Description
The purpose of this investigation was to determine if there is any relationship between well depth and well yield in the fractured crystalline rocks in San Diego County, California. Previous investigations into depth versus yield relationships in fractured crystalline rocks in different locations, based on either water well or pressure test data, suggest that there is a logarithmic decrease in the rate of increase of well yield with increasing depth. Most authors go further suggesting a maximum well depth, beyond which wells should not be drilled if sufficient yield has not been obtained. These studies support the conceptual arguments for a decrease in the increase of yield with increasing depth. These conceptual arguments include a decrease in the number and aperture of fractures with increasing depth due to an increase in lithostatic pressure. This investigation, relying on water well data, differed from previous investigations, in that a subset of wells had information on the depths of water producing fractures as well as the amount of water contributed by those fractures. In contrast most of the previous investigations centered around data sets with information on only the total depth and total yield of each well. By submitting the San Diego County water well data to the same analysis applied by previous investigators at different locations, it was possible to show, erroneously, that the wells in San Diego County also showed a decease in the increase of well yield with increasing depth. Further analysis of the subset of data showed that there is no evidence for an increase or decrease in the incremental well yield or the frequency of water-bearing fractures with depth for wells in San Diego County. Furthermore, the analysis showed that all water well data sets subjected to an analysis of depth versus yield must include more information than total well depths and total well yields. Topography and the depths of water producing fractures as well as the amount of water contributed by those fractures must be considered. As found in numerous other studies, this investigation found that wells in valleys have a higher percentage of wells with water-bearing fractures and yield more water per depth interval than those on slopes and hills. In San Diego County, on average, the yield per 20 foot depth interval for wells in valleys ranges between 0 and 3 gallons per minute. The average yield per 20 foot depth interval for wells on slopes and hills ranges from 0 to 1 and 0 to 0.5 gallons per minute, respectively.
