Submitted to: Geological Society of America Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 10/31/1996
Publication Date: N/A
Technical Abstract: Confining units, consisting of till, loess, and paleosol units of Wisconsinan, Illinoian, and Pre-Illinoian age, overlie bedrock aquifers in Iowa. For many hydrogeologists, these confining units are an "impermeable" mass of sediment possessing a narrow range of hydraulic conductivity (K) values and hydrogeochemical characteristics. In contrast, our research suggests that the physical and biogeochemical character of confining units is complex and that their Quaternary history provides a key to understanding processes there and in the underlying aquifers. Research funded by the USDA Management Systems Evaluation Area (MSEA) project in central and northeastern Iowa supports our suggestion. First K values (determined by slug and pumping tests) of the Quaternary units range from 10**-12 (Yarmouth-Sangamon paleosol and unoxidized Pre-Illinoian till units) to 10**-5 m/s (near-surface, factured, late Wisconsinan till units). Second, large areal and vertical variability in the distribution of Quaternary units is controlled by paleotopography. Third, Quaternary units exhibit a large range in the concentration of dissolved organic carbon (DOC; 2 to 88 mg/L) and particulate (solid) organic carbon (POC; 0.2 to greater than 2 percent). Organic carbon (C) occurrence is controlled by soil forming episodes during interglacial periods and subsequent erosion and re-deposition of sediment into paleotopographic lows. Variations in organic C help explain why some Quaternary units produce methane (about 2500 umol/L), while other units do not appear to have progressed geochemically past denitrification. Late Wisconsinan and Pre-Illinoian till units also show a range of sorption capacity for atrazine, a common corn herbicide.