WORK PLANS - Laserfiche WebLink

Home Browse Search

Total petroleum hydrocarbons as diesel (TPHd) was not detected in unsaturated soil samples collected from the former UST area;therefore,TPHd is not included in our calculations. Groundwater We also estimated the residual mass of dissolved-phase hydrocarbons in groundwater using the concentrations reported during the First ySemi-Annual 2013 samyling event (January 2013). We based our calculatioon�s on the measured square footag o each TPHg—and`TPHd isoconce-M a ion ontour depicted on the TPHg in Groundwater Isoconcentration Map-January 2013 (Figure 4) and the TPHd in Groundwater Isoconcentration Map-January 2013 (Figure 5),an assumed vertical groundwater thickness of 37 feet, and a homogenous lateral and vertical distribution of TPHg.and TPHd between each contour interval. We assigned the area within the>500 micrograms per liter(µg/1) TPHg contour a concentration of 920 µg/1 (0.92 [milligrams per liter] mg/1) based on the reported concentration for MW-13, the only sampling point within the contour. For the area between the 100 and 500 µg/1 TPHg contours, we assigned the maximum TPHg concentration of 240 µg/1 •(0.24 mg/1). As shown on Table 2, we estimate that approximately 0.31 gallon of gasoline remains in the groundwater beneath the Site. We assigned the area within the>5,000 pg/1 TPHd contour a concentration of 8,000 µg/l(8.0 mg/1) for the entire lateral and vertical distribution based on the reported concentration for MW-19. We utilized 8,000 µg/1 as a conservative value even though this concentration appears to be anomalous as the previous maximum concentration for a sample from this, well was 360 pg/l. For the area between the 1,000 and 5,000 gg/1 TPHd contours, we assigned the maximum concentration of 2,200 µg/1 (2.2 mg/1). For the area between the 100 and 1,000 µg/1 TPHd contours,we assigned the maximum TPHd concentration of 470 pg/1 (0.47 mg/1). As shown on Table 3, we estimate that approximately 4.95 gallons of diesel remain in the groundwater beneath the Site. FATE AND TRANSPORT MODELING The SJCEHD requested that modeling be performed to evaluate the fate and transport of residual petroleum hydrocarbons in groundwater at the Site. r Geocon performed modeling to simulate groundwater flow and fate and transport of TPHg (as benzene) and TPHd(as naphthalene). The purposes of the modeling effort were to predict.the downgradient extent of contaminant migration from the Site and determine if residual TPHg and TPHd in groundwater pose a threat to the nearby public water supply well. The modeling was performed using Visual Modflow Pro software, ver. 4.1.0.143, under license from Waterloo Hydrogeologic, Inc. (now known as Schlumberger Water Services). Subsurface Stratigraphy Based on the driller report for DWR Well ID No. 24090 (State Well No. iN/7E-18D1), subsurface stratigraphy in the site vicinity consists of sandy clay and light sand to a depth of 74 feet, underlain by 53 feet of hard clay from a depth of 74 to 127 feet.Underlying the hard clay at a depth of 127 feet is sand and sandy clay to a depth of 450 feet. Public Water Supply Well State Well No. iN/7E48Di was completed in March 1956 and is located approximately 700 feet south-southeast of the former USTs. According to the driller.report for DWR Well ID No. 24090, the well has a 30-inch-diameter '/4-inch-steel conductor casing extending from the ground surface to a depth of Stockton Yard Maintenance Station,TO No.IB Caltrans Contract No.06AI895,EA No.43-910206 Geocon Project No.59800-01-01 B -4- September 30,2013