The fate of disinfection byproducts during aquifer storage and recovery (ASR) is evaluated for aquifers in southern Nevada. Rapid declines of haloacetic acid (HAA) concentrations during ASR, with associated little change in Cl concentration, indicate that (HAAs) decline primarily by in situ microbial oxidation. Dilution is only a minor contributor to HAA concentration declines during ASR. Trihalomethane (THM) concentrations generally increased during storage of artificial recharge (AR) water and then declined during recovery. The decline of THM concentrations during recovery was primarily from dilution of current season AR water with residual AR water remaining in the aquifer from previous ASR seasons and native ground water. In more recent ASR seasons, for wells with the longest history of ASR, brominated THMs declined during storage and recovery by processes in addition to dilution. These conclusions about THMs are indicated by THM/Cl values and supported by a comparison of measured and model predicted THM concentrations. Geochemical mixing models were constructed using major‐ion chemistry of the three end‐member waters to calculate predicted THM concentrations. The decline in brominated THM concentrations in addition to that from dilution may result from biotransformation processes.

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2. Map showing location of surface-water data collection sites. 3. Cation-anion diagrams showing chemical character of water during low-flow conditions at sites in North River basin.. 11 4. Diagram showing daily discharge and'maximum and minimum temperature, specific conductance, dissolved oxygen,

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