U.S. Geological Survey
Water-Resources Investigations Report 99-4230

Prepared in cooperation with the
BUREAU OF RECLAMATION

Deposition of Selenium and Other Constituents in Reservoir Bottom Sediment of the Solomon River Basin, North-Central Kansas

By Victoria G. Christensen

CONTENTS

Abstract
Introduction
Purpose and Objectives
Basin Description
Methods

Sediment-Core Collection, Processing, and Analysis
Quality Control
Trend Analysis

Results of Reservoir Bottom-Sediment Analysis

Sediment-Core Lithology and Grain Size
Age Dating and Depositional Patterns of Bottom Sediment
Concentrations and Trends in Selected Chemical Constituents

Selenium
Arsenic
Strontium
Nutrients and Total Organic Carbon

Bottom-Sediment Volume and Mass in Webster Reservoir
Selenium, Arsenic, and Strontium Loads in Webster Reservoir

Comparison of Depositional Trends to Irrigation and Constituent Loads
Summary
References
Supplemental Information

FIGURES

Figure 1. Maps showing location and near-surface geology of the Solomon River Basin, north-central Kansas
Figure 2. Maps showing approximate location of sediment-coring sites in Kirwin Reservoir, Webster Reservoir, and Waconda Lake
Figure 3. Photographs showing gravity corer mounted on pontoon boat and core preparation
4-11. Graphs showing:

Figure 4. Depth-normalized cesium-137 profiles in selected bottom-sediment cores collected in May 1998 from Kirwin Reservoir, Webster Reservoir, and Waconda Lake
Figure 5. Historical inflows to Kirwin Reservoir, Webster Reservoir, and Waconda Lake
Figure 6. Depth-normalized selenium concentration profiles in selected bottom-sediment cores collected in May 1998 from Kirwin Reservoir, Webster Reservoir, and Waconda Lake
Figure 7. Depth-normalized arsenic concentration profiles in selected bottom-sediment cores collected in May 1998 from Kirwin Reservoir, Webster Reservoir, and Waconda Lake
Figure 8. Depth-normalized strontium concentration profiles in selected bottom-sediment cores collected in May 1998 from Kirwin Reservoir, Webster Reservoir, and Waconda Lake
Figure 9. Irrigated acres in Kirwin and Webster Irrigation Districts, 1957-97
Figure 10. Background and post-background selenium, arsenic, and strontium concentrations in bottom sediment from Kirwin Reservoir, Webster Reservoir, and Waconda Lake and estimated background and post-background selenium, arsensic, and strontium annual loads in Webster Reservoir
Figure 11. Background and post-background mean annual streamflow into Kirwin Reservoir, Webster Reservoir, and Waconda Lake

TABLE

Table 1. Distribution of irrigated acres in the Solomon River Basin, 1995
Table 2. Location of bottom-sediment coring sites in the Solomon River Basin, May 1998
Table 3. Mean relative percentage differences for total recoverable metals, nutrients, and total organic carbon concentrations in sequential replicate samples from bottom-sediment coring sites WAC 1 and WAC 2 in Waconda Lake, May 1998
Table 4. Mean relative percentage differences for total recoverable metals, nutrients, and total organic carbon concentrations in bottom-sediment samples from Kirwin and Webster Reservoirs, May 1998
Table 5. Sediment-recovery percentages in cores collected from Kirwin Reservoir, Webster Reservoir, and Waconda Lake, May 1998
Table 6. Mean percentage of silt and clay, percentage of sand, bulk density, and percentage of moisture in bottom-sediment samples from coring sites in Kirwin Reservoir, Webster Reservoir, and Waconda Lake, May 1998
Table 7. Statistical summary of selected constituent concentrations in bottom-sediment samples from Kirwin Reservoir, Webster Reservoir, and Waconda Lake, May 1998, and comparison to sediment-quality guidelines
Table 8. Results of trend tests on concentrations of selected constituents in bottom-sediment cores collected from Kirwin Reservoir, Webster Reservoir, and Waconda Lake, May 1998
Table 9. Core length, estimated sediment thickness, and recovery percentage for multiple cores from sediment coring sites in Kirwin Reservoir, Webster Reservoir, and Waconda Lake, May 1998
Table 10. Results of analyses of selected physical properties for bottom-sediment cores collected in May 1998 from Kirwin Reservoir, Webster Reservoir, and Waconda Lake
Table 11. Results of analyses of sediment-quality constituents in bottom-sediment cores collected from Kirwin Reservoir, Webster Reservoir, and Waconda Lake, May 1998

CONVERSION FACTORS, ABBREVIATIONS, AND VERTICAL DATUM

Temperatures in this report can be converted to degrees Celsius (°C) or degrees Fahrenheit (°F) by the equations:

Water year: Water year as used in this report refers to the 12-month period beginning October 1 and ending September 30. The water year is designated by the calendar year in which it ends; for example, the 1998 water year begins October 1, 1997, and ends September 30, 1998.

Deposition of Selenium and Other Constituents in Reservoir Bottom Sediment of the Solomon River Basin, North-Central Kansas

By Victoria G. Christensen

Abstract

INTRODUCTION

The Solomon River Basin extends across parts of 17 counties in north-central Kansas (fig. 1) and includes the Solomon River and its major tributaries--the North Fork Solomon River and the South Fork Solomon River. The Solomon River drains about 6,840 mi² of mainly agricultural land. The Solomon River Basin is underlain by strata of marine origin. Irrigation return flow in areas underlain by these strata may contain concentrations of selenium that, when introduced into a lake, reservoir, or wetland area, could bioaccumulate to levels toxic to aquatic organisms, predatory species, or potentially could affect human health through consumption of contaminated organisms (Ramirez and Armstrong, 1992; Roy and O'Brien, 1992).

