U.S. Geological Survey
Water-Resources Investigations Report 01-4085

Prepared in cooperation with the
KANSAS WATER OFFICE and the
KANSAS DEPARTMENT OF HEALTH AND ENVIRONMENT

Sediment Deposition and Trends and Transport of Phosphorus and Other Chemical Constituents, Cheney Reservoir Watershed, South-Central Kansas

Control tower and shoreline northeast of dam, Cheney Reservoir, south-central Kansas.

CONTENTS

Abstract
Introduction

Previous Investigations
Current Investigation

Description of Cheney Reservoir Watershed
Methods

Bathymetric Survey
Estimation of Bottom-Sediment Depth and Volume
Reservoir Sediment-Core Collection, Processing, and Analysis
Quality Control

Sediment Deposition in Cheney Reservoir

Calculation of Bottom-Sediment Volume and Mass
Changes in Reservoir Storage Capacity
Comparison to Sediment Deposition in Other Reservoirs in Kansas

Water-Quality Trends and Mass Transport

Phosphorus

Phosphorus Concentrations in Reservoir Bottom Sediment
Estimates of Total Phosphorus Concentrations in Surface-Water Inflow
Estimates of Phosphorus Yields in Cheney Reservoir Watershed

Nitrogen
Trace Elements
Selected Pesticides

Summary and Conclusions
References
Supplemental Information

FIGURES

1-4. Maps showing:

Figure 1. Location of Cheney Reservoir watershed in south-central Kansas and bottom-sediment coring sites in Cheney Reservoir
Figure 2. Land use in Cheney Reservoir watershed, 1993
Figure 3. Location of range lines and coring sites at Cheney Reservoir, 1997-98, and estimated surface area of reservoir segments
Figure 4. Length of bottom-sediment cores collected from Cheney Reservoir, 1997-99

Figure 5. Schematic diagram showing storage capacities in Cheney Reservoir
Figure 6. Map showing mean concentrations of total phosphorus in bottom-sediment cores collected from Cheney Reservoir, 1997-99
Figure 7. Map showing mean particle-size composition of bottom-sediment cores from Cheney Reservoir, 1997-99
Figure 8. Graph showing relation between mean percentage of bottom-sediment particles less than 0.062 millimeter in diameter and mean concentrations of total phosphorus in bottom-sediment samples, Cheney Reservoir, 1997-99
9-12. Graphs showing:

Figure 9. Relation between total phosphorus concentrations in bottom-sediment core sample and normalized depth of sample from selected coring sites in Cheney Reservoir, 1965-97
Figure 10. Relation between concentrations of radioactive cesium-137 and normalized depth of bottom-sediment core samples from selected coring sites in Cheney Reservoir, 1965-99
Figure 11. Relation between mean percentage of bottom-sediment particles less than 0.062 millimeter in diameter and mean concentrations of total ammonia plus organic nitrogen as nitrogen in bottom-sediment samples, Cheney Reservoir, 1997-99
Figure 12. Comparison of selected trace element mean concentrations in bottom-sediment cores from in-channel and out-of-channel coring sites to Threshold Effects Levels, Cheney Reservoir, 1997-99

13-17. Graph showing comparison of 1964 Bureau of Reclamation topographic data and 1998 U.S. Geological Survey bathymetric data for Cheney Reservoir:

Figure 13. Range line 1
Figure 14. Range line 7
Figure 15. Range line 8
Figure 16. Range line 9
Figure 17. Range line 10

Figure 18. Graphs showing relation between arsenic, chromium, copper, and nickel concentrations in bottom-sediment sample and normalized depth of sample at coring site M5, Cheney Reservoir, 1999

TABLES

Table 1. Location of bottom-sediment coring sites in Cheney Reservoir, south-central Kansas, 1997-99
Table 2. Analytical results and relative percentage differences for selected nutrients and trace elements for split replicate samples of bottom sediment from coring sites M2 and M3, Cheney Reservoir, south-central Kansas, 1998-99
Table 3. Estimated mean sediment deposition along range lines for both in-channel and out-of-channel subsegments in Cheney Reservoir, south-central Kansas, 1965-98
Table 4. Estimated mean in-channel and out-of-channel sediment depth, surface area, and sediment volume deposited between 1965 and 1998, Cheney Reservoir, south-central Kansas
Table 5. Estimated mean sediment bulk density in both in-channel and out-of-channel subsegments, Cheney Reservoir, south-central Kansas, 1998
Table 6. Estimated bottom-sediment volume, mean bulk density, and bottom-sediment mass in both in-channel and out-of-channel subsegments, Cheney Reservoir, south-central Kansas, 1998
Table 7. Estimated total sediment deposition and sediment yield since impoundment of Webster Reservoir, Cheney Reservoir, Tuttle Creek Lake, and Hillsdale Lake, Kansas
Table 8. Mean annual water yield from Webster Reservoir, Cheney Reservoir, Tuttle Creek Lake, and Hillsdale Lake watersheds, Kansas, 1999
Table 9. Estimated mean in-channel and out-of-channel concentrations of total phosphorus in reservoir bottom sediment by reservoir segment, Cheney Reservoir, south-central Kansas, 1999
Table 10. Annual water budgets for Cheney Reservoir, south-central Kansas, 1966-98
Table 11. Concentrations of total ammonia plus organic nitrogen as nitrogen in Cheney Reservoir bottom sediment, south-central Kansas, 1999
Table 12. Summary of mean concentrations and comparisons to sediment-quality guidelines for selected trace elements in 13 bottom-sediment samples from Cheney Reservoir, south-central Kansas, 1997-99
Table 13. Nutrients analyzed in reservoir bottom sediment from Cheney Reservoir, south-central Kansas
Table 14. Selected trace elements and miscellaneous constituents analyzed in reservoir bottom sediment from Cheney Reservoir, south-central Kansas
Table 15. Selected organic constituents analyzed in reservoir bottom sediment from Cheney Reservoir, south-central Kansas
Table 16. Radiochemicals analyzed in reservoir bottom sediment for age dating, Cheney Reservoir, south-central Kansas

CONVERSION FACTORS, ABBREVIATIONS, AND DEFINITIONS

The use of brand names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.

