Estimated subsidence in the Raymond Basin, Los Angeles County, California, for a postulated water-level lowering, 1970-2020

Abstract

One of the alternate plans for water utilization being considered by the California Department of Water Resources in the Raymond Basin in southern California involves partial mining of ground water during the period 1970-2020, and consequent substantial lowering of water levels. The Department wants to know whether land subsidence would be a problem as a result of the postulated lowering. To answer this question, to the extent that existing leveling data permit, the present study has been made at the request of and in cooperation with the Department of Water Resources. Water levels in the Raymond Basin declined to historic low levels in the mid-thirties, then rose and remained above these lows during most of the forties, fifties, and early sixties. Thus, no additional hydraulic stresses were imposed on the water-bearing deposits during this latter period to induce subsidence. Bench-mark control for the 27-year period, 1933-34 to 1961, suggested 0.2 foot of subsidence for most bench marks in and near the basin. This is attributed to network-adjustment problems rather than actual subsidence. At none of the bench marks studied is there a basis for directly relating subsidence to water-level decline. Limited leveling control during the 1961-68 period is also inconclusive. Unfortunately, no leveling control is available for the pre-1933 period of major water-level decline. Based on subsidence/head-decline ratios of nearby areas, it is concluded that subsidence in Raymond Basin would be small if Plan D pumping stresses were effected. Maximum subsidence, occurring where pumping stresses were the greatest and unconsolidated deposits are the thickest, would probably be less than 1 foot. Subsidence would occur so slowly and over such a broad area that no problems should result.