National Water Quality Assessment Program: Eastern Iowa Basins


Many synthetic organic compounds that contain chlorine atoms (organochlorine compounds) were developed and used during the period of 1940-1980 as pesticides and for industrial applications. One characteristic of many of these compounds is their stability and persistence in the environment. The compounds were transported to rivers and streams by rainfall runoff and were incorporated into aquatic organisms and adsorbed to suspended sediment. Because of their stability and persistence in the environment and toxic effects on aquatic organisms and humans, organochlorine compound use decreased during the 1970's. Many of these pesticides were banned from use in the United States during this period.

Organochlorine compounds were used extensively in the Iowa River watershed as insecticides during the 1960's and early 1970's (Sato and Schnoor, 1991). Bioaccumulation of one of the compounds, dieldrin, in roughfish resulted in a ban on commercial fishing in the Coralville Reservoir (fig 1). Monitoring for a limited number of organochlorine compounds in the Iowa River at Iowa City continued for about 15 years from 1968-82. Data before and after this period are sporadic or not available. Because of the heavy use, persistent nature, and toxic effects, there is a need to understand the past and present occurrence of organochlorine compounds in the environment to help understand their effect on humans and aquatic organisms. Knowledge of the long-term occurrence of organochlorine compounds will help evaluate the effectiveness of environmental regulations controlling chemical use.

FIGURE 1 (thumbnail) 
Figure 1 (23k .gif)

Location of the Iowa River watershed and the sediment-core site in Coralville Reservoir (slope data from Soil Conservation Service, 1993.)

Suspended sediments and adsorbed chemicals transported in rivers settle from the river in the lower velocity environment of a reservoir. Once deposited, sediments are stored for long periods. Organochlorine compounds adsorbed onto sediments then can provide insight into the historical occurrence of selected chemical compounds in the watershed. Sections of sediment cores can be dated using physical, chemical, and isotopic markers and analyzed to determine residual organochlorine compound concentrations. Plotting the chemical concentration versus age of sediments can show the trend or lack of trend in the occurrence of the compound in the reservoir sediments and in the contributing watershed.

Water-quality trends are of interest because trends can improve the understanding of the influence of human activities on water-quality, can indicate the effectiveness of environmental regulations in controlling undesirable chemical inputs, and can provide an indication of potential future water-quality conditions. A common approach for determining water-quality trends in streams is to apply statistical tests to historical data; however, historical water-quality data have several limitations. In Iowa, these include: lack of data, changes in sampling and analytical methods, and numerous measurements below detection levels. Where historical data are lacking or are inappropriate for statistical trend testing, water-quality records can be partly reconstructed using sediment cores from receiving water bodies such as reservoirs.

The purpose of this factsheet is to describe the concentrations of selected organochlorine compounds in sediment cores from the Coralville Reservoir and to relate the results to the depositional period of the sediments to indicate possible trends. A secondary purpose is to compare organochlorine compound concentrations in Coralville Reservoir sediments to concentrations in the Iowa River to evaluate whether compounds found in sediments are representative of those in the water. Sediment cores were collected from the Coralville Reservoir as part of a National Water Quality Assessment Program (NAWQA) study to use chemical analyses of dated sediment cores from reservoirs to define historical trends in water quality of the influent rivers and their watersheds in the Central and Southeastern United States (Van Metre and others, 1997).

FIGURE 2 (thumbnail) Figure 2 (16k .gif)

Suspended sediment and stream discharge in the Iowa River downstream from Coralville Reservoir at Iowa City, Iowa.


The Coralville Reservoir (fig. 1) is a flood-control and recreational-use reservoir that was formed by the construction of the Coralville dam. The reservoir, formed by impounding the Iowa River about three miles (mi.) north of Iowa City in 1958, has a surface area of about 8.4 mi2 and has a storage capacity of about 41,000 acre-feet at the normal conservation pool elevation of 683 ft. above sea level. Suspended-sediment transport in the Iowa River downstream of the reservoir has been significantly reduced since the completion of the dam. The yearly suspended-sediment load in the Iowa River at Iowa City (fig. 2) decreased from an average of 980,000 tons per year to 363,000 tons per year after the closure of the Coralville Dam (Schuetz and Matthes, 1977). Sediment deposition in the reservoir could partially account for the difference.

The Iowa River is the major source of sediment to the Coralville Reservoir. The Iowa River drains an agricultural watershed of about 3,100 mi.2 upstream of the dam (fig. 1). Greater than 90 percent of the land in the watershed is used for agricultural purposes. Marshalltown, located about 60 mi. upstream of the Coralville Reservoir, is the largest urban area (25,178 people; U.S. Bureau of Census, 1992) in the watershed.

