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Land Suitability and Water Quality Studies

Sediment at Angostura Reservoir

There are two aspects of sediment to consider from a water quality perspective. The first is sediment as a pollutant in and of itself. For example, the DENR (1996) has listed suspended solids, equivalent to total suspended sediment (TSS), as the greatest cause of water quality impairment in the Cheyenne River basin between Angostura Reservoir and the mouth of the Belle Fourche River. The other aspect of suspended sediment relates to its vehicle for the transport of contaminants. Both aspects of sediment will be addressed in this section.

Much of the TSS that is responsible for water quality impairment is attributed to erosion off the Badlands via Sage Creek (DENR, 1996), which empties into the Cheyenne River downstream from the AID. One of the characteristics of a reservoir is that it traps sediment. The accumulation of sediment in the reservoir has been addressed elsewhere. From a water quality perspective, the reservoir removes sediment from the river. There are few TSS data available for the river downstream from Angostura Dam. The only TSS samples were collected during the summer of 1950; all of the data are shown in Table 10. Samples were also collected from a site upstream from the reservoir; contemporary samples from upstream are also included in Table 10.

Table 10. Angostura Reservoir Inflow and Outflow and Suspended Sediment during the Summer of 1950
No. of 
No. of Days Stream
Susp Sed
Date Stream
Susp Sed
07/11 5 6 846 60,600 07/11/50 109 42
07/18 1 1 105 10,200 07/18/50 125 66
07/24 8 4 789 21,550 07/26/50 197 79
08/05 4 14 1,238 19,575 08/08/50 116 86
08/15 1 1 332 14,200 08/15/50 79 19
08/25 2 20 436 19,400 08/29/50 55 10
09/05 5 3 2,864 31,100 09/19/50 28 12

Most of the upstream samples were not collected on the same dates as the downstream samples. The upstream data represent an average for a period that precedes or includes the date of the downstream sample. In Table 10 the number of observations is the number of samples in the average, while the number of days is the period that was included in the average. The comparison is somewhat complicated by the fact that there is a variable travel time through the reservoir. The travel time is affected by the flow and by the reservoir content. In addition stratification within reservoir can dictate the formation of currents that can short-circuit travel through the reservoir. High concentrations of TSS will also affect the density of the inflowing water; this can also affect the way in which the water is routed through the reservoir. What all of this means is that the comparison in Table 10 is something of an oversimplification of a complex situation. Nevertheless Table 10 provides an example of what happens in the reservoir.

There are two mechanisms that can affect the TSS concentration in the reservoir. The first is settling. When the flowing water enters the reservoir, the velocity declines significantly as the inflow mixes with the water already in the reservoir. The ability to transport sediment is greatly affected by velocity. As the velocity declines, the ability to transport sediment decreases, depositing the sediment. The deposition is related to the size and composition of the particles. In general, the larger particles are deposited first, while the finer particles are transported farther in the reservoir. Some particles may be carried through the reservoir to the river downstream. This is reflected in the sediment concentrations in Table 10.

The complete inflow TSS data set is shown on Figure 11. The plot shows the inflow TSS for the period prior to sample collection downstream Line graphs of TSS at Angostura Reservoirfrom the dam. The plot is on semi-log axes because the variation in the inflow TSS concentration and difference between the inflow and outflow TSS are so large. It should be noted that there are still only 7 samples below the dam; there appear to be more on figure 11 because of interpolation between adjacent points in the data set. Since Figure 11 is only intended as a qualitative illustration, this should not be a problem.

There are several orders of magnitude of difference between the TSS of the inflow and outflow of Angostura Reservoir (Table 10; Figure 11). For design purposes, Reclamation assumes that 100 percent of the inflowing sediment will be retained in the reservoir. The average inflow TSS concentration for the data in Figure 11 is 24,800 mg/L; the average TSS in the outflow is 52 mg/L. If these are representative data, then 99.8 percent of the sediment was being retained in the reservoir.

The appearance imparted to water by suspended material is more of a consideration to most than the actual concentration of TSS in the water. The measure of the visual effect of TSS is turbidity. Turbidity data for the Cheyenne River are shown in Table 11. Table 11 also includes all of the TSS data collected at the inflow site for Angostura Dam. The samples were collected between 1949 and 1967. The turbidity samples  shown as "Above Dam" were collected at two sites by the TVA (Tennessee Valley Authority) between 1983 and 1989. The sample sites were above and below the Edgemont Uranium Mill UMTRA cleanup, but both above the reservoir. There are a large number of samples. The sample design involved hourly samples for 24 hours either once a month or following storms. Consequently the samples are in sets of 24 that are internally aggregated; in other words they are not independent samples in a statistical sense. From that perspective, there are considerable fewer than the 1500 samples shown in Table 11. There are no concurrent TSS samples in the TVA data set.

