Nonpoint source arsenic contamination of soil and groundwater from legacy pesticides.

Arsenic (As) contamination in wells is common throughout the northeastern United States. It is well documented that lead-arsenate (PbHAsO4 ) pesticides were widely used on fruit tree orchards from the 1890s to 1950s. This study evaluates the potential for As contamination of groundwater from former orchards in Connecticut, where there were over 47,000 orchards in 1935. A proximity analysis involving 189 orchards and 114 domestic wells was conducted to assess the spatial relationship between historic orchards and As in wells. Field studies were then conducted to characterize As and lead (Pb) distributions in soils and wells near historic orchards. The proximity analysis found that the wells with no detected As were further away from historic orchards and had fewer historic orchards within their vicinity when compared with wells that contained As. The field investigations found that elevated levels of As and Pb were widespread in soils from orchards established by 1951, with some As concentrations exceeding 200 ppm. In some soils, As and Pb were leachable at concentrations exceeding USEPA drinking water standards in synthetic precipitation laboratory tests. It was also found that the wells nearest to the impacted soils tended to contain the highest As concentrations, while the wells located in areas that were forested prior to 1970 contained no As. Overall, this study found that As and Pb from legacy pesticide residues are still abundant in former orchard soils and that a strong spatial relationship exists between As-contaminated wells and historic orchards. Greater consideration should be given to historic orchard soils as a potential contributing nonpoint source of As to the groundwater in Connecticut, where domestic well contamination rates are high.

Use of bacteria community analysis to distinguish groundwater recharge sources to shallow wells.

In this study, bacteria community analysis was performed to supplement a preexisting evaluation of nitrate contamination in drinking water wells at a coastal site in Old Lyme, CT. Given well usage and coastal hydrogeologic conditions, the source(s) of nitrate contamination in domestic wells could not be discerned between local septic systems or a nearby farm where organic fertilizers were used. Groundwater bacteria communities are known to be sensitive to a variety of environmental conditions. As such, they are potentially useful in distinguishing groundwater recharge sources. Groundwater samples collected from wells were analyzed using polymerase chain reactions (PCR) and 16S rRNA sequencing to determine the bacteria distributions in each well. The biostatistical analysis of the data using Bray-Curtis nonmetric multidimensional scaling and permutational multivariate analysis of variance revealed three distinct bacteria community distributions that coincided with three different areas on the site. Additionally, principal component analysis (PCA) of the water quality data revealed that wells with similar bacteria shared similar water quality, all of which was indicative of local recharge. These findings suggested that the domestic well nitrate contamination was derived from local septic systems rather than the farm. Septic indicator analysis using ultra-performance liquid chromatography-tandem mass spectrometry determined the presence of caffeine in domestic wells, which was consistent with the conclusions from the bacteria analysis, PCA, and the known hydrogeologic conditions. The low cost, ease of sample collection, and growing availability of bioinformatics laboratory services and software are conducive to the application of microbial community analysis as a supplemental tool for groundwater investigations.

Analytical Solutions of Three-Dimensional Contaminant Transport Models with Exponential Source Decay.

Despite the availability of numerical models, interest in analytical solutions of multidimensional advection-dispersion systems remains high. Such models are commonly used for performing Tier I risk analysis and are embedded in many regulatory frameworks dealing with groundwater contamination. In this work, we develop a closed-form solution of the three-dimensional advection-dispersion equation with exponential source decay, first-order reaction, and retardation, and present an approach based on some ease of use diagrams to compare it with the integral open form solution and with earlier versions of the closed-form solution. The comparison approach focuses on the relative differences associated with source decay and the effect of simulation time. The analysis of concentration contours, longitudinal sections, and transverse sections confirms that the closed-form solutions studied can be used with acceptable approximation in the central area of a plume bound transversely within the source width, both behind and beyond the advective front and for concentration values up to two orders of magnitude less than the initial source concentration. As the proposed closed-form model can be evaluated without nested numerical computations and with simple mathematical functions, it can be very useful in risk assessment procedures.

