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A WHOLE-LAKE WATER QUALITY SURVEY OF LAKE OAHE BASED ON A SPATIALLY-BALANCED PROBABILISTIC DESIGN
Citation:
Bolgrien, D W., T. Angradi, T D. Corry, S E. Miller, G. S. Peterson, J. V. Scharold, E. W. Schweiger, C W. West, AND J. R. Kelly. A WHOLE-LAKE WATER QUALITY SURVEY OF LAKE OAHE BASED ON A SPATIALLY-BALANCED PROBABILISTIC DESIGN. Presented at Missouri River Conference, South Sioux City, NE, April 21-24, 2002.
Description:
Assessing conditions on large bodies of water presets multiple statistical and logistical challenges. As part of the Upper Missouri River Program of the Environmental Monitoring and Assessment Project (EMAP) we surveyed water quality of Lake Oahe in July-August, 2002 using a spatially-balanced probabilistic design. A total of 51 sites were distributed throughout the reservoir with 36 sites in bays and 15 sites in the nominal open-water. Although bays comprise only 28% of the area of the reservoir, a disproportionate number of bays sites was designed to sample the more variable conditions in bays. Twelve of the bay sites were placed in the confluence areas of the Cheyenne, Grand, and Moreau Rivers. Using the EMAP design, data collected at a relatively small number of sites can be extrapolated with known statistical confidence to the entire reservoir. Water quality variables measured included turbidity, chlorophyll, conductivity, light attenuation, total phosphorous, nitrogen (total, nitrate+ nitrite, ammonia), silica, total organic carbon, total suspended solids, and a suite of anions and cations. Preliminary results suggest that trophic status in Lake Oahe increases from south to north. Measured chlorophyll concentrations ranging from 0.8 ug/L near Oahe Dam to 9.7 ug/L near the delta. Further analyses will reveal the cumulative area of the reservoir possessing a given range of values for each water quality variables. Compared to traditional sampling programs, the EMAP strategy increases the capacity to conduct large-scale assessments because fewer sampling locations are needed to make statistically defensible statements about extensive resources. This abstract does not necessarily reflect EPA policy.