GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 371-3
Presentation Time: 9:00 AM-6:30 PM

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


BARDSLEY, Audra I. and HAMMOND, Douglas E., Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, aibardsl@usc.edu

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.