U.S. DEPARTMENT OF THE INTERIORBUREAU OF LAND MANAGEMENT
Mercury Mine/Mill Site Remediation Projects Within The Pajaro and Panoche-San Luis Reservoir Watersheds, Central California
a) Evaluate the degree of mercury contamination at the abandoned mine/mill sites using technical specialists from the United States Geological Survey and Stanford University
List of Existing Research Studies:
The Pajaro and Panoche-San Luis Reservoir Watersheds are highly mineralized and contain over 100 separate mineral species. It is here that the State of California Gem "Benitoite" is found, named after the county it is found in (San Benito County). The New Idria Mercury Mine, located on private land, operated from the 1850´s to the 1970´s and was the second largest mercury producer in the country. The New Idria Mercury Mine was the primary processing center for many of the smaller mines located on the surrounding lands. The New Idria Mine produced over 433,000 flasks of liquid mercury (quicksilver) between 1902-1944. The other smaller mercury mine/mill sites in these watersheds produced only 4,100 flasks of mercury. A "flask" of mercury weighs 78 pounds. Some of this mercury was used in the foothills of the Sierra Nevada Range in the vicinity of the Bear and Yuba Rivers during the hydraulic gold mining days.
During the time mercury was produced from these watersheds, there were limited environmental oversight and regulatory control. The mercury production technology consisted of mining the ore (cinnabar) from surface or subsurface deposits, crushing the ore to about gravel size, roasting the gravel sized ore in ovens (retorts), passing the mercury vapors through cooling condensers, and then collecting the condensed mercury (quicksilver) in the flasks. The roasting wastes, primarily the spent ore (calcines) and retort ash, were normally disposed onto adjacent lands, including filling in of gullies and ravines. Often times the calcines were end-dumped near the retorts to form additional, level work areas. Over time, the winter rain and melting snows washed large amounts of the calcines and retort ash downslope into perennial and ephemeral streams.
The abandoned mercury mine/mill sites located on Federal land in these watersheds that are targeted for environmental cleanup are the result this simplistic mining technology of the early 1900´s. Since the calcines still contained mercury, the disposal into the riparian zones and active creeks in this watershed pose an environmental problem. Over time the calcines were eroded into the major river systems such as the San Benito River. Several miles downstream of these mines the Hernandez Reservoir was built in the 1950´s for flood control, the calcine sediment eventually were trapped by this dam and aquatic species living here, such as fish have been found to contain methylmercury, a potent and persistent toxin.
ENVIRONMENTAL/HUMAN HEALTH ISSUES
The BLM manages over 50,000 acres in this watershed, which is very popular with recreationists exploring the area on the extensive network on historic mine exploration roads and trails. This Off-Highway Vehicle (OHV) use via motorcycles and 4-wheel drive vehicles in this the highly mineralized watershed, the water quality can be impacted by these mercury-containing sediments. In 1999, BLM identified four mercury mine/mill sites (Alpine, Aurora, Larious Canyon and Xanadu) with elevated mercury concentrations in the soils. These sites were closed to vehicle use. In Spring of 2000, BLM contracted with several environmental remediation firms to restore the four identified abandoned mercury mine/mill sites. The Mercury Mine Remediation Strategy was threefold: 1) Reduce the mercury containing sediments from fluvial transport, 2) Reduce public exposure due to camping, hunting or OHV use, and 3) Cleanup projects must to implemented in a practical, cost-effective manner and on a watershed-wide scale. In 2001, two additional sites (CalMerc Mill site and Jade Mill Site) were identified as containing elevated levels of mercury in the soils. Theses site were cleaned up during
Based upon the remediation strategy for limited public access, OHV on restoration areas is not allowed, but access for camping, hiking and OHV use is allowed outside restoration areas. The implementation plan was developed to consolidate and encapsulate mining equipment and mining waste rock in geologically stable areas (outside of riparian zones), recontour, revegetate and limit public use in the restored areas.