Disinfection by-product dynamics in a chlorinated, indoor swimming pool under conditions of heavy use: national swimming competition.

Anecdotal evidence suggests that water quality in chlorinated, indoor pools deteriorates under conditions of heavy use. However, data to define these dynamics have not been reported. To address this issue, a study was performed in which water chemistry was monitored in a chlorinated, indoor pool before and during a national swimming competition, a period of heavy, intense use. NCl3 concentration was observed to double after the first day, and increased by a factor of 3-4 over the 4 days of competition. CNCHCl2 and CH3NCl2 concentrations both increased by a factor of 2-3 during the course of the meet, while CHCl3 concentration showed only a modest increase during this same period. Diurnal patterns of NCl3, CH3NCl2 and CHCl3 concentrations were observed, and these patterns appeared to depend on the Henry's law constant of the compound. Urea concentration showed a diurnal pattern, superimposed on a trend of steady increase during each day of the competition; however, the diurnal pattern of urea behavior could not be explained by reactions with chlorine, as the urea-free chlorine reaction is relatively slow. It is more likely that the overnight decrease in urea concentration was attributable to mixing of surface water with water in the deeper parts of the pool. The findings of this study provide an indication of the changes in pool water chemistry that take place in a chlorinated, indoor pool under heavy use conditions.