A COMPLEX LEGACY OF CONTAMINATION IN URBAN ESTUARINE SYSTEMS

Industrialized urban waterways have typically suffered decades of contamination, varying in source and intensity as manufacturing and transportation practices evolved. The U.S. Environmental Protection Agency designates locales with particularly severe contamination as Superfund sites. Among these, the Gowanus Canal and lower Passaic River in the New York/New Jersey harbor estuary illustrate a complex range of contamination types.

The 2.2 km Gowanus Canal, with sluggish circulation driven mostly by tides, accumulated fine-grained sediments (average thickness of 3 m) highly enriched in organic carbon (OC, mean 11 % but up to 49 %) derived from hydrocarbons, sewage, coal, char, and biomass, along with heavy metals (e.g., Pb, mean 736 mg/kg). This contrasts sharply with subjacent sediments deposited prior to mid-19th century canal construction, with mean OC and Pb contents of only 1.9 % and 14 mg/kg respectively. The canal sediments are severely contaminated by polycyclic aromatic hydrocarbons (PAHs), with a mean concentration of 11300 mg/kg (summed EPA PAHs) in the 300 m canal segment adjacent to a former manufactured gas plant (MGP). Although the underlying pre-anthropogenic sediments are low in OC and heavy metals, they are nearly as enriched in PAHs as the canal fill (mean of 6400 mg/kg in the same 300 m stretch), likely the result of subsurface migration of coal tar liquids rather than direct deposition. This occurrence complicates remediation planning.

The estuarine lower Passaic River receives freshwater flow from a 1500 km² watershed, constrained by upstream dams, with episodic storm and snowmelt pulses. A 5 m sediment core taken adjacent to a former MGP reflects further complexity due to a 1.7 m tidal range and mid-20th century navigational dredging. Mean OC and Pb values of 5.4 % and 336 mg/kg, respectively, in the core sediments approach those detected in the Gowanus Canal. Mid-core sediments show the impact of petroleum, polychlorinated biphenyl, and "dioxin" (2,3,7,8-tetrachlorodibenzo-p-dioxin) pollution. In the sandy zone encountered at the base of the core, OC and Pb values drop precipitously to 0.2 % and 13 mg/kg, while PAHs increase threefold above the mean to 284 mg/kg. As observed in the Gowanus Canal, these hydrocarbons may have originated at the nearby MGP and been emplaced via subsurface migration.