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(CSWRCB, 1989). The Chico area is situated on recent <br /> P°"° alluvial fan materials derived from volcanic sediments and <br /> s . y \ N mudflows originating in the hills to the east of Chico(DW R, <br /> J y. .` 1984).The alluvial deposits average about 40 to 50 feet in <br /> BOLINAS MESA .\`. �� thickness and consist of unconsolidated cobbles,gravel and <br /> sruov AREA stud; sand,and minor amounts of clay.These deposits support a <br /> shallow unconfined aquifer that is recharged directly by <br /> P <br /> infiltration from precipitation,local runoff,and discharge <br /> X from subsurface sewage disposal. Older alluvium imme- <br /> diately underlies the recent alluvium and extends to depths <br /> of nearly 450 feet. It is characterized mostly by thick clay <br /> layers and cemented sand and gravel.In this zone,ground <br /> =_= /- o° water occurs mainly in thin uncemented sand and gravel <br /> ------------- <br /> lenses under semiconfined conditions,recharged by vertical <br /> ---------- <br /> leakage from the overlying recent alluvium and from incised <br /> -- <br />