U.S. Geological Survey
Fact Sheet 009-02

Prepared in cooperation with the
CITY OF WICHITA, KANSAS

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Significant Findings of Water-Quality Studies and Implications for Cheney Reservoir Watershed, South-Central Kansas, 1996-2001

Larry M. Pope

CONTENTS

Overview
Nutrients
Pesticides
Bacteria
Suspended Solids
References

Water-quality issues in the Cheney Reservoir watershed were investigated from 1996-2001 as part of a cooperative effort between the U.S. Geological Survey (USGS) and the city of Wichita, Kansas. Water quality in the Cheney Reservoir watershed is important because much of the population of the area, which includes the Wichita metropolitan area, relies on Cheney Reservoir as a drinking-water source and for recreational activities. Water-quality studies conducted during the investigation addressed the transport of important water-quality constituents that included nutrients (nitrogen and phosphorus species), pesticides, bacteria, and suspended solids. Conclusions drawn for most water-quality studies conducted in the Cheney Reservoir watershed were the result of samples collected at six surface-water-quality sampling sites (five upstream and one downstream from Cheney Reservoir) and reservoir-sediment and watershed-soil studies (fig. 1).

The water-quality studies are documented in the reports referenced in this fact sheet. Reports of all of these studies are available on the World Wide Web at: http://ks.water.usgs.gov/Kansas/studies/qw/cheney/

Overview

Nutrients

• Mean concentrations of nitrate were small compared to the U.S. Environmental Protection Agency (USEPA) drinking-water criterion of 10 mg/L. Among the five surface-water-quality sampling sites upstream from Cheney Reservoir, mean concentrations of nitrate ranged from 0.43 (sampling site 2, fig. 1) to 1.6 mg/L (milligrams per liter) (sampling site 5) in surface-water samples collected during 1997-2000. However, mean concentrations of nitrate in water from four of the five sites were larger than the national mean background concentration of 0.6 mg/L, which may indicate that nitrate concentrations in streamflow in the Cheney Reservoir watershed are enriched as a result of agricultural activities (Milligan and Pope, 2001).

• Mean concentrations of total phosphorus exceeded water-quality goals. The long-term mean stream-water-quality goal of 0.10 mg/L for total phosphorus established by the Cheney Reservoir Watershed Task Force Committee was exceeded by mean concentrations of total phosphorus in water samples collected during 1997˜2000 from all five surface-water-quality sampling sites upstream from Cheney Reservoir (fig. 2). These mean concentrations ranged from 0.23 (sampling site 2, fig. 1) to 0.50 mg/L (sampling site 5) and were substantially larger than the 0.10-mg/L national mean background concentration of total phosphorus in streams, which indicates enrichment by agricultural activities or large natural concentrations in soils (Milligan and Pope, 2001). Historically (1965-98), however, the mean total phosphorus concentration in the surface-water inflow to Cheney Reservoir was 0.76 mg/L as calculated on the basis of phosphorus deposited in the reservoir sediment (Mau, 2001). The implication of these relatively large mean phosphorus concentrations is that phosphorus input to Cheney Reservoir is sufficient to produce algal blooms (excessive growth of algae), possible taste-and-odor problems in treated drinking water, and potentially could reduce the aesthetic and recreational appeal of the reservoir.

• Phosphorus concentrations have been increasing over time. An analysis of reservoir bottom sediment indicated an increasing trend (since construction of the reservoir in 1965) in total phosphorus concentrations in water from the Cheney Reservoir watershed. This trend probably is related to human activities such as fertilizer use in crop production, which more than doubled between 1965 and 1996 (Mau, 2001).

• The amount of phosphorus annually transported from the watershed varied. Annual fluctuations in the amount of phosphorus transported by streams in the watershed resulted, in large part, from variability in precipitation and resulting runoff. Wetter years produced more phosphorus transport. Most of the annual phosphorus transport to Cheney Reservoir occurred during runoff (high-flow) conditions. For example, during 1997-2000, 72 percent of the phosphorus transport in streamflow at sampling site 4 (fig. 1) occurred during runoff (Pope and others, 2002). The estimated mean annual phosphorus yield for the entire Cheney Reservoir watershed for 1997-98 was 0.20 pound per acre (Pope and Milligan, 2000). Historically (1965-98), however, the mean annual phosphorus yield of the watershed was estimated at 0.38 pound per acre (Mau, 2001). Estimated mean annual yields from watersheds of other reservoirs in Kansas have ranged from 0.02 to 1.76 pounds per acre.

• Agricultural activities accounted for 65 percent of the phosphorus transported to Cheney Reservoir. A comparison of the historical (1965-98) mean concentrations of phosphorus in sediment transported to Cheney Reservoir with the mean concentration of phosphorus in soil from 43 nonagricultural sites (fig. 1) in the Cheney Reservoir watershed indicated that agricultural activities increased the transport of phosphorus into Cheney Reservoir 2.9 times greater than that expected under natural conditions of phosphorus in soil. It was estimated that during 1965-98, 8.4 million pounds of phosphorus were transported to Cheney Reservoir. Ninety-two (92) percent of this phosphorus was deposited in the bottom sediment of the reservoir. The amount of phosphorus transported to Cheney Reservoir related to agricultural activities (65 percent of the total) was calculated from the total amount transported (8.4 million pounds) and the agricultural-enrichment factor (2.9). The implication of this large percentage of agriculturally related phosphorus is that a large potential exists for reducing phosphorus transport to Cheney Reservoir by changing agricultural activities (Pope and others, 2002).

