Modeled sulfate concentrations in North Dakota streams, 1993-2008, based on spatial basin characteristics

Sulfate concentration data collected from North Dakota streams during recent (1993–2008) years indicates generally higher sulfate concentrations across much of the State compared to concentrations during earlier years. The higher sulfate concentrations have been attributed in other studies to wetter climatic conditions, associated increases in contributing drainage areas, and rising water tables. The State’s current (2013) stream classification system, which includes a standard for 30-day average sulfate concentration, is based on earlier data and thus may not reflect natural conditions for more recent years. The U.S. Geological Survey, in cooperation with the North Dakota Department of Health and the North Dakota State Water Commission, completed a study to evaluate the relation of maximum seasonal (30-day moving average) sulfate concentrations during 1993–2008 to characteristics of the contributing basins to model expected naturally-occurring sulfate concentrations in North Dakota streams.

Sulfate concentration data for 75 stream sampling sites in North Dakota were analyzed for this study. A spatial analysis was conducted with digital data using a Geographic Information System to obtain selected basin characteristics, which were in turn used as explanatory variables in a regression analysis to model the maximum seasonal (30-day moving average) sulfate concentration. Characteristics used in the regression analysis included mean annual precipitation, mean percent soil clay content, and mean percent saturation overland flow.

Modeled sulfate concentrations generally were highest (greater than 750 milligrams per liter) in basins in western North Dakota and lowest (less than 250 milligrams per liter) in basins in the upper Sheyenne River and upper James River. Area-weighted means for the basin characteristics also were computed for 10-digit and 8-digit hydrologic units for streams in North Dakota and modeled sulfate concentrations were computed from the characteristics. The resulting distribution of modeled sulfate concentrations was similar to the distribution of estimates for the 12-digit hydrologic units, but less variable because the basin characteristics were averaged over larger areas.