Geology, groundwater flow, and water quality in the Walnut Creek watershed

An investigation of the Walnut Creek watershed was performed to characteriz e the geology, groundwater now, and water quality in geological units impac ted by agriculture, Six Quaternary lithostratigraphic units including alluv ium of the Deforest Formation, till of the Doffs and Wolf Creek Formations, and loess of the Peoria, Pisgah, and Loveland Formations, along with two p edostratigraphic 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, sed iment movement into the paleo-valley, and later erosion and deposition by t he Des Moines Lobe. Groundwater flows toward the creek under unconfined con ditions in the fractured, oxidized zone of till of the Doffs Formation, Tri tium 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 shelved NO3-N concentrations a t or above the MCL of 10 mg/L; pesticides were detected in two samples. Som e of the highest concentrations of NO3-N occur in bomb peak groundwater nea r the base of the oxidized zone, This relationship implies that NO3-N leach ing has decreased during a period of increased N application in Iowa. It ma y actually reflect an increase in tile drain efficiency, decrease in ground water recharge, and resultant isolation of NO3-N in less-mobile groundwater , Agricultural practices that further decrease NO3-N leaching will be neede d to improve the quality of shallow groundwater in the watershed.