Table 11. Suspended Solids and Turbidity at Three Sites on the Cheyenne River
Above Dam Below Dam Near Mouth
Minimum 8         < 1       10         < 1       < 1         < 1      
Median 14,000         17       42         10       390         61      
Maximum 68,200         14,625       86         25       53,900         12,000      
# of Obs. 162         1503       7         7       184         144      

FTU - Formazin Turbidity Unit
JTU - Jackson Turbidity Unit 
NTU - Nepholometric Turbidity Unit

The turbidity data that are also included in Table 11 have 3 different measurement units their equivalent abbreviations are also shown in the table. The original measure of turbidity was the Jackson candle turbidimeter (APHA, 1992); turbidity was measured in JTU. The minimum turbidity that the Jackson candle could measure was about 25 JTU (APHA, 1992), a value that would be characteristic of clear pond water (McKee and Wolfe, 1963). The currently preferred method is to use a turbidimeter (or nephelometer). The turbidimeter is standardized against a suspension of formazin polymer. An FTU is any turbidity measure against a formazin standard. The NTU is an FTU as measured on a nephelometer, which can measure down to 0.1 NTU. The drinking water standard for turbidity is 1 NTU, which is assumed to indicate the presence of bacteria. The NTU (= FTU) is equal to a JTU at 40 units. The measures will deviate from each other as one moves away from 40 units depending on the instrument used; they are nevertheless considered equivalent (Fishman and Friedman, 1989).

Table 11 contains both TSS and turbidity data for a third site near the mouth of the Cheyenne River. Specifically the site is at the USGS gage at Cherry Creek Village. Of the 144 turbidity samples collected at the third site, there are 78 with concurrent TSS data. The turbidity and TSS data show a relatively good correlation (r² = 0.65). However, when an obvious pair of outliers is dropped from the data set, the r² improved to 0.95. The equation without the outliers is:

    Turb. = (0.3201 * TSS) - 3.0294

Based on this equation, turbidity is about the TSS.

It should be noted that turbidity only relates in part to the TSS concentration. The turbidity is also affected by the shape and distribution of particles within the sample. The r² of 0.95 would indicate that the sediments are comparatively uniform in size from sample to sample and fine enough to be similarly dispersed throughout the samples.

The turbidity data shown in Table 11 for the site below the dam are not actual turbidity measurements. The turbidity data were developed using the above turbidity equation. How accurate this might be is unknown since the relationship reflects conditions well downstream from the dam. If it is accurate, then the dam releases would have a turbidity of 25 NTU or less. This is consistent with the type of clear water release that is characteristic of dams.

The TSS samples at the site at Cherry Creek were collected between June 1972 and August 1995. Obviously this is later than the TSS samples near the reservoir, but it includes the period during which the TVA samples were collected. The range of TSS in the samples collected at the site upstream from the reservoir is somewhat greater than that at the Cherry Creek gage (Table 11). The turbidity range is also much larger at the upstream site. Since the reservoir is between the two sites, most of the sediment at the Cherry Creek gage would be expected to have originated from inflows downstream from the dam.

The Cheyenne River gage near Wasta also has limited data on both turbidity and TSS. The Wasta gage is nearest gage downstream from the project area, but it is also downstream from the Sage Creek confluence. The 7 turbidity and TSS samples are shown in Table 12 along with stream flows and sample dates. The turbidity samples were collected in less than a year during 1969-70, while the TSS samples were collected over a 4-year period during 1983-87 (Table 12). The turbidity samples were collected over a relatively small range of flow (42-188 ft³/s) and show a comparably small range. The TSS samples on the other hand were collected over a very broad range of flow (64-20,300 ft³/s) and show an appropriately broad TSS range. The TSS data are consistent with the effects of storm runoff with erosion as the sediment source. It is also consistent with the DENR (1996) characterization of sediment sources and patterns downstream from Angostura Dam.

Table 12. Flow, Turbidity, and TSS in the Cheyenne River near Wasta
Sample Date 
Stream Flow (ft³/s) 
Turbidity (JTU) 
Sample Date 
Stream Flow,  (Inst-ft³/s)
TSS (mg/L)

The relationships between flow and TSS, for each of the above discussed gages on the Cheyenne River, along with one for turbidity at the Cherry Creek gage, are illustrated by the regressions in Table 13. All of the regressions are statistically significant at an of level less than or equal to 0.01; the turbidity and flow data presented in Table 12 for the gage near Wasta do not produce a statistically significant regression (r² = 0.26). The two seemingly best regression relationships shown in Table 13 are for the gages below Angostura Dam and near Wasta; however, both are based on only 7 observations. Because of the limited data sets on which these are based, the relationships represent a limited range of conditions and cannot be used as predictive tools. Alternatively, the r²-values for the other regressions are less than 0.5, indicating that the regressions explain less than 50 percent of the variation in the dependent variable (i.e., TSS or turbidity). Because of the small r²-values, these regressions are not good predictive tools either. As a generality the regressions indicate that there is a relationship between the TSS concentration, but it is more complex than what can be quantified by a simple linear regression.

