SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Mr. Benson Cuthbertson - 2 - 19 March 2013 PNA submitted the 30 March 2011 Soil Investigation Report which reported the results of 12 additional soil borings, installed to a maximum depth of 12 feet bgs, along with the results of inspection and pressure testing all remaining fuel supply lines. The Soil Investigation Report concluded that there was no on-going release of TPH, the TPH on the water table was residual from the 1996 spill, and that approximately 5,300 gallons of TPH remain in site soil and groundwater. In February 2012 PNA submitted the Light-non-aqueous Phase Liquid(LNAPL) Remedial Feasibility Study(Feasibility Study) in response to a 15 April 2011 Regional Water Board letter requesting assessment of further remedial options to enhance the rate of LNAPL recovery. The Feasibility Study reviewed existing site data and stratigraphic and hydrogeologic information obtained from a remedial investigation occurring on the former Occidental Chemical site, approximately 1000 feet to the east and downgradient of the PNA site. Although the Feasibility Study identified the Occidental remedial investigation, it did not identify the operating groundwater extraction and treatment system that is likely controlling groundwater gradient in at the PNA site. In a 5 June 2012 letter, Regional Water Board staff rejected the FS recommendation for making Monitored Natural Attenuation the preferred remedy at this site because Water Quality Objectives would not be met in a reasonable time period, and directed PNA to continue its program of skimming diesel fuel (TPHd) from the water table along with semi-annual groundwater monitoring. Free Product Removal Section 3.1.4 of the Closure Report states that free product has been removed to the maximum extent practicable. Regional Water Board staff do not concur. Since 1999, PNA has been attempting to recover diesel fuel from six 4-inch recovery wells by bailing approximately every one to two weeks. The data show that diesel was recoverable from only two to four of the wells at any time. The data also show that as recently as December 2011 over a foot of diesel fuel would accumulate in recovery wells on a weekly basis. The most recent data submitted from September 2012 shows that several inches of diesel was still present in recovery well RW-2. Regional Board staff letter of 23 April 1998 recommended using at least 6-inch diameter wells with a larger .06-inch slot size because it is more efficient to recover fuel from larger infiltration galleys. However, PNA installed 4-inch wells with a .02-inch slot size. It is Regional Water Board staffs opinion that diesel recovery could be substantially enhanced by installing larger-diameter recovery wells, installing additional recovery wells within the LNAPL plume, and either continuing the bailing program or installing automatic low flow recovery pumps. Incomplete Conceptual Site Model Section 3.1.5 of the Closure Report states that a Conceptual Site Model has been developed. Regional Water Board staff do not concur that the Conceptual Site Model is complete for the following reasons: 1. The maximum depth of investigation at the site is 17 feet bgs. Investigations to date have not defined the vertical extent of the contamination.