Smull, Erika, authorGooseff, Michael, advisorBledsoe, Brian, committee memberSingha, Kamini, committee memberWohl, Ellen, committee member2016-01-112016-01-112015http://hdl.handle.net/10217/170299Bidirectional hydrologic exchanges between surface water and groundwater along a stream reach can act to dilute nutrient levels (physical processes), and/or can facilitate biogeochemical cycling (physical and biological processes). Such exchanges thus affect nitrogen transport within stream catchments, many of which have been altered in high-elevation locations along Colorado's Front Range due to elevated nitrogen levels from industrialization in recent decades. We applied a fully informed hydrologic mass balance model and nitrate mass balance model that include gross gains and gross losses along a 1000 m study reach, to better understand nitrate removal potential for a Colorado montane zone catchment, Lower Gordon Gulch. We collected data during four synoptic stream tracer and sampling campaigns along our study reach during the 2014-2015 water year, and also analyzed near-stream riparian lateral hydraulic gradients to assess groundwater and surface water interactions from a second perspective. Three distinct hydrologic regimes are captured in our results, including two experiments during baseflow, one experiment following snowmelt, and one experiment following late-spring rainfall. Results show a transition from hydrologic sources of nitrate following snowmelt, to biological sources during rainfall, and finally to hydrologic removal during baseflow. Higher hillslope water content appears to be directly correlated with nitrate sources, and lower in-stream discharge appears to be directly correlated with in-stream nitrate removal. This finding combined with lateral hydraulic gradient results point to groundwater-surface water interactions. Our findings corroborate earlier work in montane zone streams that show preferential flow on south-facing slopes and matrix flow with greater microbial activity on north-facing slopes following snowmelt. We provide a modeling framework that separates physical from biological processes to assess the potential of such catchments to cycle nitrate, which can help scientists and environmental planners in assessing ecosystem changes in Colorado due to anthropogenic influences.born digitalmasters thesesengCopyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright.bidirectional exchangeColoradohyporheic exchangemass balancemontanenitrateText