Abstract

INTRODUCTION

Reservoirs in Kansas are affected by a variety of constituents that include suspended sediment, nutrients (phosphorus and nitrogen species), trace elements, and agricultural pesticides. Many of these constituents adhere to sediment particles and are deposited in reservoirs, which act as sinks for storage of these constituents. The large sediment concentrations common in Kansas reservoirs also are responsible for turbidity levels that can reduce the euphotic zone (that part of a water body in which there is sufficient light penetration to support photosynthesis) from several feet to a fraction of a foot. Large turbidity levels in combination with the presence of nutrients and pesticides can have harmful effects on both humans and aquatic organisms and can reduce the recreational value of the reservoir. One reservoir currently under investigation to determine the effects of these constituents, Cheney Reservoir, is located in south-central Kansas (fig. 1) and serves as both a drinking-water supply for the more than 300,000 people in the city of Wichita and as a regional recreational resource.

Cheney Reservoir is a multipurpose impoundment located on the North Fork Ninnescah River. The reservoir was constructed between 1962 and 1965 by the Bureau of Reclamation (BOR), U.S. Department of the Interior, to provide downstream flood control, to serve as a public water-supply source, to allow recreational activities, and to provide wildlife benefits. Because of the reservoir's importance to the area, there is considerable interest in maintaining acceptable water quality by mitigating contamination and preventing long-term detrimental effects to the aquatic ecosystem.

Previous Investigations

Current Investigation

DESCRIPTION OF CHENEY RESERVOIR WATERSHED

Cheney Reservoir has a surface area of about 15 miî, a mean depth of about 16 ft, a conservation-pool storage of about 152,000 acre-ft, and an additional flood-control pool capacity of about 81,000 acre-ft. The study area encompasses a contributing-drainage area of 933 miî (597,000 acres) and includes the North Fork Ninnescah River and associated tributary streams (fig. 1)

There are less than 4,000 people in the watershed, most of whom live on the approximately 1,000 farms in the area (Cheney Reservoir Watershed Task Force Committee, written commun., 1996). The six largest towns in the watershed range in population from less than 200 to slightly more than 1,200 people (Helyar, 1994).

The mean annual temperature in south-central Kansas is 56 ÷F, with a mean monthly range of 30.7 ÷F in January to 81.3 ÷F in July (Kansas Department of Agriculture and U.S. Department of Agriculture, 1997, p. 8). Mean annual precipitation (1961-90) is about 27 in., which occurs primarily during the growing season from April through September (National Oceanic and Atmospheric Administration, 1966-98).

The North Fork Ninnescah River Valley and the surrounding plains are underlain by consolidated rocks of Permian age (230 to 280 million years old) and covered by unconsolidated fluvial and windblown deposits of Pleistocene age (Zeller, 1968). The thickness of the unconsolidated deposits ranges from 0 to 40 ft and is commonly about 20 ft (Zeller, 1968). Soils in the Cheney Reservoir watershed generally are classified as clayey loam on the uplands to sand or sandy loam on low-lying areas or where slopes are less than 3 percent. Many of the soils in the watershed are subject to erosion by wind and rainfall runoff (Rockers and others, 1966).

Land use in the Cheney Reservoir watershed is predominantly agricultural (fig. 2) as previously described in other recent reports (Christensen and Pope, 1997; Pope, 1998). Crop production has fluctuated between 46 and 57 percent of the land use from 1964 to 1994, with a long-term mean of 52 percent (Kansas State Board of Agriculture and U.S. Department of Agriculture, 1964-94; Kansas Department of Agriculture and U.S. Department of Agriculture, 1995-97). The primary crop raised in the area is wheat, comprising about 63 percent of the crop production, followed by sorghum (24 percent), corn (10 percent), and soybeans (3 percent).

Irrigated crop production in the Cheney Reservoir watershed increased significantly after 1970 (Kansas State Board of Agriculture and U.S. Department of Agriculture, 1964-94; Kansas Department of Agriculture and U.S. Department of Agriculture, 1995-97). There was a nine-fold increase in irrigated crop production between 1970 and 1982 (from 0.63 to 6.0 percent). Since 1982, irrigated crop production has remained relatively constant at about 6 percent (Pope, 1998, p. 5).

Livestock production is a large and vital part of the economy in the Cheney Reservoir watershed. The mean annual number of cattle and calves in the watershed between 1964 and 1996 was 64,000 head, fluctuating between a low of 55,000 in 1976 to a high of 75,000 in 1996 (Kansas Department of Agriculture and U.S. Department of Agriculture, 1995-97). Annual inventories of hogs and sheep were considerably less than for cattle and averaged about 18,000 head for hogs and pigs and about 9,400 head for sheep and lambs for 1964-96.

METHODS

Bathymetric Survey

The bathymetric survey of Cheney Reservoir was done using a pontoon boat to which the GPS and fathometer were mounted. Range lines were established by the BOR in 1964 and used in the initial design of Cheney Reservoir (fig. 3) (Bureau of Reclamation, U.S. Department of Interior, written commun., 1993). These range lines were resurveyed during the current investigation, and the data from each range line compared to the 1964 range-line data (figs. 13, 14, 15, 16, and 17in "Supplemental Information" section at the back of this report). Information on any range lines that may have been established by the BOR in 1964 between range lines 1 and 7 was not available during this investigation. Therefore, no historical data were available in that area to compare to the 1998 bathymetric data.

