METHANE PRODUCTION AND GROUNDWATER QUALITY IN SURFICIAL AQUITARDS AND BEDROCK AQUIFERS IN NORTH-CENTRAL IOWA

Research in Iowa during the past 15 years has shown that organic C in the surficial aquitard of north-central Iowa, composed of late Wisconsin till (Dows Formation) and loess of the Peoria and Pisgah Formations, drives methane production in those units.  The aquitard contains up to 8 g C/kg and DOC concentrations up to 54 mg/L, causing depletion of electron acceptors, C oxidation, and ultimately production of methane.  Within the Des Moines Lobe, high concentrations of methane (up to 2600 μmol/L) and dissolved Fe (up to 5 mg/L) begin at depths of about 4 m; dissolved O₂, nitrate, and sulfate are absent generally below that depth.  Above-ambient concentrations of N₂O in groundwater (~ 0.5 μmol/L) suggest that denitrification is an important process.  Carbon isotopic evidence (d¹³C = -26 ‰ and ¹⁴C ≈ 16kA) from DOC and methane suggests that organic C is transported from the aquitard into the underlying Mississippian aquifer, where it promotes a similar geochemical environment for denitrification and methane production.  Preliminary simulations of the groundwater flow system indicate that transport of organic C from recharge areas into the Mississippian aquifer is possible within the time frame necessary since aquitard deposition.  Vertical transport may be inhibited in some areas by thick overlying Pre-Illinoian till units or Pennsylvanian shale.  Despite the hope of farmers to extract gas for additional on-farm income, heterogeneity and fractures in the aquifer probably preclude accumulation in enough quantity to make methane an economically viable resource.  Nevertheless, reducing conditions are still important in maintaining water quality in Iowa groundwater, particularly in the face of widespread “leakage” of nitrogen from intensive agriculture.