| Summary: | <p>Many drylands around the world are dependent on high-elevation snowmelt for aquifer recharge. Much of the groundwater in the American West relies on precipitation formed in the Rocky Mountains. Understanding how snowmelt-driven recharge may change over time is an important component to sustainably managing these groundwater resources. The San Luis Valley of southern Colorado is the highest elevation area in the United States used for industrial agriculture. Precipitation on the valley floor is 200 mm/yr, but its water resources are sustained by precipitation > 1,000 m higher in the San Juan and the Sangre de Cristo Mountains, at the southern extent of the Rocky Mountains. </p>
<p>This study asks, what are the main pathways and rates of recharge? Using chemical and isotopic tracers, what can we learn about the water’s source? To answer these questions, the following chapters examine 1) atmospheric source water 2) groundwater 3) plant-water, and 4) soil moisture. Chemical and isotopic tracers from >400 samples are used to follow water pathways from condensation, points of recharge, and along flowpaths. For near-surface moisture, chemical and isotopic tracers and electrical resistivity tomography are used to examine water-plant interactions and soil moisture movement in the unsaturated zone.</p>
<p>Over a three-year period from 2015 through 2017, >120 precipitation samples were collected and analyzed for chemistry and δ<sup>2</sup>H/δ<sup>18</sup>O in order to produce a valley floor Local Meteoric Water Line (LMWL) and a mountain pass high-elevation MWL. Chemical analysis demonstrates a pattern of acid rainfall, reaching greatest acidity (pH 3.9) over a two-week period in late summer. Rain in the early summer of 2015 showed as much as 4.3 mg/L NO<sub>3</sub>, 12 mg/L SO<sub>4</sub>, 3.1 mg/L NH<sub>4</sub>, and 4.6 mg/L K. Heavy metal analysis shows contaminants of health concern in rain, including Cr, Cd, As, Pd, W, Mo, and more than half the rainfall analyzed over three years shows the presence of uranium. </p>
<p>Recharge to the confined aquifer is snowmelt-driven. Mountain block recharge, where infiltration occurs at high elevations in the San Juan Mountains appears to contribute the most recharge to the upper confined aquifer. Of those wells sampled > 274 m deep in the eastern side of the valley, the mean isotopic signature is closer to the valley-floor LMWL, suggesting recharge primarily occurs through infiltration at the base of the mountains, as mountain front recharge and as fracture flow. The highest rates of recharge occur in long-screen wells in the deep, unconfined aquifer layer at the base of the Sangre de Cristo Mountains. </p>
<p>Old groundwater from the upper confined aquifer sampled within a 25km<sup>2</sup> area of the valley’s sump shows a 2.5‰ <sup>18</sup>O spread and sits significantly above the LMWL and high-elevation MWL. This water shows a mean deuterium excess (d-xs) of 17.6 ‰, which is nearly double the d-xs of the LMWL. This study argues that in addition to other human alterations to recharge pathways (such as dust-on-snow albedo changes to snowpack, etc.) 20<sup>th</sup> Century draining of playa wetlands and diverting of surface water have altered a source of recharge not previously considered.</p>
<p>Most rain events in the San Luis Valley are < 2 mm and, based on time-lapse resistivity inversions, do not appear to penetrate to a depth sufficient to reach the water table. Soil profiles indicate most precipitation on the valley floor does not penetrate more than 25 cm. Two native phreatophyte plants of the San Luis Valley, <em>E. nauseosa</em> and <em>S. vermiculatus</em>, show distinct patterns of water use and seasonal shifts depending on water availability. Differences between the species are most apparent where groundwater is most accessible. However, where the water table has dropped 6 m over the last decade, both <em>E. nauseosa</em> and <em>S. vermiculatus</em> survive only on near-surface snowmelt and rain. At this site, shrub density is reduced by an order of magnitude, species diversity is lower, and the percentage of exposed, bare earth reaches 45%.</p>
<p>This work presents evidence that a primary pathway for aquifer recharge to the San Luis Valley occurs at high elevation, and the mechanism by which it occurs may be changing over time. This dissertation employs traditional and non-traditional hydrological techniques to follow water in the San Luis Valley watershed as it moves through its air-surface-ground water cycle, and by placing hydrological insight within a human context, offers suggestions for the future stewardship of these significant groundwater resources.</p>
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