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Ground water generally flows to the southwest towards the Stanislaus and San Joaquin rivers. The unconsolidated sediment aquifer is composed from oldest to youngest of the Laguna, Turlock Lake containing the Corcoran Clay, Riverbank and Modesto formations overlain by floodplain deposits of the Stanislaus River. Sediment was deposited in continental, lacustrine and marsh, and flood-basin environments. The Corcoran Clay of the Turlock Lake formation is an important aquitard developed in the San Joaquin Basin. In areas where the Corcoran Clay is not well developed, the Forebay aquifer (referenced in regional literature) is separated into a shallow and deep. In the areas east of HWY 99, the shallow aquifer is developed above confining clays of the Laguna and Mehrten formations and the Corcoran Clay is absent. f SITE SPECIFIC HYDROGEOLOGY (Existing Monitoring Network) The ground water monitoring network has been updated over the years. The original monitoring wells MW-1 through MW-5, depicted on Figure 2, have been monitored since 1995. A new monitoring well MW-6 was completed in 2000. This well was positioned to assist in determining background water quality. In order to support the expansion of the percolation ponds, four additional wells MW-7 through MW-9 were completed in July 2002. Boring logs (132 through B8) from the 2001 Anderson Geotechnical work on site and the monitoring well boring logs from the available monitoring wells (1\4W-6 through MW- 9) were used to complete hydrogeologic cross-sections for the site in 2003. These sections indicate significant changes in the character of the alluvial sedimentation in the area. River channel migration of gravel deposits associated with the Stanislaus River deposition. Coarser grained deposits have been encountered along the Southern extent of the site with finer grained silt deposits dominate the northern portion of the site. Boring logs from MW-6 through MW-9 and borings B-2 and B-8 depict the poorly sorted sands and gravel while the boring logs from B-3 through B-7 are finer grained with dominant clays and silts represented in the borehole logging. These findings were confirmed by the resent borehole drilling for monitoring wells MW-10 and MW-11 that are detailed below. Ground water elevations based on the April round of water quality ranges from 79.35 to 84.31 ft. Ground water appears to be mounding beneath the site and flows predominantly toward the Stanislaus River. Local ground water flow is influenced by mounding beneath the ponds. City of Riverbank Wastewater Treatment Plant Monitoring Well Installation May 2006 DE Project No.:109-06 4 Rev.0