Sorption of 2-Naphthalene Sulfonate to Organic Carbon in Natural Saturated Porous Media

Abstract

Organic carbon (OC) in aquifer sediments serves as a key geochemical control on contaminant fate and transport, potentially immobilizing or delaying mass transport downgradient. Quantifying the abundance of OC in aquifer sediments is therefore imperative in developing a conceptual understanding of local contaminant transport. Traditional methods involve time-intensive coring and lab analyses for total OC (TOC) at distinct intervals with depth. These methods fail to account for the heterogeneity between discrete points and do not interrogate TOC along groundwater flow pathways. 2-naphthalene sulfonate (2-NS), a novel groundwater tracer, was investigated here as a possible sorptive tracer for estimating the relative fraction of OC (f_OC) along groundwater flow pathways. The time needed to equilibrate, linearity, and reversibility of 2-NS sorption to OC were studied using both natural and engineered media. The soil organic carbon partition coefficient (K_OC) of 2-NS was empirically determined for the first time to be between 24.6 and 36.5 L/kg and sorption was observed to be linear. The results from these tests were then entered into a simple analytical model for solute transport that produced breakthrough curves of 2-NS and a non-reactive tracer. The model output determined that 2-NS would take between 2.4 and 3.1 times longer than a non-reactive tracer, using the two experimentally determined K_OC values.