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-- 8860 2. If the stream is laterally homogenous (well-mixed), the sample can be taken from a single vertical near the centroid of flow. In order for the, stream to be considered well-mixed, temperature, pH, and specific conductance should be almost constant through the cross section. The entire sample, no matter which method of collection is used, should be composited in a cleaned and rinsed cOn- tainer, such as a u.s. Geological Survey churn splitter or a large, plastic bucket. All onsite measurements are made on the composite sample and all bottles prepared for lab analysis are filled from the composite sample. by pumping until the temperature, specific conductance, and pH of the pumped water stabilize. Generally, four to six well volumes must be cleared before a high percentage of aquifer water is being pumped (Gibb and others, 1981). The time required to obtain a given percentage of aquifer water can be calculated from the rate of pumping, well diameter, and aquifer transmissivity (U.S. Geological Sur- vey, 1977; Gibb and others, 1981). If the well can easily be pumped dry, a representative sample can be obtained as the well refills. The influence of well construction on water-quality samples has been discussed by Scalf and others (1981), Claassen (1982), and Barcelona and others (1983). Deter- minationof aquifer properties is described in the U.S. Bureau of Reclamation's Ground Water Mannal (1977). The primary value of aquifer characterization for ground- water samples is to determine the proBable source of flow to the well and potential sources of salinityc6ntamination. It can also help to determine the requirement for flushing the well prior to sampling. In order to obtain representative ground-water samples, monitoring wells must first be purged of standing water. This allows fresh aquifer water to enter the well. Normally the chemical characteristics of standing well water are dif- ferent from those of water in the aquifer. A well is purged Shallow wells (25 feet or less) can be cleared with a peris- taltic (suction) pump. However, due to pumping rates, this method is best used for small diameter (2 inches or less) wells. Bailers can be used for sha1low and inter- mediate depth wells. Bailers tend to oxygenate the water, however, and can therefore alter its chemistry. For inter- mediate and deep wells, submersible pumps are efficient. A combined use of bailer and peristaltic pump can be ef- fective. If a well is moderately transmissive, it can be cleared quicldy with a bailer, and then pumped with the peristaltic pump for sample collection. For a shallow well of low transmissivity, it may be more practical to bail for the sample. Since a shallow system is not generally deoxygenated, bailing will probably not cause significant oxidation. The advantages and disadvantages of differest methods of sample recovery have been discussed byGibb and others (1981), Scalf and others (1981),andClaassen (1982). ", , Ground Water, For ground-water monitoring, derming the source of a sample can be difficult., The aquifer or aquifers contribut- ing water to the monitoring well must be identified. This requires knowledge of the monitoring well construction, in- cluding drilling, casing, and development, as well as knowledge of the local geohydrology. Because this can be a complex study in itself, sampling should be concentrated on a few characteristic wells. The ground-water sample should be pumped or ,bailed into a clean, rinsed container, such as a churn splitter or plastic bucket. Mixing the sample with air should be m;n;mi7ed. If the sample is being pumped, the container can be filled from the bottom and allowed to overflow until temperature, specific conductance, and pH have stabilized. Then, after the pump line has been removed, onsite measurements and samples for chemical analysis can be taken from the water in the container. 32 33