Table 13. Summary of Regressions of TSS on Flow (Q) and Turbidity on Flow at USGS Gages on the Cheyenne River
Hot Springs
below Dam
near Wasta
at Cherry Creek
TSS-Q                      Turb-Q
0.386 0.764 0.829 0.358 0.468
r 0.621 0.874 0.911 0.598 0.684
d.f. 160 5 5 177 138
b1 0.745 1.248 1.229 0.975 1.260
b0 4.712 -2.076 -1.066 0.765 -2.909
Begin date 10/20/49 07/11/50 06/09/83 07/05/72 10/03/74
End Date 08/01/67 09/19/50 08/31/87 08/28/95 08/28/95
NOTE - the above regressions are fitted to an equation of the form: 
              TSS = exp([b1 ln{Q}] + b0)

Ten different fractions based on particles sizes have been sampled over the years at the Hot Springs gage near the reservoir inflow. The different fractions have been sampled with varying frequencies over the years. Table 14 shows the regressions of the concentrations as a function of flow; the regressions are in the same form as those in Table 13. What is interesting about the regressions based on particle size fractions is that, in general, the r² improves as the size of the particles increases for particle sizes up to 0.125 mm. The slope of the regression line also increases up to the same point. This is a reflection of the fact that the smaller particles are transported independent of flow. In other words, most any flow is high enough to carry the very finest particles. As the particles increase in size, transport becomes a factor; i.e. larger flows are needed to carry the larger particles.

Table 14. Regression of Size Fractions of Suspended Solids on Flow
Particle Size r² [%] r d.f. Prob. > r b1 b0
<2µm 6.1% 0.247 96 < 0.05 0.284 6.802
2-4µm 8.1% 0.285 142 < 0.01 0.369 5.518
4-8µm 25.2% 0.502 81 < 0.01 0.557 3.821
8-16µm 19.9% 0.446 141 << 0.01 0.654 3.966
16-31µm 32.9% 0.573 67 << 0.01 0.824 1.338
31-62µm 14.3% 0.378 144 << 0.01 0.618 3.658
62-125µm 58.4% 0.764 63 << 0.01 1.236 -2.687
0.125-0.25mm 77.2% 0.879 20 << 0.01 1.309 -2.495
0.25-0.5 mm 67.4% 0.821 53 << 0.01 1.242 -2.917
0.5-1 mm 63.8% 0.799 28 << 0.01 1.030 -1.995

The regressions shown in Table 14 are plotted individually against flows between 1 to 6,500 ft³/s on Figure 12A. The relationships are somewhat confounded by the varying number of samples in each of the particle data subsets (Table 15). The problem is that the data are published as cumulative frequency distributions. The concentrations for the individual size fractions were calculated from the total concentration by multiplying the TSS by the cumulative frequency for a fraction and subtracting the similar value for the previous fraction. Where a fraction is missing, then the calculated concentration is actually the concentration of two or more size fractions, depending on the intervening number that were not reported. Since this is not a sediment transport study, but an environmental impact analysis, this problem is not considered critical to the results.

Most of the curves on Figure 12A are similar in form. There is a rapid increase at lower flows, followed by an attenuated increase as the flow increases beyond 1,000 ft³/s. The two exceptions are the 8-16 µm (10-3 mm) fraction and the and the 0.125-0.25 mm fraction. The 8-16 µm fraction shows a nearly linear increase continuously to the 6,500 ft³/s maximum shown on the figure. The 0.125-0.25 mm fraction increases at an increasing rate at flows beyond about 2,500 ft³/s. Neither of these trends is sustainable. Each of the regressions is based on a maximum flow of 6,540 ft³/s with the lone exception of the 0.125-0.25 mm fraction, which had a maximum flow of only 3420 ft³/s. The plot of the fraction is being extended well beyond the maximum observed. In this case such an extrapolation is inappropriate. Alternatively the reason for the anomaly for the 8-16 µm fraction cannot be explained based on the data. This may be a reflection of the much smaller number of samples in the previous size fraction. The smaller data set has the effect of increasing some of the data in 8-16 µm fraction. For example the median for the 8-16 µm fraction is much larger than medians for the adjacent fractions (Table 15).

