Deep Groundwater Contributions as a Primary Control on Stream Chemistry and Apparent Age in a Large Alpine Watershed in the Southern Rocky Mountains of Colorado

Considerable advances have been made in understanding runoff generation at small-scales yet our understanding of runoff generation in large watersheds greater than 1000 km2 remains poor. Small-scale runoff mechanisms are well documented in the literature including: overland flow, subsurface runoff, bypass flow, unsaturated flow, etc. Yet, despite the great variety in runoff mechanisms observed at the hillslope scale, chemical fluxes derived from runoff often become damped as the observed watershed scale increases. Hydrologists continue to explain this behavior using small-scale runoff mechanisms which are often related to shallow subsurface features or processes despite residence time studies which indicate that chemicals can be temporally persistent in watersheds. In doing so, we ignore the importance of deep groundwater contributions. As a matter of fact, recent studies seem to reject the "increasing deep groundwater" hypothesis whereby stream chemistry would continue to increase if the contributions from deep groundwater continue to increase and have instead embraced an "integration process hypothesis". This hypothesis suggests that variations in stream chemistry follow the central limit theorem as scale increases; large scatter at small scales and convergence on a median value as scale increases. A critical problem exists with the integration process hypothesis with regards to the contributions of deep groundwater to streamflow generation and its effect on residence times. For example, if a conservative chemical constituent in deep groundwater approaches some invariant concentration as scale increases, then it would follow that a conservative environmental tracer would approach an invariant apparent age since both processes reflect mixing. This implies that the apparent age of streamflow will become scale invariant. Current research in a large alpine watershed of the San Juan Mountains of southern Colorado suggests otherwise. Deep groundwater components in this watershed appear to be important controls on cation concentrations in stream water as a consequence of the increased time for rock/water interactions. Furthermore, Carbon-14 and stable isotope data indicate that residence times in these watersheds may be underestimated. The deep groundwater component continues to be ignored in watershed hydrology and it may in fact be the primary control on streamflow generation and stream chemistry in similar alpine watersheds of the American Southwest.