Formatted Title:

Use of Radiocarbon Data to Evaluate Carbon Sources Driving Gas Ebullition in Newtown Creek

Background/Objectives:

Newtown Creek is a tidal tributary to the East River located between the boroughs of Brooklyn and Queens, New York, and is listed on the National Priorities List as a CERCLA (i.e., Superfund) site. Newtown Creek has a long history of municipal and private/industrial discharges and releases; such discharges compose the largest source of freshwater flow. During the Remedial Investigation/Feasibility Study, the significance of gas ebullition-facilitated transport as a pathway for nonaqueous phase liquid (NAPL) and contaminant movement from sediment to surface water was quantified using flux chambers. As part of these investigations, the radiocarbon contents of ebullition gases and sediment samples were evaluated to estimate the relative contributions of various carbon sources driving gas ebullition at the site.

Approach/Activities:

Gas can be generated and gas bubbles can form in sediment as a result of the anaerobic microbial decay of labile organic matter (carbon). Sources of carbon in Newtown Creek are biogenic carbon, produced from organic matter; fossil carbon, originating from organisms that died more than approximately 60,000 years ago; and bomb carbon, resulting from atmospheric nuclear testing (circa 1950–1963). Biogenic carbon is present in sediments in the form of aquatic vegetation and organisms or from discharges of organic-rich materials (e.g., fecal material). Fossil carbon is released or discharged to the sediments in the form of fossil fuels (e.g., petroleum, coal).

Carbon-14 (¹⁴C) is a radioisotope of carbon (half-life 5,730 years) created in the atmosphere. Living organisms take up ¹⁴C from the environment; as a result, their ¹⁴C content is equivalent either to atmospheric levels during their lifetime or, for marine organisms, to the ¹⁴C of dissolved inorganic carbon in water. When organisms die, uptake ceases and the ¹⁴C in tissues begins to decay. Analysis of ¹⁴C in sediments and gas ebullition was performed to estimate the relative contributions of biogenic and fossil carbon to the gas ebullition observed at the site.

The ¹⁴C of gas ebullition was analyzed in samples collected from locations throughout the site. In two locations, co-located surface and subsurface sediment samples were collected and analyzed for ¹⁴C. The ¹⁴C content in the gas was compared to the ¹⁴C profile in the sediment and to literature values for atmospheric ¹⁴C levels and potential carbon sources (e.g., marine organisms, municipal wastewater treatment plant influent, fossil carbon) to estimate the relative contributions of biogenic and fossil carbon sources to ebullition gases.

Results/Lessons Learned:

On average, radiocarbon results indicate that ebullition gases at Newtown Creek originate from a mixture of mostly biogenic carbon (76% to 90%), with some fossil carbon (10% to 24%). The sediment radiocarbon results indicate that the bacteria responsible for the gas production may preferentially metabolize more labile biogenic carbon. Sources of this biogenic carbon are likely combined sewer overflows and stormwater (containing a mixture of biogenic and fossil carbon) and, to a lesser extent, phytoplankton (100% biogenic carbon).

Stu Messur

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Use of Radiocarbon Data to Evaluate Carbon Sources Driving Gas Ebullition in Newtown Creek