The Red and Bonita Mine Site is located in San Juan County, Colorado. The portal is approximately seven miles north of the Town of Silverton, Colorado, at 10,893 feet above mean sea level (AMSL) at 37 degrees 53’49.95”N and 107 degrees 38’38.70”W. Road access is via County Road (CR) 110 from the Town of Silverton to CR53 at the abandoned Town of Gladstone. CR53 continues northward up the Cement Creek valley to the Site, approximately 0.5 mile north of Gladstone
220.127.116.11 Description of Threat
Following the installation of bulkheads in the American Tunnel, Red and Bonita Mine adit discharge rates increased to approximately 300 gpm. The pH of discharge water typically ranges from five to six standard units (su). The adit discharge water contains high concentrations of several metals that include (and their approximate concentrations measured over many years): total aluminum (4,000 parts per billion (ppb)), cadmium (35 ppb), iron (90,000 ppb), lead (60 ppb), manganese (34,000 ppb), and zinc (16,000 ppb). The discharge from the adit represents a significant release of heavy metals, including zinc, to the Animas River. The Red and Bonita Mine discharge accounts for approximately 18 percent of the zinc load in the Animas River during low flow periods at a point (sample station A72) one mile below Silverton (USGS presentation, 2013).
The results of a Screening Level Ecological Risk Assessment (February 2013) strongly suggested that the fish community in the Animas River at and below Silverton is experiencing high stress under current conditions. For example, the surface water hazard quotient for zinc in the Animas River below its confluence with Cement Creek is approximately four, which is four times the expected no-effects level. In addition, the study identified aluminum, copper, lead and zinc as major risk drivers for insectivorous birds ingesting surface water, sediment, and aquatic invertebrates from the Animas River at and below Silverton. Also, metal concentrations highly toxic to benthic invertebrates were measured in the substrate of the Animas River at and below Silverton. Recent fish population studies conducted by Colorado Parks and Wildlife found no fish in the Animas River below Cement Creek for approximately two miles.
Effects on benthic communities are most notable immediately below Cement Creek but are pronounced at least 30 miles downstream. Fish population surveys (2010), conducted by Colorado Parks and Wildlife (formerly Division of Wildlife), found no fish in the Animas River below Cement Creek for approximately two miles and observed precipitous declines in fish populations since 2005 as far as 20 miles downstream.
Surface water toxicity tests were performed by the EPA in 2012 and 2013 on Animas River surface water. The tests involved exposing commercially reared juvenile rainbow trout (Oncorhynchus mykiss
) to water collected from the Animas River under controlled laboratory conditions. Exposures lasted for 96 hours and in both 2012 and 2013, Animas River water collected one mile below Cement Creek, resulted in 100% mortality of fish.
Sediment toxicity tests using the standard test organism Hyalella azteca,
freshwater amphipod, were conducted in 2012. Following a 10 day exposure to Animas River sediments collected downstream of Cement Creek, mortality ranged from 24% at Bakers Bridge (about 30 miles downstream) to 95% at Elk Creek (about seven miles downstream). Mortality was about 64% one mile downstream from Cement Creek at station
1.1.3 Preliminary Removal Assessment/Removal Site Inspection Results
Initial removal assessment investigations of the Red and Bonita Mine in 2010 were focused on both the discharge from the collapsed adit and the contribution of metals to the discharge that occurred as water flowed over and through the waste dump. The investigation findings showed that there was relatively little addition of metals to the adit discharge water from the waste dump. Mine water drainage flows from the adit over the mine dump face at a typical rate of approximately 300 gpm into Cement Creek. The Cement Creek confluence with the Animas River is approximately seven miles downstream at the Town of Silverton.
In October 2012, sampling results and related modeling showed that the Red and Bonita Mine discharge accounted for an estimated 18 percent of the zinc load in the Animas River, approximately one mile below Silverton. The relative contribution from the individual mines varies seasonally, depending on flow conditions.
|Mine Adit Discharge 2005 to 2011
||Flow Rate (gpm)
|Red and Bonita
gpm – Gallons per minute. AMSL – Above mean sea level.
The mine adit contains yellowboy accumulations varying in thickness from 0.5 to 3 feet to at least 900 feet inby, which was the maximum extent of the investigation on the main adit cross-cut. Investigations of the mine allowed access to approximately 2,000 feet of workings. Collapsed ground within the tunnels prevented further investigation. However, based on historic information and the estimated waste dump volumes, it is estimated that there are only approximately 3,500 feet of underground workings and that the adit does not connect to other mines. The extent and depth of the precipitate, contributes to waist-deep mine drainage water in some areas, rendering mine entry very difficult. Air inside the adit was oxygen depleted in 2011 and 2012 and required active ventilation to allow for safe entry in subsequent entries in 2013. Ventilation in the mine was accomplished by installing “lay-flat” flexible vent bags and using a fan to blow outside ambient air into the mine. Following the initial ventilation for each entry, the oxygen levels were found to be acceptable without continuous fan powered ventilation. Lay-flat vent bags were left intact in the mine adit from the portal to 200 feet inby.
In addition, a water management system was constructed to control heavy-metal precipitate that became suspended in the mine discharge flow during mine entry activities. This did not include treatment to reduce dissolved metal concentrations normally present in the acid mine discharge, although concentrations were reduced during the process of removing suspended metal precipitates.
In August 2013, a second entry operation was performed to complete the investigations of the workings in an attempt to identify sources of water entering the mine and possible connections to other mine workings or significant faults or fracture systems. The results of the investigation revealed that the two primary sources of water included the main cross-cut flow and flow from a drift at 275 feet. Suitable rock for a potential bulkhead construction exists within the mine approximately 265 feet in from the portal and downstream of the two sources of flow.
In September 2014, additional evaluation was performed in the adit to determine if the proposed bulkhead location is suitable structurally and hydraulically. This assessment included performing pressurized, permeability testing of the fractures within the vicinity of the segment of the adit where a bulkhead could be placed. Packer testing demonstrated that the Red and Bonita adit at the proposed bulkhead location is composed of high-quality rock with very low permeability. Improvement of the rock through formation grouting is unnecessary based on the packer test results.
See attached photo: Water Management System and Settling Pond 2013