in cistern cisterns. water subsequently Contamination of increased, with larger the water supplies with increases resuspended in certain particulates cannot account for the increases in concentrations detected, but there are two other possibilities. The first requires an assumption cisterns was contaminated. were used with radionuclides cement that material used to construct the This is possible because local aggregate and water to manufacture The 1376, with exchange or other any water less of levels decrease with use, When water saturating the cistern walls. cisterns. could mixture the in incorporated the the interior surface area of the cistern is’ in direct contact with the water. As a the result, volume less of 1376, water in the cistern result, cistern the is radionuclide full and and dilutes lower activity water and concentrations there is no the rain increases concentration of A continuous use of water shortens the radionuclides in the residual water. contact time between this New the matrix. from is mobilized further decrease use for As the cistern walls. as observed. the water, When a the radionuclides continuously exchange between the cistern walls and the water. A second possible cause is that concentrations of radionuclides in the water will decrease as a result of dilution with lower activity rainwater and Heavy demands on the increase if the standing water is allowed to evaporate. cistern water between 1975 and 1977 reduced the inventory of available water. Rainfall continuously replenished part of the cistern supply and diluted the residual radionuclide During concentrations. this 1376, their lowest level since 1975. in an increased and 90. in the in By September 1978, cistern water approached those expected in wet deposition. the average concentrations of concentrations period, water were reduced to When water usage ceased, evaporation resulted radionuclide concentration. Some lower concentration rainwater diluted the radionuclide concentration of the cistern water, but the rate of the dilutant input was not equivalent to the evaporation Evaporation rates for standing bodies of water have been measured at and average 0.5 cm/d. 16 With this assumption, Table 2 can be explained by a process of dilution, concentration use, and rate. Bikini changes in evaporation. We are currently investigating the importance of these processes and how they may alter future radionuclide concentrations in atoll cistern water. 13