Paper No. 7
Presentation Time: 3:20 PM
HOLISTIC HYDROGEOLOGY: ELUCIDATING A > 500 KA RECORD OF CHEMICAL AND PHYSICAL INTERACTIONS IN A QUATERNARY AQUITARD/AQUIFER SYSTEM
A holistic hydrogeological approach was used to elucidate a >500 kA year record of chemical and physical interaction in a Quaternary aquitard/aquifer system in central Iowa. The “overburden” aquitard, consisting of up to 100 m of late Wisconsinan till (Dows Formation), loess of the Peoria and Pisgah Formations, (Yarmouth) Sangamon Geosol, and Pre-Illinoian till, provided the impetus for this approach. Sand and gravel aquifers composed of recent alluvium, DeForest (Holocene alluvium) and Noah Creek Formations (Quaternary outwash), and buried channel deposits of Pre-Illinoian age occur within the aquitard. Quaternary biological and sedimentary processes created and maintain the geochemistry of the aquitard and underlying aquifers. The 14 kA advance of the Des Moines Lobe – and simultaneous incorporation of a coniferous forest into till of the Dows Formation – placed up to 8 g organic C/kg in the upper part of the aquitard, producing extremely reducing groundwater. Concentrations of CH4 up to 2600 μmol/L and dissolved Fe up to 5 mg/L occur in groundwater about 4 m from the ground surface; dissolved O2, NO3-N, and SO4 are absent below that depth. As a result, NO3-N from agricultural activity is removed from groundwater in this zone and in the underlying Ames aquifer and Mississippian aquifer due to dissolved organic C translocated from the aquitard. Quaternary sedimentary processes also created the Ames aquifer, source of high quality drinking water to the City of Ames, and controlled its thickness, hydraulic parameters and spatial distribution. In Downtown Ames, the 22-m-thick, Ames (outwash) aquifer is directly overlain by 4 m of Peoria Formation loess (16,760 ± 80 RCYBP; Beta-254236), suggesting that the Ames aquifer is Pre-Illinoian (> 500kA) in age and that the Des Moines Lobe advanced onto a loess-capped, high K (10-3 m/s), sand and gravel aquifer that extends regionally in buried valleys. The unique coincidence of Pre-Illinoian and Wisconsinan outwash along present river valleys has produced a > 30-m-thick unconfined aquifer that is the envy of every water utility in the region. These examples illustrate why hydrogeologic investigations must move away from the traditional (and boring) “hydrogeology of overburden” paradigm toward a more holistic hydrogeologic approach in glaciated regions.