Selenium often is redistributed by irrigated agriculture (Sharma and Singh, 1983), and there is a concern about selenium concentrations in the Solomon River Basin where irrigation by the Bureau of Reclamation (Bureau), U.S. Department of the Interior, began in the late 1950's. Irrigation return flow also may contain large concentrations of dissolved constituents that may affect the quality of receiving stream water (Mueller and others, 1991; Engberg, 1993). Public Law 99-294, National Environmental Policy Act (NEPA) of 1986, requires that the Bureau investigate soil characteristics that may result in toxic or hazardous irrigation return flows. The Bureau, in response, has begun a Resource Management Assessment (RMA) of the Solomon River Basin to provide the necessary data for NEPA compliance before renewal of long-term water-service contracts with irrigation districts in the basin.

The study described in this report was begun in the spring of 1998 by the U.S. Geological Survey (USGS) in cooperation with the Bureau to determine if Bureau-operated irrigation (hereinafter referred to as Bureau irrigation) has increased selenium or other constituent concentrations in the basin. There are three reservoirs in the Solomon River Basin-Kirwin Reservoir on the North Fork Solomon; Webster Reservoir on the South Fork Solomon; and Waconda Lake on the main stem of the Solomon River (fig. 1). Three Bureau water-resource developments are located within the basin-the Kirwin Irrigation District, the Webster Irrigation District, and the Glen Elder Irrigation District (Bureau of Reclamation, 1984). The drainage areas upstream from Kirwin and Webster Reservoirs are not affected by Bureau irrigation, and therefore, bottom sediment from these reservoirs was used to determine background sediment concentrations of selected constituents and to evaluate the effects of other (non-Bureau) irrigation activities. Waconda Lake receives water from both the Kirwin Irrigation District and the Webster Irrigation District, and therefore, the bottom sediment from Waconda Lake was used to assess trends in chemical constituents in sediment for the entire Solomon River Basin and to help distinguish the effect of Bureau irrigation in the basin. Sediment quality is of concern because it reflects the quality of the overlying water column.

PURPOSE AND OBJECTIVES

BASIN DESCRIPTION

Near-surface rocks in the Solomon River Basin are sedimentary and range in age from Permian to Quaternary (fig. 1). The near-surface geology has a general east-west pattern, with the oldest rocks in the east and the youngest rocks in the west. The oldest near-surface rocks are the Permian-age Sumner Group (which includes the Wellington Formation and the Ninnescah Shale) at the far eastern end of the study area. Overlying the Sumner Group are marine strata of Cretaceous age that include the Cheyenne Sandstone, Kiowa Shale, Dakota Formation, Graneros Shale, Greenhorn Limestone, and Carlile Shale. These rocks are exposed at the land surface in parts of Ottawa County in the east to parts of Osborne County in the west (Moore and Landes, 1937). Overlying the Carlile Shale are the Niobrara Formation, which is exposed in much of the North Fork Solomon River Basin (Leonard, 1952), and the Pierre Shale, of which there is only one known outcrop in the basin (Moore and Landes, 1937; Ross, 1991). At the western end of the basin, parts of the Ogallala Formation occur at the land surface.

In the Great Plains (a semiarid region that includes Kansas), the main source of selenium is Cretaceous-age marine shale, specifically the Pierre Shale (McNeal and Balistrieri, 1989). The only known outcrop of Pierre Shale in the basin is upstream from Webster Reservoir (fig. 1); however, other marine rocks do contain selenium. Selenate (SeO₄^2-), the most dominant form of selenium, is highly mobile and easily leached from soils by irrigation return flows. Reservoir-bottom sediment is an integrator of the water quality upstream from the reservoirs, and changes in land- and water-use practices through time are recorded with the deposition of sediment in reservoirs (Van Metre and others, 1996, 1997).

The three Bureau reservoirs in the Solomon River Basin provide water for irrigation; municipal, industrial, and domestic use; flood control; recreation; and fish and wildlife habitat. Fishing, hunting, and water-related recreation play an increasingly important role in the economy of the basin (Bureau of Reclamation, written commun., 1996).

Kirwin Reservoir, completed in 1952, has a contributing drainage area of 1,367 mi², with a total storage capacity of 314,550 acre-ft of water allocated among flood control (215,115 acre-ft), conservation storage (89,650 acre-ft), and inactive and dead storage (9,785 acre-ft). During typical conditions, there are 5,073 acres of water-surface area that are available for recreation. Also, there are 5,923 acres of land surrounding the reservoir that are available for recreational use. The Kirwin National Wildlife Refuge, operated on public lands by the U.S. Fish and Wildlife Service, provides for the preservation of waterfowl, other animals, and plant life. Inflow to the reservoir may be affected by irrigation return flow from non-Bureau-operated irrigation areas. The Bureau-operated Kirwin Irrigation District No. 1 is located downstream from the reservoir and provides surface-water outflow from the reservoir to irrigate as much as 11,490 acres of farmland (Bureau of Reclamation, 1996), generally within 5 mi of the North Fork Solomon River.

Webster Reservoir, completed in 1956, has a contributing drainage area of 1,150 mi², with a total storage capacity of 260,740 acre-ft of water allocated among flood control (183,369 acre-ft), conservation storage (72,071 acre-ft), and inactive and dead storage (5,300 acre-ft). During typical conditions, there are 3,766 acres of water-surface area that are available for recreation. Also, there are 2,733 acres of land surrounding the reservoir that are available for recreational use. Inflow to the reservoir may be affected by irrigation return flow from non-Bureau-operated irrigation areas. The Bureau-operated Webster Irrigation District No. 4 is located downstream from the reservoir and provides surface-water outflow from the reservoir to irrigate as much as 8,500 acres of farmland (Bureau of Reclamation, 1996), generally within 5 mi of the South Fork Solomon River.

