THE INFLUENCE OF CRITICAL ZONE STRUCTURE ON ITS HYDROLOGIC FUNCTION: INSIGHTS INTO THE STORAGE AND ROUTING OF WATER THROUGH THE CRITICAL ZONE

This study combines major ion chemistry, age tracers, downhole neutron probe surveys, and hydrologic aquifer tests to understand how the structure of the critical zone (CZ) influences hydrologic flowpaths, mean residence times, and transport of weathering products to streams. In a high elevation volcanic catchment in the Jemez River Basin Critical Zone Observatory (JRB-CZO) of Northern New Mexico, groundwater was sampled from monitoring wells across 6 depths from two sites with different rock types (volcanic breccia and welded tuff). This study seeks to understand how the variable fracture density, complex mineralogy, and distinct aquifer properties across the monitoring well sites influence groundwater’s chemical composition and its input to streams. Analysis of surface water and groundwater from the small zero-order basin (ZOB) headwater catchment indicates that there are multiple, separate stores of water in the ZOB. Major ion chemistry and isotopic signatures suggest that these aquifers are chemically distinct which makes it possible to trace their contribution to streamflow draining the headwater catchment.

Differences in water table response within wells from the two sites are evident. The similar rapid response behavior of streamflow hydrographs and site one groundwater hydrograph indicate hydrologic connection between the fractured welded tuff aquifer and streamflow while the much slower, smoother hydrograph response of the perched aquifer in the volcanic breccia site does not show this same connection and suggests that the perched aquifer is recharged during spring snowmelt. Water content profiles from neutron probe measurements also suggest vertical infiltration and subsequent recharge of the perched aquifer which contrasts with evidence of lateral subsurface flow at a third monitoring well site. Tritium and carbon-14 concentrations indicate that each groundwater store has a component of modern recharge despite mixing with much older water. Deep groundwater from the welded tuff site appears to be more chemically representative of waters that contribute to streamflow and are more representative of the structure (geology, fractured aquifer, deep groundwater, long mean residence times) and function (hydrologic response, solute fluxes, and water routing) of the larger catchment.