Status Update
Bioremediation
of the
Sweetwater Pit Lake –
Four Years Following Treatment
Oscar Paulson
Facility Supervisor
Kennecott Uranium Company
P.O. Box 1500
Rawlins, Wyoming 82301-1500
Telephone: (307) 324-4924
Email:
Joe Harrington
ARCADIS G&M, Inc.
630 Plaza Drive, Suite 200
Highlands Ranch, Colorado 80129-2379
Telephone: (970) 484-4433
Email:
Jim Harrington
ARCADIS G&M, Inc.
630 Plaza Drive, Suite 200
Highlands Ranch, Colorado 80129-2379
Telephone: (970) 484-4433
Email:
21 August 2004
ABSTRACT
The Sweetwater Pit Lake, an oligotrophic lake located in Sweetwater County, Wyoming was formed by the flooding of the Sweetwater open pit uranium mine following cessation of dewatering activities in April 1983. The lake exhibited levels of dissolved selenium (0.46 ppm) above reclamation standards (0.05 ppm) and dissolved uranium (8.1 ppm) above livestock standards (5.0 ppm). The lake, containing approximately 1.2 billion gallons of water, was treated by Kennecott Uranium Company with 548.7 tons of sugars, fats, proteins, alcohols, phosphates and nitrates from 10/19 to 12/22/99. Average concentrations of dissolved selenium and uranium in the pit lake dropped to 0.004 ppm and 4.21 ppm respectively, on July 26, 2004. These results show that the pit lake treatment has remained effective for approximately four (4) years.
The pit lake treatment process accelerated and enhanced the natural processes occurring in the pit lake prior to treatment, which had been slowly precipitating metals in the sediments. The addition of nutrients dramatically accelerated biological activity in the pit lake, which in turn accelerated the ongoing natural process of biologic concentration of metals in the pit lake sediments.
INTRODUCTION
Location/History
The Sweetwater Pit lake, located in Sweetwater County, Wyoming, was formed by the flooding of the Sweetwater Pit, an open pit uranium mine which was excavated from 1979 to 1983 by Minerals Exploration Company (MEC), a wholly owned subsidiary of Union Oil Company of California (Unocal). Dewatering of the pit ceased on 4/25/83 and began flooding with water from the surrounding Battle Spring Aquifer. The facility was acquired by Kennecott Uranium Company in 1992.
The pit consists of four (4) sub pits, the C-1 to C-4 pits respectively. The C-1 Pit, the first to be excavated, was backfilled prior to cessation of dewatering. The adjoining C-2 and C-3 Pits constitute the existing lake. The C-4 Pit was never excavated below the piezometric surface. It consisted of the elevated/dry benches south of the pit lake.
Geology/Hydrology
The Sweetwater Pit was excavated into the Eocene Battle Spring Formation, a fluvial sandstone that hosted uranium mineralization. The pre-mining piezometric surface of the Battle Spring Aquifer was approximately one hundred (100) feet below ground surface (6650 feet – surface elevation versus 6557 feet piezometric surface elevation). The Battle Spring Aquifer is approximately 6,000 feet thick in the vicinity of the Sweetwater Uranium Project. Pre-mining groundwater flow was to the southwest. The Sweetwater Pit is located in the Great Divide Basin, a hydrologically enclosed basin surrounded by the Continental Divide. The basin is hydrologically enclosed for both surface and groundwater. Ground and surface water flows to the center of the basin, where the water evaporates from a series of playa lakes.
The pit was dewatered by a number of deep (approximately 650 feet) dewatering wells completed around the pit. The wells were pumped over the operating life of the pit to locally draw down the Battle Spring Aquifer so that the pit bottom was above the water table. Following cessation of mining, the dewatering wells were shut down on 4/25/83 and the pit began to flood as water from the Battle Spring aquifer filled it. The water level in the pit began to rise to reflect the local piezometric surface of the Battle Springs Aquifer.
By the mid to late 1990’s, the water level in the pit lake stabilized at approximately 6540 feet above mean sea level. The pit lake had evolved into an evaporative sink, due to the relatively high evaporation rates (approximately sixty (60) inches per year–pan; forty-two (42) inches per year–lake), low levels of precipitation (5.44 inches per year – ten (10) year average) and berming of the pit to prevent the entry of surface runoff. The presence of the sink was documented through measurement of the lake surface elevation and the elevation of the piezometric surface in the dewatering and piezometer wells surrounding the pit. The entire pit lake is encircled by the 6540.50-foot piezometric contour line (Fall 1998), demonstrating an evaporative sink.
