U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Effects of Topography on the Transport of Agricultural Chemicals near Princeton, Minnesota, 1992

Abstract

In 1991, the U.S. Geological Survey (USGS), with funding from the USGS Toxic Substances Hydrology Program, began studying the movement of water and agricultural chemicals at the Management Systems EvaluationArea (MSEA) near Princeton, Minnesota. The research is being conducted in a topographically low (lowland) site and a topographically high (upland) site within the northernmost cropped area at the 65-hectare Princeton MSEA. The sites are about 78 m (meters) apart and with a difference in land-surface elevation of 1.4 meters. Soils in the upper meter at both the upland and lowland sites are similar and are composed of about 95 percent sand and 5 percent silt and clay. The grain size is coarser between the 1.0- and 2.0-m depths at the lowland site than at the upland site, however. Total organic carbon in the upper 20 cm of topsoil is about 1.0 percent at the lowland site and about 0.6 percent at the upland site. Water sampling and monitoring equipment were installed at the upland and lowland sites following a dye-tracing and trenching study that was conducted to identify zones of preferential and retarded water movement and to collect soil samples from the unsaturated-zone.

Infiltration tests were conducted during 1992 at the upland and lowland sites following selected precipitation events or by the application of 2.5 cm of water from a linear-move sprinkler irrigation system. The movement of wetting fronts through the unsaturated zone to the water table were evaluated using time-domain reflectometry. Water samples were collected from suction lysimeters as the wetting front passed a given plysimeter elevation, and from water-table weds after the wetting front reached the saturated zone.

Wetting fronts typically penetrated deeper into the unsaturated zone at the lowland site than at the upland site during 1992. In addition, the total flux of water into the soil at the lowland site was greater than at the upland site. Following application of about 8 cm of water beginning June 29, 1992, for example, recharge through the unsaturated zone was about 1.0 cm at the lowland site compared to about 0.5 cm at the upland site, based on hydrograph analysis. For this recharge event, the estimated fluxes of nitrate nitrogen and atrazine through the unsaturated zone to the water table at the lowland site were 3.5 and 5 times greater, respectively, than the fluxes at the upland site. Differences in the movement of wetting fronts, agricultural chemicals, and tracers at the upland and lowland sites likely resulted from a combination of factors including differences in soil organic matter, grain size, porosity, sedimentary heterogeneities, antecedent moisture conditions, and topographic relief. Possible mechanisms that could cause focused recharge of water and agricultural chemicals at the lowland site include surface runoff and iron-rich layers that form a barrier to vertical water movement through the unsaturated zone at the upland site.