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To Purge or Not to Purge - Is That The Question
An On-Line Version of a Column First Published in:
Environmental Technology Nov./Dec. 1997 Vol. 7 No. 7 Pages 26-27
by: David B. Vance
Groundwater monitor wells are installed primarily as tools to measure the dissolved concentrations of contaminants in groundwater in and around facilities where the release of contaminants has occurred or where the potential exists for such a release to occur (i.e. adjacent to landfills). These wells are installed with the intention of obtaining an accurate assessment of the conditions in the formation adjacent to the well. A significant body of literature and guidance documentation exists concerning the location, drilling, installation, and development of those wells to that end. However, it has become evident that the quality of the data yielded from monitor wells is dependent upon screen length and location (addressed during design and installation), purging method, and purge volume. The question, and the subject of this column, then becomes do the methods used to sample groundwater from those wells generate representative results?
The regulatory community and those involved in the design and operation of remediation systems are seeking an accurate answer to that question. However, interest is also generated from the economic pressures to reduce the cost of sampling and monitoring procedures. Standard ground water sampling procedures involve purging three to five well volumes using a pump or bailer. This entails expense in the form of time (often for two technicians) and disposal of the purge water. These two issues have driven recent research by the scientific community, as well as the regulatory community, which is also beginning to act by introducing new protocols for the acceptable sampling of wells.
The use of screened hydraulic probes is recognized as an accurate sampling method and as the yardstick for comparison to various monitor well sampling techniques. Their practical use is of value for initial plume delineation and assessment in aid of the design of a monitor well network, however, they are impractical as a long term monitoring system. The groundwater monitor well is essential for that role.
Existing purge techniques were developed with the thought that the water in a monitor well was "static" and in time grew "stagnant". Concerns were two fold, one, the mechanisms at work within the well bore that would attenuate the contaminant concentrations such as vaporization and or biodegradation supported by oxygen diffusion. Second, the assumption that with time formation conditions will improve, contaminant concentrations will decrease, and samples are used to demonstrate that trend.
Recent studies have shown that conventional purging procedures can activate two phenomena that serve to give false results: vertical mass averaging within the well bore and the introduction of water lower in the formation through vertical (upward) flow paths. Vertical heterogeneity of contaminant concentrations within the well bore occurs when zones of striking hydraulic conductivity contrast are present. Contaminants preferentially flow through the more permeable units intersected by the well screen. Gross purging then simply mixes the entire well bore. Low flow discrete level sampling has been devised to address this specific phenomena. The induction of vertical uncontaminated groundwater flow from beneath a partially penetrating well is a problem in formations at the opposite end of the hydrodynamic spectrum, that are coarsely granular (sand and gravel) and relatively homogenous.
Field testing shows that low flow purging at specific vertical points within the well bore yields results that are similar to sampling via the conventional purging methods. Bailing is likely to produce the lowest concentrations, low flow pumping slightly higher, but both tend to produce results that are significantly lower than actual concentrations within the adjacent formation. In general, it appears that the more vigorously a well is purged the more likely it is that the samples will under represent contaminant concentrations within the adjacent formation.
It is worth while to examine the physical and chemical processes that can potentially impact a monitor well in a static or stagnant state. Processes that could effect the concentration of dissolved components or the degree of mixing of heterogenetic water within a well bore column include:
- The groundwater flow rate through the surrounding formation and its influence via advective flow within the well.
- The vertical profile of the contaminant concentration and hydraulic conductivity contrast within the adjacent formation.
- Vaporization and volatilization across the air water interface at the top of the well.
- Diffusion of oxygen through the air water interface into the well bore stimulating biodegradation.
- Diurnal and seasonal temperature fluctuations (1)(2).
- Diffusional transport vertically within the well bore as well as horizontally from the adjacent formation.
- Barometric pumping, which under some conditions can cause water level fluctuations (and mechanical mixing) in wells located in unconfined aquifers.
- Seismic activity.
- Tidal forces.
- Local heavy auto, truck, or train traffic.
In aggregate, these forces impact all monitor wells to various degrees depending upon the geologic and geographic site conditions. In some instances "static" conditions are dynamic enough to insure that water in a monitor well is representative of the adjacent formation.
Consequently, some regulatory bodies have begun to consider the use of non-purge or low purge (the EPA) methods of sampling. In California, the Cal/EPA allows non-purge sampling in cases where:
- The monitor well is screened in an unconfined (water table) aquifer.
- The screened interval extends above the top of the water table.
- The monitor wells are located in moderate to high recovery aquifers consisting of sands or gravels.
Purging is still required when:
- The well is new and has been sampled less than two times (this also assumes the well underwent an adequate development purge upon installation).
- Chlorinated hydrocarbons, PNAs, diesel, or metals are the contaminant.
- In slow recovering wells within fine grained sediments (clays and silts).
- In monitor wells associated with sparging or soil vapor extraction activity.
The desire by the regulatory and regulated community to obtain accurate results, the dawning realization of the problems associated with sampling, and the costs associated with conventional approaches are likely to drive the development and implementation of new standard protocols for the sampling of monitor wells. It also presents an opportunity for the development and application of new technology and approaches.
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Copyright 2008 David B. Vance
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