Waconda Lake, completed in 1967, has a contributing drainage area of 5,076 mi², with a total storage capacity of 963,775 acre-ft of water allocated among flood control (722,315 acre-ft), conservation storage (204,789 acre-ft), and inactive and dead storage (36,671 acre-ft). During typical conditions, there are 12,602 acres of water-surface area that are available for recreation. Also, there are 13,890 acres of land surrounding the reservoir that are available for recreational use. Inflow to Waconda Lake may be affected by non-Bureau-operated irrigation areas and the Bureau-operated Kirwin Irrigation District No. 1 and Webster Irrigation District No. 4. Waconda Lake provides water to irrigate as much as 6,000 acres of farmland in the Bureau-operated Glen Elder Irrigation District No. 8, a rural water district, and the city of Beloit, Kansas (Bureau of Reclamation, 1996).

Historically, the Solomon River Basin's economy centers around agriculture in terms of employment and income. Population has been slowly declining over the years, particularly among the rural population as mechanization of farms and a downsizing of the farm economy have eliminated many jobs (Bureau of Reclamation, written commun., 1996). The primary crop grown in the basin is wheat, with smaller acreages of alfalfa, corn, sorghum, soybeans, and sunflowers. Livestock production consists mainly of cattle, with lesser numbers of hogs and sheep.

Approximately 3 to 4 percent of the Solomon River Basin is irrigated. Water for irrigation in the basin comes primarily from ground water upstream from Kirwin and Webster Reservoirs (about 87 percent), and primarily from surface water downstream from Kirwin and Webster Reservoirs (about 80 percent) (Leonard, 1952; Joan Kenny, U.S. Geological Survey, written commun., 1999).

On the basis of 1995 irrigation data, approximately 48 percent of the total irrigated acres downstream from Kirwin, Webster, and Waconda are supplied water from Bureau-operated irrigation districts. However, no Bureau irrigation water is used upstream from Kirwin and Webster Reservoirs, making Bureau-irrigated acres only 12.1 percent of the total number of irrigated acres in the Solomon River Basin (table 1).

METHODS

Sediment-Core Collection, Processing, and Analysis

To determine trends in selenium and other selected constituents, bottom-sediment cores were collected in May 1998 from two sites each in Kirwin Reservoir, Webster Reservoir, and Waconda Lake (fig. 2). The two coring sites were compared to assess sampling variability at each reservoir. The cores were collected in the original channel bed near the dam where the sediment is least likely to be disturbed by biological activity, inflows or outflows, human activity, or wind-induced currents. A third coring site was sampled in Waconda Lake for quality assurance, and nine additional coring sites were sampled in Webster Reservoir to determine mass of sediment and selected constituents (table 2).

At each coring site, as many as four cores were collected using a Benthos gravity corer mounted on a pontoon boat (fig. 3A). The liner used in the corer was cellulose acetate butylate transparent tubing with a 2.625-in. inside diameter. Samples were identified first by site and then by the order in which they were collected (for example, the cores collected from site 1 in Kirwin Reservoir are identified as KIR 1.1, KIR 1.2, KIR 1.3, and KIR 1.4). All but one core (KIR 2.1 from Kirwin Reservoir) penetrated the entire thickness of reservoir sediment and into the original channel-bed or soil-surface material. Multiple cores were required to provide sufficient sediment material for laboratory analyses.

For each site, separate cores were analyzed for selected physical properties, total recoverable metals, nutrients, cesium-137 (¹³⁷Cs), and total organic carbon. The samples from the nine additional coring sites in Webster Reservoir (coring sites WEB 3 through WEB 11, table 2) were analyzed for selenium, arsenic, strontium, and phosphorus. Selenium and arsenic were selected for additional analysis because both are a concern for human health and because there is a natural source for both constituents (marine strata) in the Solomon River Basin. Strontium was selected for additional analysis because a previous study in the adjacent Republican River Basin identified an increasing trend in strontium concentrations in bottom sediment from three reservoirs (Kyle Juracek, U.S. Geological Survey, oral commun., 1998). Phosphorus was selected for additional analysis because reservoir sediment may contain large quantities of phosphorus as a result of inflow transport of particulate and dissolved forms (Pope, 1998) and because phosphorus may accumulate in reservoir sediment (Juracek, 1997).

Processing of core samples and analysis of sediment characteristics were conducted at the USGS laboratory in Lawrence, Kansas. Each core was extracted from the liner and split lengthwise to expose the relatively undisturbed inner part of the core for examination and sampling (fig. 3B). Depending on the total sediment thickness, as many as 12 samples were collected from each core. Sediment characteristics that were determined included a core description, grain size, bulk density, and percentage of moisture. The core description involved recording the stratigraphic layers of the sediment from the pre-reservoir material up to the current depositional layers. The presence of pre-reservoir material was determined by a change in particle-size composition of the sediment or the presence of organic matter, such as plant material, roots, root hairs, sticks, and twigs, that would indicate the pre-reservoir soil surface. Grain size, bulk density, and moisture percentage were determined according to methods presented in Guy (1969).

Chemical constituent analyses were performed by the Bureau laboratory in Bismarck, North Dakota, using U.S. Environmental Protection Agency (USEPA) (U.S. Environmental Protection Agency, 1997a) and USGS methods (Skougstad and others, 1979). Microwave-assisted digestion using nitric acid (USEPA method 3051) was used on all samples. Graphite furnace atomic absorption was used for arsenic (USEPA Method 7060A), lead, and selenium (USEPA Method 7740). Flame atomic absorption was used for strontium (USEPA Method 7780). Cold vapor atomic absorption (USEPA Method 7471A) was used for mercury. For all other metals, inductively coupled argon plasma (USEPA Method 6010B) was used. A flow injection analyzer was used for the analysis of total nitrogen (USEPA Method 350.1) and total phosphorus (USGS Method I-2601-78). A carbon analyzer was used for the analysis of total organic carbon (USEPA Method 9060).

