MONITORING WELL CONSTRUCTION FOR HAZARDOUS-WASTE SITES IN GEORGIA by William H. McLemore Department of Natural Resources Environmental Protection Division Georgia Geologic Survey Circular 5 MONITORING WELL CONSTRUCTION FOR HAZARDOUS-WASTE SITES IN GEORGIA by William H. McLemore Department of Natural Resources J.D. Tanner, Commissioner Environmental Protection Division J. Leonard Ledbetter, Director Georgia Geologic Survey William H. McLemore, State Geologist 1981 MONITORING WELL CONSTRUCTION FOR HAZARDOUS-WASTE SITES IN GEORGIA by William H. McLemore INTRODUCTION In the past year, the Georgia Geologic Survey has been called upon on several occasions either to drill or comment upon the drilling and construction of monitoring wells. Typically, these wells were designed to evaluate the hydrology and geochemistry of the groundwater regime at existing or proposed hazardous waste facilities. With the above in mind, it is appropriate that we delimit our philosophy by which we construct monitoring wells or by which we would evaluate monitoring wells constructed by others. Obviously, because monitoring wells are designed to "look-at" the ground-water regime as well as gather water samples, local geologic conditions will play an important role in actual well construction, and rigid adherence to any set of criteria is neither practical nor prudent. Rather, our only objective in summarizing our philosophy in this Circular is to establish a set of "side-boards" that would be expected in a monitoring well construction program, which nevertheless, could be modified as local conditions dictate. The approaches set forth in the following sections are based on the experiences of the author in drilling and constructing many hundreds of monitoring wells in various parts of the country. The narrative is written in simple practical terms for geologists or engineers already familiar with the science of ground-water hydrology as well as well-drilling procedures. Lengthy technical descriptions are not provided; rather, the reader is referred to numerous USGS, EPA (especially a document such as EPA Manual SW-611) and other technical documents for such descriptions. DRILLING SUPERVISION Drilling and construction of monitoring wells should be under the close supervision of an experienced geologist or engineer. Changes in lithology, bedrock voids, pinchouts and so forth are rarely appreciated or observed by drillers. Obviously, as such variations affect hydrologic conditions, the geology of any site should be well understood. For example, the author worked on a project to monitor a contamination plume at a manufacturing facility in the Coastal Plain of a midAtlantic state. At the manufacturing site, the shallow unconfined aquifer was separated from a deeper artesian aquifer by a dense clay. Also, the shallow aquifer was at a higher head than the deeper aquifer. Flow direction in the shallow aquifer was toward the northeast, whereas in the deeper aquifer, flow was toward the southwest. However, the detailed notes and observations of the field geologist indicated that the clay was thinning in a northeasterly direction; and apparently, the clay pinched-out immediately offsite and the two aquifers merged. As the site geologist had made careful observations and had taken good notes, it could be postulated that the plume was moving to the northeast, flowing over the lip of the pinchout into the deeper but lower head artesian aquifer, and thence moving to the southwest back underneath the manufacturing facility. Such an interpretation never could have been developed without having an experienced geologist continuously observing drilling operations and collecting cuttings. Based on my experience, a single well-trained (i.e., at least one year of rig time) geologist or engineer can supervise two drilling rigs as long as they are not too far apart (i.e., a walk or drive of 5 minutes or less) and no other ancillary duties are required. If water sampling, pump testing, well development and so forth are scheduled, additional personnel would be necessary. Drilling supervision by an experienced geologist or engineer also permits monitoring wells to be constructed to rather precise tolerances. By using a weighted steel measuring tape, sand/ gravel packs, seals and so forth can be placed with an accuracy of 112 foot. Similarly, a competent geologist or engineer should know the depth within 1 foot of the boring at any time. And by collecting cuttings ornotingvariations in drilling progress I speed, the geologist or engineer should be able to make accurate predictions regarding changes in lithology, in those portions of the boring where samples are not being collected. SELECTION OF THE DRILLING RIG Selection of the drilling rig is extremely important. Because placement of screens and seals is extremely crucial in properly evaluating specific geologic horizons, the drill rig must have the capability to collect samples. Also, because porosity and permeability measurements, grain size, or strength tests may be necessary, the drill rig also should have the ability to collect undisturbed samples. Cleanliness is another important criterion for the drill rig. Typically, drill rigs used for the construction of monitoring wells are contracted; and often they are covered with grease and grime from many different projects. As such grease and grime may contain a wide variety of solvents, metals or other chemicals, one cannot discount the possibility that trace amounts of contaminants may be introduced to the well. The author is aware of a situation where a drill rig was used for soil borings at a petrochemical complex in New Jersey and subsequently was employed for the construction of monitoring wells at aNew York manufacturing plant. When unusual hydrocarbon compounds were detected in some of the monitoring wells at the manufacturing plant, management became quite concerned, as these hydrocarbon compounds were neither used nor stored at the plant. Finally, after much frustration, it was recognized that the hydrocarbon compounds had been "carried" by the drill rig from the petrochemical complex to the manufacturing plant. Such a situation easily can be mitigated by using a clean drill rig. The simplest way to obtain a high level of cleanliness is to steam clean the drilling rig. Steam jennys can be found almost everywhere; and steam cleaning of the drill rig more or less eliminates the possibility of "carrying" contaminants onto the site. Thirdly, any drill rig used in the drilling and construction of monitoring wells should be free of oil and fuel leaks. Any oil or fuel leaking into the mud pad or adjacent to the bore hole will almost certainly enter the well. If this were to occur, total organic carbon (TOO) levels or detection of fuels and greases in ground-water samples might be spurious. Introduction of oils and fuels into the ground-water regime can be prevented by the site geologist or engineer performing a daily inspection of the drill rig and insisting that the driller tighten all parts, replace gaskets, and so forth. SAMPLING Sampling is especially important in any program of monitoring well construction. The reason for this is quite simple; namely, without a good understanding of lithologic variations, the screens and seals, which are so important in monitoring well construction, cannot be properly placed. Such samples also are useful in evaluating permeability, porosity, or subtle changes in facies. Moreover, an improperly constructed monitoring well can be a vehicle for interaquifer contamination. For example, as illustrated in Figure 1, an inappropriate sampling program can lead to an incorrect interpretation of stratigraphy, with resultant improper well construction. The author has found that, at those sites where comprehensive and rigid sampling programs were performed, well construction problems were minimal. Conversely, where sampling was de- 2 df!00 .\. ....;... :-::. ><-~::. _U.N. ..C...O-N. F-l. N. ED - . .. AQUIFER .;-~;:..:-:,->"--.;.,..,: ~=::~:_:?i~;j~;:r:-~~ : :.:;.-. :- :.::~ :_~_. ,-.~-: :1li!I~IIi:l:i~B~~l': ~~~Bl~i~*~l~tJ:l!~~1i~l&;~ .. \: " ' \: "' ' "' ' < '" ,. 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