Impacts of Road Salting on Water Quality in Fractured Crystalline Bedrock.

Many rural communities depend on bedrock wells as a primary water source, which raises the issue as to whether increasing amounts of salt application are affecting bedrock water quality and to what degree. Through wellbore profiling, this study investigated changes in specific conductance in two crystalline bedrock wells at the University of Connecticut in Storrs, CT, from 2003 to 2016, with particular emphasis on the impacts of increased salt application with a change in deicing practices at the university after 2009. Hourly specific conductance measurements were collected in 2014 to determine how water source may affect wellbore concentrations seasonally. Chloride was found to be highly persistent in the bedrock, with concentrations consistently increasing from 2003 to 2016 despite year-to-year variations in salt application. A dramatic increase in chloride occurred in the wellbores in response to the change in deicing practices, with an immediate response in fractures having a direct connection to the overburden. In light of the long-term implications of road salting on subsurface chloride contamination, this study argues that consideration be given to deicing management strategies to reduce road salt contamination.

Measuring Flow Rate in Crystalline Bedrock Wells Using the Dissolved Oxygen Alteration Method.

Determination of vertical flow rates in a fractured bedrock well can aid in planning and implementing hydraulic tests, water quality sampling, and improving interpretations of water quality data. Although flowmeters are highly accurate in flow rate measurement, the high cost and logistics may be limiting. In this study the dissolved oxygen alteration method (DOAM) is expanded upon as a low-cost alternative to determine vertical flow rates in crystalline bedrock wells. The method entails altering the dissolved oxygen content in the wellbore through bubbler aeration, and monitoring the vertical advective movement of the dissolved oxygen over time. Measurements were taken for upward and downward flows, and under ambient and pumping conditions. Vertical flow rates from 0.06 to 2.30 Lpm were measured. To validate the method, flow rates determined with the DOAM were compared to pump discharge rates and found to be in agreement within 2.5%.

Permeable Asphalt: A New Tool to Reduce Road Salt Contamination of Groundwater in Urban Areas.

Chloride contamination of groundwater in urban areas due to deicing is a well-documented phenomenon in northern climates. The objective of this study was to evaluate the effects of permeable pavement on degraded urban groundwater. Although low impact development practices have been shown to improve stormwater quality, no infiltration practice has been found to prevent road salt chlorides from entering groundwater. The few studies that have investigated chlorides in permeable asphalt have involved sampling directly beneath the asphalt; no research has looked more broadly at surrounding groundwater conditions. Monitoring wells were installed upgradient and downgradient of an 860 m2 permeable asphalt parking lot at the University of Connecticut (Storrs, Connecticut). Water level and specific conductance were measured continuously, and biweekly samples were analyzed for chloride. Samples were also analyzed for sodium (Na), calcium (Ca), and magnesium (Mg). Analysis of variance analysis indicated a significantly (p < 0.001) lower geometric mean Cl concentration downgradient (303.7 mg/L) as compared to upgradient (1280 mg/L). Concentrations of all alkali metals increased upgradient and downgradient during the winter months as compared to nonwinter months, indicating that cation exchange likely occurred. Despite the frequent high peaks of chloride in the winter months as well as the increases in alkali metals observed, monitoring revealed lower Cl concentrations downgradient than upgradient for the majority of the year. These results suggest that the use of permeable asphalt in impacted urban environments with high ambient chloride concentrations can be beneficial to shallow groundwater quality, although these results may not be generalizable to areas with low ambient chloride concentrations.

Application of first order kinetics to characterize MTBE natural attenuation in groundwater.

