GEOCHEMICAL EVIDENCE FOR GROUND-WATER/SURFACE-WATER INTERACTION IN A MOUNTAIN WATERSHED

In Prospect Gulch, San Juan County, Colorado, temporal and spatial monitoring of stream water quality has identified zones and mechanisms for ground-water discharge. Prospect Gulch, an alpine watershed, is dominated by hydrothermally-altered bedrock containing high concentrations of pyrite, which consumes oxygen and produces elevated concentrations of iron and other metals. Deep and shallow ground-water flow systems have been identified through geochemical sampling and hydraulic heads in multilevel monitoring wells. Deep ground water flows within the bedrock and has no oxygen, low copper concentrations, and high concentrations of iron. It discharges to a stream (surface water) at the base of the watershed and in an adjoining stream along a zone with an assumed difference in bedrock permeability, possibly a silicified vein. A shallow ground-water flow system occurs in the colluvium, which is an order of magnitude more permeable than the underlying bedrock. The shallow system is characterized by seasonal flow in perched water tables (seen in multilevel monitoring wells) that discharges very locally to streams. The shallow ground water is identified by measurable oxygen content, low iron concentrations, and elevated copper concentrations. Locally, the ground water and surface water contain drainage from mine adits and mine-waste rock with high metal content. Spatially, discharge of shallow ground water to the streams can be identified with elevated copper concentrations, lower iron concentrations (except in the mine-affected areas), and iron in its oxidized form (ferric iron). Sections of the stream receiving deep ground-water discharge can be identified with diluted copper concentrations, increasing iron concentrations, and more iron occurring in its reduced form (ferrous iron). Temporal trends in stream geochemistry are consistent with the presence of these shallow and deep ground-water flow systems. During winter months when no recharge occurs due to frozen ground, the shallow system cannot maintain saturated conditions and the deep system dominates the stream water geochemistry with low copper concentrations and high iron concentrations.