Temporal Variability in Denitrification in the Freshwater Reservoir

Denitrification is controlled by seasonally varying factor such as temperature and oxygen concentrations. In the Occoquan Reservoir, a water reclamation plant purposely discharges nitrate to reservoir to protect its water quality. This management strategy is effective during the summer months (May-September) when denitrification rates are highest, but it is inefficient during the winter when water temperatures are low, and oxygen is high. This study examined the feasibility of adding nitrate to the reservoir during the shoulder months (March-April and October), when denitrification is expected to be minimal. Field data revealed that in March and April, when the water temperature was \u3e12ºC, oxygen concentrations rapidly decreased from 10’2 mg/L, revealing a high oxygen and nitrate demand before the onset of thermal stratification. In microcosms, sediments from the reservoir depleted nitrate when oxygen values where ~2 mg/L, suggesting that denitrification occurred deeper in the aerobic sediments. Lower denitrification rates in aerobic sediments were the result of nitrate transport limitations, as nitrate travels deeper into the sediments to reach the anoxic layer, and not the result of lower denitrifying bacteria activity. These results will help reexamine the concept that denitrification occurs only under strictly anaerobic conditions, expanding the nitrogen cycle knowledg