DEVELOPING A RADIUM CHRONOMETER FOR SHALLOW GROUNDWATER: COMPARISON WITH 3H/3HE IN URBANIZED COASTAL WATERSHEDS

One result of urbanization in semi-arid watersheds has been the rise in dry season creek flow. In southern California, observed increases span 250-500% over the past 50 years. Factors contributing to this upsurge have important implications for resource management. Previously, we found that springs and weeps sampled in Orange County, CA (OC) consist of both imported municipal and local meteoric water, and convey geologically derived contaminants to chronically impaired urban creeks, mobilized by acidic conditions due to pyrite oxidation.

Our current efforts seek to evaluate the transit times of shallow groundwater feeding OC creeks, to provide insight for contaminant transport timescales. We collected paired samples at 7 urban springs for dating, using ³H/³He and a ²²⁶Ra/²²³Ra chronometer we are developing. This new radium proxy uses mass balance equations for both isotopes to evaluate transit time. Naturally occurring uranium in aquifer sediments decays though a series of unstable progeny including Ra, some of which is ejected from solids to water via alpha recoil and a portion is subsequently mobilized. Because of its shorter half-life, ²²³Ra in groundwater quickly reaches secular equilibrium with its ancestors, while ²²⁶Ra has slower ingrowth. Ra adsorption is characterized by the ²²²Rn/²²⁶Ra ratio. Eventually, groundwater ^226/223Ra should approach the activity ratio of ^238/235U in aquifer sediments, which is consistent regardless of geologic material. The ratio of groundwater ^226/223Ra to solid phase ^238/235U should allow for estimation of contact time if U distribution in the aquifer is homogenous.

Data for 3 sites indicate relatively young groundwater ages and some promise for the ^226/223Ra method. At the first site, Ra-based transit time (2.1 ± 0.3 y) is similar to ³H/³He age (5.4 ± 2.3 y). A second site was gas-stripped, possibly by N₂ produced from denitrification, precluding full age determination, but agreed qualitatively with ³H and ^226/223Ra that both indicated shorter transit time than site one. The third site had a large discrepancy between ³H/³He age (29 ± 4 y) and ^226/223Ra age (<1 y), possibly due to subsurface mixing of water with different ages or perhaps due to inhomogeneous distribution of Ra ancestors in subsurface solids. Analyses for 4 other comparison sites are in progress.