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<br />o <br /> <br />I I , <br />I <br /> I <br /> I Drawdown In <br /> confined aquifer <br /> I Orawdown In <br /> unconfined aquifer <br />I I I " <br /> <br />10 <br /> <br />Ii; <br />'" <br />~ <br />;t <br />~20 <br />~ <br />" <br /> <br />30 <br /> <br />40 <br /> <br />j j j j~ );i ~! <br />~ 8 ~. ~~ ~.; ~~ <br /> g'8 81;; -m <br /> E - 0 h <br /> s E E <br /> S S <br /> <br />DISTANCE FROM PUMPING WELL <br /> <br />Figure A-3. Comparison of drawdowns <br />after 1 year at seiected distances from single <br />welis that are pumped at the same rate in an <br />idealized confined aquifer and an idealized <br />unconfined aquifer. Note that the distances <br />on the x-axis are not constant or to scale. <br /> <br />The large differences in drawdowns and related <br />volumes of the cone of depression in the two types of aquifers <br />relate directly to how the two types of aquifers respond to <br />pumping. In unconfined aquifers (Figure A-2) dewatering of <br />the formerly saturated space between grains or in cracks or <br />solution holes takes place. This dewatering results in signifi- <br />cant volumes of water being released from storage per unit <br />volume of earth material in the cone of depression. On <br />the other hand, in confined aquifers (Figure A-1) the entire <br />thickness of the aquifer remains saturated during pumping. <br />However, pumping causes a decrease in head and an accom- <br />panying decrease in water pressure in the aquifer within <br />the cone of depression. This decrease in water pressure <br /> <br />allows the water to expand slightly and causes a slight <br />compression of the solid skeleton of earth material In the <br />aquifer. The volume of water released from storage per <br />unit volume of earth material in the cone of depression in <br />a confined aquifer is small compared to the volume of water <br />released by dewatering of the earth materials in an uncon- <br />fined aquifer. The difference in how the two types of aquifers <br />respond to pumping is reflected in the large numerical differ- <br />ence for values of the storage coefficient S in Table A-1. <br />The idealized aquifers and associated calculations of <br />aquifer response to pumping discussed here represent ideal <br />end members of a continuum; that is, the response of many <br />real aquifers lies somewhere between the responses in these <br />idealized examples. <br /> <br />. <br /> <br />Center of <br />pumping well <br /> <br />I <br /> <br /> <br />Figure A--4. The cone of depression associated <br />with a pumping weli in a homogeneous aquifer. <br /> <br />14 <br />