O2 reduction and denitrification rates in shallow aquifers

Abstract

O2 reduction and denitrification rates were determined in shallow aquifers of 12 study areas representing a wide range in sedimentary environments and climatic conditions. Zero‐ and first‐order rates were determined by relating reactant or product concentrations to apparent groundwater age. O2 reduction rates varied widely within and between sites, with zero‐order rates ranging from <3 μmol L−1 yr−1 to more than 140 μmol L−1 yr−1 and first‐order rates ranging from 0.02 to 0.27 yr−1. Moderate denitrification rates (10–100 μmol N L−1 yr−1; 0.06–0.30 yr−1) were observed in most areas with O2 concentrations below 60 μmol L−1, while higher rates (>100 μmol N L−1 yr−1; >0.36 yr−1) occur when changes in lithology result in a sharp increase in the supply of electron donors. Denitrification lag times (i.e., groundwater travel times prior to the onset of denitrification) ranged from <20 yr to >80 yr. The availability of electron donors is indicated as the primary factor affecting O2 reduction rates. Concentrations of dissolved organic carbon (DOC) and/or sulfate (an indicator of sulfide oxidation) were positively correlated with groundwater age at sites with high O2 reduction rates and negatively correlated at sites with lower rates. Furthermore, electron donors from recharging DOC are not sufficient to account for appreciable O2 and nitrate reduction. These relations suggest that lithologic sources of DOC and sulfides are important sources of electrons at these sites but surface‐derived sources of DOC are not. A review of published rates suggests that denitrification tends to occur more quickly when linked with sulfide oxidation than with carbon oxidation.