average KSS of 4.52 h-1. At a site in New Jersey that also had low iron and low TOC, the average KSS for the same carbon was 2.54 h-1.

Microbial Risk

GAC units often support microbial populations because they provide a good surface for attachment and concentration of organic carbon and nutrients (Camper and others 1987; Graese and others 1987; Camper and others 1986; 1985; Wilcox and others 1983). As a result, GAC units that treat radon have had high concentrations of heterotrophic bacteria in their effluent (Cornwell and others 1999; Kinner and others 1990; 1989). Because heterotrophic plate counts could periodically exceed 500 colony forming units per milliliter, which would violate the proposed GWDR, the treated water would need to be disinfected before distribution (EPA 1992c). Either chlorination or ultraviolet disinfection could be used. Unlike the situation with aeration systems, it is less likely that disinfection byproducts would be formed if chlorination were used after a GAC unit. Disinfection byproducts result from the reaction between chlorine-based disinfectants and naturally occurring organic matter in the water. GAC can sorb the low levels of naturally occurring organic matter in the groundwater (Cornwell and others 1999) until it becomes saturated (Kinner and others 1990). Before saturation, the risk posed by disinfection byproducts would be minimal. Thereafter, it could be similar to that of an aeration system that uses chlorine-based disinfection.

Precipitate Formation

In groundwaters that have high levels of iron, precipitates might accumulate on the top of the GAC bed. This reduces the hydraulic head and contaminant-removal efficiency of the GAC and makes it a poor choice for radon treatment for these types of raw water. Although some iron precipitation has been observed in field evaluations of GAC units that treat radon with low iron (Cornwell and others 1999; Kinner and others 1990; 1989), the problem is usually less acute than in aeration treatment; the water is not usually oxygenated and exposed to atmospheric conditions, so, much less oxidation of the iron occurs. If iron precipitates do form, they present the same problems outlined for aeration systems. Pretreatment to remove the iron before the water enters the GAC is unlikely to reduce the disposal issue unless sequestering agents are used to prevent precipitation. Pretreatment, such as with ion-exchange, would just accumulate the long-lived radionuclides on the resin, also presenting a disposal problem.

In addition, Lowry and others (1990), Lowry and Brandow (1985) and Dixon and Lee (1988) have noted that backwashing releases sorbed radon to treated water, although this has not been observed in all cases (Cornwell and others 1999; Kinner and others 1990; 1989). Desorption of the nongaseous radon progeny has not been observed during backwashing (Lowry and others 1990). In this

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