Determination of Impact of Sediment Oxygen Demand on the Souris River Dissolved Oxygen

Upper portion of the Souris River in the United States, a 43.4 mile reach from the Canadian border to Lake Darling, has been on the North Dakota Section 303(d) list for dissolved oxygen (DO) impairment due to DO concentrations below the standards (5 mg/L) and required the development of a pollutant specific total maximum daily load (TMDL). Analysis of historical river flow and water quality data indicated that the low DO conditions occurred during winter months when the river was covered by ice and flow rate was low. An extensive field sampling was carried out to determine the nature and extent of the impairment. Sediment oxygen demand (SOD) was identified as the main cause of DO depletion. Experimental studies showed that SOD rate is proportional to the percentage of organic matter in the sediment. QUAL2K model was employed to simulate river flow and DO conditions. Model simulation results showed that sediment organic reductions of 31%, 39% and 55% were required to maintain a minimum DO of 5 mg/L in the river, if DO level in the upstream of the impaired reach could be raised to 8 mg/L, 7 mg/L and 6 mg/L, respectively.