To calculate mean and median constituent concentrations for samples with concentrations less than the laboratory detection limit and for constituents with multiple detection limits, a value of one-half the highest detection limit reported for that constituent in each reservoir was arbitrarily assigned. Mean and median values were not calculated when there were more than five samples in a core with concentrations less than the laboratory detection limits.

Cesium-137 (¹³⁷Cs) concentrations were determined by Quanterra Laboratory in Denver, Colorado. The concentration of ¹³⁷Cs was determined to assist in age dating the samples. ¹³⁷Cs is a radioactive isotope that is a by-product of nuclear weapons testing. Measurable concentrations of this isotope first appeared in the atmosphere in about 1952, peaked during 1963-64, and have since declined (Holmes, 1998). ¹³⁷Cs strongly sorbs to fine-grained sediment, making it useful in many studies to age date sediment that has been exposed to atmospheric fallout (Callender and Robbins, 1993; Van Metre and others, 1996, 1997; Benninger and others, 1997; Wallbrink and others, 1998). It also can be used to demonstrate that the sediment is undisturbed if a relatively uniform decrease in ^{137Cs concentration follows the 1963-64 peak (Pope, 1998).}

Quality Control

Without quality-control data, bottom-sediment data cannot be interpreted adequately because errors associated with sample data are unknown. For this study, within-site variability as well as site-to-site variability were determined. Within-site variability of data associated with all phases of sample collection and analysis was analyzed through the collection of sequential, replicate bottom cores from Waconda Lake coring site 2 (WAC 2, fig. 2), which was immediately adjacent to Waconda Lake coring site 1 (WAC 1, fig. 2). The cores from coring site WAC 2 were collected within 15 minutes of collecting the final core from coring site WAC 1. The very small change in location from site WAC 1 to site WAC 2 (table 2) was due to drifting of the pontoon boat.

Cores from coring sites WAC 1 and WAC 2 were segmented into 12 sample intervals of various lengths, according to changes in physical characteristics (fig. 3B). Each core segment was homogenized and then analyzed for selected physical characteristics, total recoverable metals, nutrients, ¹³⁷Cs, and total organic carbon concentrations to provide information on the variability of the constituents that were analyzed for all in-channel, near-dam cores. The relative percentage difference between the two sample concentrations from coring sites WAC 1 and WAC 2 was defined by the following equation:

The mean relative percentage differences for all total recoverable metals, nutrients, and total organic carbon in the sequential replicate samples (coring sites WAC 1 and WAC 2) are listed in table 3. The mean relative percentage differences for constituent concentrations between coring sites WAC 1 and WAC 2 ranged from 1.66 percent for manganese to 8.56 percent for selenium, with a mean relative percentage difference for all constituents of 3.70 percent. A typical quality-control objective for precision of replicate samples is a maximum relative percentage difference of 20 percent (Taylor, 1987). Not only does the sequential replicate bottom-sediment sample represent within-site variability but also includes any sample contamination incurred during collection and preparation. Percentage differences need to be taken into account when evaluating the variability between bottom-sediment cores from the same site. It should be noted that relative percentage differences were not calculated for chemical constituents that were not detected and that selenium showed the greatest variability among all constituents that were analyzed. Selenium concentrations were close to their method detection limit, which may account for their increased variability.

No standard method exists for obtaining representative samples of bottom sediment in reservoirs. Therefore, to gain some knowledge of the amount of variability between sites in the original channel, site-to-site variability was analyzed through the examination of constituent concentrations in samples from two coring sites each in Kirwin and Webster Reservoirs. The two sites in each reservoir were located in the original channel bed near the dam. The two sites in Kirwin Reservoir (KIR 1 and KIR 2, fig. 2) were about 200 ft apart, whereas the two sites in Webster Reservoir (WEB 1 and WEB 2, fig. 2) were about 1,000 ft apart. Relative percentage differences were determined using equation 1. Mean relative percentage differences between each site in Kirwin Reservoir for total recoverable metals, nutrients, and total organic carbon ranged from 2.15 percent for copper to 11.5 percent for chromium. In Webster Reservoir, the differences ranged from 1.91 percent for barium to 14.2 percent for chromium (table 4). The mean relative percentage difference for all constituents in table 4 was slightly larger for Webster Reservoir (5.85 percent) with sites 1,000 ft apart than for Kirwin Reservoir (5.43 percent) with sites 200 ft apart. On the basis of these comparisons, there is not substantial variability in sediment chemistry even over the fairly large distance between sampling sites.

Trend Analysis

RESULTS OF RESERVOIR BOTTOM-SEDIMENT ANALYSIS

Sediment-Core Lithology and Grain Size

The length of sediment cores collected as part of this study ranged from 1.4 to 8.0 ft (table 9 in "Supplementary Information" section). The cores from all three lakes appeared stratified, with alternating dark and light layers. These layers became less distinct at the top of the core. The deeper stratified layers could be correlated easily among the cores; the shallower layers were less easily correlated. This may account for a larger variance among concentrations of selected constituents in shallower sediment samples as compared to the deeper samples. In addition, the deeper layers tended to be thicker.

