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Mr. Fran Forkas January 13, 1989 Page 3 on Table 2 . It is particularly evident during the period that well 20-16B was monitored. The westerly hydraulic gradient evident in Table 2 may be a localized effect of the pits to the west of the plant from which soil was excavated for construction of Route 5. Figure 3 is a site plan of the treatment plant facilities . Shown on the figure are the proposed locations of the monitoring wells. MONITORING WELLS A minimum of four wells are required for groundwater monitoring. One well is required for sampling upgradient, . or background, water quality and three are required downgradient ' for water quality at the points of compliance. The recommended locations for these wells are shown on Figure 3 . Wells WSM01 would be used for background water sampling and Wells WSM02 through WSM04 would be used to monitor the downgradient groundwater quality. Well WSMO1 is to be located at the northern boundary of the City property. In their letter of April 16, 1986, The Regional Water Quality Control Board cautioned that the upgradient well had to be located at an "adequate horizontal distance from the ponds and irrigated areas to establish background water quality. " While the mounding effect may cause localized northerly groundwater movement, the prevailing direction is southerly. With the monitoring well at the north' property line, the groundwater data is most likely to be representative of the background quality. The monitoring wells should be constructed to a depth of 20 feet . The DWR well data indicate that water levels range from near the surface on the north side to about 10 feet below grade on the south side. Well WSM01 should be located at the side of the access drive near entry gate. At this point the entry road slopes down to the field from the- frontage road. Constructing the well as close as practical to the drive would allow for easy access to the well during the wet season but out of the way of field equipment, and the top of the casing would be about 7 feet above seasonal high water level . Appendix C provides a general description of the construction requirements for the monitoring wells . The monitoring wells should be constructed of 2-inch diameter PVC pipe to facilitate the use of commonly available sampling equipment.