Agricultural effects on shallow ground-water quality were the focus of two land-use studies conducted in the WMIC during Cycle 1. Water-quality and geohydrologic data were collected between September, 1993 and September, 1994 from 56 wells and 2 springs, in two agricultural areas in the study unit. These data were used to study the effect of land use and geohydrology on shallow ground-water quality. The two areas chosen for study had similar agricultural land uses but different geohydrologic characteristics (click thumbnail to the right or see figure 2 in Saad, 1997). The till land-use study area (area 1) is located in the southeast part of the WMIC. The sand and gravel land-use study area (area 2) is located along the western boundary of the WMIC. Water samples were analyzed for the same constituents as the major aquifer study wells in addition to oxygen and hydrogen isotopes, uranium, and chlorofluorocarbons. All sampled wells and springs were located down gradient from farm fields having similar crop-rotation patterns, mainly corn and alfalfa.

Area 1 is characterized by unsorted mixtures of sand and clay surficial deposits (till) overlying carbonate bedrock, and area 2 by sand and gravel deposits overlying sandstone or crystalline bedrock. The depth to water was significantly deeper and hydraulic conductivity significantly higher in area 2. Results of water-quality analyses indicate that agricultural land use has affected the groundwater quality of both study areas, however, Wisconsin ground-water-quality enforcement standards were exceeded in only 22 percent of samples for dissolved nitrate and 2 percent of samples for dissolved atrazine and its breakdown products.

The more permeable and lower organic matter content of surficial deposits in area 2 resulted in significantly higher concentrations of dissolved nitrate and atrazine (and its breakdown products) in shallow ground water than area 1. Ground-water recharge dates (estimated using chlorofluorocarbons and tritium) showed that historic patterns of atrazine concentrations in ground water mimic historic patterns of atrazine use on corn.

Beginning in Cycle 2, the land-use area 2 wells were chosen to be part of the NAWQA ground-water trends networks that are to be sampled decadally. During summer 2002, water samples from 26 wells in the network plus two nearby reference wells (located in forested areas) were analyzed again for major ions, nutrients, organic carbon, pesticides, and trace elements.

Beginning in fall 2004, a subset of 5 land-use study wells plus one reference well, chosen to represent the water quality of the entire network and background conditions, were sampled quarterly. The subset will also be sampled biennially from 2005 to 2013. The decadal, biennial, and quarterly sampling strategy is designed to help understand the factors that affect long-term and short-term trends and seasonal variability in ground-water quality.

A detailed analysis of trends in ground-water quality for this study area is described by Saad (2008). The paper describes trends in nitrate and atrazine concentrations in ground-water based on data collected from the Area 2 land-use and flowpath studies.

Results of Cycle 1 and Cycle 2 decadal samples can be found under Data.  Cycle 1 data are also published in Saad (1997) and Cycle 2 data are also published online at: http://pubs.er.usgs.gov/pubs/wdr/wdrWI021 (tip: search for “land-use study”).

References:

Saad, David A., 2008, Agriculture-Related Trends in Groundwater Quality of the Glacial Deposits Aquifer, Central Wisconsin: Journal of Environmental Quality 37:S-209-S-225, doi:10.2134/jeq2007.0053 and http://jeq.scijournals.org/cgi/content/abstract/37/5_Supplement/S-209.

Saad, D.A., 1997, Effects of Land Use and Geohydrology on the Quality of Shallow Ground Water in Two Agricultural Areas in the Western Lake Michigan Drainages, Wisconsin: Water-Resources Investigations Report 96-4292

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