Before a comparison could be made between the length of the cores collected and the Bureau's sedimentation data, the effect of core shortening had to be determined. Core shortening is a phenomenon associated with the use of a gravity corer and has been estimated to shorten the core by as much as 50 percent (Emery and Hulsemann, 1964). Core shortening also has been observed in a previous study in Kansas that collected sediment using a gravity corer (Juracek, 1997). Hongve and Erlandsen (1979) attribute the shortening to compression of sediment and loss of water content. This shortening effect also has been attributed to the effects of friction of the sediment against the inner wall of the core liner (Emery and Deitz, 1941; Blomqvist and Bostrom, 1987). There is also the possibility that the core-shortening phenomenon is due to a loss of sediment from the core column. If this occurs, there may be an effect on bulk density and related calculations.

In this study, the amount of core shortening was determined by measuring the distance from the water surface to the top of the sediment with a weighted tape and confirming this distance with a depth finder. When the gravity corer was released into the reservoir, the distance from the water surface to the tip of the gravity corer was determined by marking and measuring the distance against the cable holding the coring device. The difference between this depth and the depth to the sediment (as measured by weighted tape) was determined to be the thickness of sediment, in situ. Once the sediment core was retrieved, its length was measured, and this was divided by the thickness of sediment and multiplied by 100 to determine the recovery percentage (table 5). In most cases, any original channel-bed or land-surface material in the liner was less than 0.1 ft and did not affect the calculation of the recovery percentage. The average recovery percentage for cores from all three reservoirs was 68 percent. Table 9 in the "Supplemental Information" section at the end of this report includes lengths of all cores collected during this study, uncorrected for original channel-bed or land-surface material thickness or core shortening.

The lengths of cores KIR 1.4 and KIR 2.1 used for analysis of core lithology and grain size from Kirwin Reservoir were 7.9 and 3.2 ft, respectively (table 9). Core KIR 2.1 was the only core collected during the study that did not reach original land-surface material, which accounts for the shorter core length. When visually compared to the other three cores collected from coring site KIR 2, it was evident that the deepest three samples were missing from core KIR 2.1. The percentage of silt and clay was equal to or greater than 97.7 percent in all samples from both Kirwin cores (table 10 in "Supplemental Information" section). The mean percentages of silt and clay determined for coring sites KIR 1 and KIR 2 were 99.4 and 99.5 percent, respectively (table 6). The mean percentage of sand was less than 1.0 percent for both coring sites. Bulk densities for samples from cores KIR 1.4 and KIR 2.1 ranged from 27.5 to 50.4 lb/ft³ (table 10), with the larger bulk density in samples from the bottom (deeper) parts of each core. The percentage of moisture in samples from cores KIR 1.4 and KIR 2.1 ranged from 46.8 to 65.5 percent, with the shallower (top) samples having a larger percentage of moisture (table 10).

The lengths of cores WEB 1.1 and WEB 2.1 used for the analysis of core lithology and grain size for Webster Reservoir were 7.0 and 7.2 ft, respectively (table 9). The smallest percentage of silt and clay in all samples was 98.9 percent, and the percentage of sand in all samples was less than or equal to 1.1 percent (table 10). Bulk density ranged from 21.1 to 48.6 lb/ft³ (table 10). Mean bulk-density values for the two cores were 35.4 and 34.8 lb/ft³ (table 6). Larger bulk-density values generally occurred in the deeper samples. The percentage of moisture ranged from 46.6 to 72.9 percent, with samples at the top of the cores generally having a larger percentage of moisture (table 10). The additional cores collected in Webster Reservoir, because they were collected both in and out of the original channel bed, had a wider range in grain size, bulk density, and percentage of moisture. In general, grain sizes and bulk-density values were larger in out-of-channel cores as compared to in-channel cores, and the percentage of moisture was smaller in out-of-channel cores.

The lengths of cores WAC 1.1 and WAC 3.2 used for the analysis of core lithology and grain size for Waconda Lake were 6.1 and 7.0 ft, respectively (table 9). The percentage of silt and clay in samples from both cores was greater than 99 percent, with less than 1 percent sand (table 10). Bulk density ranged from 21.6 to 47.0 lb/ft³, with larger bulk-density values generally occurring in deeper samples (table 10). The percentage of moisture ranged from 46.3 to 73.1, with shallower samples generally having a larger percentage of moisture.

The cores from Kirwin, Webster, and Waconda that were collected within the channel and near the dam had mean percentages of silt and clay greater than 99 percent (table 6). In-channel core bulk-density values were generally larger in deeper samples, and the percentage of moisture was generally smaller when compared to shallower samples. The cores from Webster Reservoir that were collected outside of the original channel bed (coring sites WEB 6 through WEB 11) generally had larger sand percentages, larger bulk-density values, and smaller moisture percentages when compared to samples from in-channel cores.

Age Dating and Depositional Patterns of Bottom Sediment

Kirwin Reservoir, the oldest of the three reservoirs in the basin, was completed in 1952. This completion date coincides with the first occurrence of ¹³⁷Cs in the atmosphere. Therefore, the date of deposition of the sediment in the bottom of cores from Kirwin Reservoir was assumed to be 1952 (fig. 4). To make comparisons between coring sites and among the different reservoirs, core-sample intervals were normalized. The peak in ¹³⁷Cs concentration in sediment cores KIR 1.2 and KIR 2.3 from Kirwin Reservoir occurred near the normalized sample interval of 0.5 (dimensionless) (fig. 4). The date 1963-64, the peak of ¹³⁷Cs atmospheric deposition (Holmes, 1998), was assigned to this interval. The ¹³⁷Cs concentration following the peak shows a smooth decline in concentration, which would indicate that the sediment has been relatively undisturbed by biological activity, inflows or outflows, human activity, or wind-induced currents. One additional date, the date of sampling (1998), was assigned to the top of the core.