Water Chemistry
The pit lake, prior to treatment, exceeded (0.526 ppm – 10/5/99) the reclamation standard (0.05 ppm) for dissolved selenium and exceeded (8.51 ppm – 10/5/99) the Wyoming Department of Environmental Quality (DEQ) livestock standard (5.00 ppm) for dissolved uranium. With the exception of these two (2) parameters, the pit lake met all the other reclamation and livestock standards. Most, if not all, of the selenium was present in the plus six (+6) selenate state.
Limnology
The lake is an oligotrophic lake (maximum depth: approximately 120 feet) containing 1.2 billion gallons of exceptionally clear water. Secchi disk readings of 57 to 61 feet were obtained prior to treatment. The lake developed a thermocline in the late Spring/early Summer of each year at a depth of approximately twelve (12) to seventeen (17) meters. The thermocline breaks down and the lake mixes in late September of each year. The lake freezes in late November to early December of each year with ice out usually occurring in late March. Lateral variability in pit lake chemistry is minimal.
Biology
The lake contained few, if any, plants growing along shore due to the lack of shallows prior to reclamation. No species of fish were observed in the lake. Limited algal growth was present. Naturally occurring microbes capable of reducing selenium were observed in the water as early as 1991. The surface lake water contained 3.3 x 104 cells per milliliter of denitrifying bacteria, 1.3 x 103 cells per milliliter of selenium reducing bacteria and 1.5 x 103 cells per milliliter of sulfate reducing bacteria. The lake water averaged 4.4 x 105 cells per milliliter of total bacteria. The pit lake, during the summertime, exhibits a diurnal fluctuation in dissolved oxygen of up to 5 milligrams per liter due to algal oxygen production.
Bottom Sediments
Pre-treatment sampling of the bottom sediments was completed in September 1999. The sampling revealed varved sediments on the lake bottom; the number of varves approximating the age of the lake. The bottom sediments were enriched in uranium and selenium concentrated by ongoing biologic activity as indicated by the elevated carbon concentrations. Figure 3. Pre Treatment Sediment Concentrations of Selected Elements shows the pre-treatment concentrations of selenium, uranium, carbon, sulfur and iron at four (4) locations on the pit lake bottom. Dissolved selenium and uranium concentrations have not returned to pre-treatment levels due to sequestration of the metals in iron, carbon and sulfur rich sediments on the bottom.
TREATMENT AND RESULTS
Initial Testing
Two (2) phases of preliminary testing were conducted prior to treating the pit lake. The first phase of testing consisted of bench scale laboratory tests conducted in the spring of 1999, in which nutrients were added to samples of water from the pit lake. Naturally occurring microbes in the lake water metabolized the nutrients and respired on the selenium and uranium, removing it from solution, causing substantial drops in water concentration. Larger scale barrel tests were conducted on site from 8/18 to 10/1/99 that confirmed the laboratory bench scale test results. Three (3) of the seven barrels were inoculated with a non-native microbial inoculum. Little difference was detected in either the rate or extent of reduction, based upon the presence or absence of the non-native microbial inoculum.
Initial Monitoring
Stratified sampling of twenty (20) points at four (4) surface locations in the pit lake was completed on September 3, 1999 to establish baseline concentrations of dissolved uranium and selenium in the pit lake. These results are shown in Figure 1. Dissolved Selenium Concentrations Before, During and After Treatment and Figure 2. Dissolved Uranium Concentrations Before, During and After Treatment. Samples of the bottom sediments at the same four (4) locations were taken as well. The results of these samples are listed in Figure 3. Pre Treatment Sediment Concentrations of Selected Elements and Figure 4. Post Treatment Sediment Concentrations of Selected Elements.
In addition, a raft with four (4) trolls hung at 1.5, 6, 11 and 14 meters respectively and capable of measuring pressure, temperature, pH, dissolved oxygen and oxidation/reduction potential (ORP) was installed floating over the deepest portion of the lake. The trolls were set to record data on an hourly basis. In July 2001 the raft was replaced by a six (6) foot long by two (2) foot in diameter polyethylene buoy.
Treatment
Treatment commenced on 10/19/99 and continued until 12/22/99, and consisted of the addition of 548.7 tons of sugars, fats, proteins, alcohols, phosphates and nitrates. The treatment was performed by a contractor, Green World Science, Inc., which possesses two (2) patents (5,632,715 and 5,710,361) for insitu metals immobilization. Some of these materials were added using a hydroseeder. Other materials such as the alcohols were added via a pipe from the lake’s edge. No mixing problems were encountered when adding the nutrients. Stratified sampling showed that the nutrients mixed well throughout the pit lake. The treatment was a cost effective solution for this site.
