Geology, Groundwater Flow, and Water Quality in the Walnut Creek Watershed

Abstract

Abstract An investigation of the Walnut Creek watershed was performed to characterize the geology, groundwater flow, and water quality in geological units impacted by agriculture. Six Quaternary lithostratigraphic units induding alluvium of the DeForest Formation, till of the Dows and Wolf Creek Formations, and loess of the Peoria, Pisgah, and Loveland Formations, along with two pedostratigraphic units, the Farmdale and Sangamon Geosols, were identified. The thickness and spatial distribution of the units were controlled by the formation of a paleo‐valley before the advance of the Des Moines Lobe, sediment movement into the paleo‐valley, and later erosion and deposition by the Des Moines Lobe. Groundwater flows toward the creek under unconfined conditions in the fractured, oxidized zone of till of the Dows Formation. Tritium data indicate that recent recharge occurs at the water table and that the 1963 bomb peak occurs near the bottom of the oxidized zone. Fifty‐five percent of groundwater samples taken in 1995 showed NO 3 ‐N concentrations at or above the MCL of 10 mg/L; pesticides were detected in two samples. Some of the highest concentrations of NO 3 ‐N occur in bomb peak groundwater near the base of the oxidized zone. This relationship implies that NO 3 ‐N leaching has decreased during a period of increased N application in Iowa. It may actually reflect an increase in tile drain efficiency, decrease in groundwater recharge, and resultant isolation of NO 3 ‐N in less‐mobile groundwater. Agricultural practices that further decrease NO 3 ‐N leaching will be needed to improve the quality of shallow groundwater in the watershed.