National Water Quality Assessment Program: Eastern Iowa Basins

Major Ground-Water Findings from the National Water-Quality Assessment Program (NAWQA) in Iowa

By Stephen J. Kalkhoff,

Abstracts from the Iowa Groundwater Association Fall 2000 Meeting in Iowa Groundwater Quarterly, vol. 11, no. 4. p.31-32.

Ground-water samples were collected from 124 wells in the Wapsipinicon, Iowa, Cedar, and Skunk River basins in 1996 through 1998 as part of the Eastern Iowa Basins (EIWA) study unit of the U.S. Geological Survey's National Water-Quality Assessment Program (NAWQA). The EIWA study unit is one of 59 study units across the Nation designed to assess the status and trends in the quality of the Nation's ground- and surface-water resources and to link the status and trends with an understanding of the natural and human factors that affect the quality of water.

Samples were collected primarily from the alluvial aquifers because of the aquifers' direct hydraulic connection with rivers and streams and because alluvial aquifers are one of the most important sources for domestic, municipal, and industrial water supplies in the study area. Monitoring wells were installed in agricultural and urban areas of the alluvial aquifers to assess the quality of the most recently recharged water in relation to land use. Existing domestic wells screened in alluvial aquifers and the Silurian/Devonian aquifer were sampled to assess deeper and older ground water. A number of major findings from the study are listed below

Nitrate concentrations generally decreased with depth in the alluvial aquifer. Biological denitrification may result in decreased nitrate concentration with depth, but it is also possible that the deeper water infiltrated during years when less fertilizer was used for crops.

Pesticides were detected in alluvial aquifers underlying both agricultural and urban areas, but shallow ground water in agricultural areas contained greater concentrations than in urban areas. A greater variety of pesticide compounds were detected in urban areas indicating a more diverse usage. Pesticides most frequently found in alluvial aquifers underlying urban areas include atrazine (50 percent of the samples) and metolachlor (23 percent of samples), herbicides used primarily for crops. Prometon, a persistent herbicide that is used for weed control in rights-of-way and around buildings was the second most commonly detected pesticide (40 percent of the samples) in urban areas. Concentrations of prometon were most frequently less than 0.1 mg/l.

With the exception of atrazine and metolachlor and prometon in urban areas, pesticides were infrequently detected in alluvial aquifers. Pesticide break down products generally were more commonly detected in the alluvial aquifers than their parent compounds. The increased presence of break down compounds indicates that many pesticides break down in the soil and that the resulting break down compounds are transported to the shallow aquifers.

MTBE, a common gasoline additive used to increase the octane content or ensure cleaner burning was detected in 23 percent of urban alluvial wells. Concentrations exceeded the drinking-water advisory for taste and odor in samples from 6 percent of the wells.


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