Monitoring Well Intake Design
April 15, 2008
The intakes of monitoring wells should be designed and constructed to:
- accurately sample the aquifer zone the well is intended to
- minimize the passage of formation materials (turbidity) into the
- ensure sufficient structural integrity to prevent the collapse of the intake structure.
The goal of a properly completed monitoring well is to provide low turbidity water that is representative of ground water quality in the vicinity of the well. Monitoring wells completed in rock often do not require screens, though wells completed in unconsolidated sediments do require screens.
Screen LengthThe selection of screen length usually depends on the objective of the well. Piezometers, for example, generally are completed using short screens (2 ft. or less), as are wells where only a discrete flow path, such as thin gravel interbedded with clays, is monitored. To avoid dilution, well screens should be kept to the minimum length appropriate for intercepting a contaminant plume, especially in a high-yielding aquifer. The screen length generally should not exceed 10 feet. If construction of a water-table well is the objective, either for defining flow gradient or detecting the presence of floating non-aqueous phase liquid (NAPL), then a longer screen spanning the water table is acceptable, to account for NAPLs or seasonal water-table fluctuations. Do not use screen lengths that create a conduit for contaminant transport across hydraulically separated geologic units.
Screen Slot SizeWell screen slot size should be selected to retain at least 90 percent of the filter pack material in artificially filter packed wells, or a minimum of 50 percent of the formation material in naturally packed wells, unless it can be demonstrate that turbidity-free water can be obtained using a larger slot size. Although this is a higher percentage than usually is required in a production well, the low withdrawal rates and the infrequent use of a monitoring well necessitate the higher percentage exclusion. Filtering a sample subsequent to its collection is not the solution for dealing with turbidity in an improperly designed well. Furthermore, well screens should be factory-slotted. Manually slotting casing as a substitute for screens should not be accepted under any conditions.
Filter Packs/Pack MaterialThe annular space between the borehole wall and the screen or slotted casing should be filled in a manner that minimizes the passage of formation materials into the well. The driller generally should install an artificial filter pack around each well intake. As discussed above, wells in rock often do not require screens, and thus do not require filter packs. However, they are the exception; most wells will require filter packs and a screened length of casing.
An artificial filter pack is appropriate in most geologic settings. In particular, an artificial filter pack should be used when:
- the natural formation is poorly sorted;
- a long screened interval is required
and/or the intake spans highly stratified geologic materials of widely varying
- the natural formation is a uniform fine sand, silt or clay;
- the natural formation is thin-bedded;
- the natural formation is poorly cemented sandstone;
- the natural formation is highly fractured or characterized by
relatively large solution channels;
- the natural formation is shales or coals that will act as a constant
source of turbidity to ground water samples;
- the diameter of the borehole is significantly greater than the diameter of the screen.
Using natural formation material as filter pack is recommended only when the natural formation materials are well sorted and relatively coarse-grained.
Filter pack material should be chemically inert. Sands should be analyzed for cation-exchange capacity and volatile organic compounds (VOCs) to determine whether it will interact with analytes of concern in the ground water. Commercially available pea gravel may be acceptable for use in gravel aquifers; however, because the filter pack should be chemically inert, the pea gravel itself should not be chemically active or coated with a chemically active metal oxide. Filters constructed from fabric should not be allowed, as they tend to plug and may be chemically reactive.
Filter pack material should be installed in a manner that prevents bridging and particle-size segregation. Filter pack material installed below the water table generally should be tremied into the annular space. Allowing filter pack material to fall by gravity into the annular space only is appropriate when wells are relatively shallow, when the filter pack has a uniform grain size, and when the filter pack material can be poured continuously into the well without stopping.
At least 2 inches of filter pack material should be installed between the well screen and the borehole wall. The filter pack should extend at least 2 feet above the top of the well screen. In deep wells, the filter pack may not compress when initially installed; consequently, when the annular and surface seals are placed on the filter pack, the filter pack compresses sufficiently to allow grout into, or very close to, the screen. Consequently, filter packs may need to be installed as high as 5 feet above the screened interval in monitoring wells that are deep (i.e., greater than 200 ft.). The precise volume of filter pack material required should be calculated and recorded before placement, and the actual volume used should be determined and recorded during well construction. Any discrepancy between the calculated volume and the actual volume requires an explanation.
Prior to installing the annular seal, a 1-foot to 2-foot layer of chemically inert fine sand may be placed over the filter pack to prevent the intrusion of annular or surface sealants into the filter pack. The entire length of the annular space that is filled with filter pack material or sand effectively is the monitored zone. Therefore, if the filter pack or sand extends from the screened zone into an overlying zone, a conduit for the possible transport of contaminants is created between the two zones.
This article is provided through the courtesy of the California Environmental Protection Agency. It is excerpted from the agency’s “Guidance Manual for Ground Water Investigations.”