Nitrate Addition Pilot - Onondaga Lake
2011 Pilot Test Results
The first year of a three-year nitrate addition pilot test in Onondaga Lake has been completed successfully and has met its objective: to demonstrate the ability to maintain nitrate concentrations in the hypolimnion (i.e., water deeper than 30 ft.) at levels sufficient to inhibit release of methylmercury from lake sediment to the overlying waters during stratification. Methylmercury concentrations measured in deep waters during 2011 were lower than during any recent prior year as a result of adding nitrate.
Methylmercury release from Onondaga Lake bottom sediment occurs when oxygen and nitrate are depleted from overlying water during summer stratification. Stratification is a natural process in Onondaga Lake and other temperate lakes whereby upper waters are warm and well mixed during the summer while lower waters remain cool and isolated. Dissolved oxygen and nitrate concentrations in the lower waters naturally decline in a gradual manner as stratification continues through the summer. Depletion of oxygen and nitrate eventually results in the release of methylmercury from bottom sediments to the overlying deep waters in the middle of the lake. At fall turnover, when the upper waters cool and mix completely with the lower waters, fish are potentially exposed to methylmercury that has accumulated in the lower waters. The presence of oxygen and/or nitrate in the lower waters limits methylmercury release, and therefore reduces the exposure of fish to methylmercury.
The first year of nitrate addition consisted of continuous applications of a diluted calcium nitrate solution (calcium nitrate) to bottom waters of the lake during 40 non-consecutive days between June 30 and October 10, 2011. The liquid calcium nitrate was a commercially-available and commonly-used product that was mixed with lake water to an appropriate density for placing the nitrate near the lake bottom. A self-propelled barge approximately 40 ft. long and 24 ft. wide was used to conduct each of the nitrate applications. The target dose for each daily application was 4,800 gallons of calcium nitrate (2.3 metric tons of nitrate-nitrogen) at water depths of 42 to 62 ft. (13 to 19 meters), or 7 to 17 ft. (2 to 5 meters) above the lake bottom. The dose could be easily controlled and modified to meet target nitrate levels in the lake water. Results indicate that adding nitrate did not significantly affect nitrite levels.