Simulation of Selenium Discharge from Subsurface Drainage Systems
Investigators (most current known information)
Selenium, salinity and sodium concentrations were measured in a collapsed drift geologic formation to assess the potential impact of the soil chemical components on return flow water quality from the proposed Lake Andes-Wagner Irrigation Project in south central South Dakota. Soil samples were collected from 0.0-0.5, 0.5-1.0, 1.0-2.0 and 2.0-3.0 m depths every 200 m along a 26.7-km transect. No trends were detected in the magnitude of chemical concentrations along the transect. Spatial correlation (structure) was found to occur for sodium adsorption ratio (SAR) values with distances of 500 m or less and for selenium and salinity parameters at soil sample spacings of 200 m or less. Selenium concentrations tend to be similar and the highest for the 1.0-2.0 m and 2.0-3.0 m depths while the 0.0-0.5 m depth contains the smallest concentrations. Mean total selenium concentrations are 894, 965, 1346 and 1674 ppb for the four soil depths. Readily available selenium is about 8% of the total selenium in the top 0.5 m soil layer, 22% in the 0.5-1.0m layer, 37% in the 1.0-2.0 m layer and 38% in the 2.0-3.0 m layer. Log normal probability density functions can be used to describe selenium concentrations and profile SAR values for the soil. Soil profile electrical conductivity values can be described by a normal probability function.
A two parameter Gamma Distribution Function was used for the development of a transfer function to describe the discharge of selenium from a single subsurface drain line. Relationships were developed for the two parameters of the Gamma Function and geometries of subsurface drainage systems. The frequency of drain line spacings anticipated for the Lake Andes-Wanger Project area was defined by a log normal probability density function. A Monte Carlo simulation was then used to obtain an estimate of selenium discharge from a region that contains a variety of drain line spacings. The basic assumption associated with the simulation results is that piston or bulk water flow controls the movement of selenium to the drain line and the influence of diffusion, dispersion and chemical reactions in the soil need to be defined before accurate estimates of selenium discharges from subsurface drainage systems are possible.
Articles in Journals
DeBoer, D.W., J.A. Doolittle, A.G. Bly and R.A. Kohl. 1994. "Salinity and selenium distributions in glacial till." ASAE Paper No. 9421009. ASAE. St. Joseph MI.
DeBoer, D.W. and W. Warrick. 1994. "Selenium concentrations in subsurface drainage water." ASAE Paper No. 942625. ASAE. St. Joseph MI.