WORK PLANS - Laserfiche WebLink

Home Browse Search

West Clay Property - 2 - 23 August 2022 associated grab groundwater samples. In conclusion of the Assessment Report, AGI recommended the development of a work plan for the installation of a monitoring well cluster approximately 50 feet south of the intersection of West Jefferson Street and South Harrison Street. Staff does not concur with the scope of this recommendation because the installation of a monitoring well is premature without additional field investigation. Staff recommends the extent of the plumes be determined by collecting additional grab groundwater samples prior to the installation of any wells. Based on AGI's proposed well location and grab groundwater sample results from boring CPT-1, Staff is concerned the proposed well location is within the boundaries of the plumes and will not define the plumes to the water quality objective. Alternatively, Staff requests a work plan to fully delineate the offsite extent of the Site's dissolved hydrocarbon plumes. Staff recommends additional investigations be conducted in the following areas: • The plume is not delineated south of soil borings B16 and B18. Grab groundwater samples were only collected from these borings at depths of 54 feet below surface grade (bsg) and 46 feet bsg, respectively. Boring B19, approximately 40 feet north of B18, contained total petroleum hydrocarbons as diesel (TPHd) at 2,900 micrograms per liter (fag/L) in a grab groundwater sample collected from 85 feet bsg to 90 feet bsg. Based on the results of the B19 sample and the shallow depths of the B16 and B18 grab samples, the dissolved TPHd plumes within deeper Hydrogeologic Units have not been delineated south of B19. • Based on the extent of several dissolved hydrocarbon plumes at UST sites in the area, historical groundwater flow and plume migration has been typically towards the east. Due to the historical direction of groundwater flow/plume migration and the presence of residential properties limiting the area of investigation, Staff recommends borings be advanced along South Lincoln Street to potentially achieve delineation to water quality objectives. • The plume is not delineated north and east of boring CPT-1. Boring CPT-1 grab groundwater samples contained TPHd at concentrations ranging from 853 pg/L to 1,370 pg/L. AGI has interpreted that the dissolved hydrocarbon plumes originating from the UST No. 3 area migrated away from the Site towards the northeast. Based on this characterization, along with the rationale for recommended boring location selection previously provided, Staff recommends additional investigations occur along West Jefferson Street to the north and South Lincoln Street to the east to potentially achieve plume delineation to water quality objectives. Staff recommends that grab groundwater samples be collected from the 40 feet bsg to 70 feet bsg, 70 feet bsg to 90 feet bsg, and 90 feet bsg to 170 feet bsg intervals which coincide with the intervals in which previous detections of TPHd occurred.