Electrical Conductivity of Agricultural Drainage Water in Iowa

Abstract

<p>Assessing the effectiveness of management strategies to reduce agricultural nutrient efflux is hampered by the lack of affordable, continuous monitoring systems. Generalized water quality monitoring is possible using electrical conductivity. However environmental conditions can influence the ionic ratios, resulting in misinterpretations of established electrical conductivity and ionic composition relationships. Here we characterize specific electrical conductivity (k₂₅) of agricultural drainage waters to define these environmental conditions and dissolved constituents that contribute to k₂₅. A field investigation revealed that the magnitude of measured k₂₅ varied from 370 to 760 µS cm^-1. Statistical analysis indicated that variability in k₂₅ was not correlated with drainage water pH, temperature, nor flow rate. While k₂₅ was not significantly different among drainage waters from growing and post-growing season, significant results were observed for different cropping systems. Soybean plots in rotation with corn had significantly lower conductivities than those of corn plots in rotation with soybeans, continuous corn plots, and prairie plots. In addition to evaluating k₂₅ variability, regression analysis was used to estimate the concentration of major ions in solution from measured k₂₅. Regression results indicated that HCO₃^-, Ca²⁺, NO₃^-, Mg²⁺, Cl^-, Na²⁺, SO₄^2- were the major drainage constituents contributing to the bulk electrical conductivity. Calculated ionic molal conductivities of these analytes suggests that HCO₃^-, Ca²⁺, NO₃^-, and Mg²⁺ account for approximately 97% of the bulk electrical conductivity.</p>