Given the above dates, it may be possible to calculate depositional rates for sediment before 1963-64 and after 1964. However, these depositional rates may be misleading because of the core-shortening phenomenon discussed in the previous section. Because the cause of the core shortening is unknown, it is not possible to accurately correct for it. Additionally, although it is possible to verify that the entire sediment thickness has been penetrated and the deepest layers are present, it is not possible to verify that the shallowest layers are present. Because the gravity corer descends with great force, it may shift the shallowest, less consolidated layers so that they are not recovered in the core liner. Therefore, average depositional rates calculated on the basis of core length are not presented herein, but depositional patterns as they relate to reservoir inflows are discussed in more general terms.

In Kirwin Reservoir, it appears from the ¹³⁷Cs profiles (cores KIR 1.2 and KIR 2.3, fig. 4) that the 1963-64 peak is at approximately the center of each core. Because the normalized sample intervals between 0 and 0.5 represent approximately the first 12 years of sediment deposition and the normalized sample intervals between 0.5 and 1.0 represent approximately 34 years of sediment deposition, one might conclude that the depositional rate prior to 1963-64 was more than twice the depositional rate after 1964. However, because of the core shortening and recovery phenomena discussed previously, a depositional rate cannot be accurately established. There is evidence, however, that indicates that deposition was, in fact, greater in the early years of the reservoir. A visual examination of the cores from Kirwin Reservoir reveals thick layers of sediment in the deepest intervals. This is consistent with other reservoir studies (Ritchie and others, 1986; Callender and Robbins, 1993), which demonstrated that sedimentation rates decreased with reservoir age. Historical streamflow data for the inflow to Kirwin Reservoir indicate that much of the early sediment deposition in the lake may have been caused by floods during the 1957 and 1960 water years (fig. 5).

Webster Reservoir was completed in 1956, and although the first atmospheric deposition of ¹³⁷Cs cannot be assigned, the 1963-64 peak is evident near the 0.5 sample interval (fig. 4). As with Kirwin Reservoir, the ¹³⁷Cs concentration following the 1963-64 peak shows a relatively smooth decline, indicating presumably undisturbed sediment. The top of the cores may be assigned a date of 1998 (the date of sampling). The cores from Webster Reservoir (cores WEB 1.3 and WEB 2.3, fig. 4) also showed thicker layers of sediment in the deeper samples compared to the shallower samples. Similar to data derived from cores from Kirwin Reservoir, the greater sedimentation rate of the initial deposits in Webster Reservoir may have been caused by floods in the 1957, 1960, and 1962 water years (fig. 5).

Because Waconda Lake was completed in 1967, neither the first incidence of ¹³⁷Cs (in 1952) nor the peak (1963-64) is evident in sediment cores WAC 1.2, WAC 2.2, and WAC 3.4 from Waconda Lake (fig. 4). The only dates that can be assigned to the cores from Waconda Lake are the date of dam completion (1967) to the bottom of the core and the sampling date (1998) to the top of the core. However, a relatively smooth decrease in ¹³⁷Cs concentrations is evident for the three sediment cores. This would indicate that the bottom sediment has been relatively undisturbed. Upon visual examination of the Waconda Lake cores (WAC 1.2, WAC 2.2, and WAC 3.4, fig. 4), it appears that there also was significant sedimentation in the early years of the reservoir, as indicated by a very thick sedimentation layer in the deepest core samples. This may be due to large inflows in 1969 and 1974 (fig. 5).

In addition to large inflows in the years following reservoir completion, the greater sedimentation rates observed in the three reservoirs before 1964 may be due, in part, to other factors. These include a change in sediment source, changes in land use, or the development of small-scale impoundments or reservoirs within the basin that trap sediment before it reaches Kirwin Reservoir, Webster Reservoir, and Waconda Lake.

Concentrations and Trends in Selected Chemical Constituents

Table 7 shows a statistical summary of selected chemical constituents analyzed in bottom-sediment samples from the three Solomon River Basin reservoirs. Concentrations of selenium, arsenic, strontium, and phosphorus in samples from the additional cores collected from Webster Reservoir (coring sites WEB 3-11) are not included. The USEPA (1997b) has established TELs and PELs for eight of the constituents listed. The median arsenic and copper concentrations exceeded the TELs for samples from all three reservoirs. Cadmium concentrations exceeded the TEL in four samples from Kirwin and Webster Reservoirs. Nickel concentrations exceeded the TEL in 11 samples from Webster Reservoir. Concentrations of chromium, lead, and mercury either were not detected or did not exceed TELs in samples from any reservoir. PELs were not exceeded for any of the eight constituents in samples from any of the three reservoirs.

Increasing trends in selenium, arsenic, manganese, strontium, and phosphorus concentrations, determined using Spearman's rho correlation and Kendall's tau trend test, occurred in bottom-sediment samples from at least one reservoir (table 8). Decreasing trends in aluminum and magnesium concentrations occurred in bottom-sediment samples from Webster Reservoir. Decreasing trends in iron concentrations occurred in bottom-sediment samples from Kirwin and Webster Reservoirs. No increasing or decreasing trends occurred in concentrations of barium, beryllium, boron, cadmium, chromium, copper, lead, nickel, vanadium, zinc, nitrogen, or total organic carbon. Mercury was not detected in bottom sediment from any of the three reservoirs in the basin. Of the chemical constituents analyzed in core samples from the three reservoirs, arsenic, selenium, and strontium were the only constituents that showed an increasing trend in concentrations in samples from more than one reservoir.

Selenium

In Kirwin Reservoir, selenium concentrations in samples from bottom-sediment cores ranged from less than 0.3 to 2.2 mg/kg (table 11 in "Supplemental Information" section). A significant increasing trend (p=0.006) was evident in only one of the two Kirwin cores (core KIR 1.3, fig. 6). In Webster Reservoir, the selenium concentrations in bottom-sediment cores ranged from 0.3 to 4.0 mg/kg (table 11). A significant increasing trend (p=0.001 and p=0.035) occurred in both Webster bottom-sediment cores. In Waconda Lake, selenium concentrations in samples from bottom-sediment cores ranged from less than 0.4 to 3.4 mg/kg (table 11). Significant increasing trends were evident in two of the three cores (cores WAC 2.1, p=0.031, and WAC 3.1, p=0.001). Core WAC 1.3 had a p value slightly higher than the specified criteria (p=0.052).

