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 Farming Systems on Ground-Water Quality at the Princeton, Minnesota, Management Systems Evaluation Area, 1991

Abstract

The 65-hectare Management Systems Evaluation Area (MSEA) near Princeton, Minnesota is one of five primary MSEA's in the Midwest Cornbelt. The Princeton MSEA program is a cooperative research effort among the U.S. Department of Agriculture-Agricultural Research Service, the University of Minnesota Soil Science Department, and the U.S. Geological Survey. The primary program objective is to evaluate the effects of ridge-tillage practices in corn and soybean farming systems on ground-water quality in a sand-plain setting. A sweet-corn and potato farming system is also being evaluated. Analyses of samples collected at the beginning of the study during April 1991 indicated that some contamination of ground water with nitrate nitrogen (nitrate-N), atrazine, and atrazine metabolizes had already occurred before implementation of the MSEA farming systems.

Chloride in potash fertilizers applied to the MSEA cropped areas in April 1991 was used as a tracer for ground water affected by the MSEA farming systems. Chloride reached ground water beneath all of the cropped areas. Analyses of ground-water samples collected during June, August, and December 1991 and during April 1992 indicated that concentrations of chloride in the upper meter of the saturated zone beneath and downgradient of the cropped areas were 20 to 50 mg/L (milligrams per liter) compared with background concentrations of 2 to 19 mg/L.

Because the nitrogen and potash fertilizer application rates on the potato-cropped area were 1.4 and 4.5 times greater, respectively, than on the other cropped areas, nitrate-N and chloride concentrations were greatest beneath the potato-cropped area. Nitrate-N concentrations in the upper meter of the saturated zone beneath the potato-cropped area (22-47 mg/L) were greater than concentrations measured upgradient (6-18 mg/L) from the cropped area. Nitrate-N concentrations were similar upgradient, beneath, and downgradient of the other cropped areas. Thus, additions of nitrate-N as a result of MSEA activity were difficult to distinguish from background concentrations. Concentrations of nitrate-N in the saturated zone across the entire MSEA typically decreased from 15 to 25 mg/L near the water table to 5 to 10 mg/L two meters below the water table.

Atrazine and atrazine metabolites de-ethylatrazine (DEA) and de-isopropylatrazine (DIA) were detected in 75, 97, and 76 percent, respectively, of the 124 wells sampled at the Princeton MSEA during the four sampling periods from June 1991 through April 1992. The concentrations of these compounds in the saturated zone varied temporally. Atrazine was detected in 51 percent of the 315 ground-water samples collected in concentrations ranging from less than 0.01 µg/L (micrograms per liter) to 0.54 µg/L. The median concentration of atrazine was the detection limit of 0.01 µg/L. DEA was detected in 74 percent of the samples at concentrations ranging from less than 0.03 µg/L to 1.00 µg/L. The median concentration of DEA was 0.08 µg/L. DIA was detected in 10 percent of the samples in concentrations rangingfrom less than 0.06 µg/L to 0.66 µg/L. The median concentration of DIA was less than the detection limit of 0.06 µg/L. Alachlor was detected in 2 percent of the samples. Concentrations of DEA in April 1992 increased with depth from less than 0.06 µg/L near the water table to 0.08 to 0.30 µg/L between 1 and 2 m below the water table. This trend with depth was not noticeable prior to April 1992 and could indicate that water recharging the aquifer beneath the cropped areas during 1991 had lower DEA concentrations than older ground water.

Because some of the applied fertilizers and herbicides and herbicide metabolites existed in shallow ground water at the onset of this study, it is difficult to assess the affects of MSEA farming systems on water quality without additional data collected over several years. None of the detections of herbicides or herbicide metabolites can be conclusively linked to the MSEA farming systems at this time. Because some atrazine and atrazine metabolites were detected in ground water that was recharged through the MSEA cropped areas, however, the possibility that some of these chemicals leached from the MSEA farming systems to ground water cannot be discounted. Use of conservative tracers like chloride and bromide to delineate annual recharge fronts beneath the MSEA cropped areas should make it easier to identify the affects of MSEA farming systems on ground-water quality as older ground water affected by previous farming practices moves horizontally and vertically away from the cropped areas.