Background: This study aimed at evaluating the water quality of thermal spring of Sarein in the province of Ardabil, with emphasis on bacteria S. aureus as one of the organisms involved in skin and eye infections, in the first half of 2011.

Methods: In this study, 85 composite samples were collected from all thermal spring pools of Sarein during May to September 2011; in the peak number of swimmers. The concentration of free residual chlorine, temperature, pH, turbidity, H₂S, ORP, DO and EC were measured in-situ in the sampling sites by portable devices. Total and fecal Coliforms, Staphilococcus aurous, fecal Streptococci and HPC tests were done according to the Standard methods for examination of water and wastewater.

Results: Data showed that the maximum contamination of total coliforms were occurred in August (MPN/100=309.82), fecal Coliforms in July (MPN/100=69.59), minimum contamination of samples were related to fecal streptococci in August and September, peak of HPC in September with 4774.12 cfu/mL, and Staphylococcus aureus with 1149.35 cfu/100mL in August were recorded. Results of averages comparing showed no significant difference between the months of sampling and contamination with bacteria S. aureus.

Conclusion: Data showed that 3.53%, 9.41%, 84.71%, and 81.18% of samples were contaminated to total Coliform, fecal Coliform, HPC and Staphylococcus aureus, respectively which were higher than standard limits that it was an alarm for the occurrence of skin diseases and infections to swimmers.