Arsenic

In Kirwin Reservoir, arsenic concentrations in samples from bottom-sediment cores ranged from 4.6 to 10 mg/kg (table 11 in "Supplemental Information" section). In Webster Reservoir, arsenic concentrations in bottom-sediment cores ranged from 8.0 to 15.1 mg/kg (table 11). In Waconda Lake, arsenic concentrations in bottom-sediment cores ranged from 5.4 to 13.1 mg/kg (fig. 7, table 11). A significant increasing trend in arsenic concentrations was not evident in the Kirwin cores. There was, however, an increasing trend in the cores from Webster Reservoir (core WEB 1.2, p=0.010, and core WEB 2.4, p=0.010) and Waconda Lake (core WAC 1.3, p=0.012, and core WAC 3.1, p=0.002).

Strontium

Strontium concentrations in core samples from Kirwin Reservoir ranged from 74 to 219 mg/kg (table 11 in "Supplemental Information" section) and showed a significant increasing trend in both cores (p=0 and p=0.017) (fig. 8). Strontium concentrations ranged from 109 to 263 mg/kg in cores WEB 1.2 and WEB 2.4 from Webster Reservoir (table 11) and showed a significant increasing trend (p=0). Strontium concentrations in Waconda cores WAC 1.3, WAC 2.1, and WAC 3.1 ranged from 68 to 336 mg/kg (table 11). Two of the three bottom-sediment cores from Waconda Lake had significant increasing trends (core WAC 1.3, p=0.005, and core WAC 2.1, p=0.001).

Nutrients and Total Organic Carbon

Bottom-Sediment Volume and Mass in Webster Reservoir

Additional sediment cores were collected from seven sites (WEB 3 through WEB 11, fig. 2B) in Webster Reservoir to estimate the mass of bottom sediment. Of the three reservoirs in the Solomon River Basin, complete bathymetric data were available only for Webster Reservoir. An extensive sedimentation survey of Webster Reservoir was completed by the Bureau in 1996. This survey showed very little sediment deposition in Webster Reservoir given the size of its drainage area (Ferrari, 1997). Previous surveys by the Bureau in 1968 and 1979 verify the small accumulation of sediment. In addition, during this study, several attempts to collect sediment outside the original channel bed failed because there was no sediment or there was too little sediment for analysis, confirming findings of previous bathymetric surveys. The 1996 survey estimated that a total of 1,267 acre-ft of sediment has accumulated since dam closure in May 1956; approximately 81 percent of the sediment was deposited in the original channel bed (Ferrari, 1997). As with previous surveys, the 1996 survey showed very little sediment deposition outside of the original channel bed, except for some accumulation near the north bank due to bank erosion.

The sediment volume determined by the Bureau's 1996 sedimentation survey of Webster Reservoir was used to estimate the 1998 volume and mass of sediment for the reservoir. The Bureau's estimated annual loss in storage capacity was 31.7 acre-ft per year; therefore 31.7 acre-ft was added to the 1996 sediment volume to estimate 1997 volume, and 31.7 acre-ft was added to the 1997 volume for 1998. The final estimated sediment accumulation in the reservoir for 1998 was 1,330 acre-ft.

Bulk density, the weight per unit volume of the oven-dried bottom sediment, was calculated for all sites in Webster Reservoir using methods described by Guy (1969). Bulk density generally was higher for the out-of-channel cores; the mean out-of-channel bulk density was 91.0 lb/ft³ (samples containing original material were not included), and the mean in-channel bulk density was 48.9 lb/ft³. Because of the differences in bulk density, mass was calculated separately for in-channel and out-of-channel components. On the basis of the Bureau's estimate that 81 percent of sediment accumulation was in the original channel bed, 1,080 acre-ft of sediment was multiplied by the mean bulk density of all in-channel samples (sites WEB 1-5, fig. 2B) to estimate the in-channel sediment mass. Similarly, the remaining 253 acre-ft of sediment accumulation was multiplied by the mean out-of-channel bulk density to estimate the out-of-channel sediment mass. These two components were summed to arrive at the total mass of sediment accumulation in Webster Reservoir since dam closure in 1956 of 3,300 million lb. It is important to recognize that the core-shortening phenomenon may affect bulk-density calculations and thus the calculation for total mass. The mass calculation presented here has not been corrected for core shortening.

Mean annual sediment deposition was calculated by dividing the total mass of sediment accumulation by 42 (the number of years since dam closure). Mean annual sediment deposition was calculated as 78,600,000 lb/yr. This estimate does not reflect probable differences in sedimentation from year to year. In addition, because mean bulk density was used in the calculation, mean annual sediment deposition may be understated if the core-shortening phenomenon discussed previously was due to a loss of sediment in the core.

Selenium, Arsenic, and Strontium Loads in Webster Reservoir

Comparison of Depositional Trends to Irrigation and Constituent Loads

The possible effect of irrigation on selenium, arsenic, and strontium concentrations in bottom sediment was assessed first by comparing the annual change in irrigated acres in Kirwin and Webster Irrigation Districts (fig. 9) to selenium, arsenic, and strontium concentration profiles in cores from Waconda Lake (figs. 6, 7, and 8), which receives water from these Bureau-operated irrigation districts. The amount of land irrigated with water from Kirwin and Webster Irrigation Districts, which depend on available water supply, has not consistently increased through the years. The concentrations of selenium, arsenic, and strontium in reservoir bottom sediment, however, have increased since completion of Waconda Lake in 1967. Therefore, constituent concentrations in Waconda Lake bottom sediment cannot be explained entirely by Bureau irrigation.

