One of the principal questions in hydrology is how and when water leaves the critical zone storage as either stream flow or evapotranspiration. We investigated subsurface water storage and storage selection of the Southern Sierra Critical Zone Observatory (California, USA) within the age-ranked storage selection framework, constrained by a novel combination of cosmogenic radioactive and stable isotopes: tritium, sodium-22, sulfur-35, and oxygen-18. We found a significant positive correlation between tritium and stream flow rate and between sulfur-35 and stream flow rate, indicating that the age distribution of stream flow varies with stream flow rate. Storage selection functions that vary with stream flow rate are better able to reproduce tritium concentrations in stream flow than functions that are constant in time. For the Southern Sierra Critical Zone, there is a strong preference to discharge the oldest water in storage during dry conditions but only a weak preference for younger water during wet conditions. The preference of evapotranspiration for younger water, constrained by oxygen-18 in stream water, is essential to parameterize subsurface storage but needs to be confirmed by isotopic or other investigations of evapotranspiration. This is the first study to illustrate how a combination of cosmogenic radioactive isotopes reveals the hydrochronology and water storage dynamics of catchments, constrains the subsurface architecture of the critical zone, and provides insight into landscape evolution.Plain Language Summary Watersheds store water underground in soils and weathered bedrock.How long it takes for water to flow through the subsurface to feed streams is difficult to measure but important to understand how watersheds function. We used a novel combination of isotopic tracer methods to study the mixture of water ages in Providence Creek, a stream in the southern Sierra Nevada, California (USA). We studied naturally occurring radioactive isotopes of hydrogen (tritium), sodium-22, and sulfur-35. The abundance of these isotopes decreases because of radioactive decay as water spends more time underground. Each of these isotopes has a distinct half-life (12.3 years, 2.6 years, and 87 days, respectively). By using this combination of isotopes we were able to tease out the mixture of water ages in the stream. This level of detail helps us understand how the subsurface "selects" water from storage to generate stream flow. We find that Providence Creek has a strong preference to remove the oldest water in storage during dry conditions but shows only a slight preference to remove younger water during wet conditions. Based on the age mixtures of stream water, we estimate that the Providence Creek watershed stores 3 m of water in the subsurface.

Cosmogenic sulfur-35 in water as dissolved sulfate ((35)SO4) has successfully been used as an intrinsic hydrologic tracer in low-SO4, high-elevation basins. Its application in environmental waters containing high SO4 concentrations has been limited because only small amounts of SO4 can be analyzed using current liquid scintillation counting (LSC) techniques. We present a new analytical method for analyzing large amounts of BaSO4 for (35)S. We quantify efficiency gains when suspending BaSO4 precipitate in Inta-Gel Plus cocktail, purify BaSO4 precipitate to remove dissolved organic matter, mitigate interference of radium-226 and its daughter products by selection of high purity barium chloride, and optimize LSC counting parameters for (35)S determination in larger masses of BaSO4. Using this improved procedure, we achieved counting efficiencies that are comparable to published LSC techniques despite a 10-fold increase in the SO4 sample load. (35)SO4 was successfully measured in high SO4 surface waters and groundwaters containing low ratios of (35)S activity to SO4 mass demonstrating that this new analytical method expands the analytical range of (35)SO4 and broadens the utility of (35)SO4 as an intrinsic tracer in hydrologic settings.

