This article provides a review of both "long-term" and "short-term" methods for assessing the time of release of organic residues into the environment. The long-term release describes two methods that are capable of measuring release ages to tens of thousands of years. The first is the radiometric carbon-14 method ( 14 C), which extends to approximately 50,000 years before present (YBP). The second method depends on the rate of amino acid racemization, which has been used to age date proteinaceous organic matter >100,000 YBP. The short-term methods describe release times of crude oil, refined petroleum fuels, and chlorinated solvents, and generally can only be used to estimate release times post-1940. The procedure used to estimate time of release of the petroleum fuels and chlorinated solvents depends on the following five approaches: (1) changes in refining technology; (2) temporal changes in government regulation concerning fuel or solvent composition, including use of additives; (3) estimate rates of chemical or biological alteration (weathering) based on temporal concentration changes of individual compounds; (4) changes in 206 Pb/ 207 Pb isotope ratios in lead additives between 1964 and 1984; and (5) time for onset of corrosion in pipes and underground storage tanks from the time of their installation. It is recommended that, where possible, multiple approaches be used to establish the time of release of an organic residue.
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