Data for Arsenic Variability and Groundwater Age in Three Water-Supply Wells in Southeast New Hampshire

Three wells in New Hampshire were sampled bimonthly over three years to evaluate the temporal variability of arsenic concentrations and groundwater age. All samples had measurable concentrations of arsenic throughout the entire sampling period and concentrations in individual wells varied, on average, by more than 7 µg/L. High arsenic concentrations (>10 µg/L) were measured in wells KFW-87 and SGW-93, consistent with the high pH and low dissolved oxygen typically found in bedrock wells. Lower arsenic concentrations (<10 µg/L) at well SGW-65 were consistent with lower pH typical of the glacial aquifer. The well producing the oldest water, public bedrock well SGW-93, was not the well with the highest arsenic concentrations; however the groundwater age generally increased at this well over time with arsenic concentrations. Arsenic concentrations at the private bedrock well, KFW-87, which had the highest concentrations among the three wells covaried with groundwater depth (rho=-0.53, p=0.029), suggesting flushing during recharge events. Arsenic concentrations in the public supply wells, SGW-93 and SGW-65, correlated significantly with one another by sample date (rho=0.77, p<0.001). Similarly, the old fraction of water in the public glacial well and the young fraction of water in the public bedrock well correlated significantly by sampling date, suggesting that some of the water captured by the glacial aquifer well may originate in the bedrock aquifer, as no other drivers of arsenic variability were observed in the glacial well. A direct relation between groundwater age and arsenic could not be determined with the available data and age model results for this time period at any of the wells, however, pumping rate and depth to water appeared to be indicative of arsenic concentration changes over time. This data release documents four Microsoft Excel tables that contain data for understanding arsenic variability related to three water-supply wells in southeast New Hampshire. Table_1_GF_AgeInterpretations.xlsx contains dissolved gas modeling results, environmental tracer concentrations (tritium, tritiogenic helium-3, sulfur hexafluoride, carbon-14, and chlorofluorocarbons (CFCs)), and results for the mean age of groundwater by calibration of lumped parameter models to tracer concentrations (Jurgens and others, 2012). Dissolved gas modeling and environmental tracer results were averaged when multiple dissolved gas models and tracer concentrations were computed in tables 2 and 3. In cases where age was modeled with a binary lumped parameter model (BMM), the mean age was computed from the mean age and fraction of the two components in the mixture. Please see the processing steps below and the main manuscript for additional details on the results presented in this table. Table_2_GF_DissolvedGasModeling.xlsx contains detailed information on the calibration of dissolved gas models to dissolved gas concentrations (neon, argon, krypton, xenon, nitrogen, oxygen, carbon dioxide, methane, hydrogen, and nitrous oxide). Calibration was done using methods described by Aeschbach-Hertig and others (1999) with modifications to include nitrogen gas (Weiss 1970). In most cases, a single set of noble gas data (neon, argon, krypton, and xenon) were used to determine recharge conditions (recharge temperature, excess air or entrapped air, and fractionation). In cases where noble gas data were not available, multiple analyses of nitrogen and argon (collected sequentially on the same sample date) were used to determine recharge conditions. Table_3_GF_ComputedTracerConcentrations.xlsx contains detailed information on calculations of environmental tracer data. Dissolved gas models were paired with sulfur hexafluoride and helium isotopes (3He/4He) and helium to determine concentrations of tritiogenic helium-3 (from decay of tritium; Solomon and Cook, 2000). Multiple tracer concentrations were computed when sites had multiple dissolved gas model results and analyses for sulfur hexafluoride or helium isotopes. Table_4_GF_ConcentrationsAndValues.xlsx contains values of selected physiochemical parameters collected during well purging and selected chemical concentrations from filtered samples collected on various dates at each well. The table also contains physical characteristics, depth to water, and pumping rate, of each well that were calculated from continuous data. Depth to water was calculated as the minimum monthly values at KFW-87 and pumping rate was calculated as the arithmetic mean between each sampling date at SGW-65 and SGW-93.

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