ABSTRACT:

The western U.S. is experiencing shifts in recharge due to climate change, and it is currently unclear how hydrologic shifts will impact geochemical weathering and stream concentration-discharge (C-Q) patterns. Hydrologists often use C-Q analyses to assess feedbacks between stream discharge and geochemistry as a result of abundant stream discharge and chemistry data. Chemostasis is commonly observed, indicating that geochemical controls, rather than changes discharge, are shaping stream C-Q patterns. However, few C-Q studies investigate how geochemical reactions evolve along groundwater flowpaths before groundwater contributes to streamflow, resulting in potential omission of important C-Q controls such as coupled mineral dissolution and clay precipitation and subsequent cation exchange. Here, we use field observations—including groundwater age, stream discharge, and stream and groundwater chemistry—to analyze C-Q relations in the Manitou Experimental Forest in the Colorado Front Range, USA, a site where we’ve previously observed chemostasis. We combine field data with laboratory analyses of whole rock and clay X-ray diffraction and soil cation-extraction experiments to investigate the role that clays play in influencing stream chemistry. We use Geochemist’s Workbench to identify geochemical reactions driving stream chemistry and subsequently predict how climate change will impact stream C-Q trends. We show that as groundwater age increases, C-Q slope and stream solute response are not impacted. Instead, primary mineral dissolution and subsequent clay precipitation drive near-perfect chemostasis for silica and aluminum and enable cation exchange that buffers calcium and magnesium concentrations, leading to weak chemostatic behavior for divalent cations. The influence of clays on stream C-Q highlights the importance of delineating geochemical controls along flowpaths, as upgradient mineral dissolution and clay precipitation enable downgradient cation exchange. Our results suggest that geochemical reactions will not be impacted by future decreasing flows, and thus where chemostasis currently exists, it will continue to persist despite changes in recharge.

ABSTRACT:

The western U.S. is experiencing increasing rain to snow ratios due to climate change, and scientists are uncertain how changing recharge patterns will affect future groundwater-surface water connection. We examined how watershed topography and streambed hydraulic conductivity impact groundwater age and stream discharge at eight sites along a headwater stream within the Manitou Experimental Forest, CO USA. To do so, we measured: 1) continuous stream and groundwater discharge/level and specific conductivity from April to November, 2021; 2) biweekly stream and groundwater chemistry; 3) groundwater chlorofluorocarbons and tritium in spring and fall; 4) streambed hydraulic conductivity; and 5) local slope. We used the chemistry data to calculate fluorite saturation states that were used to inform end-member mixing analysis of streamflow source. We then combined chlorofluorocarbon and tritium data to estimate the age composition of riparian groundwater. Our data suggest that future stream drying is more probable where local slope is steep and streambed hydraulic conductivity is high. In these areas, groundwater source shifted seasonally, as indicated by age increases, and we observed a high fraction of groundwater in streamflow, primarily interflow from adjacent hillslopes. In contrast, where local slope is flat and streambed hydraulic conductivity is low, streamflow is more likely to persist as groundwater age was seasonally constant and buffered by storage in alluvial sediments. Groundwater age and streamflow paired with characterization of watershed topography and subsurface characteristics enabled identification of likely controls on future stream drying patterns.