Does “non-detect” sorption matter to contaminant transport in groundwater?

Abstract Sulfolane is a groundwater-miscible pollutant that is widely reported to exhibit negligible sorption onto minerals. Yet sub-monolayer uptake can evade detection in conventional batch tests while still affecting plume evolution over transport distances. Here we combine synchrotron soft X-ray spectroscopy, electronic-structure calculations, and transport modeling to test whether “undetectable” sorption can matter at the aquifer scale. Using O K-edge X-ray absorption spectroscopy (XAS) in a liquid flow cell with silicon nitride windows, we observe a sulfolane signal after flushing with water, consistent with slow desorption or irreversible retention on experimental timescales despite negligible sorption inferred from batch measurements. Complementary electronic-structure calculations support the plausibility of sub-monolayer sulfolane association on hydrated motifs representative of calcite and iron hydroxides, for which laboratory studies have reported no detectable uptake. We quantify transport implications of low-capacity interactions using two frameworks: (i) a capacity-limited storage model that represents finite interfacial capacity and (ii) a kinetic sink model that treats uptake as rate-limited removal rather than instantaneous equilibrium partitioning. Across soils spanning specific surface areas and site densities, even sub-monolayer surface excess can attenuate dissolved concentrations over kilometer-scale distances under plausible parameter combinations. Under sustained releases, the solid phase can accumulate inventories comparable to the dissolved mass while progressively saturating capacity along the flow path. This distributed surface reservoir is poised to desorb under changing conditions (e.g., dilution or geochemical shifts), sustaining long-term residual aqueous concentrations even when equilibrium sorption appears negligible. Although focused on sulfolane, the same mechanism may apply to other mobile groundwater contaminants.