Regional oxygen reduction and denitrification rates, San Joaquin Valley, USA

Abstract:

Rates of oxygen and nitrate reduction are key factors in determining the chemical evolution of groundwater. Little is known about how these rates vary in regional groundwater settings, as few studies have focused on regional datasets with multiple tracers and methods of analysis that account for effects of mixed residence times on apparent reaction rates. This study provides insight into residence times and rates of O₂ reduction and denitrification using a novel approach of multi-model residence time distributions (RTDs) applied to a data set of atmospheric tracers of groundwater age and geochemical data from 141 well samples in the Central Eastern San Joaquin Valley, CA. The residence time distribution approach accounts for mixtures of residence times in a single sample to provide estimates of in-situ rates. Tracers included SF₆, CFCs, ³H, He from ³H, ¹⁴C, and terrigenic He. Parameter estimation and multi-model averaging were used to establish RTDs with lower error variance than produced by individual RTD models. The set of multi-model RTDs was used in combination with NO₃⁻ and dissolved gas data to estimate zero order and first order rates of O₂ reduction and denitrification. Results indicated that these rates followed approximately log-normal distributions. Rates of O₂ reduction and denitrification were correlated and, on an electron milliequivalent basis, denitrification rates tended to exceed O₂ reduction rates. Results indicate that the multi-model approach can improve estimation of age distributions, and that, because of the correlations, relatively easily measured O₂ rates can provide information about trends in denitrification rates, which are more difficult to measure.