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.

Shallow Groundwater Quality and Geochemistry in the Shields River Basin, South-Central Montana

Water samples were collected from 33 domestic wells, 2 springs, and 3 streams in the Shields River Basin (Basin) in southwest Montana. Samples were collected in 2013 to describe the chemical quality of groundwater in the Basin. Sampling was done to assess potential impacts to water quality from recent exploratory oil and gas drilling and to establish baseline water quality conditions. Wells were selected in areas near and away from oil and gas drilling and in areas susceptible to contamination. Water samples from surface water sites were collected in October to characterize base flow conditions. Physical characteristics of the land surface, soils, and shallow aquifers were used to assess groundwater susceptibility to contamination from the land surface. This analysis was completed using GIS. Samples were analyzed for major ions, trace metals, water isotopes of oxygen and hydrogen. A subset (24) of samples were analyzed for tritium and organic constituents (GRO, DRO, BTEX, methane, ethylene, and ethane). One sample exceeded the human health drinking water standard for selenium. Dissolved methane and ethylene gas were detected in six samples at concentrations less than 0.184 milligrams per liter. Three locations were resampled in 2014, and no methane or ethylene was detected. Shallow groundwater and streams are generally calcium- or sodium-bicarbonate type water with total dissolved solids concentration less than 300 milligrams per liter. Some wells produce either sodium-chloride or sodium-sulfate type water suggesting slower flow paths and more rock-water interaction. Tritium concentrations suggest that older water (TU< 0.8), recharged prior to the mid-1950’s, is generally sodium type, whereas younger water (TU > 4) is generally a calcium type. Water-quality data from this study were compared to available historic data in the Basin. Additionally, the USGS Produced Waters Geochemical database was queried for chemical data of produced waters from reservoir rocks throughout Montana and the surrounding states. Comparisons to historic and produced water chemical data suggest no impact to shallow groundwater quality from exploratory oil and gas drilling.