Line graphs of suspended solids versus flow

Table 15. Summary Statistics for Flow and Suspended Solids (mg/L) Regressions
Susp Sed
Susp Sed
Part. Size
Susp Sed
Part. Size
Minimum 18 8 32 124 18 66
Median 413 14,000 443 6,449 443 2,497
Maximum 6,540 68,200 6,540 38,190 6,540 39,476
# of Obs. 162 162 98 98 144 144
Stream Susp Sed Stream Susp Sed Stream Susp Sed
Flow, Part. Size Flow, Part. Size Flow, Part. Size
ft³/s 4-8µm ft³/s 8-16µm ft³/s 16-31µm
Minimum 32 46 18 10 32 12
Median 484 1,444 455 3,146 608 651
Maximum 6,540 12,060 6,540 53,932 6,540 63,426
# of Obs. 83 83 143 143 69 69
Stream Susp Sed Stream Susp Sed Stream Susp Sed
Flow, Part. Size Flow, Part. Size Flow, Part. Size
ft³/s 31-62µm ft³/s 62-125µm ft³/s .125-.25mm
Minimum 18 8 42 3 88 12
Median 453 1,452 669 246 1,040 670
Maximum 6,540 56,745 6,540 30,987 3,420 2,096
# of Obs. 146 146 65 65 22 22
Stream Susp Sed Stream Susp Sed
Flow, Part. Size Flow, Part. Size
ft³/s .25-.5mm ft³/s 0.5-1 mm
Minimum 24 1 24 3
Median 653 216 1,040 143
Maximum 6,540 3,808 6,540 1,245
# of Obs. 55 55 30 30

Figure 12B shows a comparison of the sum of the concentrations in the various size fractions from Figure 12A and the concentrations from the TSS-flow regression shown in Table 13. The results are similar at the lower flows, although the summed TSS increases somewhat more rapidly. This is a reflection of the effect of the very fine sediment fraction (< 2 µm), which shows a very rapid increase at flows less than 1,000 ft³/s (Figure 12A). The two curves diverge greatly at higher flows and differ by about 12,000 mg/L at 6,500 ft³/s. At higher flows the sum of the fractions gives a much better estimate of the TSS than does the regression equation. The measured maximum TSS was 68,000 mg/L (Table 15). The maximum TSS from the sum of the fractions is 65,200 mg/L, while the TSS-Q regression gives a TSS of over 77,000 mg/L.

The above provides the basis for the evaluation of the effects of the various contract renewal alternatives. Alternatives that increase the storage, and consequently the amount of time that the water remains in the reservoir, relative to existing conditions, the more TSS will be removed from the water. The reverse will also be true and alternative that decrease the storage will increase the TSS in the releases relative to existing conditions. However, as long as the dam remains in place, some sediment would still be removed relative to the TSS of a free-flowing river.


Reclamation collected bed sediment samples on August 4, 1997. The NIWQP had only sampled bed sediments on 1 date during the 1988 reconnaissance study of the Angostura Unit, and no sediments were sampled during the 1994 verification study. The purpose of sampling the Cheyenne River sediments in 1997 was to update and expand the database developed by the NIWQP. The NIWQP database include 9 sites between the Edgemont gage and Red Shirt (see Greene et al., 1990; Table 23). Reclamation resampled 4 of the NIWQP sites, and sampled 3 other NIWQP sites where water or biotic samples had been collected, but no sediment sample. Sample locations and site numbers, which were assigned their NIWQP equivalent where appropriate, are as follows:

4a. Angostura Reservoir - Inflow Site
4b. Angostura Reservoir - Horsehead Creek Arm
4c. Angostura Reservoir near Dam
6 Angostura Canal
6S Angostura Canal - split
9 Topeska's Pond
10 Iron Draw
11 Kimmie's Pond
12 Cheyenne River near Custer County Road 656 Bridge
14 Cheyenne River near Fairburn (Red Shirt)
CRRC Cheyenne River at Rapid Creek
CRCC Cheyenne River at Cherry Creek

The two additional sites (CRRC and CRCC) that were sampled downstream from the NIWQP study area were added to supplement the data and extend the study area to near the confluence with the Missouri River. The complete set of Reclamation results are shown in Table 16.

The NIWQP evaluated sediment contamination by comparing the sample data against a baseline for Western U.S. soils that was published by Shacklette and Boerngen (1984). The rationale for this is that most of the stream and lake sediments were derived from eroded soils and the chemical composition of the sediment should reflect that origin. The baseline is shown in Table 17 for the elements analyzed in any samples that are to be used in this report, including those collected by the NIWQP, Reclamation, the OST, and the CRST. The Shacklette and Boerngen (1984) baseline does not include all of the elements that were analyzed in the above referenced studies, e.g. cadmium was not included.

A baseline that is used in exploration geochemistry is based on crustal abundance values (CAV). These are average concentrations in the earth's crust; concentrations that are greatly above these average CAV's are indicative of mineralized areas. In the Cheyenne Basin, many of the tributaries to the river originate in the Black Hills, which are predominantly dolomite [CaMg(CO3)2]. Other parts of the drainage basin are composed of a variety sedimentary rocks. For this reason abundance values for a variety of sedimentary rocks are also included in Table 17. Table 17 also illustrates the variability in the chemical composition of natural materials.