Results
The two graphs, Figure 1.Dissolved Selenium Concentrations Before, During and After Treatment and Figure 2. Dissolved Uranium Concentrations Before, During and After Treatment,contain the stratified pit lake sampling results before, during and after treatment.
Figure 4. Post Treatment Sediment Concentrations of Selected Elements, shows the concentrations of selenium, uranium, carbon, sulfur and iron in the pit lake sediments on 2/22/00 and 1/23/02, following completion of the addition of the nutrients.
The post treatment sediment sampling results show greatly increased concentrations of uranium and selenium. The dissolved selenium and uranium precipitated from the pit lake became entrained in the lake sediments. Dissolved selenium and uranium concentrations have not returned to pre-treatment levels due to sequestration of the metals in iron, carbon and sulfur rich sediments on the bottom.
CONCLUSIONS
- Elevated levels of Selenium and Uranium were successfully (and economically) reduced utilizing an insitu microbiological treatment.
- Dissolved selenium concentrations dropped from 0.46 parts per million before treatment to 0.004 parts per million (average of twenty-one (21) samples taken on 7/26/04) after treatment. Dissolved uranium concentrations dropped from 8.1 parts per million to 4.21 parts per million (average of twenty-one (21) samples taken on 7/26/04) after treatment.
- On 2/22/00 and 1/23/02, following treatment, the sediments were resampled. The results showed greatly increased levels of selenium and uranium in the sediments following treatment. Table 3 contains the analysis results.
- The pit lake treatment process accelerated and enhanced the natural processes occurring in the pit lake prior to treatment, which were slowly precipitating metals in the sediments. The addition of nutrients accelerated biological activity, which in turn accelerated the ongoing natural process of biologic concentration of metals in the pit lake sediments.
- The addition of nutrients to a system containing indigenous microbes, capable of respiring on dissolved metal oxides (electron acceptors such as dissolved selenate and uranyl ions), can, on a large scale, promote the growth and metabolic activity of these microbes, resulting in the reduction and removal by precipitation of the dissolved metals from the system. Dissolved selenium and uranium concentrations have not returned to pre-treatment levels due to sequestration of the metals in iron, carbon and sulfur rich sediments on the bottom.
- The dissolved concentrations of uranium and selenium have continued to remain below treatment goals (0.05 parts per million selenium and 5.0 parts per million uranium) for over four (4) years following the first ice out (March 25, 2000) after completion of treatment in December 1999.
REFERENCES
Horne, A. and C. Goldman. 1994. Limnology, 2nd Edition, McGraw Hill, New York, New York.
Kennecott Uranium Company. DEQ/LQD Permit to Mine – Annual Report 11/1/99 – 10/31/00.
Kloos, K., U. Husgen, and H. Bothe. 1998. DNA-probing for genes coding for denitrification, nitrogen fixation, and nitrification in bacteria isolated from different soils. Biosciences, 53, 69-81.
Li, J., K. Purdy, S. Takii and H. Hayashi. 1999. Seasonal changes in ribosomal RNA of sulfate reducing bacteria and sulfate reducing activity in a freshwater lake sediment. FEMS Microbiology Ecology, 28, 31-39.
Maiers, D., P. Wichlacz, D. Thompson and D. Bruhn. 1988. Selenate reduction by bacteria from a selenium-rich environment. Applied and Environmental Microbiology, 54, 2591-3.
Mikell, A., C. Smith and J. Robinson. 1996. Evaluation of media and techniques to enumerate heterotrophic microbes from Karst and Sand Aquifer Springs. Microbial Ecology, 31: 115-24.
Oremland, R., J. Switzer Blum, C. Culbertson, P. Visscher, L. Miller ,P. Dowdle and F. Strohmaier. 1994. Isolation, growth and metabolism of an obligately anaerobic, selenate-respiring bacterium, strain SES-3. Applied and Environmental Microbiology, 60, 3011-119.
Shepherd Miller, Inc. 1994. Revised Environmental Report – Sweetwater Uranium Project.
Shepherd Miller, Inc. 1999. Phase I Evaluation of Sweetwater Pit Reclamation Alternatives.
Shepherd Miller, Inc. 1999. Sweetwater Pit Lake – Limnology and Treatability Report.
Stoltz, J., D. Ellis, J. Blum, D. Ahmann, D. Lovely and R. Oremland. 1999. Sulfurospirillum barnesii sp. nov. And Sulfurospirillum arsenophilum sp. Nov., new members of the Sulfurospirillum clade of the epsilon Proteobacteria. International Journal of Systematic Bacteriology, 49, 1177-80.
