly different parameter values than the other two models. Predictions of q during the test period depended on calibration methodology; models calibrated with multiple targets simulated q more accurately than the model calibrated on h alone (r = 0.79 compared to r = 0.49). Based on the results of these simulations, we conclude: (1) Post-calibration prediction is important in assessing the value of different data types in automated calibration; (2) inverse-solution uniqueness is not a requirement for accurate h predictions; (3) relatively simple models can predict with reasonable accuracy transient ground water flow in a complex aquifer, and parameters governing this prediction can be estimated by nonlinear regression methods that incorporate both h and q data; (4) addition of C data to the calibration did not improve model predictive capacity because the information in the C data was similar to that in the q data, from the perspective of model calibration (the subsurface chemical signal in question was controlled mainly by seepage of high-chloride canal water into the low-chloride ground water system)., Introduction Application of numerical models of ground water flow almost always involves some sort of calibration to identify values for one or more model parameters. Calibration is generally an iterative [...]

Data from a large-scale canal-drawdown test were used to estimate the specific yield ([s.sub.y]) of the Biscayne Aquifer, an unconfined limestone aquifer in southeast Florida, The drawdown test involved dropping the water level in a canal by about 30 cm and monitoring the response of hydraulic head in the surrounding aquifer. Specific yield was determined by analyzing data from the unsteady portion of the drawdown test using an analytical stream-aquifer interaction model (Ziotnik and Huang 1999). Specific yield values computed from drawdown at individual piezometers ranged from 0.050 to 0.57, most likely indicating heterogeneity of specific yield within the aquifer (small-scale variation in hydraulic conductivity may also have contributed to the differences in [s.sub.y] among piezometers). A value of 0.15 (our best estimate) was computed based on all drawdown data from all piezometers. We incorporated our best estimate of specific yield into a large-scale two-dimensional numerical MODFLOW-based ground water flow model and made predictions of head during a 183-day period at four wells located 337 to 2546 m from the canal. We found good agreement between observed and predicted heads, indicating our estimate of specific yield is representative of the large portion of the Biscayne Aquifer studied here. This work represents a practical and novel approach to the determination of a key hydrogeological parameter (the storage parameter needed for simulation and calculation of transient unconfined ground water flow), at a large spatial scale (a common scale for water resource modeling), for a highly transmissive limestone aquifer (in which execution of a traditional pump test would be impractical and would likely yield ambiguous results). Accurate estimates of specific yield and other hydrogeological parameters are critical for management of water supply, Everglades environmental restoration, flood control, and other issues related to the ground water hydrology of the Biscayne Aquifer., Carl H. Bolster (1,2), David P. Genereux (3), and James E. Saiers (1) Introduction Predictions of ground water flow and solute transport rely on accurate estimates of aquifer parameters. Parameters [...]

Horizontal drilling with hydraulic fracturing (HDHF) relies on the use of anthropogenic organic chemicals in proximity to residential areas, raising concern for groundwater contamination. Here, we extensively characterized organic contaminants in 94 domestic groundwater sites in Northeastern Pennsylvania after ten years of activity in the region. All analyzed volatile and semi-volatile compounds were below recommended United States Environmental Protection Agency maximum contaminant levels, and integrated concentrations across two volatility ranges, gasoline range organic compounds (GRO) and diesel range organic compounds (DRO), were low (0.13 ± 0.06 to 2.2 ± 0.7 ppb and 5.2–101.6 ppb, respectively). Following dozens of correlation analyses with distance-to-well metrics and inter-chemical indicator correlations, no statistically significant correlations were found except: (1) GRO levels were higher within 2 km of violations and (2) correlation between DRO and a few inorganic species (e.g., Ba and Sr) and methane. The correlation of DRO with inorganic species suggests a potential high salinity source, whereas elevated GRO may result from nearby safety violations. Highest-concentration DRO samples contained bis-2-ethylhexyl phthalate and N,N-dimethyltetradecylamine. Nevertheless, the overall low rate of contamination for the analytes could be explained by a spatially-resolved hydrogeologic model, where estimated transport distances from gas wells over the relevant timeframes were short relative to the distance to the nearest groundwater wells. Together, the observations and modeled results suggest a low probability of systematic groundwater organic contamination in the region. [ABSTRACT FROM AUTHOR]

