Marty D. Frisbee

Is there a geomorphic expression of interbasin groundwater flow in watersheds? Interactions between interbasin groundwater flow, springs, streams, and geomorphology

Interbasin groundwater flow (IGF) can play a significant role in the generation and geochemical evolution of streamflow. However, it is exceedingly difficult to identify IGF and to determine the location and quantity of water that is exchanged between watersheds. How does IGF affect landscape/watershed geomorphic evolution? Can geomorphic metrics be used to identify the presence of IGF? We examine these questions in two adjacent sedimentary watersheds in northern New Mexico using a combination of geomorphic/landscape metrics, springflow residence times, and spatial geochemical patterns. IGF is expressed geomorphically in the landscape placement of springs and flow direction and shape of stream channels. Springs emerge preferentially on one side of stream valleys where landscape incision has intercepted IGF flow paths. Stream channels grow toward the IGF source and show little bifurcation. In addition, radiocarbon residence times of springs decrease and the geochemical composition of springs changes as the connection to IGF is lost.

Climate Change and the Fate of Desert Springs

Springs are integral components of the unique web of life in desert ecosystems of the western United States. Many desert springs would not exist without local mountains to intercept and store water from rainfall and snowmelt, and many desert aquatic ecosystems would not exist without the springs, illustrating the connectivity between landscape processes (the realm of geoscientists) and ecosystem functioning (the realm of ecologists). On a human scale, early exploration, inhabitation, and survival in the arid and semiarid western United States would not have been feasible without springs. People living there today continue to value springs as dependable sources of water for irrigation, livestock, drinking, and recreational and economic uses (e.g., hot springs). Unfortunately, some desert springs may be less resistant to the effects of climate change than others. How can this resistance be quantified?

Field estimates of groundwater circulation depths in two mountainous watersheds in the western U.S. and the effect of deep circulation on solute concentrations in streamflow

Estimates of groundwater circulation depths based on field data are lacking. These data are critical to inform and refine hydrogeologic models of mountainous watersheds, and to quantify depth and time dependencies of weathering processes in watersheds. Here we test two competing hypotheses on the role of geology and geologic setting in deep groundwater circulation and the role of deep groundwater in the geochemical evolution of streams and springs. We test these hypotheses in two mountainous watersheds that have distinctly different geologic settings (one crystalline, metamorphic bedrock and the other volcanic bedrock). Estimated circulation depths for springs in both watersheds range from 0.6 to 1.6 km and may be as great as 2.5 km. These estimated groundwater circulation depths are much deeper than commonly modeled depths suggesting that we may be forcing groundwater flow paths too shallow in models. In addition, the spatial relationships of groundwater circulation depths are different between the two watersheds. Groundwater circulation depths in the crystalline bedrock watershed increase with decreasing elevation indicative of topography-driven groundwater flow. This relationship is not present in the volcanic bedrock watershed suggesting that both the source of fracturing (tectonic versus volcanic) and increased primary porosity in the volcanic bedrock play a role in deep groundwater circulation. The results from the crystalline bedrock watershed also indicate that relatively deep groundwater circulation can occur at local scales in headwater drainages less than 9.0 km2 and at larger fractions than commonly perceived. Deep groundwater is a primary control on streamflow processes and solute concentrations in both watersheds.

What is the source of baseflow in agriculturally-fragmented catchments? Complex groundwater/surface-water interactions in three tributary catchments of the Wabash River, Indiana, USA

Some conceptual models suggest that baseflow in agriculturally-fragmented watersheds may contain little, if any, groundwater. This has critical implications for stream quality and ecosystem functioning. Here we: 1) identify the sources and flowpaths contributing to baseflow using 222Rn and 87Sr/86Sr, and 2) quantify mean apparent ages of groundwater and baseflow using multiple isotopic tracers (CFC, SF6, 36Cl, and 3H) in four small (0.08 to 0.64 km2) tributary catchments to the Wabash River in Indiana, USA. 222Rn activities and 87Sr/86Sr ratios indicate that baseflow in three catchments is sourced primarily from groundwater; baseflow in the fourth is dominated by a source similar to agricultural runoff. CFC-12 data indicate that springs in one catchment are discharging significant proportions of water that recharged between 1974 (42 +/- 2 years) and 1961 (55 +/- 2 years). Those same springs have 36Cl/Cl ratios between 1381.08 +/- 29.37 (x 10-15) and 1530.64 +/- 27.65 (x 10-15) indicating that a substantial proportion of the discharge likely recharged between 1975 (41 years) and 1950 (66 years). Groundwater collected from streambed mini-piezometers in a separate catchment have CFC-12 concentrations indicating that a large proportion of the recharge occurred between 1948 (68 +/- 2 years) to 1950 (66 +/- 2 years). Samples collected in September 2015 after above-average summer rainfall did not show significant decreases in mean apparent age. The relatively old ages observed in three of the catchments can be explained by geological complexities which are likely present in all four catchments, but overwhelmed by flow from the shallow phreatic aquifer in the fourth catchment.