Reservoir Sediment Studies in Kansas
An understanding of the quantity and quality of sediment deposited in a reservoir is necessary for effective reservoir and basin management. Sedimentation affects both the useful life of a reservoir for such important purposes as flood control and water supply as well as its aesthetic quality. Sediment quality is an important environmental concern because sediment may act as a sink for water-quality constituents and as a source of constituents to the overlying water column and biota. Once in the food chain, sediment-derived constituents may pose an even greater concern due to bioaccumulation. An analysis of reservoir bottom sediments can provide historical information on sediment deposition as well as magnitudes and trends in water-quality constituents from the basin that are associated with sediment such as phosphorus, trace elements, and some pesticides. The U.S. Geological Survey (USGS), in cooperation with Federal, State, and local agencies, has completed a number of reservoir sediment studies in Kansas (fig. 1) using a combination of bathymetric surveying, sediment coring, chemical analysis, and statistical analysis. Specific objectives of the studies were to: (1) estimate total sediment volume and mass, (2) estimate annual sediment deposition and yield from the basin, (3) determine the occurrence and trends of constituents, (4) estimate annual constituent loads and yields from the basin, (5) assess sediment quality, and (6) provide a baseline for future assessments. Information from reservoir sediment studies may be used to: (1) partly reconstruct historical sediment- and water-quality records, (2) determine if sediment and water quality are changing (possibly due to changes in human activity in the basin), (3) provide a warning of potential future water-quality problems, (4) provide a baseline for future assessments to measure the effectiveness of implemented best-management practices (BMPs) in a basin, and (5) assist in the development and evaluation of total maximum daily loads (TMDLs). Selected results from several reservoir sediment studies are presented in the following sections. Results indicated that decreases in total water-storage capacity at normal pool elevations due to sedimentation ranged from less than 5 percent at Cheney Reservoir (south-central Kansas), Hillsdale Lake (northeast Kansas), and Webster Reservoir (north-central Kansas) to about 55 percent at Crystal Lake, a small impoundment in east-central Kansas. Decreases in storage capacity at Perry and Tuttle Creek Lakes (northeast Kansas) were in the range of 20 to 35 percent. Sedimentation has decreased the water-storage capacity of most of the large Federal reservoirs at an average annual rate of less than 1 percent. Mean annual net sediment yield for the large Federal reservoirs ranged from 0.03 (acre-ft/mi²)/yr for Webster Reservoir to 1.59 (acre-ft/mi²)/yr for Perry Lake. Table 1 provides a comparison of sedimentation for several Kansas reservoirs. Figure 2 depicts the thickness of sediment deposited in the submerged Big Blue River channel in Tuttle Creek Lake. A statistically significant positive correlation was determined for the relation between sediment yield and mean annual precipitation (Juracek, 2004). Phosphorus is an important nutrient because, if concentrations are too large, algal growth may become excessive and cause taste-and-order problems for water suppliers. Also, excessive algal growth may be detrimental to aquatic life in, and discourage recreational use of, a lake. For Hillsdale Lake, an analysis of reservoir sediment was used to determine that the mean annual phosphorus load input was 7 percent from point sources and 93 percent from nonpoint sources (Juracek, 1997, 1998). This information helped regulators and local groups identify the most important sources of phosphorus in the basin. A comparison of mean annual net phosphorus yields for several reservoirs in Kansas is provided in table 2. Mean annual net phosphorus yields, which paralleled the sediment yields, ranged from 26 (lb/mi²)/yr for Webster Reservoir to 3,000 (lb/mi²)/yr for Perry Lake. Some small reservoirs have possible increasing trends in nitrogen or phosphorus deposition (Juracek, 2004). Parts of the Republican and Solomon River Basins have soils rich in selenium, a trace element that has had serious effects on waterfowl in California and other parts of the West. Possible increasing trends over time in selenium concentrations that appear to correlate with increasing irrigation development in the basins (fig. 3) were determined from reservoir sediment data (Christensen and Juracek, 2001). This finding could be an important consideration in the long-term effects of some irrigation projects in western Kansas and Nebraska. Sediment-quality guidelines adopted by the U.S. Environmental Protection Agency (USEPA) allow for the assessment of reservoir sediment with respect to level-of-concern concentrations of various trace elements and organochlorine compounds (including polychlorinated biphenyls and several pesticides). Two such level-of-concern concentrations adopted by USEPA are referred to as the threshold-effects level (TEL) and the probable-effects level (PEL). The TEL represents the concentration below which toxic biological effects rarely occur. In the range between the TEL and PEL, toxic effects occasionally occur. The PEL represents the concentration above which toxic effects usually or frequently occur. The guidelines are used by USEPA as screening tools and are not enforceable. An assessment of the sediment in Kansas reservoirs with respect to USEPA guidelines indicated that several constituents may be of concern. For most of the reservoirs studied, arsenic, copper, and nickel typically were detected at concentrations that exceeded the TELs. Copper concentrations exceeded the PEL at Bronson City Lake, Crystal Lake, and Gardner City Lake. The elevated copper concentrations at these three reservoirs are due to the application of copper sulfate to control algal blooms. The depositional trend for copper in Crystal Lake is shown in figure 4. At Perry Lake and Gardner City Lake, nickel concentrations typically exceeded the PEL. Chromium, lead, and zinc were detected at concentrations that exceeded the TELs at some of the reservoirs. Cadmium and mercury concentrations typically were less than the TELs at all reservoirs studied (table 3). Likewise, organochlorine compounds typically either were not detected or were detected at concentrations less than the TELs (Pope, 1998; Christensen and Juracek, 2001; Mau, 2001; Juracek and Mau, 2002; Mau, 2002; Juracek, 2003, 2004). At Crystal Lake (completed in 1879), baseline concentrations [in the deepest (oldest) part of the sediment core] for arsenic, chromium, copper, lead, nickel, and zinc are either similar to, or substantially larger than, the respective TELs (Juracek, 2004). This finding indicates the possibility that for certain trace elements in certain areas, baseline concentrations may equal or exceed the TELs prior to the effects of human activity. DDE, a degradation product of DDT, had a depositional pattern at Perry Lake and Tuttle Creek Lake (fig. 5) that reflected the history of DDT use. DDT was used extensively in agriculture during the 1950s and 1960s. Then, following the ban of DDT in 1972, its use declined. The depositional history of DDE depicted in figure 5 documents the effects of an important change in human activity in the Tuttle Creek Lake Basin. The detection of DDE in the recently deposited sediments of several reservoirs indicated that DDT use was widespread in eastern Kansas (Juracek, 2004).