Dawn Mining Company
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1.0 Site Identification
|Type of Site: ||Uranium Recovery Facility |
|Location: ||Ford, WA |
|License No.: ||WN-I043-2 |
|Docket No.: || |
|License Status: || |
2.0 Site Status Summary
The Dawn Mining Company uranium millsite covers about 800 acres. Uranium ore processing operations occurred between 1956 and 1982. The facility has been owned by Dawn Mining Company since the start of operations through to the present and has had a Washington State radioactive materials license since 1968. After Washington State Department of Health approved the decommissioning and closure plan in 1995, Dawn Mining Company has progressed with decommissioning consistent with the approved plan. The approved closure plan has four major components: 1) tailings area groundwater corrective action; 2) process and treatment water evaporation; 3) demolition of mill and support buildings, characterization and cleanup of contaminated soils and disposal of generated waste into an on-site lined facility; and, 4) covering, contouring and revegetating waste disposal and borrow areas. The status of DMC's progress through the Closure Plan components is provided below.
Tailings Disposal Areas
There are four tailings disposal areas, including three above-grade unlined impoundments (denoted as TDA-1, TDA-2 and TDA-3, covering about 110 acres, and used between 1956 and 1981) and one below-grade lined facility (denoted as TDA-4, covering about 20 acres, and used in 1981 and 1982).
DMC constructed five passive evaporation ponds (EP-1, 2, 3, 4 and 5) with 60 mil High Density Polyethylene (HDPE) in 1995 over the unlined tailings. Initially the ponds contained a total of 600 acre feet of contaminated process solutions. In order for final radon barrier construction to proceed on schedule, DMC constructed a new lined evaporation pond (EP-6) in 2013. DMC transferred the remaining 90 acre feet of contaminated water to the new pond in the summer of 2013.
TDA-4 is a lined, below-grade impoundment operated for uranium mill tailings disposal from September 1981 until November 1982, resulting in only a small portion of the impoundment's total capacity being used for tailings disposal. The remaining capacity has been used for disposal of stabilizing fill and contaminated materials (such as construction debris, equipment and soils).
In 2013, DMC started decommissioning the five passive evaporation ponds and constructing the final radon barrier. In 2017, DMC completed installation of the final radon barrier soils over tailings disposal areas TDA-1, TDA-2, TDA-3 and TDA-4. In 2015 and 2017, DMC conducted radon flux measurements over about 70% of the covered waste. Radon flux measurements were all below 20 pCi/m2s (maximum of 2.42 pCi/m2s and average of 0.57 pCi/m2s). DMC will install rip-rap erosion protection over the final radon barrier embankments and complete radon flux monitoring in 2018.
DMC decommissioned and buried the mill building in TDA-4 in 2003. Between 2013 and 2017, DMC demolished the remaining structures, septic system, power poles and wiring, and disposed the materials into TDA-4. The only structures remaining onsite as of 2018 are a portable trailer used as an office and a storage shed located adjacent to EP-6.
Soil Characterization and Cleanup
DMC has conducted soil characterization and cleanup at the millsite since the 1980s. The site is being decommissioned with concentration based criterion established 10 CFR Part 40, Appendix A and in Washington State uranium and thorium milling regulations, WAC 246-252. The soil regulatory standard as a result of by-product material, averaged over areas of 100 square meters, must not exceed the background level by more than 5 picocuries per gram radium-226 averaged over the first 15 centimeters below the surface and 15 picocuries per gram radium-226 averaged over 15 centimeters thick layers more than 15 centimeters below the surface. In addition, site specific dose analyses have been conducted to establish appropriate natural uranium and thorium soil cleanup levels. The table below lists the maximum allowable soil radionuclide concentrations (above background) and applies the most restrictive concentrations in all millsite areas (both land to be transferred to the Department of Energy and land to be released for restricted use).
| ||Maximum Allowable Soil Radionuclide Concentrations Above Background (pCi/g) |
|Radionuclide ||Surface Soils (top 15 cm) ||Sub-surface Soils (below 15 cm) |
|Natural Uranium ||170 ||530 |
|Thorium-232 ||4.1 ||15 |
|Thorium-230 ||14.2 ||42 |
|Radium-226 ||5.0 ||15 |
DMC performed site-wide characterization of surface soils in 2014 and 2015, including establishing a correlation between gamma exposure rate and surface soil radium-226. The correlation allows DMC to use gamma exposure rate measurements as a surrogate measurement of radium-226 activity in surface soils (top 15 centimeters). During cleanup activities in 2015, DMC discovered soils contaminated with bunker oil on the west side of the demolished mill building, likely caused by a leakage along the path of a previous oil transfer pipeline which supplied bunker oil from a storage tank to the mill boiler. DMC completed excavation of the oil contaminated soils in 2016 and backfilled of this area in 2017.
