Central Nebraska Basins NAWQA Study Unit

The occurrence of agricultural chemicals in shallow ground water beneath glaciated areas, northeast Nebraska

The ground-water status component of NAWQA is intended to identify the status of the water quality of the nations ground water and to provide a base line for water-quality information (2003). Identification of ground-water status can enable resource managers and polycymakers to assess the value of programs and decide if specific conservation and farming practices need to become implemented. A national ground-water trends network includes sampling sites (wells) initially visited as part of (1) major aquifer studies or (2) studies of shallow ground water beneath specific land uses (urban and agriculture).

In the Central Nebraska Basin and Range (CNBR) study area of the NAWQA program initial agricultural land-use effects to study the movement of agricultural land-use effects study began in 2002 with the installation of 31 monitoring wells (including 2 reference monitoring wells) in what is predominantly the lower Elkhorn River Basin (map) and area with a land use of greater than 20 percent corn production and 20 percent soybean production. In 2003, these 31 sites were sampled for water-quality constituents, including agricultural chemicals.

Sampling locations were randomly selected throughout the study area and during the summer of 2002, 31 monitoring wells, including two reference or background wells were installed. Soil samples were collected from the soil column removed during drilling to determine how soil characteristics varied with depth. In addition soil samples were analyzed for nutrients, chloride, and physical properties related to permeability and soil moisture. Ground-water samples, collected from each monitoring well in spring 2003, were analyzed for major ions, nutrients, trace elements, dissolved organic carbon (DOC), metabolites pesticides and metabolites. In addition, one-quarter of the samples were analyzed for about 120 stable isotopes and indicators of the residency age of the ground water. Beginning in October 2004, samples will be collected quarterly from a subset of these wells, and beginning in 2006 this subset will be sampled once every 2 years to monitor trends.

The results of the sample analyses will be evaluated to address four questions: (1) Occurrence – which constituents were detected and at what frequency? (2) Magnitude – which constituents occurred in the largest concentrations, and which were at concentrations of concern relative to water-quality criteria? (3) Spatial distribution – are there spatial patterns of constituent occurrence and do the patterns correlate with the factors that affect shallow ground-water quality? (4) Background – how do the levels of constituent concentrations compare with those found at sites in non-cropland areas (reference sites)?

Contact:
Greg Steele, Ronald B. Zelt

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