POSSIBLE ANTHROPOGENIC SOURCE OF ATMOSPHERIC CARBON DIOXIDE FROM THE DECAY OF BETX IN GROUNDWATER AT PETROLEUM IMPACTED SITES

As global warming intensifies due to increased greenhouse gas concentrations in the atmosphere, research into sources and sinks of greenhouse gases is fundamental to establish sound scientific criteria by which we examine Earth system processes. Carbon dioxide has traditionally been the focus of scientific research into global warming because it is not only a major greenhouse gas but also one of the most abundant greenhouse gases. A potential anthropogenic source of carbon dioxide is from the breakdown of petroleum products at hazardous waste sites.

The most common form of groundwater contamination in the US is due to the release of petroleum. As of March 1996 there were approximately 300,000 confirmed petroleum releases from underground storage tanks (USTs). The breakdown of aromatic hydrocarbons, such as benzene, toluene, ethylbenzene, and xylenes (BTEX) from petroleum-impacted sites could be a potential additional source of carbon dioxide to local groundwater. When the pCO₂ of water exceeds the pCO₂ of the atmosphere, carbon dioxide volatilizes. Such a condition occurs when there is a substantial amount of CO₂ added to a local aquifer at concentrations that increase pCO₂. One such source derives from the decay of BTEX.

Estimates of carbon dioxide that could potentially be released to the atmosphere from sites impacted with BTEX were calculated using information obtained from the literature regarding the reduction and oxidation of BETX along with the decay constants for these chemicals and other properties that influence the phase partitioning of organic compounds. Approximately 6 to 8 moles of carbon dioxide are released for every mole of BTEX reduced in groundwater. The calculated rates of decay are based only on natural attenuation of BTEX. Sites where active remediation is currently being implemented will experience faster rates of decay. In addition, local geology affects the rate of volatilization and whether volatilization occurs at all. This potential source of atmospheric carbon dioxide has not been previously quantified. However, as petroleum releases become more prevalent and active remediation at petroleum-impacted sites becomes more efficient, the decay of BTEX and other organic compounds could become a more substantial source of anthropogenic carbon dioxide to the atmosphere.