Methyl tertiary butyl ether (MTBE) was a gasoline oxygenate that became widely used in reformulated gasoline as a means to reduce air pollution in the 1990s. Unfortunately, many of the underground storage tanks containing reformulated gasoline experienced subsurface releases which soon became a health concern given the increase in public and private water supplies containing MTBE. Many states responded to this by banning the use of MTBE as an additive, including Connecticut. Although MTBE dissipates by natural attenuation, it continues to be prevalent in groundwater long after the Connecticut ban in 2004. This study estimated the rate of the natural attenuation in groundwater following the Connecticut ban by evaluating the MTBE concentration two years prior to and two years after the MTBE ban at eighty-three monitoring wells from twenty-two retail gasoline stations where MTBE contamination was observed. Sites chosen for this study had not undergone active remediation ensuring no artificial influence to the natural attenuation processes that controls the migration and dissipation of MTBE. Results indicate that MTBE has dissipated in the natural environment, at more than 80% of the sites and at approximately 82% of the individual monitoring wells. In general, dissipation approximated first order kinetics. Dissipation half-lives, calculated using concentration data from the two year period after the ban, ranged from approximately three weeks to just over seven years with an average half-life of 7.3 months with little variability in estimates for different site characteristics. The accuracy of first order estimates to predict further MTBE dissipation were tested by comparing predicted concentrations with those observed after the two year post-ban period; the predicted concentrations closely match the observed concentrations which supports the use of first order kinetics for predictions of this nature.

An unsteady state tracer method for characterizing fractures in bedrock wells.

Evaluating contaminants impacting wells in fractured crystalline rock requires knowledge of the individual fractures contributing water. This typically involves using a sequence of tools including downhole geophysics, flow meters, and straddle packers. In conjunction with each other these methods are expensive, time consuming, and can be logistically difficult to implement. This study demonstrates an unsteady state tracer method as a cost-effective alternative for gathering fracture information in wells. The method entails introducing tracer dye throughout the well, inducing fracture flow into the well by conducting a slug test and then profiling the tracer concentration in the well to locate water contributing fractures where the dye has been diluted. By monitoring the development of the dilution zones within the wellbore with time, the transmissivity and the hydraulic head of the water contributing fractures can be determined. Ambient flow conditions and the contaminant concentration within the fractures can also be determined from the tracer dilution. This method was tested on a large physical model well and a bedrock well. The model well was used to test the theory underlying the method and to refine method logistics. The approach located the fracture and generated transmissivity values that were in excellent agreement with those calculated by slug testing. For the bedrock well tested, two major active fractures were located. Fracture location and ambient well conditions matched results from conventional methods. Estimates of transmissivity values by the tracer method were within an order of magnitude of those calculated using heat-pulse flow meter data.

Using domestic well records to determine fractured bedrock watersheds and recharge rates.

This study presents an approach for delineating groundwater basins and estimating rates of recharge to fractured crystalline bedrock. It entailed the use of completion report data (boring logs) from 2500 domestic wells in bedrock from the Coventry Quadrangle, which is located in northeastern Connecticut and characterized by metamorphic gneiss and schist. Completion report data were digitized and imported into ArcGIS(®) for data analysis. The data were processed to delineate groundwater drainage basins for the fractured rock based on flow conditions and to estimate groundwater recharge to the bedrock. Results indicate that drainage basins derived from surface topography, in general, may not correspond with bedrock drainage basins due to scale. Estimates of recharge to the bedrock for the study area indicate that only a small fraction of the precipitation or the amount of water that enters the overburden recharges the rock. The approach presented here can be a useful method for water resource-related assessments that involve fractured rock aquifers.

Effects of Road Salt on Connecticut's Groundwater: A Statewide Centennial Perspective.

This study examined the extent to which development and road salting has affected Connecticut's groundwater. We gathered water quality data from different time periods between 1894 and the present and analyzed the data using maps generated with ESRI ArcGIS. Historical reports illustrate a statewide baseline trend of decreasing chloride concentration northward across the State (average, 2 ppm). Since then, statewide chloride concentrations in ground water have increased by more than an order of magnitude on average. Analysis indicates spatial correlation between chloride impacts and major roadways. Furthermore, increases in statewide chloride concentration parallel increases in road salt application. Projected trends suggest that statewide baseline concentrations will increase by an amount equal to five times background levels between the present and the year 2030. The analytical process outlined herein can be readily applied to any region to investigate salt impacts on large spatial and temporal scales.

Low-flow hydraulic conductivity tests at wells that cross the water table.