Estimation of Bottom-Sediment Depth and Volume

Cheney Reservoir was divided into six segments and each segment was defined by range lines (fig. 3). Each segment was separated further into an in-channel and out-of-channel subsegment because of differences in sediment depth in these areas.

In-channel areas were determined by calculating the mean of the two in-channel widths from the 1964 range lines (prior to any sediment deposition) that defined each segment. The mean in-channel width for each segment then was multiplied by the in-channel length in each segment to determine the in-channel area. Mean in-channel depth of sedimentation for each reservoir segment was calculated as the average of the mean differences between reservoir-bottom elevations determined by the 1964 and 1998 surveys for the in-channel parts of the range lines that bracket the reservoir segment (figs. 13, 14, 15, 16, and 17in "Supplemental Information" section at the back of this report). A corresponding procedure was used to determine the mean out-of-channel depths.

The segments closest to the dam and farthest upstream from the dam were defined by range lines on one side only (range lines 1 and 10). The dam and upstream shoreline represented the other boundary side of these two segments, respectively. Depth of sedimentation for these two segments was determined using the depth of sedimentation calculated from range lines 1 and 10, respectively, and applying the depth to the entire segment. Finally, the in-channel and out-of-channel results from the product of area and depth for all segments were summed to provide an estimate of total sediment volume in the reservoir.

Reservoir Sediment-Core Collection, Processing, and Analysis

Sediment mass (in pounds) is a product of sediment volume (cubic feet) and sediment bulk density (pounds per cubic foot). Sediment bulk density was determined at selected locations in the reservoir between August 1997 and May 1999. A gravity corer was used to collect sediment cores from 13 in- and out-of-channel sites in 1997 (sites P1 through P13) and 10 out-of-channel sites (sites M1 through M10) in 1998-99 (table 1, fig. 3). The liner used in the corer was cellulose acetate butyrate transparent tubing with a 2.88-in. outside diameter and a 2.63-in. inside diameter. Depending upon the types of analyses, one to three sediment cores were collected at each site to provide sufficient material.

During this investigation all of the collected bottom-sediment cores penetrated to pre-reservoir material. This was evident by a change in physical appearance of the sediment, differences in sediment grain-size composition, and the presence of pre-reservoir organic material such as root hairs, plant material, or small sticks. The presence of pre-reservoir material indicated that a complete sediment record had been collected from each core.

The sediment cores were processed at the USGS laboratory in Lawrence, Kansas. The plastic liners were cut longitudinally in two places and 180÷ apart. The cuts were partially completed using a rotary handsaw and finished with a retractable razor knife set at a depth to minimize penetration of the sediment core. The cores were split in half by pulling a tightly held nylon string through the length of the two cuts and allowing the halves to separate. Depending upon the type of analysis as well as available bottom material in each core, the cores then were subsectioned into equal length intervals, and sediment samples from each interval were removed for physical and chemical determinations. Analyses of selected bottom-sediment cores included mean concentrations for the entire core. In these instances, equal volumes of sediment from each interval were removed from the sediment core, placed in a mixing bowl, and mechanically homogenized. A sample of the homogenized mass then was sent to the USGS National Water-Quality Laboratory (NWQL) in Denver, Colorado, for chemical analysis.

Sediment bulk densities were determined for selected sediment cores. These sediment cores were subsectioned, weighed, and oven dried to remove all moisture. Each dried subsectioned core was reweighed, and the weight divided by the respective subsection volume to determine bulk density. The mean bulk density for the entire sediment core was determined from the subsectioned bulk densities. Sediment-core mean bulk densities were used to calculate an average bulk density for each reservoir segment (fig. 3). Sediment cores were not collected for the reservoir segment located between range lines 7 and 8 (fig. 3). Mean bulk densities from upstream and downstream sediment cores closest to the segment were averaged to determine bulk densities.

Subsections of some of the cores collected during the investigation (sites P1-P13) were analyzed for bulk density and percentage of sand and silt and (or) clay at the USGS laboratory in Lawrence, Kansas, according to methods presented in Guy (1969). Subsections of other cores collected during the investigation (sites M1-M10) were analyzed for percentage of sand and silt and (or) clay at the USGS sediment laboratory in Iowa City, Iowa, according to methods presented in Guy (1969). Core subsections from selected sites (M1-M10) were analyzed at the NWQL for total phosphorus and nitrogen and selected trace elements according to methods presented in Fishman (1993) (tables 13-14, in "Supplemental Information" section at the back of this report). In addition, organophosphate insecticides, chlorophenoxy-acid herbicides, organochlorine insecticides, and triazine and acetanilide pesticides were analyzed from selected core subsections according to procedures in Foreman and others (1994) (table 15, in "Supplemental Information" section at the back of this report).

The use of a gravity corer, such as used during the investigation described in this report, may produce a "core-shortening" phenomenon (Blomqvist, 1985). Core shortening refers to the difference between the depth of sediment penetration by the sample tube and the length of the core inside the sample tube. This phenomenon has been documented at length and is described as being caused by friction of the sediment against the inner wall of the sample tube as the corer penetrates the sediment (Emery and Hulseman, 1964; Blomqvist and Bostrom, 1987). The friction causes the core sample to underrepresent the entire thickness of the penetrated sediment bed, presumably through a thinning of individual sediment layers during the penetration process. Compression of the sediment and loss of water content have been suggested as mechanisms that would explain core shortening in very soft muds (Hongve and Erlandsen, 1979). If compression of sediment is a dominant physical mechanism to explain the core-shortening phenomenon, then sediment densities and, ultimately, calculations of total mass presented in this report may be affected. However, for most sediment cores collected from Cheney Reservoir, very soft muds were evident only in the upper few inches of the cores. Although core shortening affects the length of the retrieved sediment core (fig. 4), the entire core sample represents a complete sediment profile and, thus, a potential record of historical trends in water-quality constituents.

