Laserfiche WebLink
Ground Water Sampling <br /> provides a lower sampling rate and less agitation than direct line or centrifugal <br /> pumps <br /> 2.3.4 Gas Contact Pumps <br /> Gas contact sampling devices include gas-lift and gas-drive devices <br /> Gam Lim c <br /> tAn air or gas lift pump allows collection of ground-water samples by bubbling air <br /> or gas at depth in the well Sample transport occurs primarily as a result of the <br /> reduced specific gravity of the water being lifted to the surface Water is forced <br /> up a discharge pipe, which may be the outer casing or a smaller diameter pipe <br /> inserted into the well Air or gas lift methods can result in considerable sample <br /> agitation and mixing in the well, and are not permitted for collecting samples for <br /> chemical analysis The considerable pressures required for deep sampling can <br /> result in significant redox and pH changes <br /> Gas-Drive Pumps <br /> Gas drive(gas displacement)pumps are distinguished from air lift pumps by their <br /> method of sample transport Gas displacement pumps force a column of water <br /> under linear flow conditions to the surface without extensive mixing of the <br /> pressurized gas and water A vacuum can also be used to assist the gas The <br /> disadvantages of a gas drive pump are that the drive gas comes into contact with <br /> the water and therefore,can be a source of contamination,also, the pump can be <br /> difficult to clean <br /> 23.5 Packer Assemblages <br /> A packer assembly provides a means by which to isolate and sample a discrete <br /> interval in the subsurface Hydraulic-or pneumatic-activated packers are wedged <br /> against the casing wall or screen allowing sample collection from an isolated <br /> portion of the well The packers deflate for vertical movement within the well and <br /> inflate when the desired depth is attained Packers are usually constructed from <br /> some type of rubber or rubber compound and can be used with submersible, gas <br /> lift, and suctionpumps <br /> If pumps are operated at a low rate, a packer assembly allows sampling of low- <br /> yielding wells, and wells that would otherwise produce turbid samples A number <br /> of different samplers can be placed within the packers depending upon the <br /> analytical specifications for sample testing One disadvantage is that vertical <br /> movement of water outside the well is possible with packer assemblages, <br /> depending upon the pumping rate and formation properties Another possible <br /> disadvantage is that the packer material may contribute undesirable organic <br /> constituents to the water sample <br /> L15 <br />