Our focus is strictly on the design and location of the monitoring wells. conventional drilling rigs, or expedited assessment vs. Since the well casings are 4-inch inside diameter, if needed, they can accommodate free-product recovery (and other remediation technology) equipment.įor purposes of the following discussion, we'll assume that there is no problem with sampling or well installation techniques - this isn't a discussion of push technologies vs. Quarterly gauging of the water levels indicates that water table fluctuations should remain within the screened interval so that none of the wells will go dry. Analysis of ground water samples from each of the four wells indicates (we'll assume) that they are essentially free of dissolved hydrocarbons. Each boring has a continuous log plus undisturbed samples at 5-foot intervals.Īnalysis of soil samples from each boring indicates only minor amounts of residual contamination near the tank field. Sounds familiar, doesn't it? With respect to the above scenario, conventional wisdom holds that the stratigraphy, water table and ground water flow direction all are well defined. Finally, after being allowed to recover for at least 24 hours past development, the well is ready for water level measurement and liquid sample collection. Later, the newly installed well is “developed” using either pumping or surging techniques. The top of the well is fitted with a locking, watertight well cap. The wellhead itself is protected either with a flush-mount cover or steel surface casing. The remaining annular seal-to-land surface is sealed with a bentonite-grout slurry. On top of the sand is a bentonite seal that is 2 feet to 5 feet in thickness. The screened portion is backfilled with coarse (#2) sand to a level that is a foot or two above the top of the screen. Sufficient lengths of casing and screen are installed such that the screened portion extends 5 feet to 10 feet below and above the water table to allow for seasonal variation. The casing and screen consist of a 4-inch inside diameter Schedule 40 PVC pipe with factory-threaded couplings and factory-cut slots (0.020 inch). When the boring finally reaches the water table, it is advanced another 5 feet to 10 feet, and then the casing and screen are installed. The on-site geologist also logs cuttings between the undisturbed samples. Later, the jar with the sample containing the highest reading from each borehole will be sent to a laboratory for analysis. A geologist records the lithologic information for each sample and screens each interval with an organic vapor meter/analyzer for the presence of petroleum hydrocarbons.īeginning with the first detection of organic vapors, soil samples are placed in labeled jars and stored on ice in a cooler. Each sample is 18 inches to 24 inches in length. As a result, in plan, these four points outline a compressed and elongated quadrilateral, not a square.Īt each location, the drill rig advances a 10- to 12-inch diameter bit in 5-foot increments and then stops to allow for undisturbed soil samples to be collected. However, due to the locations of overhead and underground utilities, the tank field, the pump islands, the waste oil tank and the station building, the locations for the monitoring wells must be shifted somewhat from the originally intended locations at the property corners.
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