Water-Quality Characteristics and Mass-Balance Modeling of the French Gulch Drainage, Breckenridge, Colorado
By Jeffrey B. Bails
The water quality of the surface and groundwater within French Gulch, near Breckenridge, Colorado is affected by historical mining activities in the lower parts of the drainage. The Wellington-Oro mining complex, located two miles upstream from the confluence of French Gulch with the Blue River, is the largest contributor to the trace-element loading in the surface and groundwater. In surface water, concentrations of dissolved (filtered through a 0.45 micron capsule filter) zinc and cadmium downstream from the mine site exceeded the State's chronic stream standard throughout water year 1996. Dissolved lead concentrations in surface water at the mouth of French Gulch during water year 1996 exceeded the State's chronic stream standard at all times except at the beginning of high-flow conditions (March, April, and May). Samples collected at the gaging station at the mouth of French Gulch during water year 1996 show variation in the water composition as a result of changes in the streamflow. Differences in the zinc and cadmium ratios for the surface-water sites indicate variation in the source for these constituents between low- and high-flow conditions.
The groundwater in French Gulch, as with the surface water, shows large variations in water composition due to the effects of the Wellington-Oro mining complex. Unaffected water in each of the three hydrologic units (porphyry, shale, and alluvium) are classified as calcium-bicarbonate type water and have concentrations of dissolved cadmium, iron, lead, and manganese that are orders of magnitude lower than the calcium-sulfate type mining-affected water. Seeps in French Gulch have dissolved major-ion and trace-element concentrations that are generally greater than the unaffected sites and less than the mining-affected sites, indicating a combination of affected and unaffected water as possible sources.
Mass-balance modeling of the water-rock interactions shows the effect that the host rock, ore mineralogy, and more importantly, the extensive exposure of these minerals caused by blasting, tunneling, and stoping has on water quality. Dissolved major-ion and trace-element concentrations in the affected groundwater within the different hydrologic units (porphyry, shale, and alluvium) can be primarily attributed to the presence of sulfide ores and alteration mineralogy. Surface-water trace-element loading around the Wellington-Oro mining complex can be accounted for through the input of less than one percent of the mine pool water. Seep waters within French Gulch were able to be modeled as the result of combinations of affected surface and groundwaters. Seep S-KDS also was modeled as an intermediate composition between the unaffected and affected porphyry water, indicating the effect of the exposure of ore minerals, due to blasting, tunneling, and stoping, has on the dissolved major-ion and trace-element concentrations of the affected waters.