This study focuses on characterizing the contributions of key terrestrial pathways that deliver dissolved organic carbon (DOC) to streams during hydrological events and on elucidating factors governing variation in water and DOC fluxes from these pathways. We made high‐frequency measurements of discharge, specific conductance (SC), and fluorescent dissolved organic matter (FDOM) during 221 events recorded over 2 years within four Vermont (USA) watersheds that range in area from 0.4 to 139 km2. Using the SC measurements, together with statistical information on discharge, we separated the event hydrographs into contributions from three terrestrial pathways, which we refer to as riparian quickflow, subsurface quickflow, and slow‐flow groundwater. The pathway discharges were used as input to a mixing model that closely approximated sub‐hourly streamwater DOC concentrations as measured with the FDOM sensors. Subsurface quickflow, comprised of pre‐event water, was the leading contributor to streamwater DOC fluxes, while riparian quickflow, comprised of event water, was the second‐leading contributor to streamwater DOC fluxes, despite comprising the smallest proportion of streamflow yield among the three end‐member pathways. Fixed‐effects regression analysis revealed that the relationship between DOC fluxes from the end‐member pathways and event magnitude was consistent across the four watersheds. This analysis also showed that DOC fluxes from the quickflow pathways increased significantly with temperature and varied inversely, but weakly, with catchment antecedent wetness. We believe that our approach, which leverages in‐stream sensors that enable high‐frequency measurements over extended periods, may be applicable for evaluating controls on DOC export from other watersheds within and beyond our study region. Key Points: Calculations of a mixing model reproduce sub‐hourly observations of streamwater dissolved organic carbon (DOC) concentrations in four watershedsStreamwater specific conductance can be used to quantify discharge from terrestrial flowpaths and constrain the DOC mixing modelDOC fluxes from principal flow paths vary discernibly with event magnitude, temperature, and antecedent wetness [ABSTRACT FROM AUTHOR]

The rapid expansion of unconventional oil and gas development (UD), made possible by horizontal drilling and hydraulic fracturing, has triggered concerns over groundwater contamination and public health risks. To improve our understanding of the risks posed by UD, we develop a physically based, spatially explicit framework for evaluating groundwater well vulnerability to aqueous phase contaminants released from surface spills and leaks at UD well pad locations. The proposed framework utilizes the concept of capture probability and incorporates decision‐relevant planning horizons and acceptable risks to support goal‐oriented modeling for groundwater protection. We illustrate the approach in northeastern Pennsylvania, where a high intensity of UD activity overlaps with local dependence on domestic groundwater wells. Using two alternative models of the bedrock aquifer and a precautionary paradigm to integrate their results, we found that most domestic wells in the domain had low vulnerability as the extent of their modeled probabilistic capture zones were smaller than distances to the nearest existing UD well pad. We also found that simulated capture probability and vulnerability were most sensitive to the model parameters of matrix hydraulic conductivity, porosity, pumping rate, and the ratio of fracture to matrix conductivity. Our analysis demonstrated the potential inadequacy of current state‐mandated setback distances that allow UD within the boundaries of delineated capture zones. The proposed framework, while limited to aqueous phase contamination, emphasizes the need to incorporate information on flow paths and transport timescales into policies aiming to protect groundwater from contamination by UD. Key Points: Capture probability is used to assess domestic well vulnerability to aqueous phase contamination from unconventional oil and gas developmentMultiple groundwater model conceptualizations provide more conservative risk estimates than a single calibrated modelThe approach provides a scientifically defensible basis for groundwater and human health protection regulations such as setback distances [ABSTRACT FROM AUTHOR]

