The installation of ground water monitoring wells at wastewater treatment facilities may be essential when such facilities have the potential for degrading ground water quality. Discharge to ground water at these facilities is caused by seepage or direct discharge of effluent through the soil profile. The necessity for monitoring wells and/or ground water discharge plans is based on a number of factors, including the soil type, depth to ground water, strength of the waste, mobility of the waste, and general quality of the ground water. If the potential for ground water degradation is high, monitoring wells are necessary to ensure that these discharges are not degrading ground water quality beyond the ground water quality standards, especially when they are in close proximity to water supplies and other facilities that can be impacted by ground water contamination.
The following factors should be considered when selecting an appropriate drilling method:
- type of formation
- depth of drilling
- depth of desired screen setting
- types of pollutants expected
- location of drill site, i.e., accessibility
- design of monitoring well desired
- availability of drilling equipment
Drilling MethodsHollow-stem augering one of the most desirable drilling methods for constructing monitoring wells. No drilling fluids are used, and disturbance to the geologic material penetrated is minimal. Auger rigs generally are not used when consolidated rock must be penetrated, and drilling depths usually are limited to less than 150 feet. In formations where the borehole will not stand open, the monitoring well can be constructed inside the hollow-stem augers prior to their removal from the borehole. The hollow-stem has the advantage of allowing continuous in situ geologic sample collection without removal of the augers.
Solid-stem augering is most useful in fine-grained, unconsolidated materials that will not collapse when unsupported. The method is similar to hollow-stem, except the augers must be removed from the borehole to allow insertion of the well casing and screen. In situ geologic samples are difficult to collect when using a solid-stem. In many cases, it is necessary to rely on the cuttings, which come to the surface, for geologic sampling. This is an undesirable method because the exact depth at which the cuttings come from is not known.
Cable-tool drilling one of the oldest methods used in the water well industry. Even though the rate of penetration is slow, this method offers many advantages for monitoring well construction. With the cable-tool method, excellent formation samples can be collected, and the presence of thin permeable zones can be detected. As drilling progresses, a casing normally is driven, which provides temporary support for the borehole, allowing construction of the monitoring well within the casing.
Air rotary drilling air is forced down the drill stem and back up the borehole to remove the cuttings. This type of drilling is particularly well suited for fractured rock formations. If ground water monitoring for volatile organics is planned, the drilling air must be filtered to ensure that oil from the air compressor is not introduced to the formation being monitored. Air rotary should not be used in highly contaminated environments because the water and cuttings blown out of the hole are difficult to control and can pose a hazard. Where volatile compounds are of interest, air rotary can volatilize these compounds, causing water samples withdrawn from the borehole to be unrepresentative of in situ conditions. The use of foam additives to aid cuttings removal can introduce organic contaminants into the monitoring well.
Mud rotary drilling probably the most popular method used in the water well industry. However, mud rotary drilling does have some disadvantages for monitoring well construction. With mud rotary, a drilling fluid is circulated down the drill stem and up the borehole to remove cuttings. The drilling mud creates a wall on the sides of the borehole that must be removed from the screened area by development procedures. With small-diameter wells, complete removal of drilling mud is not always achieved. The ion-exchange potential of most drilling muds is high and may effectively reduce the concentration of trace metals in water entering the well. In addition, the use of biodegradable organic drilling muds can introduce organic components to monitoring well samples.
DecontaminationPrior to entering the work site, the condition of the drill rig and equipment shall be such that it is not a potential source for monitoring well contamination. Leaking equipment seals or leaking tanks containing fluids should not be brought on-site.
All steam-cleaning must be done on-site or at an approved off-site location. Any equipment that will not be used at the site should be removed prior to entering the site. The drill rig must be steam-cleaned with water that is from a potable source. All drill rods, bits, casing, samplers, pipe wrenches, etc., should be laid on supports and cleaned until all visible signs of grease, oil, mud, etc. are removed. Cleaned equipment should not be handled with soiled gloves. Surgeon's gloves or new clean cotton work gloves should be used.
The use of new painted drill bits and tools is not recommended because paint chips could be introduced into the borehole. All water tanks, pumps, mud pans, hoses, including hoses and tanks used to transfer water from the potable source, must be cleaned. Fittings on the drilling equipment may be lubricated with non-petroleum-based substances, such as vegetable oil. Every precaution must be taken to prevent contamination of the well with oil, grease or any other substance. Lubricants must not be used on drilling and sampling tools or fittings.
At a minimum, casing and well screen should be washed with detergent and rinsed thoroughly with clean water. To ensure that these materials are protected from contamination prior to placement in the borehole, materials should be covered (with plastic sheeting or another material) and kept off the ground.
All pumps used in well development must be steam-cleaned prior to developing each monitoring well. Pumps that leak or may otherwise cause contamination must not be used. Electrical tape should not be used to attach wires to the discharge pipe of submersible pumps. Wire should be attached using stainless steel or plastic clips. Air compressors used for development must be equipped with operable oil traps and a filter. Nitrogen gas, if used for development, must be regulated and passed through an oil trap and filter before it enters the well.