Big Sandy DOT Mitigation Wetland 



Climatic setting. The net annual precipitation balance for the Big 

 Sandy site (from the MAPS database) is -26 inches, placing it 

 toward the middle of the range for sample sites with which it is 

 classified. Mean monthly precipitation shows a well-defined June 

 maximum. The three-year moving average of annual total 

 precipitation remained near or above the long-term average for most 

 of the 1975-1990 period. The annual total in 1993 (the year prior 

 to sampling) exceeded the long-term average by about 5 inches. 



Geologic setting. The WET07 site is underlain by fine-grained 

 paludal deposits capping the in-filled channel of the preglacial 

 Missouri River. Geologic mapping shows stratified ice-contact 

 deposits (kame terrace and esker) immediately upslope of the site, 

 which may receive ground-water discharge from these sediments. 100 

 feet or more of alluvial fill underlies the paludal deposits in 

 this area; this alluvium provides ground-water of variable water 

 guality. The bedrock surface underlying the surficial deposits is 

 comprised of the late Cretaceous Claggett Shale Judith River 

 Formation. 



Hydrologic type. Although located along a major intermittent(?) 

 stream, the immediate sample site is an excavated basin reportedly 

 lacking surface water outflow. Inflow is reported to be from 

 ground-water sources. 



Basin characteristics. The catchment to wetland area ratio for 

 this site is very high if Big Sandy Creek contributes to the 

 wetland 's water balance. The reported lack of surface water inflow 

 indicates the site may not be integrated into the Big Sandy 

 drainage. Artificial excavation as a highway mitigation site is 

 inferred to create disturbed and probably transient substrate 

 conditions. 



Water chemistry. Water of the Big Sandy wetland was of a 

 relatively dilute (particularly for a Great Plains site), 

 moderately alkaline sodium bicarbonate character. The WET07 

 analysis is at the lower end of the dissolved solids range reported 

 for ground-water from surficial deposits in the area, and 

 considerably more sodic in composition than similarly dilute 

 ground-water samples. This distinction suggests shallow 

 groundwater may not be the primary water source for the wetland. 

 Mineral equilibria indicate oversaturation with carbonate species. 



Nitrogen (nitrate plus ammonium), phosphorous (total and 

 orthophosphorous ) and total organic carbon concentrations are all 

 toward the lower end for the class including WET07. Selenium was 

 reported at the analytical detection limit; water column selenium 

 was reported at only four other sample sites . Arsenic and boron 

 concentrations were toward the low end of the range for site of 

 this class. Most other trace elements were below detection limits. 



