THE RELATION BETWEEN CONFINING BEDS AND THE VERTICAL EXTENT OF DEEP PERCOLATION IN THE SAN JOAQUIN VALLEY, CALIFORNIA

Historically, only laterally extensive clay layers were normally mentioned as confining beds in the San Joaquin Valley. In particular, the widespread Corcoran Clay has been noted for many decades beneath the western and central parts of the valley. In addition, the shallower A-Clay has been extensively discussed. The east side of the valley was often considered to be underlain by an unconfined aquifer. When many of the U.S. Geological Survey reports were done in the 1950’s and 1960’s, most of the test holes and wells on the east side of the valley were less than about 300 feet deep. Coarse-grained deposits were often predominant. However, in recent decades many holes have been drilled to depths of 800 feet or deeper. This deeper information indicates that there are significant thick confining beds beneath most parts of the east side of the valley.

Evidence on the vertical extent of deep percolation from irrigation can be obtained by evaluating concentrations of salinity, nitrate, DBCP, and 1,2,3-TCP in the groundwater. The uppermost part of the groundwater beneath irrigated parts of the valley usually has higher total dissolved solids (TDS) concentrations than are in the deeper groundwater. In general, one or more significant confining beds separate the upper higher TDS groundwater from the deeper lower TDS groundwater. Nitrate concentrations in the shallow groundwater beneath irrigated areas are often due to historical irrigation and fertilizer practices. The vertical distribution of nitrate generally is similar to that of TDS in these areas.

Beneath parts of the east side of the valley, DBCP and 1,2,3-TCP are also present in the shallow groundwater. DBCP was first applied to control nematodes in about 1950 and its use was subsequently banned in 1977. Concentrations of these two trace organics delineate the vertical extent in the groundwater of deep percolation that originated in recent decades.

Understanding of the relation between significant confining beds, the extent of deep percolation, and geochemical factors that affect naturally derived constituents in the groundwater has been used to successfully design and develop new public supply wells in the valley for more than three decades.