Geohydrology of Monitoring Wells Drilled in Oasis Valley near Beatty, Nye County, Nevada, 1997

Water-Resources Investigations Report 98-4184

 By Armando R. Robledo, Philip L. Ryder, Joseph M. Fenelon, and Frederick L. Paillet


Description of Stratigraphic Units in Boreholes

Lithologic samples collected from each borehole were visually analyzed for mineralogic content and lithology. Seven distinct units, listed from oldest to youngest, were identified from the samples: (1) Ammonia Tanks Tuff; (2) Tuff of Cutoff Road; (3) tuffs, not formally named but informally referred to in this report as the "tuff of Oasis Valley"; (4) lavas also informally named in this report as the "rhyolitic lavas of Colson Pond"; (5) Tertiary colluvial and alluvial gravelly deposits; (6) Tertiary and Quaternary colluvium; and (7) Quaternary alluvium. A general description of each unit based on work by Scott Minor and others (U.S. Geological Survey, written commun., 1998) is given below.

The Ammonia Tanks Tuff (Tma) [11.45 Ma] is a member of the Timber Mountain Group. The tuff was erupted from the Timber Mountain caldera complex and is described as a light- to medium-gray-brown, metaluminous, welded ash-flow tuff. It is compositionally zoned and grades from a volumetrically dominant rhyolite (abundant sanidine, common quartz and plagioclase, sparse biotite and sphene, and rare clinopyroxene) in its lower part to a crystal-rich trachyte (abundant sanidine and biotite, common plagioclase and quartz, and sparse clinopyroxene and sphene) in its upper part. Sanidine and large (as much as 0.2 in. in diameter) equant and euhedral quartz are characteristic of the unit. It locally contains basaltic xenoliths as large as several meters across. In general, the tuff is distinguished by high quartz and mafic contents and sparse sphene. In a north-trending fault zone on the western side of the Oasis Valley, this unit has undergone intense hydrothermal alteration and brecciation.

The Tuff of Cutoff Road (Tfc) [11.1 Ma] also is a member of the Timber Mountain Group. The tuff is described as a light-pinkish-gray to light-yellowish-tan, moderately welded ash-flow tuff that forms a single unit. It has a pumiceous base with white, pale pink, and tan pumice clasts from about 0.5 in. to as large as 1 ft in diameter. The tuff contains 5 to 15 percent phenocrysts with the following mineralogy: abundant sanidine; common plagioclase and biotite, sparse hornblende and quartz, and common accessory sphene (Fridrich and others, 1994). Small lithic fragments within the tuff are moderately abundant. Locally, this tuff unit has undergone intense hydrothermal alteration.

The "tuff of Oasis Valley" is a grayish-white to light brown, non- to moderately welded, pumiceous and shard-rich sequence of ash-flow and air-fall tuffs. It contains pink, yellow, tan, and light-gray pumice clasts ranging from less than 0.5 in. to greater than 4 in., that are commonly zeolitized. This unit is bracketed stratigraphically between the 11.1 Ma Timber Mountain lavas and tuffs and the 9.5 Ma "rhyolitic lavas of Colson Pond." It locally contains reworked ash from the Rainbow Mountain volcanic sequence to the south and west (C.J. Fridrich, U.S. Geological Survey, oral commun., 1998). The "tuff of Oasis Valley" contains 5-15 percent phenocrysts of plagioclase, sanidine, quartz, biotite, iron-titanium oxides, and some clinopyroxene, hornblende, and accessory sphene.

The "rhyolitic lavas of Colson Pond" (9.5 Ma) consist of multiple rhyolitic lava domes, lava flows, and ash-flow tuff sheets. The unit is well exposed at, and appears to be limited to, the mouth of Thirsty Canyon and the area near the present-day Colson Pond (less than 1 mi southwest of well ER-OV-01). The rhyolitic lavas are flow-banded and bluish-gray to black where glassy. Where the lavas are devitrified (and locally spherulitic), they are bluish-gray, pale reddish-brown, and medium brown in color. Based on argon-40/argon-39 isotope dating, the age of the unit is estimated to be approximately 9.5 Ma (D.A. Sawyer, U.S. Geological Survey, oral commun., 1998). The lavas contain approximately 5 to 15 percent phenocrysts of plagioclase, sanidine, biotite, some quartz, and lesser amounts of clinopyroxene and hornblende.

The Tertiary gravels (Tgs) are principally colluvial and alluvial gravel with some beds of sand and tuffaceous sand. The alluvial gravels are medium-gray to brownish-gray, subangular to well-rounded, poorly to moderately well-sorted, and well-bedded to nonbedded. Clasts within the colluvium and alluvium consist primarily of Paleozoic carbonate and clastic (siliceous) rocks and Tertiary volcanic rocks; the clasts include boulders as large as 10 ft in diameter. The gravels are well cemented by carbonate minerals (calichified) and locally form resistant ledges. North of Beatty Wash, the unit intertongues locally with volcanic rocks of local origin (Swadley and Parrish, 1988).

The Tertiary and Quaternary colluvium (QTc) consists of unsorted, nonbedded, unconsolidated to moderately consolidated, angular pebble-to-boulder talus. Landslide deposits and sandy slope-wash and alluvial deposits also are included within this unit. The unit is present along or at the base of moderate to steep bedrock slopes.

The Quaternary alluvium (Qal) is poorly sorted, poorly bedded, unconsolidated to weakly consolidated, gravel, gravelly sand, and sand. Gravel clasts are angular to rounded and consist mainly of Miocene tuff and lava of local provenance. Sand grains are poorly sorted, fine to very coarse, and locally silty and clayey. The alluvium forms channel deposits of active streams, terrace deposits along some large washes, and extensive alluvial fans that commonly form the lower part of fan aprons flanking bedrock uplands. Localized sandy-silty-clayey paleo-stream channels occur throughout the alluvium. Additionally, localized zones of reddish hydrothermal alteration or subaerially weathered surfaces are present throughout the deposit.

