Summary of the Major Water-Quality Findings from the
Eastern Iowa Basins Study Unit of the
National Water-Quality Assessment Program


By Stephen J. Kalkhoff, U.S. Geological Survey, Iowa City, IA

Published in the Iowa Groundwater Quarterly, volume 11, number 3, Fall 2000, pp. 20-21

An integrated assessment of the water quality in streams and aquifers in the Wapsipinicon, Iowa, Cedar, and Skunk River basins was conducted 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. Over 90 percent of the land in the EIWA study unit is used for agricultural purposes, while forested areas account for only 4 percent and urban areas about 2 percent of the land.

Surface-water samples were collected monthly and during selected storm events from six sites in medium-sized basins (125 to about 400 mi2) and five sites in large river basins (2,300 to 12,500 mi2). The medium-sized basins were selected to be representative of various physical features, hydrogeology, and agricultural landuse (row crops and concentrated animal feeding operations) that may affect water quality. The large river sites were selected to determine the integrated effects of combinations of landuse and hydrogeology on river-water quality.

Ground-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.

Surface- and ground-water samples were analyzed for a wide variety of chemical constituents (major ions, nutrients, and pesticides) commonly associated with agricultural and urban activities. Because they were not expected to occur in rivers and streams, volatile organic compounds (VOC's), commonly comprising fuels, solvents, and other industrial compounds were only analyzed in ground-water samples. The age of the ground water, important information needed to relate ground-water quality to land use, was determined using both tritium and chlorofluorocarbons (Freon?) age-dating methods.

Results from the EIWA NAWQA study build on previous and ongoing research and water-quality monitoring programs in Iowa and provide new insights into the relation between the quality of the State's water resources and human activities. The major findings from the study are listed below.

Major Stream and River Findings

Nitrogen and phosphorus concentrations in streams in the agricultural study area rank as some of the highest in the "cornbelt" as well as the Nation and were higher than the drinking-water standard in many samples. These conditions reflect intensive use of fertilizer on cropland and dense populations of livestock in some basins.

• Nitrate concentrations in 22 percent of the samples exceeded the U.S. EPA drinking water standard of 10 milligrams per liter (mg/L). Standards were most frequently exceeded during the month of June soon after spring fertilizer application. Though many of the streams sampled are currently not used for drinking water supplies, the Cedar and Iowa rivers are the direct or indirect source for Cedar Rapids and Iowa City--two of the three largest cities in the study area.

• The highest nitrate concentrations occurred in medium-sized streams draining basins with the most intensive row-crop agriculture and in a stream draining a basin with both intensive row-crop agriculture and dense concentrations of large-scale animal feeding operations. Nitrate concentrations in these streams were greater than 10 mg/L almost 50 percent of the time. Conversely, nitrate concentrations were lowest in basins that had greater percentages of pasture, grassland, and forest.

• Total phosphorus concentrations frequently (75 percent of the samples), exceeded the 0.1 mg/L goal to minimize algal growth in rivers. Total phosphorus concentrations were greatest in streams and rivers that drain basins with more highly erodable soils and in large river basins that contain the largest cities and towns.

• The large amounts of nitrogen and phosphorus that are transported to the Mississippi River from eastern Iowa represents an economic loss to farmers and a potential environmental threat to downstream waters. The estimated annual loss of from 10 to 27 lb/acre of nitrogen and from 0.4 to 0.7 lb/acre of phosphorus represent a potential loss in crop yield or the cost of additional fertilizer needed to compensate for that flushed from the fields. Nutrients transported to the Mississippi River from the basin likely reach the Gulf of Mexico where they contribute to eutrophication and hypoxia.

• Although the use of herbicides and insecticides in the study area are among the most intense nationwide, herbicide concentrations in streams were not among the highest 25 percent Nationally and insecticide concentrations were in the lowest 25 percent Nationally. Break down compounds, whose widespread occurrence has only recently been discovered and for which little is known of human and environmental impacts, generally comprised the majority of the pesticide compounds present in rivers and streams.

• The most commonly used herbicides were the most frequently detected and were generally present in the greatest concentrations. Atrazine and metolachlor were detected in all samples. Concentrations generally ranged from 0.1 to 1.0 micrograms per liter (μg/L). Atrazine concentrations exceeded the 3.0 μg/L drinking water standard in about 10 percent of the samples; exceedances occurred mainly during late-spring runoff.

• Acetochlor, a conditionally registered herbicide that is intended to replace a number of other commonly used herbicides, was frequently detected, but usually (75 percent of the samples) at concentrations less than 0.1 μg/L. Mean annual acetochlor concentrations did not exceed the 2.0 μg/L registration requirement at any site but concentrations did exceed this level in about 3 percent of the individual samples. The maximum concentration (10.6 μg/L) measured during the study exceeded the level that would trigger requirements for biweekly sampling by water-supply systems.

• Break down products of alachlor, metolachlor, and acetochlor are present in relatively high concentrations throughout the year suggesting that they are more persistent than their parent compound.

• Carbofuran and chlorpyrifos, insecticides that have been identified as posing a high risk to aquatic insects and mussels, were present in up to 60 percent of the monthly samples during the summer when these insecticides are normally applied. Concentrations generally did not exceed aquatic life standards.

Major Ground-Water Findings

Compared to surface water, ground water in the Eastern Iowa Basins had substantially lower nutrient concentrations and less frequent detections. However, land use had a substantial effect on the quality of shallow ground water. Pesticide break down compounds were some of the most commonly detected constituents in shallow ground water. Nitrate and methyl tert-butyl ether (MTBE) exceeded standards in some of the samples. In contrast, deeper bedrock aquifer systems, which are generally more protected by thick clay and shale layers, typically had low nitrogen concentrations and low frequency of pesticide detections.

• 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.

• Nutrients move from ground water to streams by natural drainage and tile lines. Nitrate concentrations in 24 of 25 medium-sized streams exceeded 10 mg/L during the sampling period in May 1998 when streamflow originated primarily from ground-water inflow. Nitrate concentrations consistently exceeded 10 mg/L in water from a representative tile-line draining to the Iowa River.

• 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.

The major findings of this study are currently being compiled and will be summarized in U.S. Geological Survey Circular 1210 "Water Quality in the Eastern Iowa Basins Iowa and Minnesota, 1996-98" This report will be published and available by the end of December 2000. A digital version of the report will be available on the EIWA Web page . Other reports prepared as part of the project, in addition to links to both National and local sites (still to be established) containing the data collected during 1996-98, are also available on this site.


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