Historical and Chemical Assessment of the Sources of PAHs in Soils at a Former Coal-Burning Power Plant, New Haven, Connecticut

Abstract

Soils from the manmade island in the Mill River (New Haven, CT) where a former power plant (English Station) operated for nearly 100 years had been previously shown to contain polycyclic aromatic hydrocarbons (PAHs) at concentrations that exceed the state's Department of Environmental Protection (DEP) Remediation Standard Regulations (RSRs) numerical cleanup criteria for industrial/commercial properties. Understanding the source(s) of these PAHs was important for property development and environmental management decisions. This study was designed to assess the industrial use history of the study area and conduct detailed chemical and organic petrographic characterization of the organic matter in the site's soils, particularly the PAHs, in order to determine their likely source or sources. Historical documents revealed that the English Station property is a largely manmade island and consists of sediments dredged from the Mill River, on various occasions from the late 1800s to at least 1936, to improve navigability. Throughout this time period, and as early as 1861, a manufactured gas plant (MGP) operated immediately downriver from the English Station site. The land use in the vicinity of the site and along the Mill River historically was industrial. In 1886, the English Station property was filled sufficiently to support operations of coal and wood storage yards. In 1901, and until the 1960s, the site was used for electric power generation via separate coal-fired power plants. In the 1960s, the boilers were converted to burn residual fuel oil rather than coal and operated until they were decommissioned in 1992. Sixty-six soil samples from the site were collected and, after assessing the detailed nature of the total (extractable) petroleum hydrocarbon (TPH), a subset was further analyzed using a combination of detailed PAH “fingerprinting” and petrographic analyses. The concentration and distribution (lateral or depth) of TPH in the soils analyzed appears unrelated to either the depth or location of the samples. Five distinct hydrocarbon sources could be recognized, namely: (1) tars associated with manufactured gas production (MGP), (2) residual (high boiling) petroleums of an unspecific origin, (3) particulate coal, (4) bottom ash (clinker) associated with coal combustion, and (5) “background” hydrocarbons associated with naturally occurring organic matter (e.g., plant debris). Organic petrographic analysis generally confirmed the hydrocarbon fingerprinting characterizations and also revealed coal and coke particles associated with the MGP tars in deeper soils, suggesting they share a common or contemporaneous source. The normalized distribution of the 44 PAH analytes provided detailed fingerprints that readily distinguished the five source materials. Because of the high concentration of PAH in MGP tars, and the widespread occurrence of MGP tar in the soils studied, the source of PAH in most soils studied, even those containing various mixtures, is largely attributable to the MGP tars. The residual petroleums, which appear physically associated with MGP tar in most soils, would appear to be the second most common source of PAH in the soils studied. The PAH derived from particulate coal and bottom ash are limited to specific areas of the site. Considering the area's industrial and dredging history, along with the widespread distribution of these PAH sources in the site soils, it is evident that the dredged sediments used to construct the island property contained MGP tars and petroleum wastes discharged from historic MGP and/or other industrial operations on the Mill River. The more recent coal combustion operations on the English Station property have contributed lower amounts of PAHs in the form of particulate coal and bottom ash, but with a limited distribution. The combination of the historical information with the chemical and petrographic data provided a stronger basis to establish PAH sources than either approach would have alone. Thus, this study demonstrates the value of combining extensive historical research and appropriate contaminant assessment methods in determining the source of contamination at sites with a long operational history. As a result of this study, the property owner's request for a widespread polluted fill variance from RSR compliance with the pollutant mobility criteria was granted by the DEP.