An Investigation into the Effects of an External Electron Acceptor on Nutrient Cycling at the Sediment-Water Interface of the Occoquan Reservoir

Abstract

Water supply reservoirs are often subject to accelerated nutrient enrichment from urban sources. Cultural eutrophication due to such enrichment requires the development of efficient management and remediation strategies to protect drinking water sources. This study investigates the effects of using nitrate as part of a management strategy to control nutrient cycling in the Occoquan Reservoir in northern Virginia, USA. A novel aspect of the study is that the reservoir is part of an indirect potable reuse system where the source of nitrate is the product water from an advanced water reclamation facility (WRF).

Field and laboratory studies showed that nitrate at a concentration greater than 1 mg/L N was effective in controlling the release of phosphorus, iron, and manganese from the sediments after the depletion of oxygen from the hypolimnetic waters of the reservoir. However, when the nitrate concentration above the sediment-water interface was less than 1 mg/L N, phosphorus, iron, and manganese release from the sediments was evident. Experiments revealed that the presence of nitrate decreased sediment ammonium release, but did not completely prevent it during anoxic periods. Results also showed that changes in the effective depth (ED) value along the length of the reservoir promoted higher denitrification rates in the upper reaches of the reservoir, thereby decreasing the downstream transport of nitrate. During periods of hypolimnetic anoxia, a nitrate-N input from the WRF of at least 10 mg/L N is needed to maintain an oxidized environment above the sediment-water interface. Therefore, decreasing the nitrate input to the reservoir will likely result in the deterioration of the surface water quality in the reservoir.

Finally, the ED concept was proven to be an effective method to simulate different segments of the reservoir in laboratory-scaled experiments. Similarities between the field and laboratory results suggests that the environment that existed in the waters of the reservoir was closely replicated in the experimental setup, and provides confidence that laboratory results can be extrapolated to natural reservoir conditions.