Data Collection and Interpretation

Cores penetrating the entire sequence of reservoir bottom sediment were collected November 4, 1993 approximately 1.5 mi. upstream of the dam in the former channel of the Iowa River (fig. 1). The cores were collected in the late fall following a summer of record flooding in the watershed. The core site was selected as representative of the deep depositional zone containing relatively homogeneous fine-grained sediments. Sediment cores were collected by a gravity corer from a pontoon boat. The gravity corer has a diameter of 2.5 inches (in.) and is 10 feet (ft.) long (fig. 3). Discrete horizontal slices of this sediment core were analyzed for organochlorine compounds (Foreman and others, 1994). Other cores collected from the same location were analyzed for cesium-137, major and minor elements, and grain size. A subsequent bottom sediment sample was collected with an Ekman dredge from the core site in November 1996 to document the deposition of organochlorine compounds since 1993.

The core sediments were dated using both physical and chemical markers. The top of the core was assigned the sampling date (November 1993). The bottom of the core, determined on the basis of observed physical differences between the sediment and the original channel-bed material, was assigned the reservoir impoundment date (1958). Concentrations of radioactive cesium-137, a by-product of atmospheric nuclear weapons testing, were used to identify the sediments deposited during the early 1960's. Significant concentrations of this isotope occurred in the atmosphere beginning in about 1952, and peaked in 1963-1964. Ages of the sediments were then assigned based on these markers and assuming a uniform rate of annual sediment deposition (table 1).

Table 1

FIGURE 3 (thumbnail)
Figure 3 (31k .gif)

Removing a bed-sediment sample from the gravity corer.

Sediment Deposition

The average depositional rate at the core site was calculated to be about 1.9 inches per year. This rate was calculated by dividing the total length of the core (68 inches) by age of the deepest interval (35 years).

The physical character of the sediments, as indicated by the core material, changed after completion of the Coralville Dam. Before the dam was closed, greater than 70 percent of the channel-sediment material was silt- or sand-sized material. Upon closure of the dam, 70-80 percent of the reservoir-sediment material was clay-sized material.

Organochlorine Compounds

Thirty-one organochlorine compounds were analyzed in ten selected intervals from the core. Nine compounds were found in trace or quantifiable concentrations in at least one core interval. These compounds include aldrin, three chlordane compounds, dieldrin, polychlorinated biphenyls (PCB's), and three DDT degradation compounds. Only dieldrin and DDE were detected in the sediment sample collected in November 1996.


Aldrin is an insecticide that was used extensively in Iowa to control corn rootworms and cutworms. Aldrin use decreased between the mid 1960's and 1970's due to increased insect resistance (Sato and Schnoor, 1991). The manufacture and use of aldrin was discontinued in 1976. Aldrin degrades to form dieldrin. Detectable aldrin concentrations were present in sediments deposited in the late 1960's and in a core interval that corresponds to deposition in about 1992 (fig. 4).


Chlordane was used to control rootworms and cutworms until its use was restricted for agriculture in the mid 1970's. Chlordane use continued in urban areas to control termites until the late 1980's. Both chlordane isomers, cis- and trans-chlordane, were detected in all intervals of the sediment core. Chlordane was not detected in the bottom-sediment sample collected in 1996. Cis-chlordane and the chlorodane degradation compound, trans-nonachlor, were present in only trace amounts. Trans-chlordane was detected in trace concentrations from the oldest and in the youngest sample from the core. Concentrations peaked at 2.2 micrograms per kilogram (mg/kg) in the mid-1970's (fig.4).


Use of the organochlorine insecticide DDT began in 1939. It's usage peaked in the 1960's and widespread usage continued until about 1970, just before it was banned in 1972. DDT breaks down to DDE and DDD which also are toxic and are very resistant to further chemical breakdown. Trace concentrations of DDD were detected in all intervals of the core. DDE was detected in all intervals sampled(fig. 5). The greatest concentration (3.4 mg/kg) occurred within the first 10 years after completion of the reservoir. Subsequent concentrations decreased and then peaked again in sediments deposited in the mid 1970's. The overall trend has been one of decreasing concentrations with time. The DDE concentration from sediments deposited in 1996 was 1.2 mg/kg.


Dieldrin is a contact insecticide that was used extensively for corn. It is also a degradation product of aldrin. Dieldrin was not present in Coralville sediments deposited before the mid-1960's. Dieldrin concentrations in sediments increased to 1.6 mg/kg around 1975, decreased during the late 1970's and 1980's and then peaked again in 1993 (fig. 5) following a summer of record flooding. The concentration of dieldrin in sediment deposited in 1996 decreased to 1.4 mg/kg. The dieldrin concentration from a Coralville reservoir sediment sample collected in 1970 was 2.7 mg/kg (Morris and Johnson, 1970)

Total PCB's

PCB's have been used as plasticizers, hydraulic lubricants in gas turbines and vacuum pumps, in heat transfer systems, and as dielectric fluids in electrical capacitors and transformers. PCB's are generally a mixture of many closely related compounds known as congeners that differ in their physical, chemical, and biological characteristics. PCB production peaked in the United States in 1968 and essentially ended in 1979 when new uses were banned. Trace amounts were detected in the oldest sediments from Coralville Reservoir (fig. 4).