Root system architecture (RSA) can significantly affect plant access to water, total transpiration, as well as its partitioning by soil depth, with implications for surface heat, water, and carbon budgets. Despite recent advances in land surface model (LSM) descriptions of plant hydraulics, descriptions of RSA have not been included because of their three-dimensional complexity, which makes them generally too computationally costly. Here we demonstrate a new, process-based 1D layered model that captures the dynamic shifts in water potential gradients of 3D RSA under different soil moisture conditions: the RSA stencil. Using root systems calibrated to the rooting profiles of four plant functional types (PFT) of the Community Land Model, we show that the RSA stencil predicts plant water potentials within 2% to the outputs of a full 3D model, under the same assumptions on soil moisture heterogeneity, despite its trivial computational cost, resulting in improved predictions of water uptake and soil moisture compared to a model without RSA in a transient simulation. Our results suggest that LSM predictions of soil moisture dynamics and dependent variables can be improved by the implementation of this model, calibrated for individual PFTs using field observations. [ABSTRACT FROM AUTHOR]

Understanding the processes controlling the transfer of organic matter from terrestrial to aquatic ecosystems is of fundamental importance for the aquatic sciences. Over the course of a full year, fluorescence, absorbance and bioavailability of dissolved organic matter (DOM) were characterised in Bigelow Brook, a forested headwater stream in Massachusetts, USA. Parallel factor analysis (PARAFAC) identified a four-component model to describe observedDOM fluorescence (C1-C4). Component C2 exhibited the characteristics of a more humic-like fluorophore, with a potentially more reduced redox state and increased with discharge, whereas more fulvic-like (C1) and protein-like (C3, C4) fluorophores decreased. Under both dark and light-exposed conditions, percentage bioavailable dissolved organic carbon (%BDOC) increased with discharge (R2=0.37 and R2=0.56). C2 and specific absorptivity (SUVA) were reduced following BDOC incubations, whereas C1, C3 and C4 increased. These changes to DOM characteristics with increasing discharge were observed under both baseflow and stormflow conditions, indicating that with rising watertable, loading from a large riparian or hyporheic pool of organic matter is likely occurring. Other headwater streams, where loading is controlled by hillslope processes, are likely to exhibit a similar pattern of increasing export of more humic and bioavailable DOM during hydrologic events. [ABSTRACT FROM AUTHOR]

Exchange of water and solutes between contaminated soil matrix and bulk solution in preferential flow paths has been shown to contribute to the long-term release of dissolved contaminants in the subsurface, but whether and how this exchange can affect the release of colloids in a soil are unclear. To examine this, we applied rrainfall solutions of different ionic strength on an intact soil core and compared the resulting changes in effluent colloid concentration through multiple sampling ports. The exchange of water between soil matrix and the preferential flow paths leading to 2 each port was characterized on the basis of the bromide (conservative tracer) breakthrough time at the port. At individual ports, two rainfalls of a certain ionic strength mobilized different amounts of colloids when the soil was pre-exposed to a solution of lower or higher ionic strength. This result indicates that colloid mobilization depended on rainfall solution history, which is referred as colloid mobilization hysteresis. The extent of hysteresis was increased with increases in exchange of pore water and solutes between preferential flow paths and matrix. The results indicate that the soil matrix exchanged the old water from the previous infiltration with new infiltrating water during successive infiltration and changed the pore water chemistry in the preferential flow paths, which in turn affected the release of soil colloids. Therefore, rainfall solution history and soil heterogeneity must be considered to assess colloid mobilization in the subsurface. These findings have implications for the release of colloids, colloid-associated contaminants, and pathogens from soils. [ABSTRACT FROM AUTHOR]

