Influence of variable precipitation on coastal water quality in southern California.

Objectives: To examine the consequences of changing precipitation levels on southern California's recreational coastal water quality, and compare the responses of watersheds with differing levels of urban development.

Methods: The geo-temporal relationship for six years (2000-2005) of precipitation levels, discharge rates for the ten primary waterways, and coastal water bacteria concentrations at seventy-eight southern California beaches were examined.

Results: Precipitation levels, river-creek discharge rates, and coastal water bacteria concentrations were significantly correlated (p < 0.01) for all ten watersheds investigated. Water bacteria concentrations significantly increased with higher levels of precipitation across 95% of the seventy-eight beaches investigated. A heavily developed watershed had significantly higher median bacteria concentrations (186 cfu) in the adjoining coastal waters compared to an undeveloped watershed (10 cfu) of similar size.

Conclusions: Precipitation and ensuing runoff strongly control the rate of polluted water delivered to most beaches in southern California. Variable precipitation generates a greater response in coastal water bacteria concentrations in developed watersheds compared to undeveloped areas. Projected declines in regional precipitation as a consequence of climate change may result in less contaminated water delivered to coastal waters, thus decreasing risk of water associated illnesses during winter months.