Hydrostratigraphic characterization of a semi-arid climate alluvial fan; Fortymile Wash, Nevada

Ressler, T.R.; Ridgway, K.D.; Stamatakos, J.A.; Sharp, J.M.J.

Abstracts with Programs - Geological Society of America 32(7): 412


Information on the structural and stratigraphic control of groundwater within the alluvium south and southeast of Yucca Mountain is needed for groundwater models used in performance assessment calculations of the proposed nuclear repository at Yucca Mountain. Fortymile Wash is a large alluvial fan system located east-southeast of Yucca Mountain in southwestern Nevada. Fortymile Wash extends south-southwest from Yucca Mountain into the Amargosa Desert, and is rimmed in its upper reaches by smaller transverse fans emerging from the surrounding uplands. The modern channel of Fortymile Wash is entrenched, providing several substantial outcrops of alluvium along the longitudinal axis of the fan. A detailed investigation of the outcrops of alluvium was completed to study sedimentary architecture and hydraulic properties of the alluvium. The exposed alluvium was subdivided into facies according to sedimentary features and deposit geometry. Gamma ray and density measurements from the outcrops were used to develop simulated wireline logs for comparison to wireline logs from recently completed boreholes within Fortymile Wash. Laboratory permeability tests on sediment samples and in-field air minipermeameter measurements were completed to investigate the variation in permeability between the different identified facies. The results will be used to develop a conceptual model of the hydrostratigraphy of the Fortymile Wash alluvium to assist in development of revised groundwater flow models.