There are two common types of ground water sampling equipment used in direct-push technology (DPT) methods – tools for obtaining one-time grab samples, and wells installed using DPT for short-term or long-term sampling. Most sampling devices used with DPT tools are composed of stainless steel or other inert metals. Well screen materials may be composed of stainless steel, polyvinyl chloride, polyethylene or polytetrafluoroethylene. DPT ground water sampling tools are of three basic types – exposed screen samplers, closed screen samplers and ground water profilers.
Exposed Screen SamplersExposed screen samplers, sometimes referred to as mill-slotted well point samplers, consist of a well screen that allows the influx of ground water, and a riser pipe that allows the extraction of a sample. In practice, exposed screen samplers are driven to the approximate sample depth below the ground surface. A ground water sample is taken by extracting water with either a bailer or tubing/pump combination. This necessitates purging and development prior to sampling.
Because the exposed screen can be pushed to different depths, exposed point samplers have an advantage in taking multi-level water samples without having to remove the tool string. This can result in significant time savings, especially where three-dimensional plume mapping is a data-quality objective.
Exposed screen samplers may have significant disadvantages. Clogging of the well screen can occur when probing through silty or clay-rich soils, as well as with ground water high in suspended solids. For this reason, they commonly are used in geologic formations composed of sands and gravels. If soil is contaminated above the saturated zone of interest, drawdown by the tool string and/or sloughing of contaminated soil produce false-positive results. In addition, contaminated ground water trapped in the well point bore also can be drawn from one zone to another, potentially biasing sampling results at the point of interest. The initial ground water withdrawn from the sampler can be turbid because of the disturbance of the formation while using DPT tools. In addition, because of the small screen diameter, development of the formation can be difficult.
Closed-screen SamplersClosed-screen samplers are protected, sealed sampling devices consisting of a well screen housed within a protective sheath to which an expendable drive point, drive rod(s) and drive head are attached. The assembly initially is driven with the outer casing in place.
Rubber O-rings keep the device watertight, eliminating the threat of formation fluids entering the screen before deployment and assuring sample integrity. Once the desired depth is reached, the outer casing is retracted to expose the screen to formation water.
After a sample is obtained, the expendable drive point is left in place and the sampling assembly is removed. Screen length can vary from 1 foot to 5 feet.
Because the screen is exposed after the tool has been placed at the target depth, susceptibility of the screen to clogging is reduced. The O-ring seals make the sampler watertight, and reduce the likelihood of cross-contamination. In addition, closed-screen samplers can be configured to leave the screen and riser in place as a temporary monitoring device.
Turbid samples may be caused both by the disturbance of the formation while driving DPT tools, and by the high initial entrance velocity of water into the sampler before the water level inside the sampler reaches equilibrium with the formation water. As with exposed screen samplers, development of the formation can be difficult due to the small screen diameter. A problem also can occur if the screen remains within the outer casing when it is retracted, and therefore fails to be exposed to the formation and ground water.
Ground Water ProfilersIn situations where discrete vertical profiling is desired, a ground water profiler may be used. The profiler is used to detect vertical variations in contaminant concentrations at a single location. Like an exposed screen sampler, a ground water profiler has sampling portholes or a screen through which samples are taken. The profiler is pushed to the desired sampling depth, and a probe rod is used to knock out the expendable drive point.
A sample is taken with a peristaltic or other small-diameter pump through a tube positioned within the screened area.
Once the sample is taken, the sampler may be advanced to the next sampling depth. To prevent plugging of the screen or sampling portholes, the pumping direction of the pump may be reversed and a small amount of water injected into the screen to maintain a positive pressure on the screen and prevent clogging of the sampling ports. Only clean, potable water of known chemical quality transported from offsite should be used. The pump flow is reversed once the next sampling depth is reached, and another ground water sample is taken from that depth. Once all samples are taken, the probe hole may be sealed by pumping grout through the profiler.
Quick, easy vertical contaminant profiling is available with a ground water profiler. Similar to exposed screen samplers, ground water profilers can take multi-level water samples without having to remove the tool string, thus providing a quick and efficient way of obtaining three-dimensional plume mapping. Ground water profilers have an advantage over exposed screen samplers, however, in that pumping water into the sampler can reduce or eliminate soil clogging of the screen.
Whenever multiple depths are sampled, there is a possibility of drawing contamination to deeper depths that could potentially bias results. While pumping water into the screen during driving may decrease the chances of this happening, the practice can create some problems of its own. Even though the water is pumped into the sampler under low pressure, some water may be introduced into the formation. Altern-atively, when the next sample is withdrawn, the clean water may mix with the formation water and cause samples to be biased low. Care should be taken to note the volume of water pumped into the sampler to ensure that the same volume of water is withdrawn. In addition, because of the small diameter and because multiple samples are obtained from one pushing event, it is unlikely that the formation can be sufficiently developed prior to obtaining the sample.
This article is provided through the courtesy of the Ohio Environmental Protection Agency’s Division of Drinking and Ground Waters. It is excerpted from its “Technical Guidance for Ground Water Investigations.”