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A-3 3-foot-thick seal of bentonite pellets is placed above the sand pack. The well then is grouted with bentonite-cement slurry or neat cement, from the top of the bentonite seal to the surface. Groundwater monitoring wells commonly are completed below grade and protected with a water- tight locking cover. A typical well construction is illustrated on Figure A-1. The top of casing (TOC) is surveyed by a licensed surveyor, and all depths to water are measured in relation to the surveyed mark on the TOC. Monitoring Well Development. Groundwater monitoring wells are developed by surging, bailing, or pumping until clean, relatively sediment-free water is produced from the well. The length of development time varies, depending upon field conditions. Development water is contained in 55-gallon barrels, which are stored temporarily on site until disposal. GROUNDWATER SAMPLING Prior to collecting a sample of groundwater from a well, the well is purged by removing three or more well volumes of water, using either a pump or a bailer. A well volume is defined as the amount of groundwater in the well casing and the sand pack in the annular space surrounding the casing, assuming a sand porosity of 35 percent. The pH and electrical conductivity of the water are measured periodically during the purging. When these parameters become relatively stable, it is assumed that fresh formation water has entered the well casing. The groundwater sample is obtained with a teflon or disposable bailer equipped with a bottom-emptying valve. To release water from the bailer with minimal aeration, the protrusion on the bottom-emptying device actuates the bottom check valve and regulates the flow into the sample bottle. The sample bottles are obtained precleaned from the analytical laboratory, and it is specific with respect to size and material to the type of analysis to be performed. The bottle is carefully filled to the very top, in order to create a positive meniscus, and sealed with a teflon-lined cap (septa). These precautions aid in eliminating air from the sample. The sample is visually inspected to ensure that no air bubbles remain within. Depending on the type of chemical analysis required, samples are preserved with acid and/or they are cooled to 4 degrees Celsius. Samples then are labeled, stored, and transported in cooled ice chests to the analytical laboratory within the permissible holding time. QUALITY ASSURANCE PLAN Proper collection and handling are essential to ensure the quality of samples. Proper documentation of sample collection and handling procedures is essential to verify the integrity of the data.