Using Specific Conductance As a Tracer of Preferential Flow Through Glacial till Soils of Iowa.

In agricultural areas with soils developed on glacial till deposits, the presence of artificial subsurface drainage networks provides an efficient flowpath for water movement to the stream. In this study, the soil preferential flow network (i.e., macropores, desiccation cracks) was shown to be an important water flowpath to the subsurface drain for a field in north-central Iowa, part of a 31 km² watershed of the South Fork Iowa River. A continuous specific conductance (SC) monitor in a subsurface drain outlet, in combination with precipitation measurements, was used to quantify the importance of preferential flowpaths to the subsurface drain over the course of two years. Within a few hours after measurable rain or large snow melting events, the subsurface drain SC decreased (at times <200 µS/cm). These fast SC decreases occurred frequently throughout the year, giving evidence of a persistent preferential flow network. Continuous stream SC and discharge also were measured. The pattern and timing of the change in SC were similar for both the stream and subsurface drain, suggesting that soil preferential flow, via subsurface drains to the stream, is important for the entire watershed. Based on continuous and discrete SC measurements, the characteristic mean SC values for precipitation (12 µS/cm), overland flow <12 h (91 µS/cm), a temporary pond (129 µS/cm), overland flow >12 h (264 µS/cm), unsaturated zone (456 µS/cm), subsurface drain (654 µS/cm), stream (681 µS/cm), and groundwater >2.8 m (863 µS/cm) were also determined. The characteristic mean SC values demonstrate that the dissolved ionic concentrations increase with a longer contact time and a higher degree of interaction with the soil. Given that preferential flow has a short contact time with soils, the degree of natural attenuation for dissolved agrichemicals (such as nitrate, atrazine, metolachlor) is expected to be small in fields with substantial subsurface drainage.