Developing Methods to Test the Influence of Critical Zone Development on Watershed Hydrology and Biogeochemistry

The Boulder Creek Critical Zone Observatory in the Front Range of Colorado, USA, is designed to study the development and function of the near-surface weathered profile. The critical zone is the interface between bedrock and the atmosphere, where water and terrestrial ecosystems drive chemical transformations, and where weathering and erosion transform landscapes and shape the critical zone itself. In Boulder Creek catchment, erosion rates and processes vary dramatically over the 2600 m elevation range from the Colorado piedmont to the headwaters at the continental divide. The topographic, climatic, erosional and ecologic variations result in a critical zone that ranges from thin, fracture-dominated, weathered profiles truncated by glacial erosion to slowly eroding, deeply weathered mantles. Quantifying these variations in critical zone development, and understanding how erosion and weathering processes produce these variations, are primary goals of the Boulder Creek CZO. Against this backdrop, we will use hydrochemistry to examine how critical zone development influences fluxes of water, solutes, and nutrients to streams. We expect that reaction progress will be low in glacially truncated critical zone profiles in the headwaters, since water residence times are expected to be low in the thin fracture-dominated weathered zone. In contrast, we expect deeply weathered profiles on post-Laramide low-relief surfaces to yield long residence time water that approaches saturation with respect to minerals present. Dissolved organic matter (DOM) will likely show the most evidence of microbial processing in the deeply weathered profiles. We will use a suite of tools to test these expectations. Water will be collected from streams, wells and soil water samplers arrayed in three subcatchments. Our headwater site is coincident with the Niwot Ridge LTER high elevation site and will be managed jointly. In the case of dry regolith, we will use laboratory soil water extracts to test compositions of subsurface water. Major element concentrations in these waters will be analyzed with respect to mineral suites present as determined from quantitative XRD. The Proterozoic age of the crystalline parent rocks in this area should make strontium isotopes a useful tool in deconvolving mineral weathering sources. DOM sources will be identified with fluorescence spectroscopy. Isotopic sampling and hydrologic simulation will constrain water residence times within our study subcatchments. Rates of processes will be constrained by elemental and mineral budgets for individual soil profiles, and by solid and solution fluxes at the watershed scale. Through standardized sampling within three subcatchments, we hope to identify how critical zone development- the size and state of the reactor- affects the fluxes out of the critical zone.