Stratigraphic and lithologic units in the boreholes were identified by analysis of cuttings and interpretation of geophysical logs (table 4). The principal geophysical logs used in the determination of intervals were caliper, natural gamma, conductivity, induction resistivity, and resistance.

Geophysical Logs

Geophysical logs collected from each borehole are presented in figures 6 through 16. A geologic log of the major stratigraphic units is shown with the geophysical logs. Horizontal scales for each type of geophysical log are the same for all boreholes with the exception of the induction resistivity log for borehole ER-OV-03a2 and ER-OV-03a3 (fig. 8), which is expanded 10 times. The vertical scale for each set of geophysical logs is different among boreholes.

Water Levels

Two sets of water-level measurements are presented in table 5. Depths to water ranged from about 18 to 350 ft below land surface. Regional ground-water flow through the Oasis Valley study area is primarily to the south (Laczniak and others, 1996). Water levels measured in Oasis Valley (table 5) generally match the regional water-level contours shown by Laczniak and others (1996, pl. 1). Little temporal variation was observed in water levels. For example, sets of measurements made more than 3 months apart (October 27, 1997, and February 10, 1998) were all within 0.8 ft of each other. A strong downward vertical gradient in the borehole with nested wells ER-OV-03a2 and ER-OV-03a3 may result from a fault that separates a shallow flow system monitored by well ER-OV-03a3 from a deeper flow system monitored by well ER-OV-03a2.

Ground-Water Flow in Well ER-OV-06a

Flow-producing fractures or bedding planes in six water-bearing zones (A-F in fig. 5) were identified in borehole ER-OV-06a with the aid of acoustic televiewer, temperature, caliper, and heat-pulse flowmeter logs (Hess, 1986). Interpretation of the flowmeter log (fig. 5) was difficult because of the large scatter in the data, primarily in the interval where flow was greatest. Most of the scatter is attributed to the rugged borehole (as shown on the caliper log), which did not allow a good seal between aquifer zones. The probable inflow or outflow points shown in table 6 are taken as the center of the zones where inflow or outflow actually occurs as inferred from inflections on the temperature log. Ambient upflow in the well was approximately 0.5 to 0.6 gal/min, which was only partly reduced when water was injected into the well. The temperature log supports the interpretation that the greatest ambient flow (both inflow and outflow) is in the interval between 210 and 340 ft. In this interval, the temperature log is nearly isothermal because the flow is so fast that it does not cool much as it travels up the borehole.

Although transmissivities of the water producing zones cannot be directly calculated from the flowmeter data, differences of inflow rates that are proportional to transmissivity may be used to determine the relative transmissivities of the zones (Paillet, 1998). When flow is measured in a borehole, the amount of flow entering from a given bed or zone is proportional to the product of two unmeasured variables--the transmissivity of the zone and the change in hydraulic head across the borehole wall. [The change in head is defined as the hydraulic head within a zone (which was not measured) minus the hydraulic head in the borehole.] Relative transmissivity for a given zone is determined from the following equation that describes the change in flow, deltaQ, for a given fracture zone:

deltaQ = delta i*T

where deltai is the change in gradient from the fracture zone to the borehole between ambient and injection conditions, and T is the transmissivity of the fracture zone. If it is assumed that the change in head difference or gradient between the borehole and the aquifer at each zone is the same, then the change in flow is directly proportional to the difference in transmissivity of the fracture zones. This difference between inflow under two head conditions is used to determine the relative transmissivity.

To determine relative transmissivities for each zone in borehole ER-OV-06a, the differences of inflows and outflows between ambient and steady injection conditions were calculated (table 6). During ambient conditions, the water level in the borehole was at equilibrium with the surrounding water-bearing zones, whereas during steady injection conditions, a constant rate of 0.39 gal/min of water was injected into the borehole. During injection conditions, flows into the borehole from all zones systematically decrease. That is, more water flows out of the borehole (which is expressed mathematically as a decrease of inflow) and less water flows into the borehole. To make the net difference between ambient and injection inflows (column 6 of table 6) a positive number, injection inflows are subtracted from ambient inflows. For example, in zone C, the ambient inflow (-0.20 gal/min) minus the injection inflow (-0.37 gal/min) equals 0.17 gal/min. Because this difference (43.6 percent) is a large percentage of the total difference between ambient and injection inflows, the relative transmissivity of zone C is ranked high relative to other zones in the borehole.

The flowmeter analysis is based on a "difference of a difference"; therefore, errors in measurements are magnified. That is, the inflow or outflow within a zone is calculated as the difference in flow between the top and bottom of the zone. Then the difference between two such sets of data (at different head conditions in the well) is used to estimate the relative transmissivity. Thus, the relative transmissivities of zones are rough estimates. These estimates were especially subject to error in the 150- to 320-ft interval (which includes zone C) where the borehole is rugged, and a great deal of scatter in the flow measurements occurs.

The zones of greatest transmissivity in borehole ER-OV-06a were in zone C (in the "rhyolitic lavas of Colson Pond") and zone D (near the top of the "tuff of Oasis Valley"). Transmissivities in the remaining four zones were relatively low. In zone A, the difference between ambient and injection data is zero, implying no transmissivity. However, the zone was clearly transmissive because it accepted flow (fig. 5 and column 4 in table 6); the transmissivity was just an order or two of magnitude smaller than that of some of the other zones.

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