Remediating Contaminant Plumes in Groundwater with Shallow Excavations Containing Coarse Reactive Media.

A groundwater flow and mass transport model tested the capability of shallow excavations filled with coarse, reactive media to remediate a hypothetical unconfined aquifer with a maximum saturated thickness of 5 m. Modeled as contaminant sinks, the rectangular excavations were 10 m downgradient of an initial contaminant plume originating from a source at the top of the aquifer. The initial plume was approximately 259 m long, 23 m wide, and 5 m thick, with a downgradient tip located approximately 100 m upgradient of the site boundary. The smallest trench capable of preventing offsite migration was 11 m long (measured perpendicular to groundwater flow), 4 m wide (measured parallel to groundwater flow), and 3 m deep. Results of this study suggest that shallow trenches filled with coarse filter media that partially penetrate unconfined aquifers may be a viable alternative for remediating contaminated groundwater at some sites.

Nitrate Concentration Patterns Over Space and Time in a Regionally Sloping Sedimentary Aquifer, Texas, USA.

Nitrate concentrations in a regionally sloping sedimentary aquifer were compiled every 4 years from 1999 to 2015. Sampled wells ranged from 14.6 m deep in the outcrop zone to 1031.4 m deep in the confined zone, with a median depth of 192.1 m. Approximately 3.6% of 138 samples collected in 2015 exceeded the drinking water standard of 44.3 mg/L; while low, this percentage was highest among five sampling years. All observed exceedances were in relatively shallow wells in the outcrop zone. Generally, the spatial pattern of nitrate occurrence persisted through time. Shallow wells tended to have higher nitrate concentrations, in addition to larger increases or decreases in concentration over time. Maximum concentrations ranged from 30.5 mg/L in 1999 to 100.5 mg/L in 2015. Over any 4-year period, the maximum concentration increase was 42.1 mg/L, and the maximum decrease was 41.0 mg/L, both occurring in wells in the outcrop zone. This study has land management implications for sloping sedimentary aquifers. Practices that alter nitrogen inputs and dilution processes in unconfined zones may produce large fluctuations in nitrate concentration over short time periods.

A reconnaissance analysis of groundwater quality in the Eagle Ford shale region reveals two distinct bromide/chloride populations.

The extraction of oil and natural gas from unconventional shale formations has prompted a series of investigations to examine the quality of the groundwater in the overlying aquifers. Here we present a reconnaissance analysis of groundwater quality in the Eagle Ford region of southern Texas. These data reveal two distinct sample populations that are differentiable by bromide/chloride ratios. Elevated levels of fluoride, nitrate, sulfate, various metal ions, and the detection of exotic volatile organic compounds highlight a high bromide group of samples, which is geographically clustered, while encompassing multiple hydrogeological strata. Samples with bromide/chloride ratios representative of connate water displayed elevated levels of total organic carbon, while revealing the detection of alcohols and chlorinated compounds. These findings suggest that groundwater quality in the Western Gulf Basin is, for the most part, controlled by a series of natural processes; however, there is also evidence of episodic contamination events potentially attributed to unconventional oil and gas development or other anthropogenic activities. Collectively, this characterization of natural groundwater constituents and exogenous compounds will guide targeted remediation efforts and provides insight for agricultural entities, industrial operators, and rural communities that rely on groundwater in southern Texas.

Exceedance Frequency Analysis of Contaminants in Streams Under Dry-Weather Conditions in Denton, Texas.

Percentages of dry-weather stream samples exceeding water quality criteria for ten parameters were compiled for mixed land use watersheds in north-central Texas. Most problematic were total suspended solids (TSS), total dissolved solids (TDS), ammonia, nitrate, phosphorus and copper. Nutrients had much higher exceedance frequency at a sampling station impacted by wastewater discharge. Whereas, TSS and TDS exceedance frequency was highest in predominantly agricultural and rangeland watersheds, and urbanized watersheds respectively. Total dissolved solids was most often exceeded in urbanized watersheds. For several parameters, especially TDS, TSS, ammonia and copper, median concentrations were below water quality thresholds in most watersheds, but exceedance frequency was high. For example, median TSS was less than its threshold in every watershed, but exceedance frequency was higher than 10 % in four of five watersheds - and nearly 43 % in one watershed. This pattern reflects the skewed nature of water quality data; often times, many observations cluster around the lowest values, causing the median to be relatively low, but several (high) outliers form the right-hand tail of the distribution. Results of this study indicate a need to examine exceedance frequency in addition to traditional descriptive measures to better understand dry-weather stream quality in watersheds.

