Timothy T. Bartos

Hydrogeology, groundwater levels, and generalized potentiometric-surface map of the Green River Basin lower Tertiary aquifer system, 2010–14, in the northern Green River structural basin

In cooperation with the Bureau of Land Management, groundwater levels in wells located in the northern Green River Basin in Wyoming, an area of ongoing energy development, were measured by the U.S. Geological Survey from 2010 to 2014. The wells were completed in the uppermost aquifers of the Green River Basin lower Tertiary aquifer system, which is a complex regional aquifer system that provides water to most wells in the area. Except for near perennial streams, groundwater-level altitudes in most aquifers generally decreased with increasing depth, indicating a general downward potential for groundwater movement in the study area. Drilled depth of the wells was observed as a useful indicator of depth to groundwater such that deeper wells typically had a greater depth to groundwater. Comparison of a subset of wells included in this study that had historical groundwater levels that were measured during the 1960s and 1970s and again between 2012 and 2014 indicated that, overall, most of the wells showed a net decline in groundwater levels. The groundwater-level measurements were used to construct a generalized potentiometric-surface map of the Green River Basin lower Tertiary aquifer system. Groundwater-level altitudes measured in nonflowing and flowing wells used to construct the potentiometric-surface map ranged from 6,451 to 7,307 feet (excluding four unmeasured flowing wells used for contour construction purposes). The potentiometric-surface map indicates that groundwater in the study area generally moves from north to south, but this pattern of flow is altered locally by groundwater divides, groundwater discharge to the Green River, and possibly to a tributary river (Big Sandy River) and two reservoirs (Fontenelle and Big Sandy Reservoirs). Introduction The Wyoming Landscape Conservation Initiative (WLCI) is a program created to “implement a long-term, science-based program of assessing, conserving, and enhancing fish and wildlife habitats while facilitating responsible energy and other development through local collaboration and partnerships” (Bowen and others, 2014, p. 2). The role of the U.S. Geological Survey (USGS) in the WLCI program is “to conduct science and perform technical-assistance activities that help to assess and monitor trends in overall ecosystem conditions, focal habitats, and species of concern; evaluate the effectiveness of habitat enhancement or restoration projects; and provide support to conservation planners and decisionmakers” (Bowen and others, 2014, p. 2). The WLCI study area includes much of southwestern Wyoming, including all or parts of Lincoln, Sublette, Fremont, Sweetwater, and Carbon Counties (Bowen and others, 2014, fig. 1). A study was completed as part of ongoing USGS contributions to the WLCI program with the objective to improve understanding of the primary groundwater resources used in a part of the northern Green River structural basin. Natural gas is currently (2015) extracted from tight (low permeability) gas reservoirs in a deep Late Cretaceous-age geologic formation (Lance Formation) (Law, 1984; Law and Spencer, 1989) that underlies the shallow groundwater resources that exclusively provide water to rural livestock, domestic, and industrial wells in the area (Clarey and others, 2010). Expansion of naturalgas development in the study area is expected in the future (EnCana Oil and Gas [USA], Inc., 2011; Bureau of Land Management, 2011). Drilling into the Lance Formation requires penetration of overlying aquifers that compose the regionally extensive and heterogeneous aquifer system contained within 2 Generalized Potentiometric-Surface Map of the Green River Basin Lower Tertiary Aquifer System, 2010–14 Tertiary rocks (known as the Green River Basin lower Tertiary aquifer system) that supplies water to these wells. To improve understanding of this locally and regionally important aquifer system, the USGS, in cooperation with the Bureau of Land Management (BLM), completed the following: (1) measured groundwater levels in wells completed in the aquifer system, (2) evaluated measured groundwater levels and equivalent groundwater-level altitudes in relation to well depths and lithostratigraphic/hydrostratigraphic unit designation, (3) compared newly measured groundwater levels with historical groundwater levels where possible, and (4) constructed an updated generalized potentiometric-surface map. Purpose and Scope The purpose of this report is to describe groundwater levels and the generalized potentiometric-surface of the lower Tertiary aquifer system in the northern Green River structural basin. The hydrogeology of the area also is summarized. Construction of a generalized potentiometric-surface map of the regional Green River Basin lower Tertiary aquifer system in the northern Green River structural basin Wyoming was the conclusion of an effort by the USGS to measure groundwater levels during 2010–14 (Sweat, 2013). The complex geology in the Green River structural basin greatly influences groundwater levels and movement of groundwater in the lower Tertiary aquifer system. Nomenclature of the interfingering and intertonguing Tertiary lithostratigraphic units representing many different depositional environments (lithostratigraphy) is very complex and has been repeatedly revised in the northern Green River structural basin. Similarly, the classification of these lithostratigraphic units as hydrogeologic units (hydrostratigraphy) also is very complex and also has been repeatedly revised; consequently, a summary and synthesis of past and current science related to the lithostratigraphy and hydrostratigraphy of the northern Green River structural basin is provided herein. Methods of Investigation The methods used to inventory and select existing wells for measurement of groundwater levels in the study area are described in Sweat (2013). In general, existing wells were selected that included information about the depth of the well, the open or screen/perforated interval(s) of the well, the type of surface seal, and the groundwater level at the time of well completion (Sweat, 2013). Groundwater-level measurements were made during 2010–14 using a steel tape, electrical tape, or pressure gauge using protocols and quality-control procedures described in Cunningham and Schalk (2011). Description of Study Area The study area is described in this section of the report. Brief descriptions of the geographic setting, climate, and geologic setting in the vicinity of the study area are presented. A summary and synthesis of past and current science related to lithostratigraphy and hydrostratigraphy is provided in the following “Hydrogeology” section. Geographic Setting and Climate The study area is located in parts of Sublette, Lincoln, and Sweetwater Counties in western Wyoming about 68 miles (mi) northwest of Rock Springs, Wyoming, and about 18 mi south of Pinedale, Wyo. (fig. 1). Most land is administered by the BLM (fig. 1), and one of the primary land uses is livestock grazing. The study area is located in an area of increasing energy development and includes a current gas development project (Jonah Infill Development Project) and a developing natural gas project (Normally Pressured Lance Natural Gas Development Project) on the BLM lands. Much of the study area is a rolling grass-, sagebrush-, and shrub-covered (greasewood-saltbrush) plain with intervening ridges, buttes, badlands, and ephemeral and perennial drainages. This vegetation is sparse in much of the study area and greatest near perennial streams. The study area includes critical habitat for the Greater Sage-Grouse (Centrocercus urophasianus), elk (Cervus elaphus), pronghorn (Antilocapra americana), mule deer (Odocoileus hemionus), and feral horses (Equus caballus) (Duke and others, 2011). Most of the study area is located east of the primary drainage in the Green River structural basin and drainage basin (Green River Basin), which is the perennial southward flowing Green River (fig. 1). Several prominent perennial tributaries to the Green River are present along the margins of the study area, including the southand southwest-flowing New Fork and Big Sandy Rivers. Two reservoirs (Fontenelle and Big Sandy Reservoirs) are present in the southwestern and southeastern parts of the study area, respectively (fig. 1). Climate in the study area is affected strongly by altitude and orographic effects of surrounding mountain ranges (Martner, 1986; Curtis and Grimes, 2004). In the southeastern part of the study area at Farson, Wyo. (fig. 1), the mean annual maximum temperature is about 55 degrees Fahrenheit (°F), and the mean annual minimum temperature is about 20 °F (period of record is January 1, 1915–December 31, 2005; Western Regional Climate Center, 2014a). Temperature in the northwestern part of the study area is similar, as the mean annual maximum temperature at Big Piney, Wyo., is about 53 °F, and the mean annual minimum temperature is about Description of Study Area 3

