@article {faulkner_modeling_2023,
	title = {Modeling the dynamic penetration depth of post-1950s water in unconfined aquifers using environmental tracers: Central Valley, California},
	journal = {Journal of Hydrology},
	volume = {616},
	year = {2023},
	month = {01/2023},
	pages = {128818},
	abstract = {The penetration depth of post-1950s recharge (D-1950) in aquifers is a marker that is frequently used to identify groundwater that is susceptible to anthropogenic contamination. Here, we compute D-1950 values at wells, interpolate them in space, and project them across time to map the moving front of modern recharge in four dimensions in the Central Valley aquifer system, California, USA. Tracers of groundwater age (tritium, carbon-14, noble gases, sulfur hexafluoride, and chlorofluorocarbons) were collected at 650 wells spatially distributed throughout the Central Valley and were fit to a lumped-parameter model that assumes a logarithmic age-depth profile in the aquifer. For samples where tritium was present (>0.3 tritium units), the model was used to predict D-1950 at wells screened above or across the modern-premodern interface (n~=~484). Wells with samples where tritium was absent (<=0.3 tritium units) were used to define the depth beyond which groundwater is completely premodern (n~=~166). Predicted D-1950 values were below the depth of screen bottoms for wells where groundwater is completely modern, and above the depth of screen tops for wells where groundwater is completely premodern. The interpolated surface of D-1950 is dynamic, less prone to extreme values, and produces maps with lower interpolation errors due to a higher spatial density of wells than maps based on the depth of premodern groundwater. Between 2005 and 2025, D-1950 is expected to deepen by 11 and 12~m in the northern and southern parts of the Central Valley, respectively. Areas where D-1950 increases rapidly are likely to see increases in nitrate and other anthropogenic contaminants associated with the downward moving front of modern water.},
	keywords = {Environmental tracers, Groundwater age, Lumped-parameter model},
	issn = {0022-1694},
	doi = {10.1016/j.jhydrol.2022.128818},
	url = {https://www.sciencedirect.com/science/article/pii/S0022169422013889},
	author = {Faulkner, Kirsten E. and Jurgens, Bryant C. and Voss, Stefan A. and Dupuy, Danielle I. and Levy, Zeno F.}
}
@article {2996,
	title = {Over a third of groundwater in USA public-supply aquifers is Anthropocene-age and susceptible to surface contamination},
	journal = {Communications Earth \& Environment},
	volume = {3},
	year = {2022},
	month = {07/2022},
	pages = {1 - 9},
	abstract = {The distribution of groundwater age is useful for evaluating the susceptibility and sustainability of groundwater resources. Here, we compute the aquifer-scale cumulative distribution function to characterize the age distribution for 21 Principal Aquifers that account for ~80\% of public-supply pumping in the United States. The aquifer-scale cumulative distribution function for each Principal Aquifer was derived from an ensemble of modeled age distributions (~60 samples per aquifer) based on multiple tracers: tritium, tritiogenic helium-3, sulfur hexafluoride, chlorofluorocarbons, carbon-14, and radiogenic helium-4. Nationally, the groundwater is 38\% Anthropocene (since 1953), 34\% Holocene (75 {\textendash} 11,800 years ago), and 28\% Pleistocene (>11,800 years ago). The Anthropocene fraction ranges from <5 to 100\%, indicating a wide range in susceptibility to land-surface contamination. The Pleistocene fraction of groundwater exceeds 50\% in 7 eastern aquifers that are predominately confined. The Holocene fraction of groundwater exceeds 50\% in 5 western aquifers that are predominately unconfined. The sustainability of pumping from these Principal Aquifers depends on rates of recharge and release of groundwater stored in fine-grained layers.},
	keywords = {Environmental impact, hydrology, Sustainability},
	isbn = {2662-4435},
	doi = {10.1038/s43247-022-00473-y},
	url = {https://www.nature.com/articles/s43247-022-00473-y},
	author = {Jurgens, Bryant C. and Faulkner, Kirsten and McMahon, Peter B. and Hunt, Andrew G. and Casile, Gerolamo and Young, Megan B. and Belitz, Kenneth}
}
@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.}
}