Wagner, M., R. Amann, H. Lemmer and K. Schleifer. 1993. Probing activated sludge with oligonucleotides specific for proteobacteria: Inadequacy of culture-dependent methods for describing microbial community structure. Applied and Environmental Microbiology, 59, 1520-25.
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Location / Iron / Sulfur / Carbon / Uranium / Selenium
(%) / (%) / (%) / (mg/kg) / (mg/kg)
A 2 cm / 3.23 / 0.32 / 1.111 / 156 / 4.21
A 4 cm / 2.84 / 0.27 / 0.73 / 78 / 1.34
A 6 cm / 2.36 / 0.22 / 0.272 / 47 / 1.28
B 2 cm / 2.46 / 0.26 / 0.723 / 68 / 0.68
B 4 cm / 2.27 / 0.24 / 0.458 / 72 / 0.37
B 6 cm / 2.18 / 0.21 / 0.167 / 64 / 0.24
C 2 cm / 3.02 / 0.28 / 0.211 / 113 / 0.58
C 4 cm / 2.86 / 0.23 / 0.142 / 98 / 0.49
C 6 cm / 2.42 / 0.2 / 0.073 / 65 / 0.29
D 2 cm / 2.86 / 0.27 / 0.824 / 67 / 0.31
D 4 cm / 2.74 / 0.22 / 0.148 / 72 / 0.33
D 6 cm / 2.58 / 0.19 / 0.067 / 84 / 0.37
Figure 4. / Post Treatment Sediment Concentrations of Selected Elements
February 22, 2000 / January 23, 2002
Location / Iron / Sulfur / TOC / Uranium / Selenium / Location / Depth / Iron / Sulfur / TOC / Uranium / Selenium / Moisture
(mg/kg) / (%) / (%) / (mg/kg) / (mg/kg) / (Inches) / (mg/kg) / (mg/kg) / (%) / (mg/kg) / (mg/kg) / (%)
A 1 cm / 16,356.4 / 0.16 / 1.25 / 2138.6 / 360.4 / A / 0 to 1 / 16500 / 1260 / 0.33 / 1040 / 70.5 / 48
A 2 cm / 20,037.8 / 0.4 / 1.61 / 1805.3 / 249.5 / A / 1 to 2 / 16800 / 1050 / 0.34 / 1280 / 167.0 / 47
A 3 cm / 19,305.0 / 0.5 / 1.29 / 416.9 / 89.3 / A / 0 to 2 / 16600 / 1260 / 0.38 / 1110 / 129.0 / 46
B 1 cm / 24,318.7 / 0.46 / 2.45 / 660.4 / 249.5 / B / 0 to 2 / 18600 / 2090 / 0.78 / 2830 / 290.0 / 55
B 2 cm / 18,811.3 / 0.21 / 1.09 / 432.1 / 93.8 / C / 0 to 2 / 11500 / 1240 / 0.72 / 4490 / 526.0 / 51
B 3 cm / 16,620.9 / 0.07 / 0.36 / 95.9 / 33.4 / D / 0 to 2 / 19400 / 1630 / 0.78 / 1690 / 186.0 / 61
C 1 cm / 18,356.6 / 0.14 / 0.61 / 283.2 / 54.0 / E / 0 to 2 / 19800 / 1820 / 0.77 / 1400 / 198.0 / 59
C 2 cm / 22,846.4 / 0.06 / 0.52 / 51.7 / 71.2 / F / 0 to 1 / 13700 / 2080 / 0.73 / 2120 / 254.0 / 56
C 3 cm / 21,032.5 / 0.08 / 0.59 / 686.4 / 94.1 / F / 0 to 2 / 15400 / 1760 / 0.59 / 1050 / 137.0 / 52
D 1 cm / 18,431.8 / 0.35 / 2.99 / 307.5 / 155.6 / G / 0 to 1 / 18000 / 2900 / 1.10 / 2460 / 345.0 / 65
D 2 cm / 18,891.3 / 0.11 / 2.24 / 1435 / 117.7 / G / 1 to 2 / 14800 / 1320 / 0.51 / 521 / 90.7 / 50
D 3 cm / 18,835.2 / 0.07 / 1.32 / 1545.1 / 43.5 / G / 0 to 2 / 15700 / 1450 / 0.51 / 740 / 106.0 / 51
H / 0 to 2 / 13400 / 1090 / 0.56 / 984 / 116.0 / 54
All constituents reported on a dry weight basis.
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