A Comprehensive Analysis of Groundwater Quality in The Barnett Shale Region.

The exploration of unconventional shale energy reserves and the extensive use of hydraulic fracturing during well stimulation have raised concerns about the potential effects of unconventional oil and gas extraction (UOG) on the environment. Most accounts of groundwater contamination have focused primarily on the compositional analysis of dissolved gases to address whether UOG activities have had deleterious effects on overlying aquifers. Here, we present an analysis of 550 groundwater samples collected from private and public supply water wells drawing from aquifers overlying the Barnett shale formation of Texas. We detected multiple volatile organic carbon compounds throughout the region, including various alcohols, the BTEX family of compounds, and several chlorinated compounds. These data do not necessarily identify UOG activities as the source of contamination; however, they do provide a strong impetus for further monitoring and analysis of groundwater quality in this region as many of the compounds we detected are known to be associated with UOG techniques.

Comparison of permeable reactive barrier, funnel and gate, nonpumped wells, and low-capacity wells for groundwater remediation.

This modeling study compared the performance of a no-action and four active groundwater remediation alternatives: a permeable reactive barrier, a funnel and gate, nonpumped wells with filter media, and a low-capacity extraction and injection well. The simulated aquifer had an average seepage velocity of 0.04 m d(-1), and the initial contaminant plume was 58 m long and 13 m wide. For each active alternative, mass transport modeling identified the smallest structure necessary to contain and remove the contaminant plume. Although the no-action alternative did not contain the plume, each active alternative did contain and remove the plume, but with significantly different installation and operation requirements. Low-capacity pumping wells required the least infrastructure, with one extraction well and one injection well each discharging only 1.7 m(3) d(-1). The amount of time necessary to remove the contaminant plume was similar among active alternatives, except for the funnel and gate, which required much more time. Results of this study suggest that, for a modest seepage velocity and relatively narrow contaminant plume, low-capacity wells may be an effective alternative for groundwater remediation.

Shallow, non-pumped wells: a low-energy alternative for cleaning polluted groundwater.

This modeling study evaluated the capability of non-pumped wells with filter media for preventing contaminant plumes from migrating offsite. Linear configurations of non-pumped wells were compared to permeable reactive barriers in simulated shallow homogeneous and heterogeneous aquifers. While permeable reactive barriers enabled faster contaminant removal and shorter distances of contaminant travel, non-pumped wells also prevented offsite contaminant migration. Overall, results of this study suggest that discontinuous, linear configurations of non-pumped wells may be a viable alternative to much more costly permeable reactive barriers for preventing offsite contaminant travel in some shallow aquifers.

Relative efficiency of multi-transect, non-pumped, reactive well networks for removing contaminated groundwater.

Alternative networks of non-pumped wells filled with reactive media were evaluated for groundwater remediation capability. Wells were screened across the saturated zone of a simulated, unconfined aquifer with a heterogeneous hydraulic conductivity. A numerical mass transport model generated an initial contaminant plume and then simulated its movement through different networks of non-pumped wells. Two cases were investigated: (1) a linear transect of wells downgradient of the plume; and (2) a downgradient linear transect combined with a second linear transect crossing the plume's interior. Several simulations were conducted for each case to determine the smallest number of wells necessary to prevent the plume from traveling offsite. Case 2 required 11% more wells, but reduced cleanup time by 21%.

Effect of aquifer heterogeneity on non-pumped, reactive well networks for removing pollutants in groundwater.

This purpose of this study was to evaluate the impact of heterogeneity in aquifer hydraulic conductivity on configurations of non-pumped wells filled with reactive media for removing contaminant plumes in groundwater. Among one homogeneous and three heterogeneous simulated aquifers, 2-16 wells were necessary to contain a plume, with no clear relationship between degree of heterogeneity and number of wells. Generally, heterogeneous aquifers with initial plumes having broad rather than narrow downgradient margins required more wells and showed greater tendency for plumes to move around wells. Cleanup time increased up to 181 % with degree of heterogeneity in hydraulic conductivity.

Assessment of multi-gate interceptors equipped with baffles in contaminated aquifers.