Reservoir bottom sediment can undergo mixing during storms or periods of flooding. This mixing can create difficulties in the analysis of trends in water-quality constituents. To determine whether bottom sediment had been disturbed, selected sediment cores (sites P4, P13, M5) (fig. 3) were analyzed for cesium-137 by Quantera Laboratory in Denver, Colorado (table 16, in "Supplemental Information" section at the back of this report). Cesium-137, a by-product of the thermonuclear-weapons testing era of the 1950s and early 1960s, was widely dispersed by atmospheric deposition and potentially sorbed to soil particles (primarily clay). Detectable cesium-137 concentrations in sediment began about 1952 and peaked in 1963-64, followed by a steady decline in concentrations (Holmes, 1998). Identifiable peaks other than the 1963-64 peak in cesium-137 concentrations from the bottom-sediment core profile may be indicative of turbulence and resuspension of sediment. Analysis of sediment for chemical trends would not be appropriate under these conditions.

A chemical marker (consisting of calcium carbonate and barium sulfate) was placed over a 30-ft by 30-ft area on the Cheney Reservoir bottom in 1998. The intent was to deposit this minimally degradable material as a layer on top of the reservoir bottom sediment and return several years later to this location and remove a sediment core. The calcium carbonate and barium sulfate would be identified by distinct layering in the sediment core, and any sediment above the marker attributed to sediment deposition since 1998.

Quality Control

Within-site variability was evaluated through the collection and analysis of sequential replicate samples collected at coring sites P1 and P2 (fig. 3; Pope, 1998). Two replicate samples at coring site P1 were subsectioned into 0.2-ft intervals for a total of six subsample pairs. The two replicate samples at coring site P2 were not subsectioned because there was insufficient sediment (Pope, 1998). Each core subsection was homogenized and subsequently analyzed for total phosphorus concentration. Relative percentage differences between replicate concentrations were calculated as the absolute value of the quotient of the difference in replicate concentrations divided by the summation of replicate concentrations, multiplied by 100.

The range in relative percentage differences in total phosphorus concentrations from coring sites P1 and P2 was small, with median values of 2.3 and 1.3 percent, respectively. The results indicated that within-site variability in the quality of deposited sediment is small in Cheney Reservoir and probably not a concern (Pope, 1998).

The analytical precision and reproducibility of analytical results from bottom-sediment cores were determined by analyzing split replicate samples. Split samples were prepared by homogenizing like core subsections, then splitting each homogenized subsection into two samples. By homogenizing the sample and then splitting it into two samples, both samples should have been chemically equivalent, and any differences could be attributed to variability within the laboratory procedure.

Analysis of split replicate samples from coring sites M2 and M3 showed little or no variability among the nutrients and trace elements analyzed (table 2), with median relative percentage differences of 1.7 (coring site M2) and 3.1 percent (coring site M3). The relative percentage difference in total phosphorus was 6.0 percent in the sediment core from coring site M2 and 14 percent in the core from coring site M3. Split replicate samples analyzed for total phosphorus in 1997 showed similar relative percentage differences that varied from 0.1 to 6.4 percent (Pope, 1998).

SEDIMENT DEPOSITION IN CHENEY RESERVOIR

Calculation of Bottom-Sediment Volume and Mass

Total sediment volume deposited in Cheney Reservoir from 1965 through 1998, was approximately 7,100 acre-ft, not including sediment removed from the reservoir through the outlet works at the dam (tables 3 and 4). Of this amount, 1,075 acre-ft, or 15 percent, were attributed to in-channel sedimentation, and 5,990 acre-ft, or 85 percent, were attributed to out-of-channel sediment deposition. The mean annual sediment deposition rate for 1965 through 1998 (34 years), calculated on the basis of 7,100 acre-ft of total sediment deposition, was 209 acre-ft/yr, or 0.22 (acre-ft/yr)/miî of watershed, when divided by the 933 mi2 of contributing-drainage area in the Cheney watershed.

The total mass of reservoir bottom-sediment material was determined by multiplying the bottom-sediment volume (tables 3 and 4) by the mean sediment bulk density for each range-line segment tables 5 and 6). Separate calculations were done for the in-channel sediment and the out-of-channel sediment. Mean bulk densities for both in-channel and out-of-channel subsegments were greater in the upstream part of the reservoir where larger sediment particles tended to settle out faster than the finer sediment material found in the downstream part of the reservoir. Mean bulk densities ranged from 24.0 to 52.0 lb/ft (1.0 to 2.3 million lb/acre-ft) for in-channel reservoir subsegments, and from 20.0 to 104.0 lb/ft (900,000 to 4.5 million lb/acre-ft) for out-of-channel subsegments (table 6). The area-weighted mean bulk densities for both the in-channel and out-of-channel subsegments (table 6), calculated to account for areal differences of the various segments, were 1.6 and 2.7 million lb/acre-ft, respectively (table 6). Area-weighted mean bulk densities (table 6) were calculated as the sum of the products of bulk density (tables 5) times surface area (table 4) for the individual reservoir segments, divided by the sum of surface areas (table 4). Area-weighted mean bulk densities were calculated separately for the in-channel and out-of-channel areas.

