The City has been challenged to explain why the San Diego County Department of Environmental Health issued a beach advisory just before the Fourth of July holiday. The advisory, which remains in effect, warns swimmers to avoid water contact in the area near Avenida del Sol in South Beach. The City made an educated guess that the elevated levels of enterococci bacteria were a result of the decaying kelp that washed ashore in the area where the high levels were recorded. Decaying kelp is normal for this time of year when ocean temperatures rise. Many strands of kelp die and release from the kelp beds offshore. Regional, scientific studies tie enterococci to environmental sources, specifically, kelp. Here is an excerpt from one study:
"While there is no evidence showing that enterococci can grow in ambient oligotrophic waters (environments that offer very low levels of nutrients), experiments showed enterococci can grow in sands (Yamahara, Walters, and Boehm, 2009) as well as in water augmented with decaying kelp (Byappanahalli, Shively, Nevers, Sadowsky, and Whitman, 2003; Imamura, Thompson, Boehm, and Jay, 2011)."
The exact reason for the high levels of enterococci bacteria in the area in late June and early July may not ever be known. However, it is very unlikely caused by sewage from the Tijuana River for many reasons, the least of which is that beaches closer to the border did not test positive for any sewage contaminants. In fact, the City of Coronado's waters tested only for enterococci, not fecal coliform or total coliform. Fecal matter is always present in human waste (sewage) contaminated waters. Additionally, the ocean currents in the area during the dates of the elevated enterococci readings were flowing north to south during the entire time of the elevated samples, (see images below). This is compelling evidence that whatever caused the higher levels of bacteria at the South Beach location could not have originated in Tijuana. According to Keith Kezer with the County Department of Environmental Health:
"It is unlikely that the advisory at Avenida del Sol is related to sewage impacts associated with the Tijuana River Valley or Mexico. A routine sampling at these locations, which ranges from one to three times each week, has not detected water quality indicative of sewage impacts from Mexico and has almost exclusively been below state established beach water quality health standards."
A sewer discharge always leaves a trail of bacteria. If the source of the contamination was Tijuana sewage, the concentration of contamination near the source would be highest and progressively become more diluted as it traveled away from the source. There is no means for contamination from the mouth of the Tijuana River Estuary to travel 10 miles north without contaminating all points in between.