Funnel-and-gate structures with three gates, two funnels (collinear with gates), and two perpendicular flow-directing vanes (baffles) were assessed for capturing contaminated groundwater in a hypothetical unconfined aquifer. Simulated structures, anchored into an underlying aquiclude, were 35 m wide. One 5-m wide gate occupied the center, and two 3-m wide gates occupied the ends, of each structure. Both homogeneous and heterogeneous (with respect to hydraulic conductivity) aquifers were modeled, with baffles at various positions along funnels in alternative configurations. A contaminant transport model, accounting for advection and hydrodynamic dispersion, tested the capability of various structures for capturing contaminant plumes. Based upon modeling results: (1) structures with baffles performed up to 17% better (homogeneous case), but also up to 48% worse (heterogeneous case), than structures without them; (2) the most effective baffles generally occupied interior portions of funnels; and (3) small (1-m) shifts in the locations of baffles resulted in up to a 33% increase (homogeneous case) in remediation timeframe.

Funnel-and-gate remediation systems augmented with passive filter wells.

The objective of this study was to evaluate the ability of funnel-and-gate structures augmented with passive wells containing filter cartridges to capture contaminated groundwater in hypothetical, homogeneous and heterogeneous, unconfined aquifers. Perpendicular to groundwater flow, linear structures were 15 m wide, 1 m thick, and keyed into the base of the aquifer. Gates occupied 4 m of the total width of each simulated structure; one gate was 5 m from a contaminant plume's leading tip, while others occupied cross-gradient margins of the plume. Results suggest a modest reduction in remediation timeframes, up to 425 d per well added in these simulations; however, incremental benefits are highly variable and case specific.

Viability of longitudinal trenches for capturing contaminated groundwater.

Using a groundwater flow and mass transport model, this study compared the capability of trenches with permeable backfill for capturing hypothetical contaminant plumes in homogeneous and heterogeneous unconfined aquifers. Longitudinal (parallel to groundwater flow), as well as conventional transverse (perpendicular to groundwater flow) trench configurations were considered. Alternate trench configurations intercepted the leading tip of an initial contaminant plume and had identical length, equal to the cross-gradient width of the plume. A longitudinal trench required 31% less time than its transverse counterpart to remediate a homogeneous aquifer. By contrast, in simulated heterogeneous aquifers, longitudinal remediation timeframes ranged from 41% less to 33% more than transverse trenches. Results suggest that longitudinal trenches may be a viable alternative for narrow contaminant plumes under low-groundwater velocity conditions, but may be impractical for plumes with wide leading tips, or in complex heterogeneous aquifers with divergent flow.

Elevated fluoride and selenium in west Texas groundwater.

Fluoride and selenium concentrations, along with total dissolved solids and depth of intake, were compiled, mapped, and analyzed for 634 water wells in the High Plains Aquifer, northwest Texas. Approximately 19% of fluoride observations exceeded the maximum contaminant level (MCL) for drinking water. Additionally, 4% of selenium observations exceeded the MCL for drinking water, and 19% exceeded the recommended limit for irrigation water. Concentrations were considerably higher in the southern part of the study area, especially in relatively deep public supply and irrigation wells. Though human activity may influence fluoride and selenium levels, natural sources largely account for patterns observed in this study.

Evaluation of reactive well networks for remediating heterogeneous aquifers.

Alternative configurations of non-pumped wells filled with reactive media were evaluated for removing hypothetical contaminant plumes. All wells were screened across the saturated zone of a simulated unconfined aquifer. Three heterogeneous hydraulic conductivity distributions (cases) were considered. A mass transport model accounting for advection and hydrodynamic dispersion produced an initial contaminant plume for each case. Two reactive well configurations were evaluated for each case. In one configuration, evenly spaced wells occupied a linear transect perpendicular to regional groundwater flow, located downgradient of the contaminant plume. A second configuration involved the same number of wells, but along evenly spaced, nonlinear flow lines originating from the downgradient boundary of the contaminant plume. Mass transport modeling simulated contaminant plumes moving through the aquifer and wells. Results suggest that nonlinear configurations, which take into account local flow variations near the downgradient boundary of a contaminant plume, more efficiently reduce contaminant concentrations and better control offsite migration.

Distribution of arsenic concentrations in groundwater of the Seymour Aquifer, Texas, USA.

The purpose of this study was to investigate arsenic concentrations in the Seymour Aquifer. Discontinuous alluvium of the aquifer occupies a broad, semi-arid region of northern Texas, USA. Throughout the formation's outcrop, permeable deposits and unconfined conditions may facilitate downward travel of contaminants applied to the land surface. Past agricultural practices are a potential source of arsenic to the aquifer. However, of 64 water samples analyzed from 2001-2004, only one exceeded the maximum contaminant level (MCL) of 10 microg/l for arsenic in drinking water. The median arsenic concentration was 2.7 microg/l, and 34% of samples had arsenic concentrations less than 2 microg/l. No relationship between arsenic concentration and well depth was observed.

