Assessing Temporal and Spatial Variations of Gasoline-Impacted Groundwater Using Relative Mole Fractions and PIANO Fingerprinting

Abstract

The release of gasoline at retail service station sites can result in the presence of gasoline-derived constituents dissolved in ground water. Due to regulatory requirements or remediation objectives the concentrations of these constituents often are monitored regularly for several years or decades. This study aims to demonstrate the utility of a large historic dataset for gasoline-impacted groundwater through the evaluation of spatial and temporal trends over 8 years of quarterly benzene, toluene, ethylbenzene, and o, m, and p-xylene polymers (BTEX) data from a plume emanating from a service station site. A novel aspect explored herein was to convert the historic, time-series BTEX concentration data into relative mole fractions in a corresponding four-component (BTEX), hypothetical non-aqueous phase liquid (NAPL), i.e., “NAPL-GW fingerprints”, in order to: 1) minimize the influence of factors affecting absolute BTEX concentrations over the 8 years of data collection (e.g., groundwater elevation in relation to smear zone), and 2) allow for the comparison of the NAPL-GW fingerprints of groundwater to fresh gasoline as a means of recognizing the impact of new releases on groundwater. More clearly than with using absolute concentration profiles, the NAPL-GW fingerprints revealed that gasoline impacted groundwater on-site during two separate time intervals. These impacts also could be recognized in off-site locations, albeit to different degrees at different downgradient locations. Specifically, an earlier release(s) appears to have impacted the entire study area whereas a later (apparently smaller volume) release only had impacted groundwater on-site and immediately off-site before attenuating. The historic dataset evaluation was supplemented by a survey of current groundwater using conventional paraffin, isoparaffin, aromatic, naphthene, and olefin (PIANO) fingerprinting that focused on the utility of C3- and C4-alkylbenzenes in fingerprinting different types of dissolved gasoline. PIANO fingerprinting revealed molecular differences reflecting two types of gasoline impacting different locations – as revealed by distinct solubility-based ratios among C3- and C4-alkylbenzenes. Collectively, the study demonstrates a means by which the timing and type of gasoline impacting groundwater may be determined using dissolved phase data only.