The water, colloid, and bed sediment data presented in this report demonstrate that the source of iron, aluminum, copper, lead, zinc, arsenic, and cadmium in the Animas River watershed is from the drainage basin upstream from site A-72 at Silverton. Partitioning of the metals between the dissolved, suspended colloidal, and bed sediment phases is dependent upon the source of the metals, the pH of the water, and the flow rate of the streams. Cement Creek has the lowest pH at low flow and carries most of its metal load in the dissolved phase. Mineral Creek carries most of its metal load in the colloidal phase whereas most of the metals in the upper Animas River (above the confluence with Cement Creek) resides in the colloidal component of the bed sediments. Downstream from the confluence of Cement Creek in the mixing zone, most of the metals transported in the dissolved metal load of Cement Creek are partitioned to the colloidal phase. Along the stream courses of upper Mineral Creek and the Animas River upstream from the confluence of Cement Creek, colloids aggregate, settle, and become an integral component of the bed sediments where they are stored until high-flow (snowmelt) runoff.
At low flow, dissolved zinc and aluminum concentrations exceed the acute aquatic-life standard at site A-72 whereas the concentrations of the other metals do not exceed the chronic aquatic-life standard. Dissolved manganese concentrations are just below the chronic aquatic-life standard. However, colloidal concentrations of copper, iron, zinc, and aluminum, if bioavailable as suggested by recent research, could exceed one or both of these aquatic-life standards at site A-72. There is a substantial loss of iron and aluminum colloids and their sorbed metal loads to the bed sediments in the Animas Canyon reach between Silverton and Bakers Bridge. More than 50 percent of the zinc in the bed sediments from Mineral Creek, more than 75 percent of the zinc in the bed sediments from Cement Creek, and more than 50 percent of the zinc in the bed sediments of the Animas River above the confluence with Cement Creek is in the form of sphalerite. Zinc concentrations in sphalerite from the bed sediments of the Animas River above the confluence with Cement Creek are about 1,000 ppm, from Cement Creek are about 800 ppm, and from Mineral Creek are about 400 ppm.
Three water and colloid samples collected at high flow during snowmelt runoff indicate that dissolved zinc concentrations far exceed the acute aquatic-life standard at site A-72 and are still twice that level as far downstream as Bondad. Metals stored in the colloidal phase or component of the bed load are flushed downstream in the water column during high flow as is indicated by the high colloidal iron, aluminum, and sorbed copper, lead, and zinc concentrations. Any remedial action that removes acidic metal-bearing discharges Mineral and Cement Creeks and the upper Animas River or its tributaries should lessen the cumulative effect of potential metal toxicity on aquatic life.
At low flow, the lead-isotopic data indicate that Mineral Creek contributes 5 to 10 percent of the total mass of metals in the bed sediments at site A-72. Using mass balance calculations, the metals contributed from Cement Creek make up less than 40 percent of the metals in the bed sediments at site A-72. Fifty to 60 percent of the metals at site A-72 are derived from the Animas River upstream from the confluence with Cement Creek. Below site A-72, the lead-isotopic data are used to document the dilution of the metals derived from the Animas River Basin above Silverton by metal contributions from the tributaries. At Durango 75 km downstream, about 80 percent of the metals in the colloidal component of the bed sediments were derived from the Animas River watershed above Silverton whereas at Aztec, 57 percent of the metals were derived from this upstream source. No measurable contamination was found from the La Plata mining district at the headwaters of Junction Creek.
Sediment transport, as suggested by the lead-isotopic data and the normalized copper, lead, and zinc data from bed sediments from the Animas River indicate some differential transport of the colloidal phase of the bed sediments. The chemical data from the waters and colloids indicate sorption of dissolved metals as a function of pH which changes with downstream distance. This colloidal component of the bed sediments dominates the bed sediment chemistry. Loss of the colloidal component to the bed sediments reduces pore space between detrital grains in the bed, introduces toxic levels of metals into the food chain habitat for aquatic invertebrates, and, if bioavailable as suggested by recent research, may cause chronic toxicity to fish.
From the lead-isotopic data, we have identified five or more discrete mineralization events in the Silverton caldera. However, the dominant lead-isotopic signature in the bed sediments of both Cement Creek and the upper Animas River is from the vein-type ore mineralization in the Eureka district. Lead-isotopic data from stream sediments from Browns Gulch on the west side of the Mineral Creek drainage indicates that vein-type ore in the Eureka district extends to the west of Cement Creek. Exploration for concealed, undiscovered vein-type ore between Prospect and Minnesota Gulches could lead to the discovery of new ore reserves. The surface expression of this structure may be obscured by the hydrothermal alteration associated with Red Mountain No. 3.