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

8-5ter of the screened interval; in wells drilled in rock, welldiameters may be as little as 2 inches larger than thescreened interval.(4) Regulations in 40 CFR 264:F requirethat the annular space between the well casing and theborehole be sealed to prevent contamination of theground water and/or sample. Both cement grout andbentonite are effective agents that are commonly usedfor sealing monitoring wells. If portland cement is used,special care should be taken to minimize shrinkage, aswell as to prevent migration of the grout into adjacentformations. Alternatively, a grout mixture of portlandcement, sand, bentonite and water can be used. Ifbentonite is used, a 3to 5-foot seal of bentonite pelletsmust be placed between the well casing and theborehole. A base of sand may also be necessary aroundand above the screen. Installation of sealing agents isbest accomplished with a tremie pipe; pouring andtamping may, however, be adequate for shallow wells ofsmall diameter.(5) Well development is necessary toensure the free flow of water into the sampling interval,to purge drilling fluids and other contaminants, and toeliminate clay, silt and other fines which could contributeto water turbidity and interfere with chemical analysis. Indeveloping the well, ground water within the casing isrepeatedly forced in and out of the sampling interval byflow reversal or surge. The well is then pumped orbailed until a volume of clear water equal to that requiredfor operation of the sampling program is obtained. If thewell cannot be adequately developed, it should bereplaced with a new well.g. Federal regulations for both existingfacilities and new facilities require that a ground-watersampling and analysis plan be prepared which detailsprocedures and techniques to be followed in collecting,preserving, shipping and analyzing samples.(1) Water level measurements arerequired each time a sample is collected. Suchmeasurements are nec-TM 5-814-7essary to detect seasonal changes or other fluctuationsin the water table which could affect flow direction andthe well’s ability to yield a representative sample.(2) Before a sample is withdrawn,standing water should be purged from the well. This isan important procedure, since such water can havesubstantially different chemical characteristics from theground water to be sampled, due to dissolution of gases;leaching or adsorption of casing, screen or groutmaterials; and/or biological activity within the well. It isgenerally recommended that wells be completelyevacuated before sampling. High-yield wells should, ifpossible, be pumped dry twice and allowed to recoverbefore sampling; one complete evacuation is sufficientfor lowyield wells. If complete evacuation is not possible,a volume of water equal to 4 to 10 times the amount ofstanding water should be withdrawn. The exact volumeto be withdrawn will depend on site-specific conditions.(3) A variety of sampling devices areavailable, including bailers, portable pumps, air-liftsampler and suction pumps. Care should be taken tochoose equipment that will not contaminate the sample,particularly when trace elements are to be analyzed. Allequipment should be thoroughly cleaned beforeintroduction into a monitoring well. Once a samplingdevice has been chosen, the same equipment andsampling procedure should be used in subsequentsampling, if values are to be compared.(4) Accepted procedures for preservingand protecting ground-water samples during shippingand while awaiting laboratory analysis should befollowed. All samples should be firmly sealed, clearlylabelled and packed in compatible containers that willprevent breakage, spills and contamination. Thesampling schedule and methods of analysis should beaccording to the regulations in 40 CFR 264:F and theguidelines presented in the RCRA ground-watermonitoring guidance manual.CANCELLED