Wells with screens and sand packs that cross the water table represent a challenging problem for determining hydraulic conductivity by slug testing due to sand pack drainage and resaturation. Sand pack drainage results in a multisegmented recovery curve. One must then subjectively pick a portion of the curve to analyze. Sand pack drainage also results in a change in the effective radius of the well which requires a guess at the porosity or specific yield in analyzing the test. In the study of Robbins et al. (2009), a method was introduced to obtain hydraulic conductivity in monitoring wells using the steady-state drawdown and flow rate obtained during low-flow sampling. The method was tested in this study in wells whose screens cross the water table and shown to avoid sand pack drainage problems that complicate analyzing slug tests. In applying the method to low-flow sampling, only a single pair of steady-state flow rate and drawdown are needed; hence, to derive meaningful results, an accurate determination of these parameters is required.

Using atmospheric tracers to reduce uncertainty in groundwater recharge areas.

A Monte Carlo-based approach to assess uncertainty in recharge areas shows that incorporation of atmospheric tracer observations (in this case, tritium concentration) and prior information on model parameters leads to more precise predictions of recharge areas. Variance-covariance matrices, from model calibration and calculation of sensitivities, were used to generate parameter sets that account for parameter correlation and uncertainty. Constraining parameter sets to those that met acceptance criteria, which included a standard error criterion, did not appear to bias model results. Although the addition of atmospheric tracer observations and prior information produced similar changes in the extent of predicted recharge areas, prior information had the effect of increasing probabilities within the recharge area to a greater extent than atmospheric tracer observations. Uncertainty in the recharge area propagates into predictions that directly affect water quality, such as land cover in the recharge area associated with a well and the residence time associated with the well. Assessments of well vulnerability that depend on these factors should include an assessment of model parameter uncertainty. A formal simulation of parameter uncertainty can be used to delineate probabilistic recharge areas, and the results can be expressed in ways that can be useful to water-resource managers. Although no one model is the correct model, the results of multiple models can be evaluated in terms of the decision being made and the probability of a given outcome from each model.

Determining hydraulic conductivity using pumping data from low-flow sampling.

Hydraulic conductivity values computed using the steady-state discharge and drawdown attained while low-flow sampling were evaluated to determine if they were equivalent to those determined from slug testing. Based on testing 12 wells, it was found that the results were statistically equivalent. Conductivity values computed using low-flow sampling parameters were also evaluated as to their reproducibility in actual practice by analyzing consultant data for three wells sampled over three quarterly monitoring periods by four field technicians. The results were found to be reproducible within about a factor of 2 or better. Since the method is based on only one pair of parameters, diligence is required in attaining steady state and in accurately measuring the flow rate and drawdown. Conductivity values computed using this approach can enhance the use of low-flow data gathered in water quality sampling, avoid the need for slug testing in a subsequent phase of investigation, and help reduce the cost of characterizing sites when multilevel samplers are used. Given the practical range of discharge in low-flow sampling, the method was found to be applicable at conductivity values somewhat greater than 10(-6) cm/s. Given the typical accuracy of water level meters and pressure transducers and a maximum discharge of 1 L/min, as mandated by regulatory guidance, the method has a calculated upper conductivity limit in the range of 10(-3) to 10(-2) cm/s.

A tracer dilution method for fracture characterization in bedrock wells.

This investigation was undertaken to develop an integrated method of downhole fracture characterization using a tracer. The method presented can be used to locate water-bearing fractures that intersect the well, to determine the ambient fracture flow rate and hydraulic head, and to calculate fracture transmissivity. The method was tested in two fractured crystalline bedrock wells located at the University of Connecticut in Storrs. The method entails injecting a tracer (uranine dye) into the well, while at the same time water is pumped out of the well. After steady-state conditions are reached, a borehole tracer concentration profile is developed. The dilution of the tracer is used to locate the inflowing fractures and to determine their flow rate. The fracture flow rate, plus the drawdown in the well, is then used to determine the fracture hydraulic head, transmissivity, and ambient flow rate.