Nutrient leaching from sediments according to different oxygen supplying conditions

Abstract:

To estimate the role of sediments controlling the water quality, many studies have adopted nutrient leaching experiments. However, the experiments were mostly performed under air-open condition, which is different from the actual condition at the bottom of fairly stagnant water body. Therefore, the results may not properly reflect the capability of the sediments supplying the nutrients to the water column. In this study, nutrient leaching experiments were conducted using various sediment samples under two different oxygen supplying conditions: one open and the other closed to the air. For this study, 5 sediment samples were collected from lakes and rivers around Kunsan, Korea. Air-closed leaching experiments were carried out using conventional BOD glass bottles (355 mL) without any head spaces. All the experiments were performed at a solid:solution ratio of 1:7.3. Each reactor was incubated at 25 ℃ for 72 hours or 120 hours. The solution was analyzed for pH, DO, Eh, NO₃-N, NH₄-N, and PO₄-P after the incubation. Silt+clay fraction, total nitrogen (TN), total phosphorus (TP), and COD of the sediments showed close correlations with each other. Air-closed condition showed much higher leached concentrations for NH₄-N and PO₄-P (up to 60 times) than the air-open condition and the concentrations increased with the incubation time. However, the differences were very small in the experiments using sandy sediments (<0.13 mg/L and < 50 ug/L for NH₄ and PO₄ concentrations, respectively). In general, the leached NH₄ and PO₄ concentrations from the air-open experiments were not correlated well with the fine fractions due to nitrification and suppression of Fe-oxide reduction in aerobic condition, respectively. Because of this reason, some air-open results showed NO₃-N concentrations higher than the concentrations obtained from the air-closed experiments. The experiments closed to air showed NO₃-N concentrations substantially decreasing with the incubation time due to denitrification. During the stratified period, it is well known that the lake bottoms are generally under anoxic condition and, thus, the air-closed leaching may reflect the actual nutrient releasing from sediments more closely.