Black Coulee National Wildlife Refuge 



Climatic setting. The net annual precipitation balance for Black 

 Coulee (estimated from the MAPS database) is -26 inches, placing 

 the site near the center of the range shown for Great Plains sample 

 sites. Mean monthly precipitation for the (inactive) Turner 

 climatic station shows a strong June peak. Annual totals for the 

 Harlem 4W station (the most relevant active station) are 

 fragmentary since the early 1970s. From 1990 through 1992 (the 

 three years prior to sampling) annual totals were below the long- 

 term average. 



Geologic setting. The Black Coulee site is underlain by till of 

 uncertain character, possibly deposited as part of a marginal 

 moraine complex ringing the Turner Plateau. Local alluvium within 

 the Black Coulee drainage is probably composed of reworked till and 

 sediments derived from the late Cretaceous Bearpaw Shale, which 

 underlies surficial deposits in the area. 



Hydrologic type. Black Coulee NWR is an artificial impoundment 

 inferred (by low salinity) to have spilled water regularly during 

 the recent past. 



Basin characteristics. Black Coulee has a catchment to wetland 

 area ratio in the mid-range for sample sites in it's class. A 

 number of other, smaller reservoirs within the basin probably 

 influence the runoff characteristics of the catchment, as may areas 

 of internal drainage within the northern portion of the mapped 

 catchment. Surficial and bedrock materials exposed in the basin 

 are generally not amenable to much ground-water flow. 



Water chemistry. At the time of sampling, water at Black Coulee 

 NWR was of a relatively dilute, moderately alkaline mixed- 

 cation/bicarbonate character with proportionately low calcium 

 concentration. Mineral equilibria indicate near-equilibrium with 

 respect to calcite and aragonite. Nutrient concentrations were low 

 to moderate in comparison to sample sites of this class. Arsenic 

 and boron concentrations were low in comparison to similar sample 

 sites, and with the exception of lead, other trace element 

 concentrations were below detection limits. Lead was reported at 

 the detection limit, one of a handful of sites with reportable 

 concentrations . 



Chemical history. No other water chemistry data from Black Coulee 

 are known. Previous sediment sampling was conducted by the U.S. 

 Fish and Wildlife Service (see below). 



Sediments. Black Coulee was at or near the high end of the range 

 for it's class in a number of trace elements, including arsenic, 

 copper, chromium, and nickel. Western U.S. mean values for soils 

 and other surficial deposits were exceeded for arsenic (by a factor 

 of about 2), boron (2), copper (1.5), and cobalt (1.5). Reported 

 values for nickel and zinc barely exceeded the regional soils 



