More on the Tijuana River Slough from San Diego County's Department of Environmental Health:

When sewage contamination in the Tijuana River moves from the estuary mouth and north along the coast, south San Diego county beaches are heavily impacted. As a result, water quality at these impacted beaches is extremely poor.

Although the Beach Report Card may show an improvement in water quality at south county beaches for the winter of 2006/2007, there are less bacterial monitoring data for these beaches compared to previous winters because the sampling effort conducted by the San Diego County Department of Environmental Health (DEH) has been reduced.

In a recent study on enteric viruses at Imperial Beach and the Tijuana River mouth(1), researchers reported a number of hepatitis A virus strains. Because untreated human fecal waste from Tijuana sewage outfalls is a major pollution source to coastal waters near the US/Mexican border, human fecal bacterial densities (E. coli and Enterocci) during wet weather exceeded state water quality standards in 86% (12 of 14) of the samples in the study. Exceptionally high concentrations of these human fecal indicator bacteria were significantly correlated with high concentrations of hepatitis A virus and enterovirus. Three strains of poliovirus were also detected(1).

To create a real time Tijuana River plume model, Scripps Institution of Oceanography compared previous monitoring data with measured hourly ocean currents from San Diego Coastal Ocean Observing System (SDCOOS). This 2-dimensional view makes tracking the movement of the plume significantly easier than the 1-dimensional real time data sets that were used previously. So far this model agrees well with water quality measurements taken this spring. The San Diego Department of Environmental Health believes that the use of this real time predictive model alleviates the need for extensive bacterial monitoring of south county beaches(2). Bacterial monitoring is thought to take too long for protecting public health; in that results are produced at least 24-hours after sample collection.

The downside of less monitoring is that it gives an incomplete water quality picture for the Beach Report Card, and State and Regional Water Board water quality assessments, impaired waters determinations and TMDL development.

1. Gersberg et al. 2006. Applied and Environmental Microbiology. 72:7438-7444

2. San Diego County Department of Environmental Health

From San Diego County's Department of Environmental Health:

As of September 1st, the County of San Diego Department of Environmental Health (DEH) suspended the use of the IDEXX Colilert 18 test method for the analysis of total coliform and E. Coli bacteria in ocean and bay water quality samples. DEH took this action after it determined that this test may not always provide an accurate measurement of E. Coli bacteria concentrations in marine waters.

A review of published research, discussion with the Colilert 18 test manufacturer, and split sample comparison of Colilert 18 (1:10 dilution) with other accepted test method data indicates that false positive E. Coli results are possible when using Colilert 18 in marine waters warmer than normally experienced in Southern California. The split sample data came from Mission Bay locations where bay water temperatures are usually several degrees warmer than coastal beaches. [The County conducted the required correlation study between a traditional test method and Colilert 18 in the winter of 2004 to validate this faster test method as an acceptable alternative, so this limitation with the Colilert 18 method was not identified at that time. Nor did the use of Colilert 18 in the summer of 2005 suggest any problem with this test method.]

Taken together, this information suggests that some of the elevated bacterial levels (E. Coli) observed in east Mission Bay during July and August of this year may have been false positives and not true measurements of the E. Coli/ fecal coliform indicator bacteria that are used in state law and regulations to make determinations about the safety of water contact. However, other test method data confirms bacterial contamination did occur during this time at some east Mission Bay locations, although we believe, not at the same frequency and extent that the Colilert 18 data indicated. It should also be noted that both the Colilert 18 data and other test method data (including non-split samples) showed great variability, and it cannot be said that all the Colilert 18 E. Coli results were false positives. The published research also indicated that the main reason for the false positive results in their studies was the presence of other families of non-coliform bacteria such as Vibro and Aremonas. These types of bacteria are widely found in the environment and some strains have been known to be pathogenic.

DEH is committed to the protection of public health by the use of science and accurate information. DEH is continuing to conduct split samples between Colilert 18 at different dilutions and other accepted test methods to determine what test method can produce accurate results in the shortest time and thereby be most protective of public health.