THE EFFECTS OF PULSED PUMPING ON LAND SUBSIDENCE IN THE SANTA-CLARA VALLEY, CALIFORNIA

Land subsidence caused by pulsed and constant pumping schemes was calc ulated, and the results were compared to determine the possible advant ages of pulsed pumping in reducing land subsidence. Pulsed pumping ref ers to pumping strategies involving alternation between periods of pum ping and recovery in a well. Subsidence was calculated using a numeric al model based on the Santa Clara Valley, CA. The model accounts for d elay in the release of water from storage in compressible interbeds as well as flow within aquifers. For these tests, the ratio of pumping p eriod length to recovery period length was either 1/1 or 1/3. Pulsing interval length (length of one pumping period plus one recovery period ) ranged from 4 to 90 days. Pumping rates were adjusted so that pulsed pumping and constant pumping removed the same volume of water over th e period under consideration. Pulsed pumping has the potential to 'con centrate' subsidence around the pumping well. For all simulations, pul sed pumping caused greater subsidence near the test well than constant pumping. Under some conditions a transition point was reached, beyond which pulsed pumping caused less subsidence than steady pumping. The distance from the test well to the transition point decreases for (a) decreasing aquifer transmissivity, (b) decreasing ratio of pumping per iod to recovery period, and (c) increasing pulsing interval length. Th e concentration of subsidence was reduced considerably when delay in r elease of water from storage in compressible aquitards was neglected, indicating the importance of including delay effects in subsidence stu dies. All pulsed pumping strategies caused a greater total volume of s ubsidence than steady pumping.