is also possible (Lowry and others 1990). In this case, 210Pb, like stable lead (Reed and Arunachalam 1994), would desorb from the GAC and enter the acid-regenerant solution. However, the spent acid could become a radioactive waste that requires special disposal.

Several authors (Cornwell and others 1999; McTigue and Cornwell 1994) explored the possibility of the GAC's being returned to the vendor (an approach used for GAC used to treat VOCs or substances that impart taste and odor to water). However, the willingness of the manufacturers to do this with radioactively contaminated GAC is not clear, especially for small quantities of GAC (less than 9,100 kg).

The best option overall with respect to disposal appears to be use of GAC in sites where the potential for 210Pb accumulation is minimized (that is, where the radon and iron concentrations in the raw water are low or the water flow rate is low. This would ensure fairly long operating times before the 210Pb reached a critical level likely to necessitate special disposal. The low radon loading would also result in lower risk of worker exposure to gamma radiation.

Long EBCT and High Cost

Bench-, pilot-and full-scale studies of GAC removal of radon have produced estimates of the KSS (adsorption-decay constant, see appendix C) for different carbons (Cornwell and others 1999; Kinner and others 1993; Lowry and others 1991; Lowry and Lowry 1987). Lowry and Lowry (1987) found that the best carbon for radon sorption was a coconut-based GAC (KSS = 3.02 h-1). This carbon has a larger percentage of micropores (0.002 µm) than other types of GAC. It is hypothesized that micropores are most effective for sorbing small molecules and atoms, such as radon gas (Drago 1998).

The cost estimates for GAC treatment have used a KSS of less than 3.02 h-1 (for example, EPA 1987b, KSS = 2.09 h-1). Recent studies by Cornwell and others (1999). specifically designed to calculate KSS values for different carbons, found that for one groundwater with low iron and TOC concentrations, the KSS ranged from 3.5 to 5.2 h-1. These higher values suggest that GAC could be a much more cost-effective option at some sites than originally thought. For example, with a raw-water radon concentration of 111,000 Bq m-3, a flow of 39 m3 d-1, and a KSS of 4.5 h-1, the EBCT and amount of GAC needed to achieve an MCL of 11,000 Bq m-3 (90% removal) would be 31 min and 0.83 m3, respectively, compared with 66 min and 1.8 m3, respectively, if the KSS were 2.09 h-1. Assuming a cost of $883 m-3 of GAC (Cornwell and others 1999), this 54% reduction translates to an $848 savings. However, Cornwell and others (1999) also suggest that a pilot study must be conducted at each site where GAC is being considered, because the KSS will likely be different for each groundwater. For example, for a low-iron, low-TOC groundwater in New Hampshire, a lignite-coal based GAC had an



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