CHEMISTRY OF SEEP DRAINAGE IN SOUTHWESTERN NORTH DAKOTA

Abstract

We analyzed effluents from subsurface drains installed to intercept flow from two saline seeps to evaluate chemical composition as a function of time and drain flow rate. The seeps were discharges of saline groundwater on upland hillside sites as contrasted to lowland or depressional sites that impound water. Electrical conductivity of the drain water ranged from 7 to 13 millimhos per centimeter. Magnesium sulfate and sodium sulfate were the predominant dissolved salts, calcium concentrations were consistently low, and nitrate concentrations were high enough to make the waters unfit for human consumption and a potential hazard to the health of livestock. The nitrate seemed to come from two sources—exchangeable ammonium of geologic origin oxidized to nitrate deep in the profile and nitrate leached from the root zone during the fallow period. Salt concentrations were not related to drain flow rate. During the 5-year study, effluent from the drains amounted to a surface-depth equivalent of 6.9 centimeters of water for the recharge area and contained the equivalent of 6100 kilograms of dissolved salt and 50 kg of nitrogen per hectare of recharge area. However, two-thirds of both the water and the salt were discharged during 1 year. Even though seeps can be hydrologically controlled by installing interceptor drains, drainage may be environmentally unacceptable because of the dissolved salt load. The best approach is to utilize the soil water while it is a nonsaline resource in the root zone of the recharge area.