Distinct stores and the routing of water in the deep critical zone of a snow-dominated volcanic catchment

Abstract

Abstract. This study combines major ion and isotope chemistry, age tracers, fracture density characterizations, and physical hydrology measurements to understand how the structure of the critical zone (CZ) influences its function, including water routing, storage, mean water residence times, and hydrologic response. In a high elevation rhyolitic tuff catchment in the Jemez River Basin Critical Zone Observatory (JRB-CZO) within the Valles Caldera National Preserve (VCNP) of northern New Mexico, a periodic precipitation pattern creates different hydrologic flow regimes during spring snowmelt, summer monsoon rain, and fall storms. Hydrometric, geochemical, and isotopic analyses of surface water and groundwater from distinct stores, most notably shallow groundwater that is likely a perched aquifer in consolidated collapse breccia and deeper groundwater in a fractured tuff aquifer system, enabled us to untangle the interactions of these groundwater stores and their contribution to streamflow across 1 complete water year (WY). Despite seasonal differences in groundwater response due to water partitioning, major ion chemistry indicates that deep groundwater from the highly fractured site is more representative of groundwater contributing to streamflow across the entire water year. Additionally, the comparison of streamflow and groundwater hydrographs indicates a hydraulic connection between the fractured welded tuff aquifer system and streamflow, while the shallow aquifer within the collapse breccia deposit does not show this same connection. Furthermore, analysis of age tracers and oxygen (δ18O) and stable hydrogen (δ2H) isotopes of water indicates that groundwater is a mix of modern and older waters recharged from snowmelt, and downhole neutron probe surveys suggest that water moves through the vadose zone both by vertical infiltration and subsurface lateral flow, depending on the lithology. We find that in complex geologic terrain like that of the JRB-CZO, differences in the CZ architecture of two hillslopes within a headwater catchment control water stores and routing through the subsurface and suggest that shallow groundwater does not contribute significantly to streams, while deep fractured aquifer systems contribute most to streamflow.

Where Is the Bottom of a Watershed?

“…Across larger river basins, Schaller and Fan () and Fan () challenged the commonly held view that catchments are closed systems, finding instead that many are regional groundwater importers or exporters. Similarly, multiple CZOs and other experimental sites operating at headwater scales have found that groundwater in fractured bedrock aquifers commonly contributes to streamflow (Brantley et al, ; Jin et al, ; Markovich, Dahlke, et al, ; McIntosh et al, ; Payn et al, ; White et al, ). Key to this discussion is the recent recognition that even at well‐studied sites, we are only beginning to detect that our “shallow views” on the hydrologic cycle may miss large fluxes of water or oversimplify our conceptual models of these systems.…”

“…There have been some advances in small borehole installation that have allowed for important discoveries related to residence time and active circulation depths in fractured rock systems (Gabrielli & McDonnell, ), but these generally extend to only 10 m or so. Additional deep well drilling and greater application of geophysical methods (e.g., White et al, ) are needed to provide spatial characterization of hydrostratigraphy down to 100–200 m. New 3‐D gridded datasets that make use of existing permeability data and data from a range of other sources are needed to improve modeling efforts at large scales. We need new tools to characterize deep flow paths as well as the vertical storage and release of deep groundwater. The stable isotopes of hydrogen and oxygen have been workhorse tracers in water studies for decades.…”

“…In many catchment hydrology studies, depth to bedrock is observed by hand‐auger well installation to a “refusal” depth or by excavating soil pits down to bedrock across headwater catchments (Jencso et al, ; Zimmer & McGlynn, ). In the CZOs, deep (tens of meters) drilling campaigns and installation of bedrock monitoring wells have helped characterize deeper groundwater stores and the dynamic hydraulic connections to streamflow (White et al, ). Still, there are only a few deep groundwater monitoring wells in CZOs with maximum depths greater than ~50 m (Küsel et al, ).…”

“…This is vital for large‐scale land surface and earth systems models where we are still iterating how best to represent the subsurface (Clark et al, ; Fan et al, ). We therefore advocate additional modeling and observational work across a more diverse set of systems, including catchments in distinct climates (e.g., Tetzlaff et al, ), those with complex CZs (e.g., Buss et al, ; Hahm et al, ; Shi et al, ; Zimmer & McGlynn, ), and those experiencing human stressors and management (e.g., Bhaskar & Welty, ; Condon & Maxwell, ; McDonnell et al, ; Perrone & Jasechko, ; White et al, ).…”

Direct Observation of the Depth of Active Groundwater Circulation in an Alpine Watershed

“…A clear need exists to directly observe and more rigorously constrain the active circulation depth in mountain watersheds by measuring hydraulic conductivity ( K ) and groundwater‐flux‐dependent variables, such as age and temperature ( T ), at discrete levels in the subsurface sufficiently deep to penetrate the inactive zone. An increasing number of hillslope nested wells are being installed at Critical Zone Observatories and other research watersheds (e.g., Salve et al., 2012; Tokunaga et al., 2019; White et al., 2019), and these have provided important information on the flow at different depths within the active system and the relative importance of different subsurface water stores in streamflow generation. However, to date these lack sufficient depth and/or adequate groundwater‐flux‐dependent data collection and interpretation to permit direct, robust delineation of the active circulation depth.…”