Passive sampling has emerged as a promising tool to assess the presence of hydrophobic organic contaminants (HOC) in water, sediment, and biota, such as polychlorinated dibenzo‐p‐dioxins/furans (PCDD/Fs) or polychlorinated biphenyls (PCBs). Previous work evaluated the ability of passive samplers to predict the bioavailability of sedimentary HOCs mostly in the laboratory, often for marine organisms. The present study assessed the use of low‐density polyethylene (LDPE) to derive freely dissolved concentrations of PCDD/Fs and PCBs in porewater in situ versus ex situ and in river water. An LDPE‐based multisampler system was deployed at 4 locations along the lower Passaic River (NJ, USA) in sediment and the water column, where sediment and benthic species samples were also collected. Good agreement was generally observed for PCDD/F and PCB concentrations comparing in situ and ex situ approaches (within 0.30–39%). Significant linear relationships were derived between log LDPE–based and log lipid–based concentrations of PCDD/Fs and PCBs. The in situ multisampler system showed promise to derive HOC concentrations in porewater and river water and to predict the bioaccumulation potential of HOCs in benthic biota. Environ Toxicol Chem 2020;39:1174–1185. © 2020 SETAC