Abstract: An Investigation of Excessive Drawdowns in Sycamore Well Field, Arvin-Edison Water Storage District, Southern San Joaquin Basin, California

James R. Ostdick, Janice M. Gillespie

Drawdowns recorded for irrigation wells in the northeast corner of Sycamore Well field during the summer of 1992 were twice as large as other wells in the field despite similar pumping rates. Two hypotheses to account for the excessive drawdown in the northeast corner were tested: (1) poor completion practices during well construction, and (2) poor aquifer quality in the northeast corner of the well field.

An analysis of well completion reports showed no apparent differences in completion techniques between the wells with large drawdowns and the normal wells. Electric log cross sections and net sand maps of the completed interval indicate that the sand thickness in the aquifer decreases toward the northeast. These observations suggested lower than normal aquifer transmissivities in this part of the field. To test this theory, data from initial pump tests conducted by the driller were used to estimate the transmissivity of the aquifer in each well using the specific capacity method. This test was completely independent from the electric log analysis, however the results were similar--the lower transmissivity values were concentrated in the northeast corner of the field. Although pumping ates reached historic highs throughout the field due to the effects of a six-year drought, these analyses suggest that the relatively low transmissivity of the aquifer in the northeast corner of the well field was responsible for the larger drawdowns observed there.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994