The total mass of in-channel bottom-sediment material was estimated to be about 1.63 billion lb by summing the masses of the six reservoir segments (table 6). Similarly, the total mass of out-of-channel bottom-sediment material was estimated to be about 13.8 billion lb. The total estimated mass of bottom sediment in the reservoir, the sum of all the in-channel and out-of-channel subsegments, was about 15.4 billion lb. Total estimated mass of bottom sediment, 15.4 billion lb, when divided by the 34 years of sedimentation, resulted in 453 million lb of bottom-sediment deposition per year, known as the mean annual sediment load.

These results indicate that 11 percent of the deposited sediment mass was located in the submerged river channel and that about 89 percent of the sediment mass was located in the out-of-channel area. Negative values for bottom-sediment volume and bottom-sediment mass (tables 4 and 6) indicate uplake areas of the reservoir where erosion of the original 1964 topography was evident (fig. 17, in "Supplemental Information" section at the back of this report). It is possible that the reservoir bottom was scoured during periods of intense inflow, and original bottom sediment was transported farther into the reservoir. The estimated bottom-sediment mass for these out-of-channel areas (rangelines 9 and 10), therefore, was subtracted from the total mass of sediment deposition in the reservoir.

Changes in Reservoir Storage Capacity

Cheney Reservoir was designed to include storage for flood control, an active conservation storage that can be varied depending on climatic conditions, and inactive conservation storage; each storage volume is defined by different elevations in the reservoir (fig. 5) (Bureau of Reclamation, 2000). The inactive conservation storage pool (between elevations 1,378 and 1,393 ft above sea level) is the volume of water located below the active conservation storage pool and normally is not used by reservoir managers except during prolonged periods of extreme drought (Morris and Fan, 1998, p. 3.8). Frequently, a part of the inactive conservation storage pool of a reservoir is allocated for sediment deposition.

The area designated for the inactive conservation storage pool at Cheney Reservoir extends from the dam to about midway between range lines 1 and 7 (fig. 3). An estimate of total sediment deposition in the inactive conservation storage pool can be determined if it is assumed that sediment is deposited evenly throughout the segment between range lines 1 and 7. If true, then one-half of the deposited sediment between these two range lines would be located in the inactive conservation storage pool, in addition to the deposited sediment located between the dam and range line 1. On the basis of these assumptions, the total volume of sediment deposited is about 4,100 acre-ft and represents 27 percent of the approximately 15,300 acre-ft of inactive conservation storage pool volume from Cheney Reservoir.

The inactive conservation storage pool commonly is designed to provide storage for 100 years of sediment deposition (Morris and Fan, 1998, p. 2.13). As of 1998, 34 years of sediment deposition had occurred, which equates to 34 percent of the design life of the reservoir. Total sediment deposited during the 34-year period (27 percent of available capacity) is less than the current volume allocated (34 percent). Therefore, design-life specifications for the reservoir are being met provided that the design-life specifications were used in the design of the reservoir.

Comparison to Sediment Deposition in Other Reservoirs in Kansas

A comparison of Cheney Reservoir and three other reservoirs in Kansas-Webster Reservoir, Tuttle Creek Lake, and Hillsdale Lake-was made by the USGS, and sediment deposition rates were calculated (Mau and Christensen, 2000). Calculated mean annual sediment deposition was about 34.5 acre-ft for Webster Reservoir, 3,000 acre-ft for Tuttle Creek Lake, and 140 acre-ft for Hillsdale Lake. Calculated mean annual sediment deposition for Cheney Reservoir was 209 acre-ft. When the comparison was made on the basis of sediment yield (mean annual sediment deposition divided by the contributing-drainage area), the Hillsdale Lake watershed w as the largest contributor of sediment, followed by the Tuttle Creek, Cheney, and Webster watersheds (table 7) (Mau and Christensen, 2000).

The calculated mean annual water yield for 1999 from the Hillsdale Lake watershed, 530 (acre-ft/yr)/miî, was almost three times larger than from the Tuttle Creek Lake watershed, 181 (acre-ft/yr)/miî, followed by Cheney and Webster Reservoir watersheds with 115 and 38 (acre-ft/yr)/miî, respectively (table 8). Sediment yield in the Hillsdale Lake watershed was three times larger than for the Tuttle Creek Lake watershed, followed by Cheney and Webster Reservoir watersheds, which suggests a close relation exists between sediment yield and water yield.

Webster Reservoir is located in northwestern Kansas (fig. 1), and although long-term mean annual precipitation in the watershed (24 in/yr) (National Oceanic and Atmospheric Administration, 1998) is similar to that for the Cheney Reservoir watershed (approximately 27 in/yr), the topography and land-use practices vary between the two watersheds. The topography of the Webster Reservoir watershed is gently sloping, and land use includes more than 800 small farm ponds that are water and sediment traps for the reservoir (Bureau of Reclamation, 1984, p. 37). The ponds serve to reduce peak streamflows and volume of suspended sediment transported into the reservoir. The number of small farm ponds in the other watersheds was unknown at the time of publication of this report (2001). Small watershed impoundments may have a substantial effect on the transport of sediment to other reservoirs as well.

