Linking land use legacies: Connecting groundwater nutrient export to historical land use using MODPATH

Abstract:

Nutrient pollution in groundwater is widespread throughout the United States, in part because excess nutrients from land-use practices have infiltrated the groundwater system. As nutrients slowly travel along groundwater flowpaths, microbial communities within the aquifer may be limited in their capacity to fully remove excess nutrients. Where groundwater flowpaths connect with surface water-interface sediments (i.e. groundwater seeps), the presence of diverse microbial communities and organic carbon allow biogeochemical processing that reduces nutrient loading to surface waters. We used handheld thermal infrared cameras and the natural temperature contrast between surface and groundwater temperatures to extensively characterize the locations of groundwater seeps (n > 300) along 26km of river length in the Farmington River watershed. We sampled water from over 100 of these observed groundwater seeps for major anions, greenhouse gases, total carbon and nitrogen, dissolved oxygen, and temperature to better understand spatial heterogeneity in groundwater seep biogeochemistry and its relationship to surrounding watershed properties. We used land cover data combined with MODPATH modeled flow lines and residence times (5-20 years) to calculate the weighted source area of land cover discharging at observed groundwater seeps. Preliminary data suggest that groundwater flowing under predominately forested land cover discharge homogenous water chemistry, while groundwater flowing under predominately developed land cover has spatially heterogeneous water chemistry. Our next steps include combining historical land cover change data (1985 - 2015) with MODPATH residence times to predict where nutrient export from groundwater sources may be elevated due to historical land use practices hidden by current land cover.