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<br />A <br /> <br />Confined and Unconfined Aquifers <br />Respond Differently to Pumping <br /> <br />1 1 <br />_I 1",,;.<<0 1 <br />1 I <br />1 1 <br />1 1 <br />I T + I <br /> T <br /> + T <br />-I . 1 + T I+- <br />L + <br /> L + <br />,_I 1 + L I+- <br />) ~ <br /> T ~ <br />I I T + 1 <br /> + T <br />-I L T +- <br /> 1 L + 1 <br /> L L <br /> + L <br />-I . 1 L I+- <br />L <br /> T . <br />I 1 + T I <br /> t__r <br />1 1 1 <br /> ~ .. <br /> <br />The markedly different response of confined and <br />unconfined aquifers to pumping (before the ground-water <br />system returns to a new equilibrium) is demonstrated by <br />calculations of drawdown resulting from a single pumping well <br />in an idealized example of each type of aquifer (Figures A-1 <br />and A-2). The numerical values used in the calculations are <br />listed in Table A-1. Inspection of these values indicates that <br />they are the same except for the storage coefficient S. Herein <br />lies the key, which we discuss further in this section. To a <br />hydrogeologist, the values in Table A-1 indicate a moderately <br />permeable (I<) and transmissive (7) aquifer, typical values of <br />the storage coefficient S for confined and unconfined aquifers, <br />and a high rate of continuous pumping (Q) for one year (~. <br /> <br /> <br />Mineral <br />grains <br /> <br />A mathematical solution was developed by Theis <br />(1940) to calculate drawdowns caused by a single well in <br />an aquifer of infinite extent where the only source of water is <br />from storage. This solution was used to calculate drawdowns <br />at the end of one year of pumping for the confined and uncon- <br />fined aquifers defined by the values in Table A-1. These draw- <br />downs are plotted on Figure A-3. Inspection of Figure A-3 <br />shows that drawdowns in the confined aquifer are always <br />larger than drawdowns in the unconfined aquifer. and that <br />significant, or at least measurable, drawdowns occur at much <br />larger distances from the pumping well in the confined aquifer. <br />For example, at a distance of 10,000 feet (about 2 miles) <br />from the pumping well, the drawdown in the unconfined <br /> <br />Figure A-f. Pumping a single well in an idealized confined <br />aquifer. Confined aquilers remain completely saturated during <br />pumping by wells (saturated thickness of aquifer remains <br />unchanged). <br /> <br /> <br />/ <br /> <br />Original ground- <br />water level (head) <br />before pumping <br /> <br />Orawdown <br /> <br />Confining unit <br />(tow hydraulic <br />conductivity) <br /> <br />Confined <br />aquifer <br /> <br />~ <br /> <br />Confining <br />unit <br /> <br />12 <br />