The contributing-drainage area for Tuttle Creek Lake is the largest of the four reservoirs investigated (9,600 miî), and it receives an average of about 32 in. of precipitation annually (National Oceanic and Atmospheric Administration, 1998). Approximately 75 percent of the contributing-drainage area to Tuttle Creek Lake is located in southeast Nebraska and the remainder in northeast Kansas. Land use in the watershed includes about 72 percent cropland, 16 percent pastureland, 10 percent woodland, and about 2 percent urban area (Nebraska Natural Resources Commission, 1983; Kansas Geological Survey, 1993). Although cropland in the watershed is substantial, mean annual sediment deposition on a per-square-mile basis of watershed is only slightly greater than for Cheney Reservoir watershed, 0.31 and 0.22 acre-ft/miî, respectively (table 7) (Mau and Christensen, 2000). The topographic similarities between the smooth plains of the Nebraska section of the Tuttle Creek Lake watershed and the flat to gently rolling topography of the Cheney Reservoir watershed, in conjunction with slight differences in precipitation, may provide a partial explanation for the similarities in sediment deposition.

The Hillsdale Lake watershed, located in eastern Kansas (fig. 1), receives an average of about 41 in. of precipitation annually (Juracek, 1997), 42 percent more than the Webster Reservoir watershed and 34 percent more than the Cheney Reservoir watershed. Approximately 40 percent of the Hillsdale Lake watershed is cultivated, 20 percent is undeveloped woodland, and 40 percent is pastureland, feedlots, and residential and urban development (Juracek, 1997). The combination of precipitation and cultivated land increases the potential for erosion that probably is a significant source of sediment deposition in Hillsdale Lake. About 56 percent of the total volume of bottom sediment in Hillsdale Lake in northeastern Kansas occurs in the in-channel area (Juracek, 1997). Differences in watershed topology, including but not limited to precipitation, land use, soil types, channel dimensions, and landslope between Hillsdale Lake and Cheney Reservoir, may account for the variation in sediment deposition (Juracek, 1997; Pope, 1998).

In a typical Kansas watershed, storm runoff increases streamflow that ultimately leads to the transport of more sediment to a downstream reservoir. Annual sediment deposition in Hillsdale Lake, on a per-square-mile of watershed basis, was more than four times greater than deposition in Cheney Reservoir, 0.97 and 0.22 (acre-ft/yr)/miî, respectively (table 7) (Mau and Christensen, 2000). Mean annual sediment load deposited in Cheney Reservoir was calculated to be about 453 million lb/yr. Annual sediment load per acre of contributing-drainage area, when divided by the 597,000 acres representing the contributing-drainage area of the reservoir, was calculated to be about 759 (lb/yr)/acre).

Large differences in sediment deposition in the four reservoirs cannot be attributed solely to differences in precipitation. Other factors such as topography, contributing-drainage area of the watershed, and differing soil types also affect the rate of sediment deposition. In an effort to reduce sediment deposition and increase the storage capacity and water quality of Cheney Reservoir, BMPs have been implemented, such as terracing, no-till practices, and the creation of buffer zones near contributing streams to the reservoir (Cheney Reservoir Watershed Task Force Committee, written commun., 1996).

WATER-QUALITY TRENDS AND MASS TRANSPORT

Phosphorus

Phosphorus Concentrations in Reservoir Bottom Sediment

Mean concentrations of total phosphorus in bottom-sediment cores from in-channel sites in Cheney Reservoir ranged from 477 mg/kg in cores from coring sites P8 and P11 to 674 mg/kg in a core from coring site P13 (fig. 6). Mean concentrations of total phosphorus in bottom-sediment cores from the out-of-channel sites in Cheney Reservoir ranged from 94 mg/kg in a core from coring site P10 at the upstream end of the reservoir to 710 mg/kg in a core from coring site M2 near the reservoir dam (fig. 6). The implications of this are that if anoxic conditions (inadequate oxygen) occur near the dam, phosphorus releases into the water column could affect the drinking-water supply. The mean phosphorus concentration in cores from all sites was 480 mg/kg. The mean in-channel phosphorus concentration was 630 mg/kg, and the mean out-of-channel concentration was 430 mg/kg.

The 1997 sediment investigation at Cheney Reservoir showed a strong linear relation between bottom-sediment particle size and total phosphorus concentration (Pope, 1998). Additional data collected during 1998-99 was added to the 1997 data in an effort to further define the relation. The regression analysis done by Pope (1998) was repeated in this investigation.

Regression analysis is one method used to determine whether a relation exists between two variables. The correlation coefficient, r, is used as a measure of the strength of the relationship and ranges from -1.0 to 1.0 depending on whether the relationship is negative (indirect) or positive (direct). The closer the correlation coefficient is to -1.0 or 1.0, the stronger the relationship between the two variables. The p-value is used as a measure of the statistical significance of a relationship. The statistical significance of a correlation increases as the p-value decreases.

The additional data collected during 1998-99 (coring sites M1-M10, (figs. 6 and 7) were used with the 1997 data to develop a curvilinear regression to represent the data. The resulting curvilinear equation (fig. 8) decreased the correlation coefficient slightly to r=0.94 (y=111.51e^0.017x , p less than 0.001) in relation to the 1997 data, but still showed a strong relation between particle-size distribution and total phosphorus concentration. The term `e' is an irrational number resulting from an infinite series, and it cannot be expressed in exact decimal or fractional form; it is approximately equal to 2.71828. The term `e' is used when a quantity increases at a rate proportional to its value. Explained another way, e is related to how much more money one would earn under compound interest than under simple interest (University of Toronto Mathematics Network, 1997).

The relation between bottom-sediment particle size and total phosphorus concentration suggests that phosphorus in bottom sediment is adsorbed to the smaller (silt- and clay-sized) particles. The silt- and clay-sized particles have large surface areas relative to their mass and, therefore, settle out of the water column much slower than sand-sized or larger material (Morris and Fan, 1998). These small particles are transported farther into the reservoir and settle out from the water column into bottom sediment in calm-water areas where the reservoir hydrodynamics are minimal. Typically, the calm, low-energy water areas are found in the deeper parts of the reservoir, especially near the dam. The bottom-sediment cores collected near the dam contained not only the largest volume of bottom-sediment material but also represented some of the largest percentages of sediment particles less than 0.062 mm in diameter (silt- and clay-sized particles) (fig. 7) and, not surprisingly, the largest phosphorus concentrations (fig. 6).