Also during 2015, DMC encountered soils with elevated gamma signature in the presence of background levels of radium-226. Subsequent analyses indicated the presence of thorium-230 and thorium-232 and daughters in surface soils that occur beneath the historic tailings line. It is likely that acidic tailings solution leaks occurred at various times in the past, resulting in precipitation of thorium-230 and thorium-232 into the soils beneath the tailings line at greater depths than radium deposition. Soils contaminated with radium were removed in the past, leaving the thorium contaminated soils in this area at or near surface. DMC conducted work in 2016 to characterize the tailings line soil contamination and installed a radon barrier soil cover over the area in 2017.
Washington State Department of Health gave DMC approval to implement their Final Status Survey Work Plan in 2016. To date, DMC has completed most of the gamma surveys, collected about 400 soil samples and conducted additional grid cleanup and re-sampling. Washington State Department of Health expects DMC to submit their Final Status Survey report in 2018 and will confirm DMC's results based on about 70 split soil samples. Washington State Department of Health will also conduct verification surveys and sampling once reclamation work is complete, including reclamation of EP-6.
Evaporation of Process Water
DMC constructed evaporation pond 6 (EP-6) in 2013 outside the waste footprint to store and evaporate remaining process solutions. As of February 2018, about 57 acre feet of contaminated water remain in EP-6. DMC continues to implement management strategies to enhance evaporation of contaminated water. Evaporation of process water from EP-6 will be a priority for DMC in 2018 and 2019.
Ore Stockpile Area
The ore stockpile area is located in the northeastern corner of the millsite and covers approximately 16 acres. DMC stockpiled and segregated ore in this area between 1956 and 1982. DMC removed remaining ore from the area in 1982 and disposed of the ore in TDA-4. DMC conducted soil characterization and removals between 2004 and 2010. The investigations confirmed that, as the ore weathered, sulfide minerals oxidized to produce sulfuric acid which leached uranium and other metals from the ore. The acidic solutions infiltrated into the vadose zone. Carbonate in the vadose zone soils neutralized the acid and uranium precipitated out as an oxy-hydroxide in the top 10 to 20 feet. Uranium forms a negatively charged ion complex with carbonate. After the initial flush of acid was neutralized, the remaining carbonate dissolved in infiltrating rain water, picked up the uranium, and migrated to the groundwater. DMC excavated out 10 to 20 feet of surface soils. Subsequent soil sampling in 2013 confirmed that DMC had essentially removed all the precipitated uranium, radium 226 and thorium 230 activity concentrations in surface soils were consistent with background levels, and no additional source removal was necessary. However, uranium remains in the unsaturated zone pore water and in the groundwater beneath the ore stockpile area.
In 2014, DMC constructed a cover over the ore stockpile area to minimize infiltration of precipitation with the objective of inhibiting further migration of uranium from the vadose zone into the groundwater and isolating any uranium that may still occur in the surface soils. The cover incorporates a vegetated soil layer and low permeability 30-mil PVC geomembrane. The slope and edges are graded so that run-off flows around the cover and run-on to the cover flows into a geomembrane-lined channel to the north and infiltrates away from the stockpile area.
3.0 Major Technical or Regulatory Issues
DMC has identified, characterized and assessed remedial alternatives for two separate groundwater contaminant plumes at the site; one associated with the unlined tailings; and, the other associated with the ore stockpile area. DMC submitted a Groundwater Protection Plan in June 2015 and informed the Washington State Department of Health of their intention to apply for alternate concentration limits (ACLs) since DMC believes that compliance with the site groundwater protection standards at the compliance wells is not practically achievable using any identified technology or combination of technologies. DMC expects to collect data to support the human health and environmental risk assessments in 2017 and 2018, and to submit an ACL application in 2018. In 2017 DMC announced resolution of the boundary between the Spokane Tribe Reservation and the DMC millsite. This confirmation of the boundary allows the ACL process to move forward.
Groundwater in the upper aquifer discharges to Chamokane Creek, which is owned by the Spokane Tribe of Indians. EPA granted authority to the Spokane Tribe to set water quality standards for the Reservation. The Spokane Tribe's uranium standard for surface water in Chamokane Creek is 0.3 pCi/L above background. Uranium activity concentration in Chamokane Creek above the millsite is about 1 pCi/L. The uranium activity concentrations in Chamokane Creek downstream do not exceed the Washington State surface water and drinking water standard of 0.030 mg/L (20.3 pCi/L). However, the uranium values exceed the Spokane Tribe's surface water standard for uranium.
Page Last Reviewed/Updated Thursday, August 23, 2018