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Ground Water Sampling <br /> measurements of temperature, pH, specific conductivity and dissolved oxygen <br />' have stabilized The actual number of casing and filter pack volumes to be <br /> removed, and the rate that they should be removed should be determined on a <br /> well-by-well basis, depending on both the hydraulic properhes of the monitored <br /> zone and the hydraulic performance of the well (Barcelona et al , 1990, <br />' Barcelona, 1985b) <br /> For wells that are shallow or screened at the water table in high yield formations, <br />' purging should draw the water from the uppermost part of the water column to <br /> ensure that fresh water from the formation will move upward in the screen <br /> Generally,the well should be purged by placing the pump intake at the air/water <br />' interface This will ensure that all of the water in the casing and filter pack is <br /> purged, and it will minimize the possibility of mixing The same effect can be <br /> achieved with a bailer that is lowered dust below the water level and then <br />' withdrawn <br /> For wells screened considerably below the water level in medium to high-yielding <br /> formations with a large amount of water present in the casing, a packer placedjust <br />' above the screened section, with a pump placed at the top of, or in the screened <br /> area can yield representative water samples in a shorter period of time This <br /> method may save money in the long run, by eliminating the need to contamenze <br />' and treat the large volumes of water that would accumulate from purging casing <br /> volumes relative to the total water column, instead of only the screened section of <br /> the well <br />' The following additional recommendations and requirements also apply to the <br /> purging of monitoring wells <br />' U Wells should be purged at rates below those used to develop the well to <br /> prevent further development of the well, to prevent damage to the well, <br /> and to avoid disturbing accumulated corrosion or reaction products in the <br /> well (Puls et al , 1990,Puls and Barcelona, 1989a,Puls and Barcelona, <br /> 1989b, Barcelona, 1985b) A low purge rate will also reduce the <br /> possibility of stripping VOCs from the water, and will reduce the <br />' likelihood of mobilizing solids in the subsurface that are immobile under <br /> natural flow conditions <br />' o Ground water should be monitored for temperature, specific conductance, <br /> and pH at regular intervals during purging, and preceding and subsequent <br /> to sampling Ground water should also be monitored for dissolved <br />' oxygen content at regular intervals during purging, and preceding and <br /> subsequent to sampling(Puls and Eychaner, 1990,Puls et al , 1990,Puts <br /> and Barcelona, 1989a,Puls and Barcelona, 1989b) A flow-through cell <br /> should be used for the analysis of temperature, specific conductance,pH <br />' and dissolved oxygen Turbidity measurements may be used by the RP <br /> to evaluate the need to redevelop monitoring wells <br /> o As soon as a well recovers sufficiently after purging, the first sample <br /> should be tested for dissolved oxygen and turbidity Samples should then <br /> . be collected and placed in containers in order of decreasing sensitivity to <br /> 1 7 <br />