Denitrification and Organic Carbon Availability in Two Aquifers

Nitrate is one of the most common ground‐water contaminants, and denitrification, the bacterially mediated conversion of nitrate into molecular nitrogen, is the only geochemical process that contributes significantly to the renovation of nitrate‐contaminated aquifers. Previous workers have shown that denitrification occurs in some, but not all, nitratecontaminated unconfined aquifers in southern Ontario, Canada. Denitrification tends to occur in aquifers that have a very shallow water table, but not in aquifers whose water table is deeper than about two to three meters. This paper describes an investigation of the importance of organic carbon in controlling the occurrence of denitrification in two shallow sandy aquifers in southern Ontario, Canada. In situ and laboratory measurements of denitrification rates using the acetylene block technique show that: (1) organic carbon availability decreases with depth below the ground surface; (2) denitrification occurs in an aquifer with a very shallow water table, but organic carbon availability limits the denitrification rate; and (3) denitrification does not occur in an aquifer with a deeper water table because there is insufficient labile organic carbon present.