Table 16. Sediment Samples - Cheyenne River and Angostura Reservoir during 1997 (All in ppm, unless otherwise noted)
4a 4b 4c 6 6S 9 10 11 12 14 CRRC CRCC
Wet Wgt. [g.] 16.30 14.28 15.73 18.52 18.52 16.65 14.81 14.59 16.13 17.61 15.73 12.10
Dry Wgt. [g.] 6.72 3.43 4.87 13.53 13.53 5.23 9.97 9.78 13.45 10.90 9.14 8.32
TOC [%] 1.26 0.78 1.31 0.55 0.62 3.19 0.39 1.13 0.09 0.31 0.66 0.41
Arsenic 5.1 7.2 16.4 13.0 10.3 10.2 6.4 10.8 21.7 4.9 15.9 104.3
Boron 20 27 19 6 5 23 4 14 9 12 12 7
Barium 192 146 232 81 47 88 84 157 615 436 561 403
Cadmium 1.3 1.5 1.4 0.6 0.6 0.7 0.3 0.7 0.2 0.6 0.5 1.9
Chromium 23.0 27.0 21.0 4.0 4.0 7.0 6.0 17.0 2.0 10.0 14.0 12.0
Copper 20.0 26.0 29.0 7.0 7.0 8.0 9.0 19.0 4.0 12.0 24.0 19.0
Lead 19 < 48 < 62 < 146 < 146 < 63 < 135 < 135 < 167 21 < 116 < 138
Molybdenum 5.9 7.3 7.5 4.0 3.0 3.9 1.9 5.4 6.9 3.4 4.7 3.9
Nickel 26.0 28.0 39.0 14.0 12.0 14.0 11.0 23.0 20.0 13.0 31.0 23.0
Selenium 1.0 0.4 1.9 0.3 0.4 1.0 0.9 0.5 0.7 0.2 0.5 0.8
Vanadium 25.0 43.0 46.0 12.0 11.0 16.0 12.0 30.0 19.0 19.0 24.0 21.0
Zinc 59.0 97.0 95.0 28.0 24.0 71.0 36.0 317.0 52.0 40.0 91.0 63.0
Mercury < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1
Thorium 11.5 15.7 13.3 7.0 9.3 9.2 8.1 10.8 4.8 10.6 11.9 10.0
Uranium 4.2 5.1 5.8 2.8 3.2 17.8 2.5 3.3 2.7 3.2 3.4 3.2
Calcium 81,297 22,709 57,734 83,481 77,696 23,348 8,196 13,901 17,437 27,619 13,860 23,286
Magnesium 22,426 10,716 6,117 4,723 3,344 2,930 1,912 6,077 1,036 4,716 6,313 5,760
Sodium 711 1,419 936 271 261 704 341 593 152 1,000 854 654
Potassium 5,802 7,120 4,338 801 632 1,507 837 2,992 308 4,039 2,522 2,051
NOTE: TOC is Total Organic Carbon. 
Wet weight is the weight of the sample as received from the field.
Dry weight is the weight of the sample after drying, but before grinding.

Table 17: Various baselines for evaluating Cheyenne River sediments [all in ppm]
-------Western States-------- Soils Baseline Crustal & Sedimentary Rock Abundance Values 
Element Lower C.L. Mean Upper C.L. Earth's Crust [CAV] Clays &
Shales Sandstones Carbonate rocks
Arsenic  1.2 5.5 22 1.8 6.6 13 1 1
Boron  5.8 23 91 9.0 100 100 35 20
Barium  200 580 1,700 390 800 580 < 10 10
Calcium  1,900 18,000 170,000 46,600 25,300 22,100 39,100 302,300
Cadmium  No Data No Data No Data 0.2 0.3 0.3 < 0.1 0.035
Chromium  8.5 41 200 122 100 90 35 11
Copper  4.9 21 90 68 57 45 < 1 4
Mercury  0.0085 0.046 0.25 0.086 0.4 0.4 0.03 0.04
Potassium  3,800 18,000 32,000 18,400 22,800 26,600 10,700 2,700
Magnesium  1,500 7,400 36,000 27,600 13,400 15,000 7,000 47,000
Molybdenum 0.18 0.85 4.0 1.2 2 2.6 0.2 0.4
Sodium  2,600 9,700 37,000 22,700 6,600 9,600 3,300 400
Nickel  3.4 15 66 99 95 68 0.2 20
Lead  5.2 17 55 13.0 20 20 7 9
Selenium  0.039 0.23 1.4 0.05 0.6 0.6 0.05 0.08
Thorium 4.1 9.1 20 8.1 11 12 1.7 1.7
Uranium  1.2 2.5 5.3 2.3 3.2 3.7 0.45 2.2
Vanadium  18 70 270 136 130 130 20 20
Zinc  17 55 180 76 80 95 16 20
Sources: Western States Baseline: Shacklette and Boerngen, 1984.
C.A.V.: Fortescue, 1992.
Sedimentary rock values: Parker, 1967.