• Substantial reduction in phosphorus transported to Cheney Reservoir may involve a combination of approaches such as reducing phosphorus application and changes in land-use, land-management, and agricultural practices. To determine if reductions in phosphorus application alone would reduce mean concentrations of total phosphorus in streams to acceptable levels (equal to or less than 0.10 mg/L), the agriculturally affected long-term (1997-2000) mean concentrations presented in figure 2 were divided by the agricultural-enrichment factor (2.9). These calculations produced estimated long-term mean concentrations of total phosphorus under natural concentrations of phosphorus in watershed soil (fig. 2). These estimated concentrations indicate that even under natural concentrations of phosphorus in watershed soil two (sampling sites 3 and 5, fig. 1) of the five surface-water-quality sampling sites in the Cheney Reservoir watershed still may not meet the long-term water-quality goal of 0.10 mg/L. The implication of this finding is that in order to meet water-quality goals for total phosphorus in the watershed, a combination of approaches such as reducing phosphorus application and changes in land-use, land-management, and agricultural practices may be involved (Pope and others, 2002).

Pesticides

• The occurrence of pesticides in surface water of the Cheney Reservoir watershed is widespread. At least one pesticide was detected in 99 percent of the streamflow samples collected between 1997 and 1999. Fifteen herbicides, one herbicide metabolite, and six insecticides were detected in streamflow samples from the Cheney Reservoir watershed. The herbicides alachlor, atrazine, and metolachlor were detected in 61, 99, and 96 percent, respectively, of all streamflow samples (Milligan and Pope, 2000).

• Median concentrations of pesticides generally were small and less than drinking-water criteria. Median concentrations of alachlor and atrazine were less than their respective Maximum Contaminant Levels (MCLs) established by the U.S. Environmental Protection Agency (USEPA) (fig. 3). An MCL has not been established for metolachlor. The variability in median concentrations of alachlor, atrazine, and metolachlor among the six surface-water-quality sampling sites probably is the result of variations in pesticide usage, amount of precipitation and runoff, topography, agricultural practices, and soil characteristics such as particle-size composition, porosity, and erodibility among the subwatershed areas represented by the sampling sites (Milligan and Pope, 2000).

• Potential adverse human health risks appear small. The implication of long-term exposure to small concentrations of pesticides in most streamflow samples for the population that relies on Cheney Reservoir as a drinking-water source is not of great concern on the basis of current (2001) water-quality criteria. However, the long-term effects of small concentrations of pesticides on aquatic life are not well known and may be an issue within the watershed (Milligan and Pope, 2000).

Bacteria

• Bacteria of fecal origin are common in streams of the Cheney Reservoir watershed. The sanitary quality of water and its use as a public drinking-water supply and for recreation were evaluated on the basis of fecal coliform bacteria densities. Fecal coliform bacteria were detected in streamflow samples from all surface-water-quality sampling sites in the watershed (fig. 4) (Mau and Pope, 1999).

• Large fecal coliform densities are common in streams during runoff conditions. Relatively large median fecal coliform densities were associated with runoff conditions at sampling sites upstream from Cheney Reservoir, and the median density at sampling site 3 was substantially larger than the 2,000-col/100 mL (colonies per 100 milliliters of water) water-quality criterion for secondary contact recreation (such as wading and fishing) established by the Kansas Department of Health and Environment (fig. 4). In contrast, median densities during base-flow (low-flow) conditions were substantially less than the 2,000-col/100 mL criterion in water from all five surface-water-quality sampling sites upstream from Cheney Reservoir. Fecal coliform densities are typically much greater in streams during runoff conditions because of nonpoint-source contributions from the watershed. These contributions can originate from deposition of fecal material by livestock and wildlife or from the use of manure as a soil amendment (Mau and Pope, 1999).

• Fecal coliform densities were much less in Cheney Reservoir than in its tributary streams. The median fecal coliform density in water samples from the outflow of Cheney Reservoir (sampling site 6, fig. 1) for 1997-98 was about 11 col/100 mL, substantially less than the 200-col/100 mL water-quality criterion for primary contact recreation (such as swimming) (fig. 4). The relatively small median density of fecal coliform bacteria in the outflow from Cheney Reservoir is attributed to the fact that bacteria transported into the reservoir by tributary streams are subject to die off and predation by other organisms. The physical process of dilution by reservoir water and deposition also play a role in decreasing bacterial densities in reservoir outflow. The implication of the relatively small median density of fecal coliform bacteria is that water flowing out of the reservoir generally is of acceptable sanitary quality for swimming and other contact activities (Mau and Pope, 1999).

Suspended Solids

References

or contact:
District Chief
U.S. Geological Survey
4821 Quail Crest Place
Lawrence, Kansas 66049-3839
(785) 842-9909
email: dc_ks@usgs.gov