Fractal dimension (FD), estimated by box-counting, is a metric used to characterize plant anatomical complexity or space-filling characteristic for a variety of purposes. The vast majority of published studies fail to evaluate the assumption of statistical self-similarity, which underpins the validity of the procedure. The box-counting procedure is also subject to error arising from arbitrary grid placement, known as quantization error (QE), which is strictly positive and varies as a function of scale, making it problematic for the procedure's slope estimation step. Previous studies either ignore QE or employ inefficient brute-force grid translations to reduce it. The goals of this study were to characterize the effect of QE due to translation and rotation on FD estimates, to provide an efficient method of reducing QE, and to evaluate the assumption of statistical self-similarity of coarse root datasets typical of those used in recent trait studies. Coarse root systems of 36 shrubs were digitized in 3D and subjected to box-counts. A pattern search algorithm was used to minimize QE by optimizing grid placement and its efficiency was compared to the brute force method. The degree of statistical self-similarity was evaluated using linear regression residuals and local slope estimates. QE, due to both grid position and orientation, was a significant source of error in FD estimates, but pattern search provided an efficient means of minimizing it. Pattern search had higher initial computational cost but converged on lower error values more efficiently than the commonly employed brute force method. Our representations of coarse root system digitizations did not exhibit details over a sufficient range of scales to be considered statistically self-similar and informatively approximated as fractals, suggesting a lack of sufficient ramification of the coarse root systems for reiteration to be thought of as a dominant force in their development. FD estimates did not characterize the scaling of our digitizations well: the scaling exponent was a function of scale. Our findings serve as a caution against applying FD under the assumption of statistical self-similarity without rigorously evaluating it first. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing of shale for gas production in Pennsylvania generates large quantities of wastewater, the composition of which has been inadequately characterized. We compiled a unique data set from state-required wastewater generator reports filed in 2009-2011. The resulting data set, comprising 160 samples of flowback, produced water, and drilling wastes, analyzed for 84 different chemicals, is the most comprehensive available to date for Marcellus Shale wastewater. We analyzed the data set using the Kaplan-Meier method to deal with the high prevalence of nondetects for some analytes, and compared wastewater characteristics with permitted effluent limits and ambient monitoring limits and capacity. Major-ion concentrations suggested that most wastewater samples originated from dilution of brines, although some of our samples were more concentrated than any Marcellus brines previously reported. One problematic aspect of this wastewater was the very high concentrations of soluble constituents such as chloride, which are poorly removed by wastewater treatment plants; the vast majority of samples exceeded relevant water quality thresholds, generally by 2-3 orders of magnitude. We also examine the capacity of regional regulatory monitoring to assess and control these risks. [ABSTRACT FROM AUTHOR]

The potential of freeze–thaw cycles to release colloids and colloid-associated contaminants into water is unknown. We examined the effect of freeze–thaw cycles on the mobilization of cesium and strontium in association with colloids in intact cores of a fractured soil, where preferential flow paths are prevalent. Two intact cores were contaminated with cesium and strontium. To mobilize colloids and metal cations sequestered in the soil cores, each core was subjected to 10 intermittent wetting events separated by 66 h pauses. During the first five pauses, the cores were dried at room temperature, and during last five pauses, the cores were subjected to 42 h of freezing followed by 24 h of thawing. In comparison to drying, freeze–thaw cycles created additional preferential flow paths through which colloids, cesium, and strontium were mobilized. The wetting events following freeze–thaw intervals mobilized about twice as many colloids as wetting events following drying at room temperature. Successive wetting events following 66 h of drying mobilized similar amounts of colloids; in contrast, successive wetting events after 66 h of freeze–thaw intervals mobilized greater amounts of colloids than the previous one. Drying and freeze–thaw treatments, respectively, increased and decreased the dissolved cesium and strontium, but both treatments increased the colloidal cesium and strontium. Overall, the freeze–thaw cycles increased the mobilization of metal contaminants primarily in association with colloids through preferential flow paths. These findings suggest that the mobilization of colloid and colloid-associated contaminants could increase when temperature variations occur around the freezing point of water. Thus, climate extremes have the potential to mobilize contaminants that have been sequestered in the vadose zone for decades. [ABSTRACT FROM AUTHOR]