Total phosphorus concentrations in bottom-sediment cores from Cheney Reservoir generally showed an increasing trend from the older deposited sediment to the newer sediment near the top of the core (fig. 9) (Pope, 1998). This suggests that the transport of total phosphorus into Cheney Reservoir has been increasing over time. Natural sources of phosphorus would be expected to result in a fairly constant concentration over time. On the basis of a previous water-quality investigation of streams in the Cheney Reservoir watershed, the increase probably is not a result of point-source discharges (Christensen and Pope, 1997).

Information on land-use characteristics in the Cheney Reservoir watershed indicates that the increase in total phosphorus in reservoir bottom sediment probably is of nonpoint-source origin and related to agricultural activities such as fertilizer and manure application to croplands or livestock production. Between 1965 and 1996, the total amount of fertilizer sold in Kansas more than doubled from 835,000 to 1,710,000 tons (Kansas State Board of Agriculture and U.S. Department of Agriculture, 1986, p. 5; Kansas Department of Agriculture and U.S. Department of Agriculture, 1997, p. 5). A similar increasing trend in fertilizer sales and application could be expected in the Cheney Reservoir watershed. Therefore, in developing a management strategy for mitigating the transport of phosphorus to Cheney Reservoir, it would be important to address the human-related sources of phosphoru and (or) to implement management practices that would restrict the movement of phosphorus from areas of application.

The depositional trend of phosphorus in the reservoir was supported by cesium-137 data. Cesium-137 data in bottom sediment from coring sites P4, P13, and M5 showed a general decline in cesium-137 concentrations from a maximum of about 1.4 pCi/g (site M5) in the older and deeper sediment to 0.3 to 0.6 pCi/g determined from sediment cores collected from more recent deposition (fig. 10). This is consistent with the known peak cesium-137 atmospheric concentrations that occurred in 1964 and the continual decrease in concentrations since that time. The cesium data in the sediment cores indicate that substantial disturbance of sediment layers has not occurred in Cheney Reservoir and, therefore, would not affect time-trend analysis.

Estimates of Total Phosphorus Concentrations in Surface-Water Inflow

Concentration calculations require knowledge of mass and volume, and both were calculated for the inflow water from the Cheney Reservoir watershed. Mass and volume were determined on the basis of total sediment deposition estimated from the 1964 topographic and 1998 bathymetric surveys of the reservoir and bulk-density determinations at selected coring sites in the reservoir. The sum of the in-channel and out-of-channel sediment deposition, 15.4 billion lb (table 6) was multiplied by the sediment volume-weighted mean phosphorus concentration in bottom sediment for the reservoir. The sediment volume-weighted mean phosphorus concentration (498 mg/kg) was calculated as the sum of the products of sediment volume times mean total phosphorus concentration (table 9) divided by total reservoir sediment volume for both the in-channel and out-of-channel subsegments in each reservoir segment (table 4). Total phosphorus mass, 7.7 million lb, was calculated by multiplying the total sediment deposition, 15.4 billion lb, by the sediment volume-weighted mean phosphorus concentration, 498 mg/kg, and dividing by a conversion factor of approximately 1.0 million. The mean annual phosphorus load, 226,000 lb/yr, was determined by dividing the total phosphorus mass by 34 years, the number of years since dam impoundment.

The change in reservoir storage was measured as the difference in monthly reservoir storage capacity and summed to determine the value for the water year (table 10). Discharge from the reservoir was measured at the USGS streamflow-gaging station on the North Fork Ninnescah River at Cheney Dam (station 07144795, (fig. 1); data on file with USGS, Lawrence, Kansas). Withdrawals from the reservoir by the city of Wichita to supplement its drinking-water supplies were measured at the Cheney Reservoir pump station (data on file with Kansas Department of Agriculture, Division of Water Resources, Topeka, Kansas).

Water budgets were determined annually using available data from water years 1966-98 and then summed (table 10). The total net inflow volume of water to the reservoir from the watershed (3.71 million acre-ft) was converted to liters (4.6 x 10¹² L) and used to calculate the total phosphorus concentration in water for water years 1966-98 from the Cheney Reservoir watershed on the basis of total phosphorus deposited in reservoir bottom sediment. The mass of total phosphorus deposited in the reservoir bottom sediment from 1965-98, 7.7 million lb (3.5 x 10¹² mg) was divided by the net inflow volume of water to the reservoir, 4.6 x 10¹² L, and the result was a mean total phosphorus concentration (that is based on reservoir bottom sediment) of 0.76 mg/L in the surface-water inflow to Cheney Reservoir.

Estimates of Phosphorus Yields in Cheney Reservoir Watershed

Nitrogen

The nitrogen associated with finer sediment particles and organic matter may be transported into reservoirs as part of the sediment load. Concentrations of mean total ammonia plus organic nitrogen as nitrogen (N) in homogenized bottom-sediment cores collected from eight out-of-channel sites in Cheney Reservoir ranged from 1,200 to 2,400 mg/kg as N, with a median out-of-channel concentration of 1,850 mg/kg as N (table 11). These values are similar to concentrations of mean total ammonia plus organic nitrogen analyzed in bottom-sediment cores collected in 1998 from Kirwin and Webster Reservoirs and Waconda Lake (Christensen, 1999). The ranges in mean concentrations of total ammonia plus organic nitrogen as N in sediment cores were 1,200 to 1,980 mg/kg for Kirwin Reservoir, 30.0 to 1,910 mg/kg for Webster Reservoir, and 704 to 3,210 mg/kg for Waconda Lake.

