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•� Quality Conti. Board a California Regional Water � y Central Valley Region Robert Schneider, Chair Arnold Dan Skopec Sacramento Main Office Schwarzenegger Acting Secretary 11020 Sun Center Drive 4200, Rancho Cordova, California 95670-6114 Governor Phone (916) 464-3291 • FAX (916) 464-4645 http://www.waterboards.ca,gov/centralvalley 21 April 2006 Mr. Michael McAnulty Atlantic Richfield Company 6 Centerpointe Drive La Palma, CA 90623 DOWNGRADIENT INVESTIGATION,APPLIED N OROS ACE STRUCTURES CORPORATION, STOCKTON, SAN J Staff of the Central Valley Regional Water Quality Control Board (Regional Water Board) have reviewed the 6 April 2006 Work Plan for Offsite Investigation Downgradient of MW -15 (Work Plan), submitted in response to our 17 February 2006 letter. We obtained further clarification of the Work Plan during a 14 April 2006 conversation with Mr. Fred Kinser of Parsons, theO consultant for Atlantic Richfield Company (ARCO). The Work Plan proposes investigating the trichloroethylene s about 400 to 550 peeme downgradient of MW -15 by taking grab groundwater sample downgradient of MW -15. Five borings will be installed in an arc to the south and east because the groundwater gradient fluctuates in those directions. The borings would be advanced by direct -push methods and obtain grab groundwater samples in three approximate zones — the water table at about 50 feet below ground surface (bgs), just above the intermediate zone at about 80 feet to 90 feet bgs, and a third sample between the two. The work plan proposes to identify the lower target zones by advancing a CPT boring through the target zones, and projecting those zones to the other borings. The CPT boring would be located near P-103, one of the borings most directly downgradient from MW -15. Samples would then be taken from boring locations P-103 and P-102 and analyzed on an overnight basis to determine if other samples should be taken from optional sample locations further downgradient. Water samples from the remaining borings would be analyzed on a normal turnaround time. Comments 1. Groundwater gradient historically varies from east to southeast. Proposed boring location P-105 is near due south from extraction well GW -7, which is near the presumed source area. Staff believes location P-105 has low probability of containing TCE. However, there is about a 200 -foot gap between proposed locations P-101 and P-102, which is close to the average downgradient direction. Staff believes that an additional sampling point is warranted at this location. lied Aerospace 2, Staff review of existing CPT logs from the eastern side of the App p property shows that geology can vary significantly over short distances. It would be difficult to project the depth to permeable zones from a single CPT log. However, if several CPT logs were obtained in the target area, correlations between logs may be California Environmental Protection Agency n- P,,,,--