Field-scale Spatial Relations Between Surface Topography, Electrical Conductivity, and Superficial Aquifer Ion Concentrations.
Francis Casey and Nathan E. Derby. North Dakota State Univ, 114 Walster Hall, Box 5638, Fargo, ND 58102
A 65 ha, pivot irrigated field, located near Oakes, ND was instrumented with 80 surface aquifer wells spaced on a 100 m grid. The well water was monitored for electrical conductivity (EC), pH, nitrate, nitrite, ammonium, phosphate, sulfate, chloride, alkalinity, calcium, magnesium, sodium, and potassium from December 1992 to April 1993. Apparent soil EC was measured using a Veris apparatus and fine resolution topography was measured using RTK. Correlations were determined between Veris soil EC, topography, and various water quality parameters. Veris EC was found to be highly correlated (inversely) to RTK ground surface elevation, probably from the close proximity of the groundwater to the soil surface resulting in evaporative accumulation of salts. This is substantiated by the slightly higher negative correlations of chloride, sulfate, sodium, and potassium to RTK elevation, i.e. higher concentrations of these in the groundwater under low spots. Sulfate is highly correlated to chloride. High sulfate, sodium, chloride, alkalinity, and EC concentrations correspond to low elevations in the field and to specific soil types in the field. Also, very localized high phosphate concentrations correspond to depressional recharge areas in the field.