Colloids play an important role in governing the transport of radionuclides in geologic environments. As naturally occurring colloidal suspensions are compositionally heterogeneous, the subsurface fate of radionuclides may be sensitive to interactions among different kinds of colloids. Therefore, we investigated the adsorption equilibrium and desorption kinetics of uranium (U(VI)) in experiments conducted with compositionally homogeneous suspensions of colloidal SiO2, ZnO, hydrous ferric oxide (HFO) or humic acids (HAs) as well as heterogeneous suspensions consisting of a colloidal metal oxide and HA. We found that interactions between HAs and ZnO or HFO greatly inhibited the sorption of U onto colloids in the heterogeneous suspensions. HA-ZnO interactions enhanced the desorption of U from the heterogeneous colloidal suspensions, while the association between HA and SiO2 or HFO inhibited U desorption. Molecular-level characterizations reveal that HFO interacted with HAs by electrostatic interactions, association with aliphatic/aromatic carbon and inner-sphere complexation with carboxyl functional groups, while SiO2 and ZnO mainly associated with HAs by weak interactions (e.g., van der Waals interactions). The present findings indicate that interactions between HA and metal-oxide colloids can substantially influence the desorption of U(VI) from these particles, thereby potentially affecting the mobility of this radionuclide in groundwater. [ABSTRACT FROM AUTHOR]

Water‐extractable organic matter (WEOM) in soil affects contaminant mobility and toxicity, heterotrophic production, and nutrient cycling in terrestrial and aquatic ecosystems. This study focuses on the influences of land use history and agricultural management practices on the water extractability of organic matter and nutrients from soils. Water‐extractable organic matter was extracted from soils under different crop rotations (an annual rotation of wheat–pea/bean–wheat–flax or a perennial‐based rotation of wheat–alfalfa–alfalfa–flax) and management systems (organic or conventional) and examined for its concentration, composition, and biodegradability. The results show that crop rotations including perennial legumes increased the concentration of water‐extractable organic carbon (WEOC) and water‐extractable organic nitrogen (WEON) and the biodegradability of WEOC in soil but depleted the quantity of water‐extractable organic phosphorus (WEOP) and water‐extractable reactive phosphorus. The 30‐d incubation experiments showed that bioavailable WEOC varied from 12.5% in annual systems to 22% for perennial systems. The value of bioavailable WEOC was found to positively correlate with WEON concentrations and to negatively correlate with C:N ratio and the specific ultraviolet absorbance of WEOM. No significant treatment effect was present with the conventional and organic management practices, which suggested that WEOM, as the relatively labile pool in soil organic matter, is more responsive to the change in crop rotation than to mineral fertilizer application. Our results indicated that agricultural landscapes with contrasting crop rotations are likely to differentially affect rates of microbial cycling of organic matter leached to soil waters. [ABSTRACT FROM AUTHOR]

The article offers the authors insight regarding the article of T. Myers "Potential Contaminant Pathways from Hydraulically Fractured Shale Aquifers." They say that the modeling framework of Myers misrepresents physical conditions driving groundwater flow and neglects critical hydrologic processes. They add that the improper representation of the hydrogeologic properties of bedrock above the Marcellus made computed flow response more unreasonable.

The risk stemming from human exposure to actinides via the groundwater track has motivated numerous studies on the transport of radionuclides within geologic environments; however, the effects of waterborne organic matter on radionuclide mobility are still poorly understood. In this study, we compared the abilities of three humic acids (HAs) (obtained through sequential extraction of a peat soil) to cotransport hexavalent uranium (U) within water-saturated sand columns. Relative breakthrough concentrations of U measured upon elution of 18 pore volumes increased from undetectable levels (<0.001) in an experiment without HAs to 0.17 to 0.55 in experiments with HAs. The strength of the HA effect on U mobility was positively correlated with the hydrophobicity of organic matter and NMR-detected content of alkyl carbon, which indicates the possible importance of hydrophobic organic matter in facilitating U transport. Carbon and uranium elemental maps collected with a scanning transmission X-ray microscope (STXM) revealed uneven microscale distribution of U. Such molecular- and column-scale data provide evidence for a critical role of hydrophobic organic matter in the association and cotransport of U by HAs. Therefore, evaluations of radionuclide transport within subsurface environments should consider the chemical characteristics of waterborne organic substances, especially hydrophobic organic matter. [ABSTRACT FROM AUTHOR]

