Management Systems and Ground Water Atrazine Concentrations

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Scattered across the northern cornbelt are extensive areas of sandy soils with shallow water tables beneath them. The highly permeable soils, combined with the relatively short distance to the water table, make these outwash sand plains particularly sensitive to contamination by atrazine and other agricultural chemicals.

How vulnerable are the aquifers beneath these areas to contamination by herbicides such as atrazine, alachlor (Lasso) and metribuzin (Sencor)? How do different farming systems affect the amounts and types of chemicals that reach ground water? Can improved water management reduce the amounts of chemical leachates in shallow ground water? These types of questions are behind the research being conducted for the Northern Cornbelt Sand Plain MSEA project.

There are two basic ways to keep atrazine out of our ground water and both are being used in the MSEA project. The first is to reduce the amount of atrazine applied, either by reducing broadcast rates or by using banded applications. The second is to keep the atrazine in the biologically active root zone as long as possible because that is where its rate of degradation is high. Once atrazine reaches the less aerobic conditions of an aquifer, degradation slows greatly and its presence can persist for years.

More specifically, banded applications of atrazine in a ridge-tilled corn and soybean farming system, combined with improved water management, will help ensure that atrazine concentrations remain below the maximum contaminant level of 3.0 parts per billion.

The MSEA project includes research sites on outwash sand plains in Minnesota, North Dakota, South Dakota and Wisconsin. The 160 acre main research site is on the 1,700 square mile Anoka Sand Plain, near Princeton in east central Minnesota. Three satellite sites are located near Oakes, North Dakota (Oakes Aquifer); Aurora, South Dakota (Big Sioux Aquifer); and Arena, Wisconsin (Lower Wisconsin River Flood Plain).

Maximum allowable contaminant levels of 3.0 parts per billion of atrazine in ground water were not exceeded at any of the four locations. Atrazine from the farming systems has not reached the surfical aquifer at Minnesota and North Dakota. At Wisconsin and South Dakota, however, aquifer samples indicate that atrazine used in the farming systems has reached the ground water. But these atrazine concentrations, when detected, rarely exceeded 1.2 parts per billion.

Conditions at the research sites

Depth to the water table in the MSEA project sites range between five and 20 feet, depending on location and time of year. Surface soil textures are sandy at the Minnesota, North Dakota and Wisconsin sites. South Dakota's site has silty clay loam, to a depth of three to four feet, underlaid with gravel outwash.

Various farming systems are being evaluated at the different sites, with ridge-tillage corn-soybean rotation common at all. The rotation, established in 1991 at all locations, uses preplant applications of atrazine and alachlor, ridge tillage, irrigation scheduling, banded herbicide applications and judicious nitrogen additions to significantly reduce chemical inputs. Atrazine was applied in a band (covering approximately one-third of the land surface) at broadcast rates of 0.75 pounds actual ingredient per acre at Wisconsin, 1.0 at North Dakota, 1.5 at Minnesota, and a range of from 3.0 to 5.0 at South Dakota.

Ridge-tillage was chosen for the MSEA project sites because it provides a nearly ideal water flow system for side-dressed nitrogen fertilizer and band-placed herbicides. Less water infiltrates and flows through the ridge of such a system than in the furrow. Corn and soybean management system blocks vary in size from 4.4 acres at the main site to 1.5 acres or less at the satellite sites. For comparison purposes, Minnesota's main site also has a continuous corn system (conventional system) and a potato-sweet corn rotation system (intensive management and input system).

To evaluate atrazine's fate as it moves through the soil and breaks down into residual chemicals, soil samples and aquifer water samples are regularly taken and analyzed. Soil samples are taken six times a year to a depth of three feet. Ground water samples are taken at least four times a year from both the surface and lower depths of the surfical aquifers.

Measuring water quality

The impact of management systems on ground water quality is being evaluated with measurements obtained through the use of multi-level ground water samplers installed at various depths in the aquifer. These samplers are placed in sampling wells located in the center and just beyond both ends ("upgradient" and "downgradient" relative to the direction of ground water flow) of each crop system block. They provide information needed to evaluate changes in ground water quality. Impacts were also measured indirectly in each field by sampling the soil to a depth of three feet before and during the growing season.

More information?

To obtain other MSEA project fact sheets, or if you have questions about the project, contact:

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