Nonpoint source contamination of surface and groundwater resources with nitrate-N (NO3-N) has been linked to agriculture across the midwestern USA. A 4-yr study was conducted to assess the extent of NO3-N leaching in a central Iowa field. Water flow rate was monitored continuously and data were stored on an internal datalogger. Water samples for chemical analysis were collected weekly provided there was sufficient flow. Twelve soil cores were collected in spring, early summer, midsummer , and after harvest for each of the 4 yr. Nitrate-N concentrations in shallow groundwater exhibited temporal trends and were higher under Clarion soil than under Okoboji or Canisteo soil. Denitrification rates were two times higher in Okoboji surface soil than in Clarion surface soil and the highest denitrificafion potential among subsurface sediments was observed for deep unoxidized loess. Soil profile NO~-N concentrations decreased with depth and were the same below 30 cm for fertilized corn (Zea mays L.) and soybean (Glycine max L. Merr.). Nitrate-N concentrations in subsurface drainage water exceeded 10 mg L-1 for 12 mo and were between 6 and 9 mg L-1 for 32 mo during the 4-yr study. The temporal pattern of NO3-N concentrations in subsurface drainage water was not related to the timing of fertilizer N application or the amount of fertilizer N applied. Total NO~-N losses to subsurface drains were greatest in 1993 (51.3 kg ha-1) and least in 1994 (4.9 kg ha-l). Most of the subsurface drainage water NO~-N was lost when crop plants were not present (November-May), except in 1993. Our results indicate that NO~-N losses to subsurface drainage water occur primarily as a result of asynchronous production and uptake of NO~-N in the soil and the presence of large quantifies of potentially mineralizable N in the soil organic matter. N ONPOINT SOURCE contamination of surface-and groundwater with NO3-N has been linked to agricultural production in the midwestern USA. This is especially true for surface waters in the upper Midwest due to extensive subsurface draining of the highly productive but poorly drained soils found in this region (Gast et al., 1978). However, the extent to which agriculture contributes to water-quality deterioration is not fully known. In some geographic regions, surface-water NO3-N concentrations in excess of the 10 mg L-1 drinking water standard frequently have been reported (Hallberg, 1986). Keeney and DeLuca (1993) found that NO3-N concentrations in Des Moines river water in central Iowa were above 10 mg L-1 for an average of 14 d per year, generally in the spring. Subsurface drainage water NO3-N concentrations exhibit yearly and seasonal variability (Kladivko et al., 1991). Nitrogen flux to subsurface drains appears to be primarilly a function of precipitation amounts and distribution, and is only slightly affected by crop N up

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The hydrogeology and redox geochemistry of CH4 in two hydrogeological settings within a late Wisconsinan till and loess sequence in central Iowa were investigated using CH4, Eh, dissolved O2, NO3‐N, NH4‐N, dissolved Fe, SO4, H2S, dissolved organic carbon (DOC), particulate organic carbon (POC), δ13C‐CH4, δ2H‐CH4, δ13C of dissolved inorganic carbon (DIC), and 14C−CH4. Depleted δ13C‐CH4 and δ2H‐CH4 values, enriched δ13C‐DIC values, and the lack of higher hydrocarbons suggest that the CH4 is microbial and is formed by CO2 reduction. Carbon 14 dates of CH4 indicate that 14,000‐ to 17,000‐year‐old POC and DOC in the 1oess and till compose the substrate for methanogenesis. Production of CH4 probably began after deglaciation at about 12,000 years B.P. and has continued to the present. Groundwater recharge, particularly in the upland topographic position at the site, has decreased CH4 concentrations in the upper part of the till.

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