Stream water concentrations of dissolved organic matter (DOM) exhibit large temporal variations during precipitation on forested, headwater catchments. We present a modeling framework appropriate for describing streamflow and event-driven export of DOM from small, forested watersheds. Our model links parametrically simple formulations for rainfall-runoff generation and soil water carbon dynamics. The rainfall-runoff formulation is developed by modifying the catchment model of Kirchner (2009) to account for hysteresis in the relationship between stream discharge and catchment water storage. Time series computations of catchment water storage are used by the soil carbon model to approximate the effects of leaching, adsorption, and mineralization on soil water DOM concentrations and the export of DOM from the terrestrial reservoir to the stream. Our findings show that this model is capable of reproducing hourly variations of stream discharge (ranging from 0.01 to 0.38 mm h-1) and stream water DOM concentrations (ranging from 1.8 to 14 mg C L-1) measured in a forested headwater stream in north central Massachusetts. Our analysis highlights the strong linkage between soil carbon dynamics and hydrological processes that govern catchment water storage and flow paths connecting the terrestrial system to the stream. [ABSTRACT FROM AUTHOR]

Fine particles are necessary for watershed biogeochemical cycling and stream metabolic processes but pose a risk to water quality if present in excess or if carrying sorbed contaminants. Despite having low settling velocities, these particles can leave suspension during downstream transport by a series of processes such as hyporheic filtration, flocculation, and biofilm entrapment. The objective of this study is to evaluate the retention of clay particles within a stream reach due to streambed filtration. To this end, we quantify hyporheic exchange through coupled formulations of turbulent surface water flow in the overlying stream and saturated groundwater flow in the streambed. Particle filtration models are informed by flow-through column experiments conducted with repacked streambed material. Results of our model simulations of hyporheic exchange demonstrate that streambed particle filtration depends on overlying streamflow and, to a lesser extent, on hyporheic zone substrate. Retention of suspended particles within the streambed sediments more than doubles between high and drought flows and varies by less than 45% across substrate grain size classes. [ABSTRACT FROM AUTHOR]

Colloid-sized particles are commonly detected in vadose- zone pore waters and are capable of binding chemicals with sorptive affinities for geologic materials. Published research demonstrates that colloids are capable of facilitating the transport of sorptive contaminants under conditions of steady pore water flow, when volumetric moisture content and pore water velocity are constant Less is known about the role of colloids in governing contaminant mobility under transient-flow conditions, which are characteristic of natural vadose-zone environments. The objective of this study is to elucidate the influences of flow transients on the mobilization and transport of in situ colloids and colloid-associated contaminants. We conducted column experiments in which the mobilization of in situ colloids and 137Cs was induced by transients associated with the drainage and imbibition of 137Cs contaminated-sediments. Our results demonstrate that substantial quantities of in situ colloids and colloid-associated 137Cs are mobilized as volumetric moisture content declines during porous-medium drainage and as volumetric moisture content increases during porous- medium imbibition. We also find that the colloid-effect on 137Cs transport is sensitive to changes in pore water ionic strength. That is, the quantities of colloids mobilized and the capacity of the these colloids to bind 137Cs decrease with increasing ionic strength, leading to a decrease of the mass of 137Cs eluted from the columns during porous-medium drainage and imbibition. [ABSTRACT FROM AUTHOR]

Fine particles are necessary for biogeochemical cycling but pose a risk to water quality if they are present in excess or carry sorbed contaminants. The objective of this work is to elucidate the effects of flow on the transport and retention of fine particles within a mountain stream and to compare particulate matter transport processes to those of a conservative solute tracer. We measured the migration of bromide (a conservative solute tracer) and micrometer-sized titanium-dioxide particles under three seasonal flow regimes in second-order stream in Connecticut. A one-dimensional transport model based upon the transient storage model was applied in inverse mode to the measured data to quantify solute and particle transport processes. Rates of particle movement by advection and longitudinal dispersion matched those of bromide. Transient, or temporary, storage influenced conservative solute transport, while particle storage was irreversible on the time scale of our experiments and likely dominated by deposition within the sediments of the hyporheic zone. Both solute transient storage and particle deposition increased with decreasing discharge. [ABSTRACT FROM AUTHOR]

