@article {2988,
	title = {The relation of geogenic contaminants to groundwater age, aquifer hydrologic position, water type, and redox conditions in Atlantic and Gulf Coastal Plain aquifers, eastern and south-central USA},
	journal = {Science of The Total Environment},
	volume = {723},
	year = {2020},
	month = {06/2020},
	pages = {137835},
	abstract = {Groundwater age distributions developed from carbon-14 (14C), tritium (3H), and helium-4 (4He) concentrations, along with aquifer hydrologic position, water type, and redox conditions, were compared to geogenic contaminants of concern (GCOC) from 252 public-supply wells in six Atlantic and Gulf Coastal Plain unconsolidated-sediment aquifers. Concentrations of one or more GCOCs in 168 (67\%) wells exceeded MCLs (maximum contaminant levels), SMCLs (secondary MCLs), or HBSLs (health-based screening levels). Human-health benchmark thresholds (MCLs or HBSLs) were exceeded in 31 (12\%) wells, and included 0.8\% for fluoride (F), 2.4\% for arsenic (As), 4\% for lead-210 (210Pb), and 6.7\% for polonium-210 (210Po). Values of pH increase with age and were outside the SMCL in 31\% of wells (23\%~<~6.5 and 7.5\%~>~8.5, SMCL). Among GCOCs with concentrations that increased significantly with groundwater age, the frequency of sentry threshold exceedances (i.e., one-half of MCL, SMCL, or HBSL) included 40\% for dissolved solids (DS), 12\% for chloride (Cl), 3.6\% for F, 4.4\% for As, and 9.5\% for 210Po. Iron (Fe) concentrations did not correlate with groundwater age, but exceeded sentry thresholds in 29\% of wells. Groundwater age, water types, redox, pH, and GCOCs varied because of unique hydrogeologic features of the aquifers (recharge locations and geometry). As expected, primarily confined aquifers had young, oxic, low to near-neutral pH water near the outcrop (recharge area), and older, reduced, high pH water deeper and farther along flow paths. However, unique aquifer hydrogeologic conditions, such as multiple-recharge zones produced anomalous patterns of young and old groundwater at varying depths and locations along flow paths. Evidence for this variability is seen in disequilibrium patterns in the progression of the chemical evolution of groundwater with hydrologic position. When hydrogeologic differences are considered, groundwater age combined with hydrologic-position data, can provide a strong basis for inferring potential occurrence of GCOCs.},
	keywords = {Arsenic, Fluoride, Iron, Lead-210, pH, Polonium-210},
	isbn = {0048-9697},
	doi = {10.1016/j.scitotenv.2020.137835},
	url = {https://www.sciencedirect.com/science/article/pii/S0048969720313474},
	author = {Degnan, James R. and Lindsey, Bruce D. and Levitt, Joseph P. and Szabo, Zoltan}
}
@article {2990,
	title = {Arsenic variability and groundwater age in three water supply wells in southeast New Hampshire},
	journal = {Geoscience Frontiers},
	volume = {10},
	year = {2019},
	month = {09/2019},
	pages = {1669 - 1683},
	abstract = {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 had a mean variation of more than 7~μg/L. The time series data from this sampling effort showed that arsenic concentrations ranged from a median of 4~μg/L in a glacial aquifer well (SGW-65) to medians of 19~μg/L and 37~μg/L in wells (SGW-93 and KFW-87) screened in the bedrock aquifer, respectively. These high arsenic concentrations were associated with the consistently high pH (median~>=~8) and low dissolved oxygen (median <0.1~mg/L) in the bedrock aquifer wells, which is typical of fractured crystalline bedrock aquifers in New Hampshire. Groundwater from the glacial aquifer often has high dissolved oxygen, but in this case was consistently low. The pH also is generally acidic in the glacial aquifer but in this case was slightly alkaline (median~=~7.5). Also, sorption sites may be more abundant in glacial aquifer deposits than in fractured bedrock which may contribute to lower arsenic concentrations. Mean groundwater ages were less than 50 years old in all three wells and correlated with conservative tracer concentrations, such as chloride; however, mean age was not directly correlated with arsenic concentrations. Arsenic concentrations at KFW-87 did correlate with water levels, in addition, there was a seasonal pattern, which suggests that either the timing of or multiple sampling efforts may be important to define the full range of arsenic concentrations in domestic bedrock wells. Since geochemically reduced conditions and alkaline pHs are common to both bedrock and glacial aquifer wells in this study, groundwater age correlates less strongly with arsenic concentrations than geochemical conditions. There also is evidence of direct hydraulic connection between the glacial and bedrock aquifers, which can influence arsenic concentrations. Correlations between arsenic concentrations and the age of the old fraction of water in SGW-65 and the age of the young fraction of water in SGW-93 suggest that water in the two aquifers may be mixing or at least some of the deeper, older water captured by the glacial aquifer well may be from a similar source as the shallow young groundwater from the bedrock aquifer. The contrast in arsenic concentrations in the two aquifers may be because of increased adsorption capacity of glacio-fluvial sediments, which can limit contaminants more than fractured rock. In addition, this study illustrates that long residence times are not necessary to achieve more geochemically evolved conditions such as high pH and reduced conditions as is typically found with older water in other regions.},
	keywords = {Age tracers, Arsenic variability, Geochemical time series, Groundwater age modeling},
	isbn = {1674-9871},
	doi = {10.1016/j.gsf.2019.01.002},
	url = {https://www.sciencedirect.com/science/article/pii/S1674987119300313},
	author = {Levitt, Joseph P. and Degnan, James R. and Flanagan, Sarah M. and Jurgens, Bryant C.}
}
