Seasonal variation of water chemistry and metals mobility in a watershed affected by acid rock drainage: case study in the Lake Creek Watershed, Colorado

The Colorado Geological Survey is conducting a NASA-funded study in Colorado’s Lake Creek watershed to determine the ability of hyperspectral remote sensing to map mineralogy related to water quality within a hydrothermally altered watershed. Two sampling events were conducted, one during high-flow conditions in May, and one during low-flow (baseflow) conditions in September. Field data include water samples, streambed mineral precipitate samples, and streamflow measurements at each station, along the 20-mile reach starting in Peekaboo Gulch and continuing down Lake Creek to its confluence with the Arkansas River. Hydrothermal alteration zones in four of Lake Creek’s tributaries — Peekaboo Gulch, Sayres Gulch, East Sayres Gulch, and La Plata Gulch — are sources of metals to the watershed. Mineral precipitation has occurred over much of the watershed in response to changes in the hydrogeochemical environment, for example by dilution and neutralization caused by influxes of clean tributaries. Preliminary data indicate that the precipitates are composed primarily of aluminum and iron oxyhydroxides, but also include concentrations of other trace metals, such as copper, nickel, and zinc, likely as co-precipitated or sorbed phases. The deposition of mineral precipitates occurs during both high and low flow, and is presumed to continue year-round. However, preliminary findings provide some interesting comparisons of hydrogeochemical characteristics between the two flow regimes. Low-flow conditions exhibit lower pH over a longer reach of the watershed than high flow, which translates to increased metals mobility and in turn deposition of mineral precipitates over a shorter reach of the watershed. Under high-flow conditions, dilution and neutralization occur higher in the watershed, causing mineral precipitation to occur farther upstream than during baseflow. Despite the overall improvement in water quality brought by high flow, the chemical loadings of most constituents increase during high flow. Water quality standards for most constituents are exceeded more frequently during low-flow conditions.