The comparison to the Western soils baseline indicates that more often than not, concentrations in the samples were below the lower confidence limit, rather than above the upper confidence limit. This is particularly true of barium and chromium among the trace elements and sodium and potassium among the more common elements; each of these elements were below the soils baseline at a majority of the sites.

There has been uranium mining and milling upstream from Angostura Reservoir. Erosion of mine and mill tailings has been implicated in loading of uranium to the reservoir sediments (Rahn et al., 1996; NRC, 1982). Uranium exceeded the upper confidence limit of the soils baseline at only one of the sites in the reservoir, the site nearest the dam (site 4c). This would indicate that the uranium was associated with the finest sediments; coarser particles would settle more quickly and be deposited nearer the inflow are of the resrvoir. The baseline was exceeded by the greatest margin in the only other site where the sample exceeded the uranium baseline, site 9 (Topeska's Pond), which is located off the river. The sample exceeded the upper limit of the baseline by a factor of slightly more than 3 (tables 10 and 11) The source for the uranium in the pond is unknown, but it seems unlikely that it came from the river or the reservoir.

Molybdenum (Mo) is the only other trace element that is elevated at more than one sample site. Mo is above the upper limit of the soils baseline at each of the reservoir sites. The sediments in the reservoir originate from areas upstream from Angostura Reservoir. Webb and Rahn (1994) indicate that the Inyan Kara Group is known to contain anomalously high concentrations of molybdenum associated with uranium deposits, some of which are adjacent to the Cheyenne River. The molybdenum could originate from either uranium mining or natural weathering of these rocks.

The reservoir sediment sample collected from the reservoir near Angostura Dam had a high concentration of selenium. Webb and Rahn (1994) also indicate that the same rocks that had the high concentration of molybdenum, contained high concentrations of selenium. The selenium could originate from the same source as the molybdenum, but in apparently finer material, since it was not deposited in the upper areas of the reservoir.

Arsenic and zinc were also above the upper confidence limit for Western soils at one site each. The elevated arsenic was observed in the sample collected from the site at Cherry Creek, which is located downstream from the mouth of the Belle Fourche River. Arsenic is a significant pollutant in the former gold fields of the Whitewood Creek basin. An estimated 100 million metric tons of finely ground gold-mill tailings were discharged to Whitewood Creek near Lead between 1876 and 1977 (Goddard et al., 1987). The primary contaminant is arsenic derived from, or still present as arsenopyrite (FeAsS), a gangue mineral common to the gold deposits around Lead (ibid.). Other elements identified in the tailings are antimony, cadmium, copper, iron, manganese, mercury, and silver (ibid.). Whitewood Creek is a tributary to the Belle Fourche River at the probable source of the arsenic in the sample from the site near Cherry Creek.

Zinc was elevated relative to the soils baseline in the sample collected from Kimmie's Pond (site 11). This site is located off stream; the site was chosen by the NIWQP as potentially receiving irrigation return flows. No water or sediment samples were collected from the site by the NIWQP, but biological samples were collected. Zinc was not elevated at the site (Greene et al., (1990).

The CAV values shown in Table 17 are based on an assumed average crustal composition that is predominantly igneous rock (95 percent - see Clarke and Washington, 1924). In an environment such as that of the AID, this may not be particularly appropriate. In the Project area, the reservoir, much of its drainage basin, and the Cheyenne River to a point a couple of miles downstream from Angostura Dam, are underlain by Mesozoic sedimentary rocks, predominantly shales and limestones (Greene et al., 1990). Most of the remainder of the Project area is underlain by Pierre Shale. Igneous rocks are confined to an area to the north of the Project area in the headwaters of Beaver Creek (see Greene et al., 1990 - Figure 2). If the sediments in the Cheyenne River are derived from the rocks in the drainage basin, the chemical composition should represent a mix of shale and limestone. Based on the data in tables 10 and 11, the sediments are principally derived from the shales. This is consistent with the conclusion of Greene et al. (1990) that their samples were similar in trace element composition to Department of Energy samples from Foxhills Sandstone and Pierre Shale surface exposures.

The purpose for sampling the sediments in this study was to expand the NIWQP data base and to compare the more recent samples. Selected elements from the 1988 and 1994 NIWQP samples and the 1997 Reclamation samples are shown on Figure 13. In 1988, the NIWQP sampled several areas in the reservoir, but the samples were composited into one sample for analysis in the laboratory; the reservoir was not sampled in 1994. Reclamation sampled three sites in the reservoir. For purposes of comparison, the geometric mean of the three Reclamation samples is plotted on Figure 13.