s u m m a r yNitrate is a major source of contamination of groundwater in the United States and around the world. We tested the applicability of multiple groundwater age tracers ( 3 H, 3 He, 4 He, 14 C, 13 C, and 85 Kr) in projecting future trends of nitrate concentration in 9 long-screened, public drinking water wells in Turlock, California, where nitrate concentrations are increasing toward the regulatory limit. Very low 85 Kr concentrations and apparent 3 H/ 3 He ages point to a relatively old modern fraction (40-50 years), diluted with pre-modern groundwater, corroborated by the onset and slope of increasing nitrate concentrations. An inverse Gaussian-Dirac model was chosen to represent the age distribution of the sampled groundwater at each well. Model parameters were estimated using a Bayesian inference, resulting in the posterior probability distribution -including the associated uncertainty -of the parameters and projected nitrate concentrations. Three scenarios were considered, including combined historic nitrate and age tracer data, the sole use of nitrate and the sole use of age tracer data. Each scenario was evaluated based on the ability of the model to reproduce the data and the level of reliability of the nitrate projections. The tracer-only scenario closely reproduced tracer concentrations, but not observed trends in the nitrate concentration. Both cases that included nitrate data resulted in good agreement with historical nitrate trends. Use of combined tracers and nitrate data resulted in a narrower range of projections of future nitrate levels. However, use of combined tracer and nitrate resulted in a larger discrepancy between modeled and measured tracers for some of the tracers. Despite nitrate trend slopes between 0.56 and 1.73 mg/L/year in 7 of the 9 wells, the probability that concentrations will increase to levels above the MCL by 2040 are over 95% for only two of the wells, and below 15% in the other wells, due to a leveling off of reconstructed historical nitrate loadings to groundwater since about 1990.

s u m m a r yIdentifying groundwater retention times near managed aquifer recharge (MAR) facilities is a high priority for managing water quality, especially for operations that incorporate recycled wastewater. To protect public health, California guidelines for Groundwater Replenishment Reuse Projects require a minimum 2-6 month subsurface retention time for recycled water depending on the level of disinfection, which highlights the importance of quantifying groundwater travel times on short time scales. This study developed and evaluated a new intrinsic tracer method using the naturally occurring radioisotope sulfur-

Identifying aquifer vulnerability to climate change is of vital importance in the Sierra Nevada and other snow-dominated basins where groundwater systems are essential to water supply and ecosystem health. Quantifying the component of new (current year's) snowmelt in groundwater and surface water is useful in evaluating aquifer vulnerability because significant annual recharge may indicate that streamflow will respond rapidly to annual variability in precipitation, followed by more gradual decreases in recharge as recharge declines over decades. Hydrologic models and field-based studies have indicated that young (<1 year) water is an important component of streamflow. The goal of this study was to utilize the short-lived, naturally occurring cosmogenic during baseflow conditions to 14.0 ± 3.4% during high-flow periods of snowmelt. Similar to SCB, the PNS in MVGB groundwater and streamflow was typically <30% with the largest fractions occurring in late spring or early summer following peak streamflow. The consistently low PNS suggests that a significant fraction of annual snowmelt in SCB and MVGB recharges groundwater, and groundwater contributions to streamflow in these systems have the potential to mitigate climate change impacts on runoff. KEYWORDSgroundwater recharge, mountain groundwater, snowmelt infiltration, sulfur-35Groundwater vulnerability to climate change in high-elevation basins has widespread implications for ecosystem health and water supply (Earman & Dettinger, 2011;Earman et al., 2015). In the mountains of the western United States, groundwater is a major component of streamflow, even during peak snowmelt conditions (e.g., Frisbee et al., 2011;Liu et al., 2004). Spatial and temporal changes in snow dynamics, such as declines in snowpack accumulation (Mote 2003;Mote et al., 2005) and earlier onset of snowmelt Knowles et al., 2006;Mote et al., 2005), are of particular concern for Sierra Nevada basins because groundwater recharge is mainly derived from snowpack for most of the southwest (Earman et al., 2006;Winograd et al., 1998). Groundwater recharge in high-elevation basins in the western United States is important for aquifer replenishment (Wilson & Guan, 2004;Manning & Solomon, 2005) and ecosystem health, yet the impact of climate change on groundwater recharge is poorly understood (Earman & Dettinger, 2011;Earman et al., 2015;Viviroli et al., 2011).Understanding climate change impacts on groundwater resources in the Sierra Nevada and other high-elevation basins is difficult because of a weak understanding of direct and indirect effects of climate change on mountain recharge processes (Earman & Dettinger, 2011;Earman et al., 2015). Current forecasts of the effects of climate change vary widely. In snow-dominated basins that are predicted to experience a shift in precipitation from snow to rain, groundwater recharge may decrease because snow is a more efficient recharging mechanism than rain (Earman et al., 2006;Meixner et al., 2016;Winograd et al., 1998 however, no data exist showing t...