Mass transport in groundwater near hanging-wall interceptors.

This study investigated mass transport near trenches designed to capture contaminated groundwater. Numerical models simulated migration of contaminant plumes toward trenches oriented perpendicular to regional groundwater flow, partially penetrating a hypothetical unconfined aquifer. Plumes originated at the top of the simulated groundwater flow system. The smallest trench necessary to capture a contaminant plume was identified for various sets of mass transport parameters. Results suggest that, in predominantly horizontal flow systems such as those simulated here: (i) vertical hydraulic conductivity has relatively little effect on downward propagation of contaminant plumes and required trench size, (ii) transverse vertical dispersivity exerts significant control on plume and trench depth, and (iii) recharge dilutes and thus reduces plume and trench width, but may induce downward vertical hydraulic gradients that deepen plumes and trenches. For all cases considered, trenches oriented perpendicular to regional groundwater flow, placed close to the leading edge of a contaminant plume, and sized slightly larger than the plume (in transverse cross-section) facilitate plume capture.

Evaluation of ecological and in vitro effects of boron on prostate cancer risk (United States).

OBJECTIVE: To determine: (1) the correlation of prostate cancer incidence and mortality with groundwater boron and selenium concentrations; and (2) the impact of boron on prostate cancer cell proliferation during co-treatment with alternative chemo-preventative agents, along with boron pre-treatment effects on cell sensitivity to ionizing radiation. METHODS: For regression analysis, data on prostate cancer incidence and mortality were obtained from the Texas Cancer Registry, while groundwater boron and selenium concentrations were derived from the Texas Water Development Board. Cultured DU-145 prostate cancer cells were used to assess the impact of boric acid on cell proliferation when applied in combination with selenomethionine and genistein, or preceding radiation exposure. RESULTS: Groundwater boron levels correlated with a decrease in prostate cancer incidence (R = 0.6) and mortality (R = 0.6) in state planning regions, whereas selenium did not (R = 0.1; R = 0.2). Growth inhibition was greater during combined treatments of boric acid and selenomethionine, or boric acid and genistein, versus singular treatments. 8-day boric acid pre-exposure enhanced the toxicity of ionizing radiation treatment, while dose-dependently decreasing the expression of anti-apoptotic protein Bcl-2. CONCLUSIONS: Increased groundwater boron concentrations, across the state of Texas, correlate with reduced risk of prostate cancer incidence and mortality. Also, boric acid improves the anti-proliferative effectiveness of chemo-preventative agents, selenomethionine and genistein, while enhancing ionizing radiation cell kill.

Heuristic for constructing minimum-well groundwater monitoring configurations at waste storage facilities.

A graphical heuristic was designed and evaluated for locating detection wells in aquifers beneath landfills. The heuristic identifies locations of a minimum number of wells on a compliance boundary to detect potential contaminant releases from a landfill. It maximizes gaps between wells while ensuring no potential releases pass between them. The solution progresses from one end of a compliance boundary to another. Heuristic solutions were compared with output from a second method, which equalizes cross-gradient distances between wells, given a specified number of wells in a monitoring network. The second approach is superior when increasing the number of wells for backup coverage or reducing them to save costs. Both approaches maintain high detection efficiency with increased seepage velocity or decreased source width relative to design values. To a lesser extent, they accommodate variations in flow direction relative to the design direction; in test cases, 20-degree variations reduced detection efficiency from 0 to 14%. Conjunctively, the approaches can identify effective candidate monitoring configurations at landfills.

Effect of setback distance on ability of gravel trenches to intercept contaminated groundwater.

Computer simulations tested the ability of gravel interceptor trenches to capture a plume of contaminated groundwater. The plume had a maximum length and width of 87 and 19 m, respectively. In alternate simulations, one-meter wide trenches were located 5, 10, 20, and 50 m downgradient of the plume. A minimum trench length and time required to capture the plume was determined for each location (setback). The plume was considered captured if it passed entirely through a trench. A 21 m-long trench captured the plume at setbacks of 5 and 10 m. Minimum trench length increased to 23 and 25 m at setbacks of 20 and 50 m, respectively. Increased contaminant spreading with distance traveled dictated longer trenches at larger setbacks. Results of this study suggest that, at settings where contaminant plumes are carefully monitored and spatially defined, passive interceptor trenches should be close to a plume's leading tip and slightly longer than the maximum width of the plume.