IDENTIFYING THE GROUND WATER COMPONENT IN SPRINGFLOW GENERATION TO QUANTIFY ERROR IN WEATHERING RELEASE CURVES IN A LARGE, ALPINE WATERSHED IN THE SOUTHERN ROCKY MOUNTAINS OF COLORADO, USA

Bedrock weathering is an important process in the geochemical cycle of watersheds and it may be the primary source of solute release to streamflow, springflow, and ground water in alpine watersheds. However, providing direct estimates of bedrock weathering is complicated and methodologies are often employed based on geochemical mass balances which provide indirect estimates of the weathering release. The geochemistry of streamflow is often used in watershed mass balance approaches; however, streamflow is an integrative process. Thus, the ground water component or the water residing in bedrock must be isolated to provide more accurate estimates of bedrock weathering using this approach. To avoid this problem, springflow is often assumed to be a proxy for ground water in watersheds lacking wells. This approach may seem especially appealing in alpine and other remote watersheds where wells are typically sparse. This is a concern since very little effort has been spent in quantifying the integrative nature of springflow. We used endmember mixing analyses (EMMA) to quantify the sources of springflow in a large alpine watershed in the San Juan Mountains of southwestern Colorado. Weathering release curves were created for sodium and calcium concentrations in well waters and springflow since these constituents appear to result from the weathering of feldspars in the volcanic bedrock. Our results indicate that the solute release of uncorrected springflow is 66 to 71 percent of the solute release from wells terminated in the local bedrock. However, when the ground water component is isolated and new weathering release curves are generated, the solute release from corrected springflow is 33 to 43 percent of the solute release from wells. Our results illustrate the integrative nature of springs and suggest that bedrock weathering rates derived from springs may drastically underestimate actual rates of weathering in bedrock aquifers.