To further assess the effect of Bureau irrigation, the mean concentrations of selenium, arsenic, and strontium in bottom sediment from Waconda Lake, which receives inflow from both Bureau- and non-Bureau-irrigated areas, were compared to mean concentrations in bottom sediment from Kirwin and Webster Reservoirs, which do not receive inflow from Bureau-irrigated areas. When comparing bottom-sediment constituent concentrations in Kirwin and Webster Reservoirs to Waconda Lake, it is important to consider the difference in geology between the reservoirs. The geology changes from primarily Quaternary and Tertiary strata upstream from Kirwin and Webster Reservoirs to primarily Cretaceous strata downstream from these two reservoirs (fig. 1). In addition, the only outcrop of Pierre Shale, a known source for selenium, in the Solomon River Basin is in a small area upstream from Webster Reservoir. Another important consideration in comparing bottom-sediment concentrations is the difference in the source of irrigation water, which is ground water upstream from Kirwin and Webster Reservoirs and surface water upstream from Waconda Lake.

The median selenium concentration in core samples from Waconda Lake is less than the median concentration in samples from Kirwin Reservoir and equal to the median concentration in samples from Webster Reservoir (table 7). The median arsenic and strontium concentrations in core samples from Waconda Lake are larger than the median arsenic and strontium concentrations in samples from Kirwin Reservoir and smaller than the median concentrations in samples from Webster Reservoir. Because constituent concentrations in bottom sediment from Waconda Lake are not generally larger or smaller than constituent concentrations in bottom sediment from Kirwin and Webster Reservoirs, Bureau irrigation, which occurs upstream from Waconda Lake, does not entirely explain increasing trends in selenium, arsenic, and strontium concentrations in bottom sediment from Waconda Lake.

The effect of total (Bureau and non-Bureau) irrigation on selenium, arsenic, and strontium concentrations was assessed by comparing background constituent concentrations (before 1963-64) to constituent concentrations after 1964 (post background) for Kirwin and Webster Reservoirs. Background concentrations for selenium, arsenic, and strontium were represented in bottom-sediment samples from Kirwin and Webster Reservoirs, which were constructed prior to the major period of irrigation development in the Solomon River Basin (early to mid-1960's). Background concentrations could not be calculated for Waconda Lake because of its 1967 completion date. ¹³⁷Cs results indicated sample intervals from Kirwin and Webster sediment cores that were deposited prior to 1963-64 and thus are representative of back-ground conditions.

Mean post-background selenium, arsenic, and strontium concentrations in core samples from Kirwin and Webster Reservoirs were larger than background concentrations in those reservoirs (figs. 10A, B, C). Because there are no Bureau-irrigated acres upstream from Kirwin and Webster Reservoirs, Bureau irrigation is not affecting the concentrations of selenium, arsenic, and strontium in bottom sediment from these reservoirs. However, other irrigation (non-Bureau) activities in the basin may be contributing to the trend to larger post-background constituent concentrations. Without sufficient historical irrigation data in the Solomon River Basin, this is difficult to verify.

Although selenium, arsenic, and strontium concentrations have increased in reservoir-bottom sediment in the Solomon River Basin, the total mass of selenium, arsenic, and strontium transported per year (annual load) to the reservoirs is also important to consider. Background and post-background mean annual loads were calculated for Webster Reservoir (figs. 10D, E, F). The post-background annual loads for selenium, arsenic, and strontium are smaller than background annual loads. In contrast, constituent concentrations have increased with time. Annual load calculations are affected by bulk-density calculations, which may be affected by the core-shortening phenomenon. It is not known whether most of the core shortening occurs in the shallow or the deep layers of a core or whether the shortening phenonmenon is due to the loss of sediment. If there is a loss of sediment in the shallow layers, mass and annual load calculations would be positively biased towards post-background annual loads (given the fact that bulk density increases with depth).

Differences in constituent loads with time may be due to several factors, including changes in streamflow, precipitation, land use, source and type of irrigation, and land-management practices. Background and post-background mean annual streamflow was examined to determine how much of the difference in background and post-background loads were due to changes in streamflow (fig. 11). For Webster Reservoir, decreases in mean annual streamflow and selenium, arsenic, and strontium loads are in agreement. However, the relative difference in background compared to post-background constituent loads is greater than the difference in background compared to post-background mean annual streamflow. Therefore, the larger constituent loads after 1964 are not completely due to changes in streamflow. Although consitutent loads were not calculated for Kirwin Reservoir and Waconda Lake, these reservoirs also had larger mean annual streamflows prior to 1964, and it is reasonable to assume that constituent loads also would be larger prior to 1964.

SUMMARY

REFERENCES

SUPPLEMENTAL INFORMATION

Table 9. Core length, estimated sediment thickness, and recovery percentage for multiple cores from sediment coring sites in Kirwin Reservoir, Webster Reservoir, and Waconda Lake, May 1998

Table 10. Results of analyses of selected physical properties for bottom-sediment cores collected in May 1998 from Kirwin Reservoir, Webster Reservoir, and Waconda Lake

Table 11. Results of analyses of sediment-quality constituents in bottom-sediment cores collected from Kirwin Reservoir, Webster Reservoir, and Waconda Lake, May 1998

For additional information about Reservoir Sediment in Kansas, please visit: http://ks.water.usgs.gov/Kansas/studies/ressed/

For additional information contact:

David Mau
U.S. Geological Survey
4821 Quail Crest Place
Lawrence, KS 66049-3839
Telephone: (785) 832-3517
Fax: (785) 832-3500
Email: dpmau@usgs.gov