Naturally Occurring Acid Rock Drainage in Colorado’s Lake Creek Watershed

Abstract

The Lake Creek watershed in Colorado is an excellent, unique, natural outdoor laboratory for studying naturally occurring acid rock drainage (ARD) and other hydrogeochemical processes involving metal transport and deposition. ARD originates in Peekaboo Gulch, Sayres Gulch, and Sayres Bowl Stream, headwater tributaries of Lake Creek that drain low-grade Cu-Mo hydrothermal systems associated with the Grizzly Peak Caldera, host of an Oligocene, calcalkaline porphyry. The watershed receives no visible contribution of ARD from mining or other anthropogenic disturbances. The pH in Peekaboo Gulch, Sayres Gulch, and Sayres Bowl Stream starts in the 2.5-3.0 range, but is naturally neutralized by freshwater tributaries to Lake Creek and ultimately attains a pH of 7.7 at the confluence with the Arkansas River, 37 kilometers downstream.

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“…At high flow, the influence of snowmelt causes dilution and neutralization of the pH to 6.7 immediately downstream from the confluence of Peekaboo Gulch with South Fork Lake Creek, and the pH stays neutral over the entire remainder of the Lake Creek reach. However, at low flow, the lack of snowmelt allows the creek to remain acidic for a much greater reach of over 11 kms, until the inflow from North Fork Lake Creek (site LC-10) raises the pH to 6.9 (note: the aberration in pH at LC-10 is explained in Bird et al, 2005). Figure 9.…”

“…From that point downstream to the confluence with the Arkansas River, TDS decreases by a factor of about 100 due to the inflows from numerous dilute, circum-neutral, freshwater tributaries that cause the stream flow to increase from <1 cfs at the headwaters to 54 cfs at the confluence during low flow, and from 8 cfs at the headwaters to over 400 cfs at the confluence during high flow (Bird et al, 2005). The abundance of mineral precipitates coating the streambeds indicates that removal of constituents by precipitation also contributes to the reduction of TDS in the watershed.…”

“…Along the Lake Creek reach, all dissolved trace-element concentrations are greatly reduced, and several drop to undetectable concentrations, coincident with the onset of sustained neutral pH conditions. Water Quality Standards Bird et al (2005) briefly discuss the concentrations of various constituents in the context of aquatic life criteria and how the chronic toxicity threshold is influenced by metals concentrations. Until recently however, no biological data were available to relate the water-and sediment-quality data to biotic communities.…”

“…16) at locations corresponding to five of the CGS water quality sampling sites in the Lake Creek watershed (see Bird et al, 2005). Additionally, the USFS had also conducted electrofishing surveys in early and mid September 2005 at selected locations in the watershed that were also in close proximity to CGS water quality sample sites.…”

“…Phase 2, which is not discussed in this paper, focuses on the mainstem of the Arkansas River. Greater detail on interpretation of the geochemistry and imagery of both phases of the project is presented in Bird et al (2005), Hauff et al (2003aHauff et al ( , 2003b and Peters et al (2004).…”