An Assessment of the Effects of Elevation and Aspect on Flux of Airborne Pollution and Water Quality in an Alpine Critical-Zone: San Juan Mountains, Colorado, USA

Abstract

The local, highly mineralized geology, along with years of mining and improper disposal of the tailings and acid-mine drainage have degraded the quality of surface water within the San Juan Mountains, specifically, the area between the towns of Silverton and Ridgway, CO. This area is a fragile ecosystem with inputs ranging from local geologic baseflow and mining to atmospheric flux (AF) of contaminants. Thus, significant questions arise: Does elevation or aspect affect the AF of pollutants, and is AF a potential source of water quality degradation in this study area?

It is assumed AF occurs on the slopes, as well as in streams, lakes, and ponds, which can be transported to nearby surface waters, further degrading them. Atmospheric deposition data were collected for aluminum, iron, manganese, nitrate, phosphate, and sulfate. Water chemistry data were collected for the same constituents as the atmospheric deposition, with the addition of temperature, dissolved oxygen, pH, and specific conductance. Samples were collected on a five-day sampling regime, over a three-week period, during a two-year period. Water quality samples were collected in-stream adjacent to the deposition-sample collectors. Collection sites were located on opposite sides of Red Mountain, at five elevations on each side, providing two different aspects.

Significant AF differences were detected between elevations for NO3 and SO4 on the northern aspect examined; however, no significant differences were found for AF on the southern aspect or between aspects. In regard to the water quality results, differences were detected between multiple elevations for all constituents examined, except PO4, on both aspects; no differences were detected between aspects for the in-situ or the lab-analyzed water-quality constituents. No runoff occurred during this study; therefore, no conclusion regarding the impact of AF on water quality could be made. Additionally, environmental stochastic modeling was performed to examine how AF may have affected water quality if runoff had occurred. Based on these calculations and the assumptions upon which they were based, it was determined that the atmospheric flux of NO3, PO4, and Fe may be sufficient to affect the surface water quality in this study area if runoff had occurred.