Sites were selected to develop a long-term ground-water monitoring network in the Oasis Valley area. The objective of the network is to define spatial changes in the chemical and isotopic character of the ground water. Information from the network may be used to (1) refine the current understanding of the ground-water flow system through a better understanding of the ground-water chemistry and subsurface geology and (2) to monitor potential changes in ground-water chemistry related to past underground testing activities at the NTS.
Seven sites were chosen in which to install single or multiple wells (fig. 2). Multiple wells, where present, were less than 30 ft from each other but screened at different depths (wells ER-OV-03a2 and ER-OV-03a3 were nested within the same borehole). The sites chosen were upgradient from the major spring-discharge area in Oasis Valley. Some wells were installed adjacent to or within fault zones that may be pathways for upward ground-water flow in the area. Twelve monitoring wells (in 11 boreholes) were installed at these sites during August through October 1997.
Two drilling methods were used--mud rotary and down-hole air hammer. The mud-rotary method is excellent for drilling deep holes and commonly is used to drill through unconsolidated or loosely consolidated sediments because the borehole walls can be stabilized or supported. During mud-rotary drilling, the borehole is drilled by the action of a rotating bit, and the cuttings are removed by the circulation of drilling fluid. The fluid used in the drilling process was a mixture of water and bentonite (sodium montmorillonite). The viscosity of the fluid was varied during drilling to control the circulation loss of drilling fluid and to stabilize the borehole wall when drilling in unconsolidated materials. Down-hole air-hammer drilling works well in conditions where the predominant material drilled is consolidated rock and where high rates of drilling are desirable. The down-hole air hammer uses a pneumatically operated hammer to break the rock into fragments, which are then transported to the surface with compressed air. Water was injected into the compressed air stream to assist in the lifting of the fragments and to control dust. Most wells were drilled for this study using only one method--either the mud-rotary or down-hole air hammer technique (table 1). However, in wells ER-OV-03c and ER-OV-03c2, the unconsolidated materials were drilled using mud-rotary techniques and the bedrock was drilled using a down-hole air hammer. For these two wells, the objective was to prevent caving or collapsing of the upper unconsolidated layers.
Most of the wells, except ER-OV-02, -03a, -04a, and -05, were drilled initially with a 57/8- or 6 3/4-in.-diameter pilot bit to depth where bedrock was contacted or the hole was deep enough to support the installation of steel surface casing. Some of the holes drilled with a pilot bit were then enlarged with a 12-in.-diameter bit to allow the installation of 6-in.-diameter surface casing (table 1). After the surface casing was installed, the borehole was drilled with a 6-in.-diameter air-hammer bit to the desired depth or to the limitations of the equipment. Wells ER-OV-03c and ER-OV-06a were planned to depths in excess of 600 ft, but limitations of the air compressor and the introduction of high volumes of water from fractures limited the completed depths to 542 ft and 536 ft, respectively.
Sample cuttings were collected from every borehole at 10-ft intervals and split three ways. Two of the splits were sent to the USGS Core Library and Data Center in Mercury, Nev., for washing and retention, and the other split was used for the lithologic analysis included in this report.
Well construction was in accordance with project requirements and State regulations (Nevada Division of Water Resources, 1997). Well-construction data for all sites are presented in table 1. Most wells extend the length of the borehole and consist of a 2.5-in.-diameter schedule-40 or schedule-80 polyvinyl-chloride riser, 0.020-in.-slot stainless-steel wire-wrapped screen, and a 10- or 20-ft sump at the bottom of the well (fig. 4, table 1). A filter pack of coarse sand was put into the borehole from the bottom to above the screen using a tremie. A bentonite annular seal was then grouted from the top of the filter pack to either (1) the bottom of the surface casing, where present, or (2) to land surface. Where surface casing was installed (table 1), neat cement was added to the hole from the bottom of the surface casing (top of the grout) to land surface. Exceptions to the typical well construction, wells ER-OV-03a2, -03a3, -03b, and -06a2, are discussed below.
Wells ER-OV-03a2 and ER-OV-03a3 were nested within a single 821-ft-deep borehole (fig. 4, see "multiple-well borehole"). On the basis of geophysical log analysis, two zones were identified for well completion: a lower zone from 580 to 640 ft below land surface and an upper zone from 88 to 160 ft below land surface. The borehole was filled with bentonite grout from 821 ft to 655 ft to seal the bottom of the hole from the lower zone of interest. A 2.5-in.-diameter deep well was screened over the lower zone, and a filter pack of sand was placed around the screen using a tremie. Then bentonite grout was added into the annular space to the upper zone of interest. A second sand filter pack was installed around a 2-in.-diameter shallow well. Finally, grout was added to the borehole from the upper filter pack to land surface.
Well ER-OV-03b was constructed of 4-in.-diameter carbon steel casing and 0.020-in. stainless-steel screen. The casing string was supported 5 ft above the bottom of the borehole, and sand was placed over the desired interval using a tremie. The casing was supported to keep the screen from collapsing under the weight of the steel casing.
Well ER-OV-06a2 differed from typical well construction in that the bottom 6 ft of the borehole was sealed with grout before well construction. The lower part of the hole was grouted to seal off upward ground-water flow from fractures observed on the geophysical logs.
Immediately after completion, all wells were developed using compressed air to alternately surge and lift out the water column inside the well. Well ER-OV-03b produced turbid water at slow rates; therefore, water was injected into the air stream to assist in development. Wells were developed until the discharge water was clear. Rates and volumes of water removed during development are listed in table 2.
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