O2 reduction and denitrification rates in shallow aquifers

O-2 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. O-2 reduction rates varied widely within and between sites, with zero-order rates ranging from <3 mu mol L-1 yr(-1) to more than 140 mu mol L-1 yr(-1) and first-order rates ranging from 0.02 to 0.27 yr(-1). Moderate denitrification rates (10-100 mu mol N L-1 yr(-1); 0.06-0.30 yr(-1)) were observed in most areas with O-2 concentrations below 60 mu mol L-1, while higher rates (> 100 mu mol 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 O-2 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 O-2 reduction rates and negatively correlated at sites with lower rates. Furthermore, electron donors from recharging DOC are not sufficient to account for appreciable O-2 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.