Groundwater flow systems at the Nevada Test Site, Nevada: A synthesis of potentiometric contours, hydrostratigraphy, and geologic structures

Abstract

Contaminants introduced into the subsurface of the Nevada Test Site by underground nuclear testing are of concern to the U.S. Department of Energy and regulators responsible for protecting human health and safety. The potential for contaminant movement away from the underground test areas and into the accessible environment is greatest by groundwater transport. The primary hydrologic control on this transport is evaluated and examined through a series of contour maps developed to represent the hydraulichead distribution within each of the major aquifers underlying the area. Aquifers were identified and their extents delineated by merging and analyzing multiple hydrostratigraphic framework models developed by other investigators from existing geologic information. A map of the hydraulic-head distribution in each major aquifer was developed from a detailed evaluation and assessment of available water-level measurements. Multiple spreadsheets that accompany this report provide pertinent water-level and geologic data by well or drill hole. Aquifers are mapped and discussed in general terms as being one of two types: alluvial-volcanic, or carbonate. Both aquifer types are subdivided and mapped as independent regional and local aquifers, based on the continuity of their component rock. Groundwater-flow directions, approximated from potentiometric contours that were developed from the hydraulic-head distribution, are indicated on the maps and discussed for each of the regional aquifers and for selected local aquifers. Hydraulic heads vary across the study area and are interpreted to range in altitude from greater than 5,000 feet in a regional alluvial-volcanic aquifer beneath a recharge area in the northern part of the study area to less than 2,300 feet in regional alluvial-volcanic and carbonate aquifers in the southwestern part of the study area. Flow directions Purpose and Scope The purpose of this report is to delineate the major aquifers beneath the NTS area and to define and describe the likely direction of groundwater flow in each of these aquifers under predevelopment conditions. Predevelopment conditions assume equilibrium or a near equilibrium state in the groundwater flow system prior to any major changes prompted by human intervention, such as pumping and nuclear testing. Groundwater flow directions are determined by constructing potentiometric surface maps that are designed to conceptualize and describe flow within and between the major aquifers in the multi-aquifer flow system of the NTS area. Maps and their component hydraulic heads can be used as calibration targets for flow models and can help identify likely groundwater flow paths. Maps are developed to delineate the spatial extent of the major aquifers throughout the NTS and surrounding area and to convey the primary direction of flow within each aquifer. These maps also show areas of recharge and likely areas of lateral leakage into and out of each aquifer. A second set of maps delineates flow systems and tributary flow systems for the regional aquif...

Estimation of Groundwater Recharge at Pahute Mesa using the Chloride Mass-Balance Method

“…Lower chloride concentrations are observed in eastern and central Pahute Mesa while higher concentrations are found in western Pahute Mesa and southwestward along Thirsty Canyon. The general direction of groundwater flow is from northeast to southwest (Fenelon et al, 2010;Stoller-Navarro, 2006) in the same direction that chloride concentration increases. Sulfate concentration also increases downgradient (Figure 16).…”

“…These rocks are grouped into four volcanic hydrogeologic units based on lithology and secondary alteration; these are lava-flow aquifers, welded-tuff aquifers, vitric-tuff aquifers, and tuff confining units (Stoller-Navarro, 2006). At least a part of the upper saturated zone is an unconfined (phreatic) aquifer (Fenelon, et al, 2010). Groundwater in Pahute Mesa generally flows in a southwesterly direction (Fenelon, et al, 2010) primarily through fractures in vitric and welded tuff aquifers (Stoller-Navarro, 2006) toward Oasis Valley.…”

“…At least a part of the upper saturated zone is an unconfined (phreatic) aquifer (Fenelon, et al, 2010). Groundwater in Pahute Mesa generally flows in a southwesterly direction (Fenelon, et al, 2010) primarily through fractures in vitric and welded tuff aquifers (Stoller-Navarro, 2006) toward Oasis Valley. Zeolitized bedded and nonwelded tuffs act as confining units that inhibit groundwater flow (Stoller-Navarro, 2006).…”

“…Blankennagel and Weir's recharge estimate is based on an approximation of the cross-sectional area of the volcanic rocks contributing to flow and the hydraulic gradient while the recharge estimates of Feeney et al and Sadler et al are based up calibration of mixing-cell models using δ 2 H as a tracer and the estimate of Spencer is based upon a mixing-cell model using 14 C as a tracer. In addition, a potentiometric contour map (Fenelon et al, 2010) (Blankennagel and Weir, 1973), 4.32 x 10 6 m 3 /yr (Rush, 1970), 2.5 x 10 6 m 3 /yr (Feeney et al, 1987), 1.65 x 10 6 m 3 /yr (Spencer, 1990), and 2.86 x 10 6 m 3 /yr (Sadler et al, 1992). The contour map of Fenelon et al (2010) shows a hydraulic head gradient from Gold Flat toward Pahute Mesa, but without an estimate of hydraulic conductivity and a cross-sectional flow area, it is difficult to determine a flow rate.…”

Nevada Test Site Environmental Report 2009

“…The USGS compiled and evaluated hydrologic data to delineate aquifers beneath the NTS and to define likely flow directions in each of the aquifers (Fenelon et al, 2010 It is the policy of NNSA/NSO to prevent pollutants, both from past and current Nevada Test Site (NTS) activities, from impacting the local groundwater. The groundwater protection goals of NNSA/NSO are to (1) prevent future groundwater contamination, (2) control existing contamination, and (3) protect groundwater quality and availability for current and future NTS missions.…”