FIELD DOCUMENTS AND WORK PLANS 1989

Home Browse Search

Gary A. Reents -6- 24 February 1989 2. H&A proposes to construct three (3) monitoring wells within the 100-foot sand layer. The sand layer varies in thickness from 5 feet at borehole 6-x (Southwest corner of south yard) to 30 feet at borehole 7-x (approximately 3600 feet downgradient). H&A proposes to construct two (2) monitoring wells for lateral plume definition along the transect C-C' (near Horner Avenue) , where the thickness of the 100-foot sand should be sufficient for screening . This transect approximates the known longitudinal limit of the chromium plume within the intermediate zone. Also, a Phase IV well (MW-423), installed in the merged 100-foot sand and intermediate zone, and located near the center of this transect, had a chromium concentration of 0.08 mg/l . The locations of these Stage 1 monitoring wells for lateral definition of the plume, within the 100-foot sand, are appropriate. In addition, H&A proposes to construct a monitoring well , within the 100-foot sand, approximately 1200 feet downgradient of the C-C' transect. The location of this well also is appropriate. 3. Within the intermediate zone the two (2 ) wells proposed along East Main Street, to define the plume' s lateral extent, are sufficient. However, the four (4) wells proposed for downgradient control are on the opposite side of Mormon Slough. This spacing (approx. 1400 ft) is consistent with the off-site spacing previously employed . However, the Mormon Slough and a public water supply well (CWS52) located at the northern bank of Mormon Slough and Oro Avnue, could significantly affect water levels and flow patterns. The four (4) monitoring wells proposed for downgradient definition could determine, based upon current data ( i .e. , potentiometric map and chromium isoconcen- tration maps) , the longitudinal limit of the plume. However, the hydraulic influence of Mormon Slough and/or CWS52 (or CWS62 to the east) could signifi- cantly alter the existing plume migration trend. Therefore, it would be prudent to locate potential State 2 monitoring well north of Mormon Slough and west of CWS 52, along the northern bank of Mormon Slough. 4. Two (2) deep wells are proposed along East Main Street to determine the downgradient extent of the chromium migration. The farthest existing downgradient deep well is MW-461, at the corner of Horner and Sinclair. This well had a chromium concentration of 0.02 mg/l . Monitoring well MW-420 located adjacent to the public supply well (CWS20-01), had less than 0.01 mg/1 of chromium. The proposed locations of the deep wells, for downgradient definition of the plume, appear appropriate. In addition, a deep well is proposed near the corner of Gertrude Avenue and Horner Avenue, to determine the lateral extent of chromium migration, and two(2) deep wells are proposed at Franklin High School at the corner of Dawes and Washington, per a request by the Department of Health Services. The well at the corner of Gertrude and Horner is adjacent to two(2) intermediate wells that have an average chromium concentration of 0.10 mg/1 . This well location appears appropriate.