Pesticides in Wyoming Groundwater, 2008-10

Groundwater samples were collected from 296 wells during 1995–2006 as part of a baseline study of pesticides in Wyoming groundwater. In 2009, a previous report summarized the results of the baseline sampling and the statistical evaluation of the occurrence of pesticides in relation to selected natural and anthropogenic (human-related) characteristics. During 2008–10, the U.S. Geological Survey, in cooperation with the Wyoming Department of Agriculture, resampled a subset (52) of the 296 wells sampled during 1995–2006 baseline study in order to compare detected compounds and respective concentrations between the two sampling periods and to evaluate the detections of new compounds. The 52 wells were distributed similarly to sites used in the 1995–2006 baseline study with respect to geographic area and land use within the geographic area of interest. Because of the use of different types of reporting levels and variability in reporting-level values during both the 1995–2006 baseline study and the 2008–10 resampling study, analytical results received from the laboratory were recensored. Two levels of recensoring were used to compare pesticides—a compound-specific assessment level (CSAL) that differed by compound and a common assessment level (CAL) of 0.07 microgram per liter. The recensoring techniques and values used for both studies, with the exception of the pesticide 2,4–D methyl ester, were the same. Twenty-eight different pesticides were detected in samples from the 52 wells during the 2008–10 resampling study. Pesticide concentrations were compared with several U.S. Environmental Protection Agency drinking-water standards or health advisories for finished (treated) water established under the Safe Drinking Water Act. All detected pesticides were measured at concentrations smaller than U.S. Environmental Protection Agency drinking-water standards or health advisories where applicable (many pesticides did not have standards or advisories). One or more pesticides were detected at concentrations greater than the CAL in water from 16 of 52 wells sampled (about 31 percent) during the resampling study. Detected pesticides were classified into one of six types: herbicides, herbicide degradates, insecticides, insecticide degradates, fungicides, or fungicide degradates. At least 95 percent of detected pesticides were classified as herbicides or herbicide degradates. The number of different pesticides detected in samples from the 52 wells was similar between the 1995–2006 baseline study (30 different pesticides) and 2008–2010 resampling study (28 different pesticides). Thirteen pesticides were detected during both studies. The change in the number of pesticides detected (without regard to which pesticide was detected) in groundwater samples from each of the 52 wells was evaluated and the number of pesticides detected in groundwater did not change for most of the wells (32). Of those that did have a difference between the two studies, 17 wells had more pesticide detections in groundwater during the 1995–2006 baseline study, whereas only 3 wells had more detections during the 2008-2010 resampling study. The difference in pesticide concentrations in groundwater samples from each of the 52 wells was determined. Few changes in concentration between the 1995–2006 baseline study and the 2008–2010 resampling study were seen for most detected pesticides. Seven pesticides had a greater concentration detected in the groundwater from the same well during the baseline sampling compared to the resampling study. Concentrations of prometon, which was detected in 17 wells, were greater in the baseline study sample compared to the resampling study sample from the same well 100 percent of the time. The change in the number of pesticides detected (without regard to which pesticide was detected) in groundwater samples from each of the 52 wells with respect to land use and geographic area was calculated. All wells with land use classified as agricultural had the same or a smaller number of pesticides detected in the resampling study compared to the baseline study. All wells in the Bighorn Basin geographic area also had the same or a smaller number of pesticides detected in the resampling study compared to the baseline study.