Evaluation of the hydraulic connection between streams and aquifers at Baker and Snake Creek near Great Basin National Park, Snake Valley, White Pine County, Nevada.

摘要

Stream channels along Baker and Snake Creek, which drain off the eastern flank of the southern Snake Range in Great Basin National Park and out into Snake Valley, were evaluated to determine their hydraulic connection with their underlying basin-fill and carbonate-rock aquifers. Characterization of this hydraulic connection included: (a) quantifying stream discharge along the creeks and analyzing variations in flow; (b) installing piezometers into the streambeds and measuring vertical head gradients from water-level measurements in both the piezometers and creeks; (c) estimating streambed hydraulic conductivities in piezometers using slug tests; (d) measuring temperature in Baker Creek, a spring, and piezometers, and analyzing variations for gain and loss rates across the streambed; and (e) doing a multiple-well aquifer test along Baker Creek. Stream discharge measurements, vertical head gradients, and streambed hydraulic conductivities were used to map gaining and losing reaches along an 8-kilometer section of Snake Creek that extends from the park boundary to the Nevada-Utah border. Snake Creek has a consistently gaining reach about 300 to 700 meters east of the park boundary where a late-Miocene fault, which juxtaposes Paleozoic footwall limestone against Miocene hanging-wall sedimentary deposits, allows Michaela spring to discharge into the creek from limestone. Snake Creek is disconnected from groundwater about 3 kilometers east of the park boundary. At this location, a recent U.S. Geological Survey monitoring well drilled 2 meters from the stream had a water level of about 60 meters below the streambed elevation, even though the creek exhibits little seepage loss.;Groundwater flow models were formulated using MODFLOW-2005 to simulate gaining and losing reaches along Snake Creek and to evaluate aquifer and streambed properties contributing to the hydraulic disconnection. Results from the models suggested that aquifer hydraulic conductivities increase eastward from about 5 meters per day in the Miocene deposits to about 20 meters per day in the Paleozoic limestone and Quaternary deposits, and streambed hydraulic conductivities decrease eastward from approximately 3 meters per day to about 0.0001 meter per day. The low hydraulic conductivity of the streambed near the Utah-Nevada border was needed because of the lack of streamflow loss in a reach where depth to groundwater exceeds 30 meters.;At Baker Creek, a second study location farther to the north along the southern Snake Range front, a multiple-well aquifer test was done to estimate streambed and underlying aquifer hydraulic properties over a 400-meter stream section. The test consisted of pumping a well 16 meters away from the creek for 4 days at a rate of 1.64 liters per second and included measurements of water levels and temperature along Baker Creek and in the test and monitoring wells. Water levels in shallow wells next to Baker Creek indicated the creek is losing in the immediate vicinity of the aquifer test. A computer program, SEAWAT, was used to couple MODFLOW-2005 with MT3DMS to estimate streambed and aquifer properties using drawdown and temperature data collected before and during the test with temperature used as a tracer during pumping. Model results suggested that the contact between the Miocene deposits and Quaternary alluvium dips into the alluvium from the north and south of the test site and a megablock is situated within the Miocene deposits immediately south of the pumped well. The initial temperature field had two temperature aureoles, one each around Baker Creek and a spring to the north of the test site that increased in temperature from beneath the creek and spring. The Quaternary alluvium had estimated horizontal and vertical hydraulic conductivities of 8.3 and 0.15 meters per day, respectively. The vertical hydraulic conductivity of the streambed for Baker Creek was 0.5 meter per day. Although the pumping rate was insufficient to produce a measurable decrease in streamflow along Baker Creek, water level declines indicated increased leakage near and downstream of the pumped well. The measured well temperatures during the aquifer test did not improve the estimates of hydraulic properties of the Quaternary alluvium, Miocene deposits, and the streambed beyond what was estimated from the measured drawdown because of uncertainty in knowing the initial distribution of temperature at the test site.