DISSOLVED NOBLE GASES AS FORENSIC TRACERS TO INVESTIGATE MOUNTAIN FRONT SPRINGS IN OWENS VALLEY, CA

Mountain front springs offer a glimpse into mountain block recharge and flow processes. However, it remains difficult to quantify spring contributing areas and to delineate groundwater flowpaths to springs, especially in high-relief, mountainous settings. It is important to quantify these parameters to improve our understanding of groundwater response to the effects of climate change, namely impacts to snowpack and snowmelt recharge. Springs along the Sierra Nevada boundary fault zone in Owens Valley, CA emerge at a significantly lower elevation (1000-1400 mamsl) than the regional divide (3600-4000+ mamsl) just a few kilometers to the west. We seek to understand where these springs are sourcing their recharge and how recharge temperature and elevation compare with other geochemical and isotopic tracers. To address these questions, 11 mountain front springs in Owens Valley were sampled during the spring of 2016 for dissolved noble gases. Noble gases can provide information about recharge conditions (temperature and elevation), interaction with deeply circulating groundwater (mantle and crustal reservoirs of helium), and groundwater age (³H/³He dating and ⁴He accumulation). Noble gas analysis was performed with the Matlab code Noble90. The study area contains terrigenic sources of ³He and ⁴He, therefore only dissolved concentrations of Ne, Xe, Kr, and Ar were used to solve for unknown parameters. Recharge elevation (H) was determined by solving for recharge temperature (T_r) at a suite of elevations from the spring head to the local peak and determining where this family of solutions intersects the line describing the local atmospheric lapse rate. Calculated T_r values were found to be insensitive to fractionation of the excess air component. Springs in central and northern Owens Valley have an average T_r of 3.7℃ corresponding to a H of 2930 mamsl. Positive correlations are observed with spring T_r and discharge temperature as well as with T_r and δ¹⁸O. In comparison, springs in the southern part of Owens Valley are further away from the regional divide and have a warmer average T_r (7.5℃) corresponding to a lower H. These springs are also warmer, ³H dead, and have older corrected ¹⁴C ages. Our results provide a baseline for evaluating how these springs will respond to decreased recharge as predicted by climate models.