Evidence of Chlorobenzene Natural Attenuation in Contaminated Sediments Using Compound Specific Isotope Analysis and High Resolution Pore Water Sampling

Abstract:

Contaminated sediments can represent a significant risk for ecosystems and hinder drinking water production if contaminants discharge to surface and ground water. Understanding of contaminant fate and the potential for natural attenuation can help protect aquatic resources.

In this study, the fate of chlorobenzene (MCB) and benzene was investigated in a contaminated canal sediment field site located in New Jersey, USA. Compound Specific Isotope Analysis (CSIA) was applied to sediment pore water samples collected with a peeper at high spatial resolution (3 cm) across the sediment – surface water interface (SWI). Samples were collected at three locations in canal sediments, all of which exhibited reducing redox conditions.

The largest concentrations were observed in the bottommost portions of the sediment profile, with concentrations ranging from 300 to 2000 µg/L for MCB, and 16 to 180 µg/L for benzene. Conversely, concentrations were below detection limit in the surface water and in the top 6 cm of the sediment.

In the zones of highest MCB concentrations, the δ¹³C values were -26.4 (location C) and -21.9 ‰ (location F), and became progressively more enriched in ¹³C while concentrations decreased, reaching -23.9 (at 12 cm below the SWI, location C) and -18.4 ‰ (at 16.5 cm below SWI, location F). Benzene was only detected in the bottom 6 cm of the sediment profiles. Benzene δ¹³C values were -27 (bottommost, i.e., 24 cm deep) to -29.7 ‰ (18 cm deep), in location C. Such significant isotopic enrichments in ¹³C (2.5 to 3.5 ‰) correlated with MCB and benzene concentration decrease are suggestive of in situ biodegradation.

In addition, benzene δ¹³C values were systematically more depleted in ¹³C than MCB, suggesting that benzene found in these zones was likely produced from MCB via reductive dechlorination.

This study combined for the first time CSIA with high spatial sampling resolution in surface water sediments. This setup enabled not only detection of contaminant loss across the SWI, but additional lines of evidence that among other mass dispersal processes, in situ biodegradation plays a significant role in contaminant attenuation.