ARCHIVED REPORTS_XR0005453 - Laserfiche WebLink

Home Browse Search

Mr Daniel K Barber,P E Ile September 9, 2003 Page 4 of 17 1 The site Iithology to the 60-foot depth is composed of Quaternary Alluvium Deposits (Qal), which predominantly consist of various gradations of sand—some clean and well-sorted, I some lower-permeability silt or clay-rich sands There appears to be a seasonal "perched water zone," as reported by Galiardo & Associates (1999), most commonly encountered at approximately 30 feet below ground surface (bgs), which is coincident with a thin clean sand horizon underlain by a less permeable clayey sand Attachment A contains available borehole geologic logs According to information from the San Joaquin County Flood Control District, historically, groundwater in the area of the site has varied between 50 and 60 feet bgs (fall 1993) and 30 to ' 40 feet bgs (fall 1987) The shallow groundwater may reflect perched conditions In 1999, first encountered groundwater during drilling at the site was at approximately 30 bgs, coincident with the likely perched zone (Gallardo & Associates, Inc , 1999) In contrast to the 1999 data, in December 2000 and June 2002, SES assisted D K Barber & Associates during ' the installation of bores and groundwater monitoring wells, where first groundwater was encountered at approximately 50 feet bgs Boreholes were geologically logged in accordance with the visual method of the Unified Soils Classification System [USCS] to identify the site stratigraphy and assess potential migrational pathways Shallow site stratigraphy consisted predominantly of silt and silty fine sand with minor clayey sand and clayey silt There were ' no definitive pathways (i e , significant variations in horizontal or vertical porosity) for potential contaminant migration noted The most notable lithologic contrast was between what appears to be a laterally-discontinuous perched zone and the underlying clayey sand Groundwater was not encountered until depths of 51 to 55 feet bgs in the June 2002 well installations, Iikely reflecting the seasonal nature of the perched zone While water levels were measured within 15 minutes after encountering groundwater (i e , full equilibration of water levels may not have been reached), groundwater was not observed to rise, indicating ' unconfined conditions Groundwater monitoring at the wells over the five monitoring events between June 2002 and June 2003 show an approximate 2-foot seasonal variation, water has ' consistently appeared at the deeper 50-foot-bgs depth, corroborating the model that the 30- feet zone is a perched zone encountered at first drilling ' The existing groundwater monitoring well network of six wells appears sufficient to characterize site-specific groundwater flow direction In the five groundwater monitoring ' events conducted in at least three wells, groundwater flow direction has been measured with a 50-degree spread—from north 3 degrees west to north 46 degrees east The hydraulic gradient has ranged between approximately 0 0021 and 0 0036 feet/foot Stellar Environmental Solutions ' SPROIECis 3aal,� RalcclfSODI�-UTwb[lud Gb�dF�dFV�EPORT�epeiber43003 dac