@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 {2993,
	title = {Timescales of water-quality change in a karst aquifer, south-central Texas},
	journal = {Journal of Hydrology X},
	volume = {4},
	year = {2019},
	month = {07/2019},
	pages = {100041},
	abstract = {Understanding the drivers and timescales over which groundwater quality changes informs groundwater management, use, and protection. To better understand timescales of water-quality change over short (daily to monthly) and long (seasonal to decadal) timescales, the U.S. Geological Survey{\textquoteright}s National Water-Quality Assessment (NAWQA) Enhanced Trends Network (ETN) program instrumented and sampled three wells in the Edwards aquifer in south-central Texas. The wells were instrumented to provide high-frequency continuous (subhourly) water-quality data (temperature, pH, specific conductance, and dissolved oxygen), which were augmented by the collection of discrete samples (about 6 per year) for a range of geochemical constituents (including selected isotopes and age tracers). ETN data (2013{\textendash}2017) are considered with data from additional sites for the same time period, and also historical records (over more than 80 years) of climatic and hydrologic conditions. During the four-year study, hydrologic conditions transitioned from very dry to very wet. Sites in the updip/unconfined part of the aquifer showed notable changes in water level and geochemistry (1) in response to rainfall/recharge events, and (2) over the multiyear dry/wet cycle. Sites in the downdip/confined part of the aquifer showed changes in water level/spring discharge over similar timescales, although the response is more muted. Geochemistry at the downdip/confined sites, however, varied slowly and minimally, indicating that the geochemical response of the deeper aquifer is decoupled from recent hydrologic responses. Changes at the updip/unconfined sites reflect mixing with recent recharge, whereas the downdip/confined sites were dominated by mineral-solution reactions resulting from longer (decadal) residence times. Mean groundwater ages interpreted from measured age tracers and lumped parameter models range from 7 to >700 years (where mixed with premodern downdip water) but were mostly modern. The aquifer is characterized by updip-to-downdip trends in geochemistry with respect to water-rock interaction and groundwater age. Fourier spectral analysis of historical records indicate hydrologic variability has occurred at dominant periods of 30 and 15 years; in conjunction with age tracers, these results provide insight into timescales at which the aquifer{\textquoteright}s public supply is vulnerable to changes in the water quality of recharge.},
	keywords = {Edwards aquifer, Groundwater age, Karst, Trends, Water quality},
	isbn = {2589-9155},
	doi = {10.1016/j.hydroa.2019.100041},
	url = {https://www.sciencedirect.com/science/article/pii/S2589915519300252},
	author = {Musgrove, M. and Solder, J. E. and Opsahl, S. P. and Wilson, J. T.}
}
