Denitrification potential of sediment from a future Mississippi River diversion site in Louisiana

Abstract

Denitrification potential was determined in surface sediment from Lake Cataouatche, the receiving basin for a future Mississippi River diversion located in the northern portion of the Barataria Basin estuary. Nitrate removal and denitrification was measured in the laboratory using static sediment microcosms flooded with lake water. Dissolved potassium nitrate (KNO3) was added to the microcosms to achieve: 1) an initial nitrate concentration similar to the mean Mississippi River concentration (~1.4 mg NO3-N l-1), and, 2) a high initial nitrate concentration to elicit a denitrification potential (~50 mg NO3-N l-1). Denitrification was determined by the acetylene inhibition technique. The denitrification potential during the most active period (day 3-10 ) of nitrate removal in September, December, and March studies ranged from 37 to 55, 29 to 60, and 34 to 111 mg N m-2 d-1, respectively. A mean denitrification potential of 49±17 mg N m-2 d-1 was estimated from the three sampling periods. The denitrification potential was 16 times greater than estimates of denitrification under nitrate concentrations similar to the Mississippi River (3.0±1.2 mg N m-2 d-1), which ranged from 1.7 to 4.1, 0.89 to 3.8, and 2.3 to 4.5 mg N m-2 d-1 in September, December, and March studies, respectively. Mean nitrate removal from the 50 mg NO3-N l-1 addition to the microcosm floodwater was estimated at 177±25 mg N m-2 d-1 (with 6.3-18.5% error using an assigned estimate of floodwater volume). However the quality of data was poor resulting from not measuring initial floodwater volumes and evaporation. The nitrate removal rate estimate is qualitative and may vary as much as 100% at the high nitrate addition rate (see Table 4). At the low nitrate addition (1.4 mg NO3-N l-1) it was difficult to assign a removal rate since many measurements were below the analytical method detection limit used. Results demonstrated that Lake Cataouatche sediment has a large capacity to remove nitrate from the water column, and also suggest that denitrification could remove a significant portion of the nitrate inputs from the Davis Pond diversion.