Bar graphs of arsenic, vanadium, selenium, and zinc concentrations at Cheyenne River sites

Arsenic in the reservoir was somewhat higher in the Reclamation sediment sample than in the NIWQP composite sample (Figure13). The sediment samples from the tributaries to Angostura Reservoir showed approximately the same arsenic concentration, but

were intermediate between the NIWQP and Reclamation sediment samples (Figure 13). The Reclamation sample from the Angostura Canal was about twice as high in arsenic as the NIWQP 1988 sample. The highest arsenic in any sample was the Reclamation sample from the county road 656 bridge, but it was only slightly higher than the NIWQP 1994 sample (Figure 13). In 1994, the NIWQP collected a sample from the site ¾ mile downstream from Angostura Dam; that sample had an arsenic concentration of 12 ppm, which is the same as the arsenic concentration in the 1988 sample from the site near Buffalo Gap. This would indicate that there is an arsenic source between the site near Buffalo Gap and the one at the county road bridge. There were no samples collected from the river between the dam and the bridge site in 1997 that could help identify the source.

The maximum arsenic in the NIWQP samples was collected from Cottonwood Creek, which had been sampled as a background site. Cottonwood Creek is downstream from the county road bridge site and could not be the source of the sediment arsenic. Arsenic in samples from the remaining two sites located in Iron Draw and the Cheyenne River near Fairburn are similar in the two sample sets (Figure 13). At each of these sites, the Reclamation samples are slightly lower in arsenic than NIWQP samples. Overall the sediments appear to have had a higher concentration of arsenic in 1997 than was the case in 1988.

In 1988, the Cheyenne River showed an increase in selenium in the downstream direction as far as Buffalo Gap (Figure 13). All of the selenium samples collected in 1994 were below the detection limit of 1 ppm. (Most of the samples collected in 1988 and 1997 were also less than 1 ppm, but the detection limits were also below 1 ppm). As was the case for arsenic, the maximum selenium concentration in the 1988 NIWQP sediment samples was observed in Cottonwood Creek. Selenium then declined at the site near Red Shirt, despite the inflow from Cottonwood Creek upstream from the site. In 1997, the results appear to be similar, although fewer sites were sampled. Selenium was barely measurable at the site near Red Shirt in 1997.

The final two elements shown on Figure 13 are vanadium and zinc. Both showed relatively high maximum concentrations in 1988, although both remained within the soils baseline. The reason for including the data in Figure 13 is to illustrate how much lower the concentrations are in 1997 than they were in 1988. This was what was expected because the 1988 samples were collected during a relatively significant drought period. This makes the above discussed arsenic results even more perplexing.

Figure 14 shows the uranium data for the NIWQP's 1988 study and the Reclamation samples collected during 1997. The NIWQP verification Bar graphs of uranium concentrations at Cheyenne Basin sitesstudy 1994 samples were all reported as < 100 ppm, which is well in excess of any of the 1988 or 1997 results. The NIWQP 1988 and Reclamation's 1997 do not show any pattern between the two years. There is an interspersion of results with some of the 1988 samples greater than the 1997 samples at half the common sites (Figure 14). The mean of the Reclamation samples from the reservoir is twice the concentration in the 1988 NIWQP composite. On the other hand the NIWQP canal and Iron Draw samples have a greater uranium concentration than the Reclamation samples. At Red Shirt the Reclamation sample once again shows the higher uranium concentration (Figure 14). About all that can be said of uranium based on the two sets of samples collected in the study area during 1988 and 1997 is that it is highly variable among the various samples sites. There is no clear pattern of uranium distribution. There is also no evidence of widespread contamination.

The OST collected sediment samples from the Cheyenne River at Red Shirt in 1996. Those data along with the NIWQP 1988 and 1994 and the Reclamation 1997 data for that site are plotted on Figure 14. The plots are shown on a logarithmic scale. There are two plots on Figure 14 to maintain a maximum of four log cycles (orders of magnitude) per plot. The upper plot (Figure 15A) includes elements with concentrations that were less than 100 ppm and ranged from a low of 0.02 ppm for mercury to a maximum of 56 ppm for boron. The lower plot (Figure 15B) includes elements that had concentrations between 10 ppm (chromium) and 1200 ppm (barium). A comparison to Table 17 shows that only the Reclamation molybdenum sample is above its respective upper 95 percent confidence limit for Western soils (15 ppm and 4 ppm).

Of the four sampled years shown on Figure 15, 1988 was one of the driest (based on the Wasta gage record), 1994 was fairly average, 1996 is another rather wet year, and 1997 was the overall wettest year in the 66 years for which there are data. If there is any relationship between mean annual flow (or particle size) and bed sediment concentrations of the elements shown, 1988 (NIWQP) should have the highest concentration and 1997 (USBR) should have the lowest. This is true for each of the elements in Figure 15B (Reclamation did not analyze the sample for manganese), but, with the exception of nickel, and possibly mercury, it is not true for any of the elements in Figure 15A. Reclamation did analyze the sample for mercury, but the analytical reporting limit was 0.1 ppm, which is greater than either of the other results shown in Figure 15A. Some of the other elements shown in Figure 15A also are affected by reporting limits. For example, the NIWQP results for both cadmium and molybdenum are actually reported as less than 2 ppm, which, in the case of cadmium, is greater than either of the other results. In this instance the actual concentrations may be greater than those of either the OST or Reclamation, but there is no way to determine this for sure. Alternatively, the molybdenum is definitely less than either of the other results.

