SEDIMENT OXYGEN DEMAND STUDY OF THE TONGUE RIVER RESERVOIR MONTANA MARCH 9-11, 1976 TECHNICAL INVESTIGATIONS BRANCH SURVEILLANCE AND ANALYSIS DIVISION U. S. ENVIRONMENTAL PROTECTION AGENCY REGION VIII ------- INTRODUCTION At the request of the Department of Health and Environmental Sciences of the State of Montana, personnel of the Surveillance and Analysis Division, Region VIII, EPA measured the oxygen demand of the sediments of the Tongue River Reservoir of Montana from 3/9-11/76. The purpose of the study was to supply data to the State to aid in determing whether the sediment oxygen demand (SOD) of the reservoir sediments was great enough to cause depletion of oxygen from the water column resulting in winter fish kills under the ice. In addition to the main thrust of the study, nutrient samples were collected to determine nutrient levels in the drainage basin and to ascer- tain whether ammonia accumulates under the ice in enough quantity to become toxic to fish. Sediment oxygen demands usually result from the decomposition of accumulated organic materials. The SOD is lowest in the winter when the water temperature is the lowest and increases to a maximum in early spring with increasing temperature and maximum dissolved oxygen (DO). The SOD rate usually decreases as summer progresses because of a loss of oxidizable sediments from the decomposition which occured in spring and early summer. However, a renewal of behthic organic deposits during the summer could cause the SOD rate to remain high or exceed the springtime rate. During this study SOD rates were measured at two locations on the reservoir: Station 1) near the dam and Station 2) approximately 4.8 kilo- meters (3 miles) upstream from the dam (Figure I). Nutrient samples were collected at the above two stations plus five additional locations (See Table II). - 1 - ------- Figure I Tongue River Reservoir ------- Figure 2 Sediment oxygen demand chamber. ------- METHODS The sediment oxygen demand rates were estimated from changes in the DO concentration of water sealed within a clear plastic A-frame shaped chamber (Figure II). Attached to the chamber was a stainless steel cutting flange which effectively sealed the water in the chamber when placed on the sediments. The water in the chamber was circulated by means of a 110 volt submersible pump. To determine the effectiveness of the chamber to sed- iment seal, a salt solution was injected into the chamber to raise the specific conductance of the water in the chamber above that of the ambient environment. The increased conductivity was then monitored during the test run. A decrease in conductivity would indicate a poor seal. Changes in the DO concentration were measured with a portable dissolved oxygen meter connected by a cable to a submersible oxygen probe mounted in the chamber. After placement of the chamber on the sediment and a short period of stabilization, the oxygen level of the chamber was monitored for approximately one hour for each test run. Several BOD bottles filled with reservoir water obtained near the bottom were incubated in the reservoir near the chamber to account for photosynthesis or respiration of planktonic algae during each test run. The SOD rates were calculated on an areal basis using the following formula: - 4 - ------- SOD = [(Cic - Cfc) ± (Ciu - Cfb3v tA Where: S0D= sediment uptake rate in gm 02/m^/day V= volume of water in the chamber in mr (.012 m3) A= bottom area of chamber in m^ (.186 m2) t= test period in days Cic= initial measured DO of chamber in mg/je, Cfc= final measured DO of chamber in mg/£ Cib= initial measured DO of BOD bottles in mg/i. Cfb= final measured DO BOD bottles in mgh The nutrient samples were collected in one liter containers and preserved with 4 ml. of mercuric chloride. Analysis of the samples was performed at the EPA, Region VIII laboratory in accordance with "Method for Chemical Analysis of Water and Wastes" U.S. EPA. - 5 - ------- RESULTS AND DISCUSSION The SOD rates taken in the Tongue River Reservoir ranged from O 0.0 to 2.13 gm 02/m /day, indicating a great deal of variation in the organic content of the sediments. Bottom water temperature at all locations during the testing was 1° C, and the dissolved oxygen concentration near the sediments ranged from 7.75 ppm to 12 ppm. Runs I and II at Station 1, near the dam, were located where the sub- strate was composed mainly of gravel covered by a layer of silt. The average SOD rate at this location was 0.59 gm 02/m^/day. While run III, located some 45 meters (50 yards) distant from the previous site showed the sediment to have a rate of 1.71 gm 02/m2/day. The reservoir bottom at this location consisted entirely of silt and mud. Apparently, the large difference in the SOD rates at the two locations can be attributed to the variation in substrate type. At Station 2, approximately 4.8 kilometers (3 miles) upstream from the dam, the SOD rates were again peculiar. Here the substrate was composed of silt and mud. Runs were performed at three separate locations; two near the east shore (runs I and II) and one near the west shore (run III). At O both sites on the east side of the reservoir the SOD rate was 0.0 gm 02/m /day, 2 while on the west side the rate was 2.13 gm 02/m /day. This indicates that the majority of the suspended sediments are deposited along the west side of the reservoir at this point. Nutrient data obtained during the study (Table II) show, generally, that levels increase with depth in the reservoir and that amounts were ------- .greatest in the Goose Creek downstream from Sheridan, Wyoming. However, nutrient levels in the reservoir at the time of the study were not great enough to have a deleterious effect on the reservoir ecosystem. - 7 - ------- TABLE 1 TONGUE RIVER RESERVOIR MONTANA 3/9-11/76 Station No. Sediment Oxygen Demand gm 02/m^/day Run I Run II Run III 1 0.57 0.62 1.71 j 2 0.0 0.0 2.13 y- - 8 - ------- TABLE II TONGUE RIVER RESERVOIR MONTANA 3/9-11/76 Nutrient Concentrations mg/l Station Locations NH3-N Reservoir Discharge 0.16 Station 1 Surface 0.04 Station 1 Mid-Depth 0.06 Station 1 Bottom 0.14 Station 2 Bottom 0.13 Decker Mine Discharge 0.14 Tongue River 0.09 Mont.-Wyo. Border Goose Creek Upstream 0.12 From Bighorn Mine Goose Creek Downstream 0.40 From Sheridan, Wyo. N02 + NO3 - N 0.24 0.13 0.21 0.27 0.23 0.11 0.18 0.24 0.22 Ortho-P 0.040 0.015 0.017 0.045 0.041 0.004 0.056 0.088 0.215 ------- |