STRONG INTERACTIONS BETWEEN STREAMFLOW AND DEEP GROUNDWATER INFERRED FROM TRENDS IN STREAM CHEMISTRY IN THREE MOUNTAINOUS WATERSHEDS IN DIFFERENT GEOLOGIC SETTINGS

The interactions between deep, watershed-scale groundwater flowpaths and streamflow have historically been poorly characterized in both field monitoring and modeling efforts. However, recent advances in this field of study indicate that contributions from deep groundwater systems have a significant influence on the magnitude and structure of geochemical compositions and apparent ages of streamflow across multiple scales in large watersheds. In fact, research in the Saguache Creek watershed, a 1600 km² volcanic watershed located in the San Juan Mountains of southern Colorado, suggest that deep groundwater is responsible for the observed increases in the concentrations of chemical constituents with increasing scale in the watershed. Research into the apparent ages of streamflow in the Saguache Creek watershed using radiocarbon and geochemical chronometer age-dating techniques indicate that apparent ages of streamflow also increase with increasing scale. More importantly, the ages of streamflow range from 150 to 300 years in the headwater subwatersheds and increase from 1000 to over 5000 years with increasing accumulated drainage area along the main stream channel. Our overarching hypothesis is that streamflow may contain significant contributions from deep, watershed-scale groundwater flowpaths that are not being detected using field monitoring and age-dating techniques traditionally used in small catchment studies. We continue to test this hypothesis by monitoring the streamflow chemistry and surface water ages across multiple scales in the Rio Hondo watershed (a crystalline-bedrock watershed greater than 300 km² located in the Sangre de Cristo Mountains of northern New Mexico) and in the El Rito watershed (a sedimentary-bedrock watershed greater than 300 km² located in the Tusas Mountains of northern New Mexico). The Rio Hondo and El Rito watersheds display similar trends in the geochemical composition of streamflow with increasing scale as those observed in the Saguache Creek watershed. These findings further illustrate the influence of deep, watershed-scale groundwater flowpaths on stream geochemistry and age. These findings also suggest that deep groundwater is an important control on the generation and geochemical evolution of streamflow in different geologic settings.