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
by
Geoffrey N. Delin (U.S. Geological Survey, Mounds View, Minn.)
and Matthew K. Landon (U.S. Geological Survey, Mounds View, Minn.)
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.
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