ARCHIVED REPORTS XR0011734 - Laserfiche WebLink

Home Browse Search

Report: Groundwater-quality Monitoring—December 13, 2001: 7500 West Eleventh Street,Tracy, CA. Page 4 OR r f� elevations were computed relative to mean sea level (MSL). Those measurements and the :.t computed groundwater table elevations are recorded in Table 1. In the period between September 11, 2001 and December 13, 2001, the groundwater table had fallen about 0.2 feet. Since May 30, 2001, the preceding seasonal maximum measured groundwater elevations, the groundwater elevation has fallen, on average, some 0.6 feet change. The groundwater elevation data presented in Table 1 was used to construct the water table contours for December 13, 2001 shown on Figure 2. The groundwater contour = configuration is similar to that seen previously where the direction of groundwater flow is in a generally northern direction but with a north-northeasterly trend in the northeastern area of the site. 2.2 Purging of Monitoring Wells r=' After the depths to groundwater were measured on December 13, 2001, a small-diameter, submersible pump was used to purge each groundwater-quality monitoring well of stagnant water. The pumped water was discharged into five-gallon pails. As each pail was filled, it was, in turn, discharged into an open-topped, 50-gallon drum equipped with a securable lid. The 50-gallon drums remain staged on the 7500 West Eleventh Street property. "rte As is SJC's normal practice when purging the shallow monitoring wells at the 7500 West Eleventh Street site, a minimum of 15 gallons of water from each well was extracted before a sample was recovered. However, to comply with a guidance letter dated September 19, 2000 issued by the SJCPHS (San Joaquin County Public Health Services 2000), during the purging procedure, the temperature, electrical conductivity and pH of the stream of purge water was monitored by periodically checking those parameters using a multi-function temperature, pH and electrical conductivity meter manufactured by Oakton. The results for each well were compiled in field notes, a copy of which is !_ included in Appendix A. Inspection of the temperature, conductivity and pH data in the field notes shows that all three parameters stabilized to within plus or minus 10% after the first few measurements were made on the purge water discharged from each well. This parametric stability is sufficient to demonstrate adequate well purging according to the criteria suggested by the SJCPHS. However, in the professional opinion of the author of this report that criterion is not sufficient to ensure adequate purging of the shallow groundwater-quality monitoring wells at the 7500 West Eleventh Street site. The submerged casing volume of each well is approximately 2.45 gallons and, if a filter pack porosity of 33% is assumed, the actual volume of stagnant water in the 8-in. ,F diameter well borings in which the wells were constructed is 14.56 gallons. sic