Regional oxygen reduction and denitrification rates in groundwater from multi-model residence time distributions, San Joaquin Valley, USA

Rates of oxygen and nitrate reduction are key factors in determining the chemical evolution of groundwater. Little is known about how these rates vary and covary 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 the characteristics of residence times and rates of O₂ reduction and denitrification (NO₃⁻ reduction) by comparing reaction rates using multi-model analytical 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 RTD 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 (tritiogenic He),¹⁴C, and terrigenic He. Parameter estimation and multi-model averaging were used to establish RTDs with lower error variances than those 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 O₂ reduction and denitrification rates followed approximately log-normal distributions. Rates of O₂ and NO₃⁻ reduction were correlated and, on an electron milliequivalent basis, denitrification rates tended to exceed O₂ reduction rates. Estimated historical NO₃⁻ trends were similar to historical measurements. Results show that the multi-model approach can improve estimation of age distributions, and that relatively easily measured O₂ rates can provide information about trends in denitrification rates, which are more difficult to estimate.