FIGURE 4 (thumbnail)
Figure 4 (10k .gif)

Aldrin, chlorane, and total PCB concentrations in a bed-sediment core from Coralville Reservoir, Iowa, 1993.

FIGURE 5 (thumbnail) 
Figure 5 (11k .gif)

Dieldrin and DDE concentrations in a bed-sediment core from Coralville Reservoir and in water from the Iowa River at Iowa City, Iowa.

Relation Between Concentrations in Water and Sediment

The general trends are similar for dieldrin and DDE concentrations in the Iowa River at Iowa City (Johnson and Morris, 1971 and U.S. EPA STORET database) and in the sediment core from the Coralville Reservoir (fig. 6). Yearly mean dieldrin concentrations in water increased from less than the detection level (0.005 mg/L) in 1968 to 0.020 mg/L in 1969 and 1970 and then decreased to 0.012 mg/L in 1972. During the same period, dieldrin concentrations in sediment increased from trace concentrations in 1969 to 1.6 mg/kg in 1975 and then decreased to trace levels in 1977. Yearly mean DDE concentrations in water peaked twice; at 0.004 mg/L in 1971 and at 0.005 mg/L in 1974. DDE concentrations in sediment peaked during this period. The peak DDE in sediment (3.0 mg/kg) in 1975 corresponds to the second yearly-mean peak of DDE in water. The correlation between the concentrations measured in the Iowa River and in the sediment core agree with previous models (Schnoor, 1981) that estimated about 40 percent of the dieldrin entering Coralville Reservoir is deposited in the bottom sediments.

Summary and Conclusions

Chemical data from sediment cores collected in a deep depositional area of the Coralville Reservoir near Iowa City, Iowa show the long term occurrence of aldrin, DDT compounds, chlordane compounds, and dieldrin in the Iowa River watershed. Even though DDT and dieldrin were banned in the 1970's, detectable concentrations are still present in the sediment into the mid 1990's; including substantial quantities of dieldrin that appear to have been transported to the Coralville Reservoir during flooding in 1993. The match between dieldrin and DDE concentrations reservoir sediments and in the river suggests that chemical data from age-dated Coralville Reservoir sediment cores can be used to evaluate the historical occurrence of selected organochlorine compounds in the Iowa River watershed.

References Cited

Foreman, W.T., Connor, B.F., Furlong, E.T., Vaught, D.G., Merten, L.M., 1994, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory-Determination of organochlorine insecticides and polychlorinated biphenyls in bottom sediment: U.S. Geological Survey Open-File Report 94-140, 78 p.

Johnson, L.G., and Morris, R.L., 1971, Chlorinated hydrocarbon pesticides in Iowa Rivers: Pesticides Monitoring Journal, vol. 4, no. 4, pp 216-219.

Morris, R.L., and Johnson, L.G., 1970, Pesticide levels in fish and bottom silts from Iowa streams: University of Iowa Hygienics Laboratory Report 71-10, 5 p.

Sato, C. and Schnoor, J.L., 1991, Applications of three completely mixed compartment models to the long-term fate of dieldrin in a reservoir: Water Research, vol. 25, no. 6, pp. 621- 631.

Schnoor, J.L., 1981, Fate and transport of dieldrin in Coralville Reservoir: Residues in fish and water following a pesticide ban: Science, vol. 211, p. 840-842.

Schuetz, J. R. and Matthes, W.J., 1977, Fluvial sediment data for Iowa: Suspended-sediment concentrations, loads, and sizes; bed-material sizes; and reservoir siltation: Iowa Geological Survey Bureau Technical Information Series Number 6, 410 p.

U.S. Bureau of Census, 1992, 1990 Census of population and housing: Summary tape file 3 on CD-ROM Iowa [machine-readable data files].

U.S. Soil Conservation Service, 1993, State soil geographic data base (STATSGO)--Data users guide, Miscellaneous Publication Number 1492, 88 p.

Van Metre, P.C., Callendar, E, and Fuller, C, 1997, Historical trends in organochlorine compounds in river basins identified using sediment cores from reservoirs: Environmental Science and Technology, in press

--S.J. Kalkhoff and P.C. Van Metre

Information on technical reports and hydrologic data related to the NAWQA program can be obtained from:
District Chief
U.S. Geological Survey
Post Office Box 1230
Iowa City, Iowa 52244

Fact Sheet FS-129-97