Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: December 10, 2002
Publication Date: December 10, 2002
Citation: TOMER, M.D., BURKART, M.R. DISCERNING EFFECTS OF CROP-MANAGEMENT HISTORY ON GROUNDWATER NITRATE CONCENTRATIONS. AMERICAN GEOPHYSICAL UNION. 2002. PAPER NO. H61C-0792. Technical Abstract: Changes in agricultural management can minimize leaching of NO3-N to groundwater. However, the time needed to realize improvements in groundwater quality after management change is uncertain. This study was conducted in two small, first-order watersheds (30 and 34 ha) in the Loess Hills of southwest Iowa. They were similarly managed from 1964 through 1995, except one received large fertilizer-N applications, averaging 446 kg ha**-1 y**-1, between 1969 and 1974. This study's objective was to determine if NO3-N from these large applications persisted in groundwater. Transects of piezometer/lysimeter nests were installed, deep cores collected, and water levels and NO3-N concentrations were measured monthly. In June 2001, 33 water samples were collected and analyzed for **3H and stable isotopes. The watershed that received large N applications had greater NO3-N concentrations in groundwater and stream baseflows. Groundwater time-of-travel estimates and tritium data support persistence of NO3-N from the historical applications. "Bomb-peak" precipitation (1963-1980) influenced tritium concentrations, especially near toeslope positions, while deep groundwater was dominated by pre-1953 precipitation. Data from analysis of deep cores suggest NO3-N may take 30 years to percolate to groundwater below the watershed's divide. Isotope data suggest runoff/infiltration processes contribute greater recharge and mixing of groundwater below the toeslope. Therefore, historical and current practices affect NO3-N concentrations in groundwater near the stream. Impacts of management systems implemented in 1996 will not be clearly discerned by monitoring groundwater for many years. In many areas, changes in agricultural practices may take decades to fully impact groundwater quality.