DEVELOPMENT AND USE OF HYDROSTRATIGRAPHIC NOMENCLATURE IN THE SOUTHWESTERN NEVADA VOLCANIC FIELD, NEVADA

Volcanic rocks of the southwestern Nevada volcanic field (SWNVF) resulted from the Miocene eruption of at least six major silicic calderas and comprise some of the principal stratigraphic units at the Nevada National Security Site (NNSS), formerly the Nevada Test Site. The volcanic rocks served as the medium for nuclear tests conducted at the NNSS and, potentially, the repository host rock for the proposed high-level nuclear waste repository at Yucca Mountain. The complex volcanic stratigraphy strongly influences groundwater flow, flow paths and travel times of radionuclides potentially released into the accessible environment from nuclear testing areas on the NNSS and, potentially, from Yucca Mountain. Hydrostratigraphic units (HSUs) are employed in regional and site-scale numerical models to spatially distribute hydraulic properties within a 3-D framework so groundwater flow and flow paths can be simulated.

Early geologic mapping at the NNSS identified major lithologic breaks and emphasized variations in the physical properties of materials. Stratigraphic studies in the 1980’s defined rock- and time-stratigraphic units based on age, composition, and correlation to caldera source areas, but not on physical and hydraulic properties. HSUs used in regional and site-scale numerical models are considerably simpler than the time-stratigraphic formations within the volcanic sequence. HSUs are defined on the basis of: (1) hydraulic and transport properties, including porosity and permeability from cores; (2) relations between secondary permeability and degree of fracturing, rock type, and degree of alteration, and (3) limited hydraulic testing. Each study and associated numerical model tends to have its own hydrostratigraphic nomenclature, accounting for local geologic variation through the addition of new HSUs. Hydraulic testing of more than 40 km³ of the SWNVF has demonstrated that a majority of the HSUs and mapped structures are hydraulically similar. This suggests that currently defined HSUs are in part defined to correspond to rock- and time-stratigraphic geologic boundaries and, by themselves, may not uniquely define the hydraulic properties.