Colloid mobilization and transport in the vadose zone have important implications to the migration of many contaminants that adsorb to mineral surfaces. The objective of this study is to elucidate the influences of transient flow events on the mobilization of in situ colloids from water-unsaturated natural sediments. Results from our laboratory column experiments demonstrate that sand pack drainage and imbibition mobilized substantial quantities of mineral colloids and that effluent colloid concentrations peaked concomitantly with the passage of the wetting and drying fronts. In these experiments, drainage events occurred first and mobilized higher concentrations of in situ colloids than the following imbibition events, and the mass of colloids mobilized was sensitive to pore water ionic strength, as well as to the rates of imbibition and drainage. The data from the column experiments were used to test a new mathematical model that is based on the assumption that colloid mobilization is driven by pore-scale changes in air-water configuration that occur during porous medium drainage and imbibition. The simulated changes in air-water configuration are predicted on the basis of the measured water retention characteristics of the porous medium. Agreement between measured and modeled concentrations of colloids is generally good, and the parameters that govern the kinetics of colloid mobilization and redeposition vary in a discernible fashion with changes in ionic strength, imbibition rates, and drainage rates. [ABSTRACT FROM AUTHOR]

Groundwater suspensions consist of colloidal particles of nonuniform size, yet most published studies of colloid straining in porous media have been conducted with monodisperse colloidal suspensions. In this technical note, we report the results of column experiments on the straining of monodisperse suspensions (3.1 or 5.1 μm microspheres) and a bidisperse suspension (containing 3.1 and 5.1 μm microspheres) within water-saturated quartz sand. The experiments are similar to those described by Xu et al. (2006) in which chemical conditions were prescribed to minimize physicochemical deposition and straining represented the dominant mechanism of colloid immobilization. We find that the straining rates of the monodisperse suspensions can be quantified with published, empirically based expressions that use the ratio of colloid diameter to sand grain diameter as the predictor variable. The straining kinetics of the bidisperse suspension cannot be quantified with these empirical expressions when the mass-averaged microsphere size is used for the colloid diameter. Analysis of the data from the experiments with the bidisperse suspension suggests that retention of the 5.1 μm microspheres constricted pore spaces with dimensions between 3.1 and 5.1 μm, which enhanced the straining of the 3.1 μm microspheres. A model that accounts for this effect and that uses one fitting parameter in coordination with parameters estimated from column experiments with the monodisperse suspensions describes the observations made in the bidisperse experiment with good success. [ABSTRACT FROM AUTHOR]

Straining may influence the mobility of colloid-sized particles within groundwater aquifers as well as within granular filters that are used in wastewater treatment. We conducted column transport experiments using latex microspheres as the colloids and quartz sand as the porous medium to investigate the response of colloid straining to changes in colloid diameter ( d p) and sand grain diameter ( d g). For these experiments the negatively charged microspheres were suspended in deionized water, and the quartz sand was thoroughly cleaned to minimize physicochemical deposition (attachment), which permitted the determination of straining in an unambiguous way. The measurements of strained (immobile phase) and effluent (aqueous phase) colloid concentrations could be described with a transport model that accounted for an exponential decline in straining rates with increasing concentrations of strained colloids. Best fit values of the model coefficient that quantified clean bed straining rates ( k o) were negligibly small for d p/ d g < 0.008 and, above this threshold, varied linearly with d p/ d g. Our findings suggest that accurate inferences on the mobility of colloid-sized particles will require consideration of the effects of straining when d p/ d g exceeds 0.008. [ABSTRACT FROM AUTHOR]