Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/1999
Publication Date: N/A
Interpretive Summary: This paper presents information about geological units and groundwater movement likely to be affected by agricultural activities in the Walnut Creek Watershed. The units are made up of very young glacial till deposited by the most recent glacial advances in the region, wind blown materials deposited between glacial advances and much older till overlying bedrock. Groundwater flow of greatest magnitude and most affected by agriculture occurs in the oxidized upper part of the younger till. Water in this unit ultimately flows to tile drains and Walnut Creek. Estimates of groundwater velocity indicate that water flows through this unit in less than 73 years. Concentrations of nitrate in the upper till were consistently at or exceeding the maximum contaminant level for drinking water of 10 parts per million. Herbicides were detected much less frequently than nitrate. The information should be useful to water supply managers, agricultural and environmental policy makers, and scientists attempting to design agricultural systems to reduce leaching of nitrate.
Technical Abstract: The distribution of Quaternary units was characterized from 17 continuous cores and groundwater flow and water quality were assessed in the units most likely to be impacted by agricultural practices. Six Quaternary lithostratigraphic units were found: DeForest Formation (Holocene) alluvium; Dows Formation (late Wisconsinan) till; loess of the Peoria Formation and Pisgah Formation (Pre-Illinoian) till. Two pedostratigraphic units, the Farmdale and Sangamon Geosols, were also identified. The distribution of the units in the watershed is controlled by a buried valley that formed before the late Wisconsinan. The primary surficial geologic unit is Dows Formation till. Groundwater flows primarily in the oxidized zone of that unit and discharges into tile drains and Walnut Creek. Residence time of groundwater in the oxidized zone in less than 73 years as determined by Tritium data and average linear velocity estimates. Groundwater is generally very young at or near the water table, and is somewhat older at the base of the oxidized zone. Concentrations of NO3-N in the till were consistently at or above the Maximum Contaminant Level (MCL) of 10 mg/L; herbicides were detected less frequently. NO3-N in shallow groundwater is derived from leaching of surface applied nitrogen; thus, tillage or fertilizer application practices that decrease leaching will be needed to improve the quality of shallow groundwater in the watershed.