An evaluation of the concentrations of total ammonia plus organic nitrogen as N in relation to the particle-size composition of bottom-sediment cores from Cheney Reservoir suggests a statistically significant relation between total ammonia plus organic nitrogen as N and percentage of silt- and (or) clay-sized particles in the sediment cores (r = 0.90, p=0.002) (fig. 11). The relation suggests, as with phosphorus, that ammonia and organic nitrogen adsorb to the smallest sediment particles, which are the silt- and clay-sized particles. These particles are transported farther into the reservoir and deposited in bottom sediment in calm-water areas.

A sediment core from site M5 at Cheney Reservoir was divided into 10 equal core subsections, and each subsection was analyzed for total ammonia plus organic nitrogen as N. A regression analysis of depth versus concentration of total ammonia plus organic nitrogen as N did not indicate a statistically significant relation between the two variables. However, synthetic fertilizer use has increased throughout the State and probably in the Cheney Reservoir watershed as well. Therefore, it is unlikely that nitrogen use is decreasing over time in the watershed.

Trace Elements

Several bottom-sediment cores from Cheney Reservoir were analyzed for trace elements (table 12). Many trace element concentrations were larger in sediment cores located farther downstream in the reservoir and may be associated with the increased percentage of silt and clay particles found farther downstream. Clay-sized particles probably contain larger concentrations of trace elements incorporated into the mineral matrix or adsorbed to the particles because of their larger surface area compared to sand- and silt-sized particles (Pope, 1998). The clay particles also tend to settle in the deeper parts of the reservoir (near the dam) where there is less disturbance to the particles from wind and wave action.

Mean concentrations of most trace elements in the Cheney Reservoir bottom sediment were generally less than the TELs and considerably less than the respective PELs (table 12). However, arsenic, chromium, copper, and nickel exceeded the TELs at several coring sites.

Mean concentrations of arsenic from 11 of 13 coring sites in Cheney Reservoir exceeded 7.2 Îg/g, the TEL for arsenic (table 12). Mean concentrations of arsenic in sediment ranged from 5.9 to 12 Îg/g. Arsenic has been used extensively as a pesticide, herbicide, and soil sterilant in agricultural soils in this country but has only a moderate bioaccumulation effect because it is not very mobile (Pais and Jones, 1997, p. 86).

Chromium concentrations exceeded the TEL in all cores except from site M9. Concentrations were about 75 percent larger in the core from in-channel site P13 compared to the mean chromium concentrations in cores from nearby out-of-channel sites M7 through M10 (fig. 3). The source of chromium is not well understood, but what is not of natural origin may be a by-product of agricultural fertilizer application (Pais and Jones, 1997).

Mean concentrations of copper in reservoir bottom sediment exceeded the TEL at four coring sites. Copper is an essential micronutrient for plants and commonly is found in soils (Pais and Jones, 1997, p. 100). The PEL for copper is 110 Îg/g, more than 4.5 times larger than the largest mean copper concentrations detected in sediment in Cheney Reservoir.

The typical nickel concentration in uncontaminated soil is 25 Îg/g although concentrations in soil can range from 1 to 200 Îg/g (Kabata-Pendias and Pendias, 1994; Pais and Jones, 1997, p. 124). Concentrations of nickel in bottom-sediment samples from Cheney Reservoir were greater than the TEL of 16 Îg/g at all coring sites but were less than or equal to the PEL of 43 Îg/g. With no apparent industrial nickel source in the watershed, it may be that phytoplankton, which are known to bioaccumulate certain trace elements including nickel, are ingesting trace amounts of nickel from the water column and subsequently depositing and concentrating the nickel in the reservoir bottom sediment. Recent research suggests that nickel is an essential micronutrient for both plants and animals; however, in large concentrations it can be toxic (Pais and Jones, 1997, p. 125).

A comparison of concentrations of selected trace elements in the sediment core from in-channel site P13 with the sediment cores from nearby out-of-channel sites M7 through M10 (fig. 12) shows larger concentrations of all trace elements in the in-channel sediment core (coring site P13). As stated previously, the in-channel sites are located in the deepest parts of the reservoir where wave action is minimal, and these sites accumulate larger amounts of silt and clay than their respective out-of-channel sites. Concentrations of constituents that adsorb to clay particles, such as phosphorus and selected trace elements, generally are larger where silt and clay percentages are large (table 12, fig. 7).

An examination of subsectioned bottom-sediment cores from coring site M5 for trends in selected trace elements was inconclusive (fig. 18, in "Supplemental Information" section at the back of this report). Arsenic, chromium, copper, and nickel concentrations varied with depth with no apparent increasing or decreasing trend. The Cheney Reservoir watershed is primarily agricultural, supporting very little commercial or industrial use. It is possible that trace element concentrations that exceeded TELs or PELs are the result of natural contributions from the sediment and not related to either point- or nonpoint-source contributions within the watershed. The Cheney Reservoir watershed does not have large urban areas, an industrial base, or large population centers; therefore, the opportunity to contribute significant trace element concentrations is limited.

Selected Pesticides

SUMMARY AND CONCLUSIONS

REFERENCES

Supplemental Information

Constituent	Method reporting limit	Units of measure- ment
Cesium-137	0.05	pCi/g

North Fork Ninnescah River above Cheney Reservoir, south-central Kansas
(photograph by Chad Milligan, U.S. Geological Survey, Wichita, Kansas).