A Combined Radio- and Stable-Isotopic Study of a California Coastal Aquifer System

Water

Peter W. Swarzenski³  Mark Baskaran¹  Robert J. Rosenbauer³  Brian D. Edwards² 

[1] Department of Geology, Wayne State University, Detroit, MI 48202, USA; E-Mail:;U.S. Geological Survey, Menlo Park, CA 94025, USA; E-Mail:;U.S. Geological Survey, Santa Cruz, CA 95060, USA; E-Mail:

关键词: radium;    radon;    groundwater;    coastal aquifer;    stable isotopes;    residence times;    desorption rate constant;   

DOI  :  10.3390/w5020480

来源: mdpi

【 摘 要 】

Stable and radioactive tracers were utilized in concert to characterize geochemical processes in a complex coastal groundwater system and to provide constraints on the kinetics of rock/water interactions. Groundwater samples from wells within the Dominguez Gap region of Los Angeles County, California were analyzed for a suite of major cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) and anions (Cl⁻, SO₄²⁻), silica, alkalinity, select trace elements (Ba, B, Sr), dissolved oxygen, stable isotopes of hydrogen (δD), oxygen (δ¹⁸O), dissolved inorganic carbon (δ¹³C_DIC), and radioactive isotopes (³H, ²²²Rn and ^{223,224,226,228}Ra). In the study area, groundwater may consist of a complex mixture of native groundwater, intruded seawater, non-native injected water, and oil-field brine water. In some wells, Cl⁻ concentrations attained seawater-like values and in conjunction with isotopically heavier δ¹⁸O values, these tracers provide information on the extent of seawater intrusion and/or mixing with oil-field brines. Groundwater ³H above 1 tritium unit (TU) was observed only in a few select wells close to the Dominguez Gap area and most other well groundwater was aged pre-1952. Based on an initial ¹⁴C value for the study site of 90 percent modern carbon (pmc), groundwater age estimates likely extend beyond 20 kyr before present and confirm deep circulation of some native groundwater through multiple aquifers. Enriched values of groundwater δ¹³C_DIC in the absence of SO₄²⁻ imply enhanced anaerobic microbial methanogenesis. While secular equilibrium was observed for ²³⁴U/²³⁸U (activity ratios ~1) in host matrices, strong isotopic fractionation in these groundwater samples can be used to obtain information of adsorption/desorption kinetics. Calculated Ra residence times are short, and the associated desorption rate constant is about three orders of magnitude slower than that of the adsorption rate constant. Combined stable- and radio-isotopic results provide unique insights into aquifer characteristics, such as geochemical cycling, rock/water interactions, and subsurface transport and mixing.

【 授权许可】

CC BY   
© 2013 by the authors; licensee MDPI, Basel, Switzerland.