Report on the Drilling of Monitoring Wells near Lehman Caves, Great Basin National Park, White Pine County, Nevada, May-June 2012

Prudic, David

The objective of this research was to gain a better understanding of the rate and direction of groundwater flow in the vicinity of Lehman Caves and the source of water to Rowland Spring, the largest spring in Great Basin National Park (GRBA). This understanding is important for an assessment of the potential for proposed pumping of groundwater in adjacent Snake Valley to deplete water resources within GRBA, and the resultant adverse effects on ecologically sensitive water-dependent habitats. This report describes the methods used to drill, construct and develop a monitoring well immediately east of the sewage lagoons, problems that occurred while drilling, the geologic materials encountered, results of slug test analyses, and results of selected water-quality measurements collected during drilling and well development. The drill hole encountered younger basin-fill deposits from land surface to a depth of 23 ft, older basin-fill deposits from 23 ft to 125 ft, residuum (clay formed from weathering of limestone) from 125 ft to 146 ft, fractured marble from 146 ft to 152 ft, and inter-fingered marble and granite below a depth of 152 ft. Water-bearing sand and gravel with clay was encountered from 45 ft to 86 ft and 100 ft to 117 ft. The fractured marble also yielded some water between 146 ft and 152 ft in depth. Three nested, 2-inch diameter PVC wells with screens were completed within the three water bearing zones encountered during drilling. The stable isotope composition of the water collected from the middle and deep wells was consistent with the stable isotope composition of Rowland Spring and Baker Creek during peak snowmelt from May through July in 2010 and 2011. However, chloride and sulfate concentrations in the middle and deep wells were considerably higher than chloride and sulfate concentrations in Baker Creek and in Rowland Spring. Chloride and sulfate concentrations in Rowland Spring are higher than Baker Creek, and likely indicate mixing of diffuse groundwater flow in the buried sand and gravel layer and the fractured marble (as indicated by water quality from the middle and deep wells) with more rapid flow of water contributed from Baker Creek, which is moving through large conduits toward Rowland Spring. Two slug tests of the middle well were done the day after well development. Estimated hydraulic conductivity for this well ranged from 3 to 5 ft/d, with a transmissivity of 30 ft2/d. A slug test of the deep well resulted in a transmissivity of 0.3 ft2/d and a hydraulic conductivity of 0.05 ft/d, indicating the fractured marble and granite at this location is less permeable than the older basin-fill deposits. Water levels in the three wells indicate an upward groundwater flow potential in the vicinity of the wells. The estimated lateral hydraulic gradient between the wells and Rowland Spring ranged from 0.03 to 0.04.