Reading Our Land Use Legacy in Groundwater: Nitrate Dynamics in the Judith River Watershed

Elevated nitrate in groundwater is common is agricultural areas where fertilizer has been added at high rates for decades. Within the Judith River Wastershed, high native soil fertility allowed for dryland wheat production without N fertilization until the 1980s, yet elevated nitrate levels were frequently observed in shallow aquifers. Dr. Stephanie Ewing presents results for soil, groundwater and surface water analyses from a hydrologically isolated strath terrace near Moccasin, MT. In context of this uniquely well constrained field setting, these observed data, along with land use history and a simple mass balance model, revel the long term development and perturbation of native soil fertility with cultivation.

Geochemical Assessment and Separation of Source Waters in the Upper Boulder River Watershed Near Boulder, MT

Environmental samples were collected at three surface water sites between 5/21/2011 and 11/21/2014 along the Upper Boulder River near Boulder Montana. The sites were located at Bernice (within the mountain block), near the High Ore drainage (near the mountain block/basin transition), and at the USGS Gauging Station near Boulder, Montana (within the basin). The parameters measured in the field were SC, temperature, and alkalinity with occasional pH measurements. We collected samples for anions, cations, and stable isotopes in the catchment. We identified endmembers by sampling snow and groundwater and determined from available data an approximate endmember for rain, snow, and groundwater. We used temporal and spatial variations of water chemistry and isotopes to generate an endmember mixing model. Groundwater was found to always be an important contributor to river flow and could increase by nearly an order of magnitude during large snowmelt events. This resulted in groundwater comprising ~20% of total river flow during snowmelt at all sites. At peak snowmelt we observed that near surface water contributions to the river were from a mixture of rain and snow. Soil water, though not sampled, was hypothesized to be an important part of the hydrologic story. If so, the endmember contributions determined in this study may be different. Groundwater may have the highest variation depending on water chemistry of shallow soil water.