relative to the small mass of them created from the decay of radon (Reed and Arunachalam 1994; Rubin and Mercer 1981) (for example, GAC can sorb lead at 6.2 × 104-1.9 × 106 µg kg-1 at a pH of 6.5).
Some groundwater also contains radium and uranium in addition to radon. Uranium can sorb directly to the GAC, but its fate is a function of the pH of the water. At a pH greater than 7, the poorly sorbed, negatively charged carbonate species of uranium, UO2 (CO3)2-2, is predominant. At a pH lower than 6.8, the neutral species, UO2CO3, is predominant and can sorb to GAC (Sorg 1988). Radium is poorly sorbed to GAC (Kinner and others 1990; Clifford 1990; Kinner and others 1989; Sorg and Logsdon 1978) because it forms a hydrophilic species, RaSO4, in water. The pattern and rate of accumulation of uranium, radium, and 210Pb in a GAC unit can be quite different if iron is present. Cornwell and others (1999) found high concentrations of these radionuclides associated with iron-rich backwash residuals from GAC units. That is because radium readily associates with ferric hydroxide and negatively charged metal oxides and hydroxides (Clifford 1990). Uranium also reacts with iron (Clifford 1990; Sorg 1988).
During operation of a GAC unit, an equilibrium is established between the radioactivity of radon and its short-lived progeny sorbed to the carbon. The primary problem resulting from retention of radionuclides is worker exposure to gamma emissions from 214Bi and 214Pb. The maximum occupational accumulated dose equivalent per year recommended for radiation workers in the United States is 50 mSv (EPA 1987a). However, EPA has stated that "there is no need to allow" workers in water-treatment facilities that remove naturally occurring radionuclides from water to receive such high annual radiation doses. It further suggests that these workers' annual accumulated dose equivalent should be "well within the levels recommended for the general public" of 1,000 µSv. Hence, EPA has recommended a maximum annual administrative control level of 1,000 µSv until more experience with such situations is gained.
Lowry and others (1988) measured the gamma-exposure fields surrounding 10 point-of-entry units treating water with radon at 96 to 28,074,000 Bq m-3 and achieving removal efficiencies of 83% to over 99%. The gamma exposure rates measured at about 1 m were considerable, in all but one case, because the radon concentration removal was very large (Cnet = 611,000 to 27,926,000 Bq m-3). Except for the 27,926,000-Bq m-3 case, the measurements are in agreement with the range of the calculations from the extended source model.
The gamma exposure to workers can be decreased by using water or lead shields around the GAC units. Lowry and others (1991; 1988) studied the effect of water shielding and lead jackets on the point-of-entry units' gamma exposure fields. For example, at site 9 (table 8.3) (28,074,000 Bq m-3) the maximum gamma-exposure rate at the unit's surface was 73 mR h-1. With a 76.2-cm water shield, this was reduced to 8.0 mR h-1. A 61.0-cm water shield reduced a maximum surface gamma-exposure rate of 4.0 mR h-1 to 0.4 mR h-1 at site 5; and at