Artificial recharge as possible causes of observed long-term change in transmissivity of the San Pedro Formation aquifers, Central Basin of the greater Los Angeles Basin, California

Transmissivity values for the aquifers of the Upper San Pedro Formation of the Central Basin of the greater Los Angeles Basin have increased since the implementation of artificial recharge. To determine if the water used for artificial recharge is related to the observed increase in transmissivity, a groundwater data collected from the Water Replenishment District of Southern California (WRD) and the United States Geological Survey (USGS) from 1995 to 2002 was compared to the historical groundwater data collected by Piper and Garrett in 1953. The results indicate that the hydrochemical facies of the groundwater of the aquifers of the upper San Pedro Formation have been altered by the implementation of artificial recharge particularly by water imported from the Colorado River and Northern California, not reclaimed waters. These waters are characterized by higher calcium, sulfate, and total dissolved solids (TDS). Although some mineral dissolution may be occurring, the evidence does not support this process as a mechanism for increased transmissivity. Therefore, the increase in transmissivity is more likely due to the physical changes to the aquifers and aquitards of the Central Basin caused by extensive drilling, seasonal fluctuations and long-term over-pumping, rather than a chemical change.