SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

f - Morita Brothers 814 E. Charter Way, Stockton Page 2 of 3 to very silty sand. Boring MW-4 along the west margin of the site, MW-7 near the southeast corner of the shop building and borings MW-5 and MW-6 on the east side of the shop building encountered fine grained soil with minor sand. Some or all of the sand between 5 and 25 feet bsg on the west side of the shop building may be excavation backfill, but this has not been demonstrated. Between 30 and 65 feet bsg, the soil horizon consists primarily of silt with minor clay or sand. Sand was encountered at 35 and 50 feet bsg only in boring MW-4, suggesting a limited vertical and lateral extent of these sand units. Primarily silt and clay were encountered between 30 and 65 feet bsg on the site. Silty sand was encountered at 70 feet, 75 feet and 80 feet bsg in the only boring to penetrate to these depths on site to date (MW-104), s uggestive of a s ignificant s and i nterval, p otentially h aving s ignificant lateral extent. This possibility is bolstered by sand encountered in the same interval on the 701 E. Charter Way site and encountered in 2 out of 3 borings advanced to sufficient depth on the 749 E. Charter Way site. Clay was encountered on the subject site in the 85-foot and 90-foot samples collected from the deep boring. A well, MW-104 was set at 87.5 to 90 feet bsg in basal clay encountered in the boring. Ground water monitoring wells have been set in three general depth intervals, herein referred to informally as A interval, B interval and C interval: • A interval - current first water depths, generally from 25 to 40 feet bsg or 30 to 50 feet bsg, monitored by MW-4, MW-5, MW-6 and MW-7; • B interval - first water depths circa 1992, generally 45 to 65 feet bsg with some latter wells screened 55 to 60 feet bsg, monitored by MW-1, MW-2, MW-3, MW-106 and MW-107; and • C interval — extending from 70 to 90 feet bsg, currently monitored only by the deep well MW-104, screened from 87.5 to 90 feet bsg in a clay zone. Impacted ground water in the A interval has been encountered in monitoring well MW-4, and is limited in lateral extent by monitoring wells MW-5, MW-6 and MW-7 toward the northeast, southeast and southwest; therefore proposed monitoring wells MW-8, MW-9 and MW-10 are not required at this time. Ground water in the B interval appears to be more intensely impacted than in the A interval. Impacted B interval ground water has been encountered in monitoring wells MW-2, MW-3, MW-106 and MW-107; the plume is delineated toward the south by monitoring well MW-1; the concentrations in MW-106 are low (TPH-g less than 100 µg/1), indicating the well may be near the eastward margin of the plume. EHD recommends that proposed monitoring wells MW-108 and MW-109 be moved approximately one-half the distance between the proposed locations and the nearest currently existing well as the screened interval appears to be primarily fine grained. EHD approves the proposed locations for monitoring wells MW-110 and MW-111. The results from ground water samples collected from MW-104, screened in the lower C (clay) interval appear to delineate the vertical extent of impacted ground water in the area of the suspected release. However, EHD is concerned that the sandy interval between 70 and 80 feet bsg may be serving as a lateral conduit and migration pathway for impacted ground water. During the ground water low of 1978-1979, the vadose zone