UFC 3-280-02A Hazardous Waste Land Disposal/Land Treatment ...

ing hydrogeologic conditions are detailed in chapter 3 ofthis manual.b. A minimum of four ground-water monitoringwells will be installed, one hydraulically upgradient of thewaste disposal unit, to provide background groundwaterquality data, and three downgradient of the facility todetect contaminant discharge. Small indoor waste pilesare the only waste facilities at which fewer wells will beconsidered.c. Upgradient wells should be installed in theuppermost aquifer at a location not likely to be affectedby the waste facility. Downgradient wells should also beinstalled in the uppermost aquifer, but along pathwayslikely to transport contaminants, should any be releasedfrom the facility. Care must be taken in locating andconstructing monitoring wells to ensure that they notserve as conduits for contaminants to enter the groundwater, or allow contaminated ground water to migrate toan uncontaminated aquifer.d. Well depth should be determined on asitespecific basis. Factors which influence well depth, aswell as the depth of the sampling (or intake) interval ofthe well casing, include ground-water levels and thebehavior of specific contaminants in the aquifer. Thesedeterminations are dependent on a detailed log ofborings and on the subsurface geologic conditions. e.The principal components of the monitoring well are thewell casing and the perforated or screened samplinginterval. A typical ground-water monitoring well is shownon figure 8-1. Details on well design and samplingmethods appear in SW-611 and in the RCRA guidancemanual on ground-water monitoring. It must be stressedthat well design must always be based on a clear anddetailed understanding of site hydrogeologic conditions.(1) One of the considerations in design ofthe well is selection of the proper well diameter, whichdepends on a number of factors, including state andfederal requirements, drilling method and subsurfaceconditions, as well as the diameter of the sampler.Monitoring wells generally have casing diameters ofeither 2 or 4 inches. The larger casing size permitsgreater flexibility in sampling methods, since an innerdiameter of 4 inches is generally required toaccommodate submersible pumps and other equipmentused for evacuation and sampling. Two-inch casingsmay be necessary or favorable in some instances,however, since they can be installed by the dry hollowstemcontinuous flight auger drilling method. Some drillrigs can install 4-inch casings but such rigs are notalways readily available.(2) Proper location of the intake, orsampling, interval of the monitoring well is extremelyimportant to ensure that it is in the path of likelycontaminants and therefore likely to yield representativesamples. Where aquifer zones are relatively thin (i.e., nomoreTM 5-814-7than 20 feet thick), the well should be perforatedthroughout the zone. In thicker aquifers, multiple wells(see figure 8-2) should be used to define water qualitystratification within the aquifer. Care should be taken toensure that the perforated interval does not providehydraulic connection between isolated aquifers.(3) Also important is the sizing of theperforations or screen. A properly sized screen,generally one designed to exclude up to 60 percent offormation materials, will prevent passage of fines fromthe formation, while allowing passage of sufficient waterfor sampling. In most cases a commercially fabricatedscreen is recommended, although a factory-slottedcasing may be adequate for some applications. Fieldperforation of well casings is not recommended.(4) Materials selected for the well casingshould be compatible with the expected contaminants tominimize the potential for interaction between the casingmaterial and the sample. Steel casings may contributeiron and other ions to the sample. Furthermore, themetallic oxides which form on a steel casing influenceconcentrations of cautions and some organic molecules.PVC pipe, unlike steel well casing, is resistant to mostchemicals, nonconductive, and chemically inert;however, PVC is not recommended for sampling certainreactive organic constituents such as ketones oraromatic compounds, which can better be accomplishedusing stainless steel or teflon. However, the finalselection of well materials should be determined by aperson knowledgeable about the probable chemicalreactions (e.g., a chemist or chemical engineer).Needed joints in PVC casings should be fashioned usingthreaded couplings instead of glue to avoidcontamination.(5) Locking caps and concrete padsshould be installed on all monitoring wells. Pads shouldbe designed to divert drainage from the casing, therebypreventing precipitation or extraneous substances fromentering the well.f. Well drilling methods, filter packing, sealingand development are the components of concern in wellconstruction, both to maintain the integrity of theborehole and to prevent contamination of samples.(1) The drilling method selected shouldavoid spreading any ground-water contamination and/orinterfering with the sample to be collected. Both dry andwet drilling methods are commonly used to constructmonitoring wells. Conventional auger drilling isCANCELLED8-2advantageous, since the potential for introducingextraneous fluids is less than with rotary drilling methods.Auger drilling is best suited to fine-grained,nonconsolidated materials; rotary (air or water) drilling isrequired for wells in cemented or consolidated materialssuch as bedrock. The maximum casing diameter inwells drilled by the standard continuous flight auger