Bar graphs showing elemental concentrations in Cheyenne River samples collected in four different years

The CRST initiated a sediment monitoring project in 1994 (CRST, undated). Results for stations in the vicinity of Cherry Creek are shown in Table 18. The Reclamation sample collected in 1997 at approximately the same site as CR-40 is also included in Table 18 for comparison. The CRST used clean-up criteria developed by the Washington Department of Ecology (DoE, 1995) for Puget Sound as evaluation criteria. The criteria include a "no adverse effect" level and a "minor adverse effect level." These are used by the DoE to evaluate whether cleanup is needed in contaminated areas of Puget Sound. None of the CRST samples from the Cheyenne River in the vicinity of Cherry Creek exceeded the DoE sediment standards (CRST, undated). The Reclamation sample collected in 1997 did exceed the DoE criterion for arsenic, but was well below each of the other criteria (Table 18).

Table 18. Sediment Criteria and CRST and Reclamation sediment data for the Cheyenne River in the vicinity of Cherry Creek

Sediment Quality Criteria 
Wash. DoE1

Cheyenne River Sioux Tribe

(Dry Weight Units) No effect Minor effect CR-21 CR-38 CR-40 CR-20 CR-39 CRCC
As, ppm 57 93 23.8 44.47 17.31 17.44 21.47 104.3
Cd, ppm 5.1 6.7 0.79 1.17 0.40 0.63 0.60 1.9
Cr, ppm 260 270 16.6 29 20.5 2 7.16 12.0
Cu, ppm 390 390 16.99 27.78 27.66 11.75 10.19 19.0
Fe, % - - - - - -  2.7 3.26 3 5.5 2 - - -
Hg, ppm 0.41 0.59 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1
Mn, ppm - - - - - -  332.9 552.9 516.5 2639.9 496.6 - - -
Ni, ppm - - - - - -  17.3 29.8 28.8 25.4 13.5 23.0
Pb, ppm 450 530 13.9 19.7 17.8 15.7 9.4 < 10.0
Zn, ppm 410 960 67.0 97.0 103.0 68.0 44.5 63.0
% solids  92.51 91.78 85.46 82.34 77.40 68.76
Sample Site Locations Site No. Date
1 Puget Sound Sediment Quality Chemical Criteria, Washington Department of Ecology

The primary reason that the CRST sampled the sediments in streams on their reservation was to evaluate the effect of floods that occurred throughout the summer of 1993 (CRST, undated). The Reclamation sample is somewhat higher than the CRST results for arsenic and cadmium, but within the range shown for the other elements (Table 18). There is no indication whether the CRST samples were sieved prior to analysis; the Reclamation sample represents only the < 0.062 mm fraction. If the CRST samples represent the total sediment, that could account for some of the difference. For example, Severson et al. (1991) observed differences among the way different elements were partitioned in different size-fractions of sediment. Arsenic was tightly clustered about the 45° line of equal distribution between fine and coarse sediments at lower concentrations, but showed a great deal of scatter at higher concentrations (> 20 ppm) of arsenic (ibid.). Despite the one excessive arsenic concentration, the sediments do not appear to be contaminated with any inorganic substances.

The partitioning of elements by size fractions is important to any flow relationship. Higher flows are capable of transporting larger sized sediments than lower flows. If an element is adsorbed to finer particles, such as clays, then it will tend to be concentrated in the fines. During periods of low flow, these elements would tend to constitute a larger part of the suspended sediments, and when deposited, a larger part of the surface layer of bed sediments. When higher flows are present in wetter years, larger particles with lower concentrations of the adsorptive elements will be present. These larger particles are less easily transported, and thus more readily deposited. The deposition of the larger particles with lower concentrations of the adsorptive elements will dilute the sediment concentrations. This is the reason that higher concentrations of some elements are expected following dry periods.

The CRST also analyzed 5 sediment samples for pesticides. No measurable pesticides were found in any of the samples, but only one of the samples was from the mainstem of the Cheyenne River (CRST, undated, Table 3). The NIWQP analyzed 5 sediment samples for atrazine and carbofuran. Samples were collected from the Cheyenne River upstream from Angostura Reservoir, above Buffalo Gap, and near Fairburn; samples were also collected from Iron Draw and Cottonwood Creek. All of the results were reported as < 0.1 ppm (Greene et al., 1990).


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