Comparison of recreational health risks associated with surfing and swimming in dry weather and post-storm conditions at Southern California beaches

Urban stormwater runoff serves as indirect transportation for pollutants of all kinds, and it contributes to the high levels of fecal bacteria contamination within coastal waters, posing a potential threat to ordinary beach goers. Eight popular beaches in Southern California underwent quantitative microbial risk assessments (QMRA) to determine the concentration of fecal indicator bacteria (enterococcus, ENT and fecal coliform, FC), and gauge the significant dangers associated with activity in these waters—specifically the risk of acquiring a gastrointestinal illness (GI) while engaged in surfing or swimming. Tseng and Jiang (2012) evaluated the difference in potential health risks between surfers and swimmers, and surveyed the corresponding threat level throughout the two distinct Southern California seasons—dry season (characterized with less than two inches of total rainfall) and storm season, which accounts for 90% of the annual rainfall. They found that higher health risks were present during storm season as opposed to dry season, and that surfers were more susceptible to illness. –Genevieve Heger
Tseng, L. Y., and Jiang, S. C., 2012. Comparison of recreational health risks associated with surfing and swimming in dry weather and post-storm conditions at Southern California beaches using quantitative microbial risk assessment (QMRA). Marina Pollution Bulletin 6, 912918

Tseng and Jiang selected eight beaches from three coastal counties based on the following criteria: popularity among surfers, proximity to weather stations, and availability of bacteriological monitoring data. Bacteriological monitoring data were collected from the responsible agency within each county, and daily precipitation data were gathered from the National Climate Data Center (NCDC) National Environmental Satellite throughout the months of January 2008 to May 2012.
Because the study strived to compare the degree of potential health risk between dry and storm season, the following terms were defined: “post-storm” is the period between 24 and 72 hours after a recorded rainfall of greater than 0.2 inches, ‘‘dry weather’’ was defined as the dates with either no recorded precipitation, or a time period of at least 72 hours following a recorded rainfall. FIB data were excluded from analysis on the dates of missing precipitation information, and on the days of recorded precipitation because the exact time of the FIB sample collection was not reported (which could have been either be before or after the rain).
In order to assess the difference in health risk between surfers and swimmers, ingested doses of seawater were estimated and evaluated for concentrations of harmful bacteria (fecal coliform and enterococcus) to determine the likelihood of acquiring a GI due to such exposure. The ingested dose of ENT or FC by surfers was given by the equation, Doral = Ioral *C, where Doral is the ENT or FC dose ingested (MPN or CFU), Ioral is the ingested seawater volume by surfers (ml), and C is the seawater concentration of ENT or FC (MPN/ml or CFU/ml). Surfer’s risk of GI from ENT was estimated by applying the exponential dose-response model (Haas et al. 1999 Stone et al. 2008), and their risk of GI from FC was estimated by applying the Beta-Poisson model (Haas et al.1999). For the comparison, the ingested seawater volume in ml by swimmers is given by the equation, Ioral;swim = T exposure   *Ringestion ; where Ioral, swim is the ingested seawater volume by swimmers,  Texposure  is the time of swimming (minutes), and Ringestionis the water volume rate of ingestion in (ml/min).
Overall, the study showed that health risks were elevated during storm season as opposed to dry season. At Santa Monica, El Segundo, San Diego and Coronado beaches, health risk associated with surfing at post-storm conditions was significantly elevated (p 0.1). It was also noted that post-storm surfing could exceed EPA risk guidelines up to 28% of time, while the chance of exceeding the risk guideline during dry weather conditions was mostly below 15% based on the ENT model. The study also indicated that the risk of GI per in-seawater event was higher for surfing than for swimming, which was attributable to the increased volume of contaminated water ingested.
Works Cited:
1. Haas, C.N., Rose, J.B., Gerba. C.P., 1999. Quantitative Microbial Risk Assessment. Wiley, New York 

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