National Water Quality Assessment Program: Eastern Iowa Basins

Water Quality Monitoring of the Cambrian-Ordovician Aquifer System in Iowa and Illinois

Kimberlee K. Barnes, U.S. Geological Survey, Iowa City, IA

Program abstracts of the National Water Quality Monitoring Council meeting, May 7-11, 2006, San Jose, CA

The Cambrian-Ordovician aquifer system in the upper Midwest United States was selected as one of 16 aquifers to be studied as part of the U.S. Geological Survey's National Water Quality Assessment (NAWQA) Program's regional assessment of ground-water quality conditions and trends. In 1990, these 16 aquifers accounted for approximately 75 percent of estimated ground-water withdrawals for drinking-water supply in the United States. In 1985, a little over half of the total fresh ground-water withdrawals from the Cambrian-Ordovician aquifer system were used for public supply. The purpose of the regional assessment is to provide additional information to supplement data collected from 1991-2001 for the NAWQA program on ground-water quality. In addition, this study will focus on the particular water-quality issue in the aquifer of mixtures of urban pesticides and radon. Initial data collection for the study began in Iowa in 2005, with subsequent sampling in Illinois scheduled for 2009.

The Cambrian-Ordovician aquifer system covers approximately 177,000 square miles in the upper Midwest. Cambrian-Ordovician rocks are present throughout Iowa except in the extreme northwestern corner. The rocks outcrop in the northeastern corner and dip southwestward under Silurian and younger age rocks. The Cambrian-Ordovician aquifer system consists of sandstone in the lower part and sandstone and shale interbedded with limestone or dolomite in the upper part creating multiple aquifer units. During the months of June, July, and August 2005, ground water from 30 public-supply wells across Iowa was sampled for nutrients, selected trace elements, pesticides, volatile organic compounds (VOCs), and radon. In addition, tritium and radiocarbon (carbon-14) concentrations will be used to assess ground-water age. Data collected from these 30 sites will be used in conjunction with data collected throughout the Midwest within the extent of the Cambrian-Ordovician aquifer system to gain a better understanding of the conditions and trends in water-quality.

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