By John A. Izbicki
U.S. GEOLOGICAL SURVEY
Fact Sheet 2004-3096ONLINE ONLY
Sacramento, California 2004
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The U.S. Geological Survey, in cooperation with the manufacturer (1Besst In2c.http://www.besstinc.com), has modified a commercially available gas-displacement sample pump to collect water at selected depths within production wells under pumping conditions. The modified pump is about 6 inches long, less than 1 inch in diameter, and is operated through repeated application and release of compressed gas. The pump is intended for use in production wells having limited access that prevents the use of traditional geophysical tools, such as wire-line bailers, used to collect depth-dependent water samples. In most cases, the production pump does not have to be removed or the well modified for insertion of the sample pump. Data collected at different depths within the production well reflect water quality at those depths under actual pumping conditions. If well-bore flow velocities are known, the quality of water in the aquifer between sample depths can be estimated.
|Figure 1. Diagram showing schematic of small-diameter pump and basic operating principles.|
The modified sample pump consists of a series of one-way flow valves connected to the surface by two 1/8-inchdiameter Teflon tubes bonded together into a single hose. One tube serves as a pressure line, and the other serves as the sample line (fig. 1). After the pump is lowered into the well to the sample depth, water enters the pump, filling both Teflon tubes to the water level in the well (Initial conditions, fig. 1). The pressure line is pressurized using compressed gas, and water is displaced from this tube into the sample line (Pressure cycle, fig. 1). A one-way flow valve at the pump intake prevents the displaced water from flowing back into the well. Once the water is in the sample line, another one-way valve prevents the water from flowing back toward the sample pump after pressure is released (Release cycle, fig. 1).
To force additional water into the sample line, pressure is alternately applied and released on the pressure line at the surface using compressed gas regulated through a control panel built by the U.S. Geological Survey. The column of water in the sample line can be forced upward to the surface through the sequential application and release of pressure. Alternately, when sufficient volume is accumulated, water in the sample line can be forced to the surface using compressed gas. With proper use, water from the pump is suitable for analysis for a wide variety of constituents, including volatile organic carbon and dissolved gasses. The pump is capable of lifting water from depths as great as 1,200 feet. The pump and hose are mounted on a motorized reel (fig. 2).
Samples from different depths within production wells can be collected by sequentially moving the pump to a different sample-collection depth without retrieving the pump from the well after each sample is collected. The pump provides an alternative to traditional sample collection methods using wire-line bailers or sample-collection methods developed by Izbicki and others (1999), both of which require sample equipment to be pulled from the well to bring each depthdependent sample to land surface.
|Figure 2 is a photograph showing a hose-reel assembly and a small-diameter sample pump.|
Depth-dependent water-quality data collected within production wells under
pumping conditions can be used to estimate the quality of water yielded to a
well from a selected depth interval using the following equation:
Ca = (CiQi - Ci+1Qi+1)/Qa , (eq. 1)
C is the concentration of a given constituent,
Q is the flow of water within the well (either as volume per unit time, as velocity, or as percent of total discharge),
i is the first sample collection and flow measurement depth,
i+1 is the second sample collection and flow measurement depth, and
a is the interval between i and i+1.
(In this example, flow in the well is presumed to be upward toward the pump intake and the first sample-collection depth is shallower than the second sample-collection depth.)
Depth-dependent water-quality data collected from production well 4N/5W-2H1
near Victorville, California, using this small-diameter sample pump are shown
in figure 3. The well is screened between 600 and 836 feet below land surface,
and the pump intake is at 685 feet. The well was pumped at 1,000 gallons per
minute. Flow was upward from the bottom of the well screen toward the pump intake.
The deepest sample collected from the well is presumed to be representative
of water in the aquifer at that depth. The next deepest sample is a mixture
of water already in the well and water that entered the well between the two
sample depths. The combination of velocity-log and depth-dependent waterquality
data can be used to resolve the mixture within the well and estimate the concentration
of a constituent in water contributed from the aquifer to the well at selected
sample depths below the pump intake during pumping (fig. 3). Results of calculations
show that, in general, arsenic concentrations increased with depth below the
pump intake but remained below the U.S. Environmental Protection Agency proposed
Maximum Contaminant Level for arsenic of 10 micrograms per liter (µg/L).
|Figure 3. Depth-dependent water-quality data collected from production well 4N/5W-2H1 near Victorville, California.|
Flow was downward in the well from the top of the screened interval to the pump intake (fig. 3). No samples were collected above the pump intake and the arsenic concentration of water yielded from the aquifer to the well above the pump intake was not calculated.
The small-diameter pump provides a means to collect depth-dependent water-quality data from specific intervals in long-screened production wells under pumped conditions. Coupled depth-dependent water-quality and velocity- log data have application to a wide range of hydrologic problems including seawater intrusion, brine invasion, naturally occurring trace-elements, and anthropogenic contamination. The smalldiameter sample pump described in this fact sheet enables more efficient collection of depth-dependent samples from production wells having limited access by increasing the ease with which water samples are collected (thereby reducing the time and cost) through eliminating the need to retrieve the sample pump from the well after each sample is collected.
1.The use of trade names in this fact sheet is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.
Izbicki, J.A., Christensen, A.H., and Hanson, R.T., 1999, U.S. Geological Survey combined well-bore flow and depth-dependent water sampler; U.S. Geological Survey Fact Sheet FS 196-99.
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