Analysis and Results

Our analysis focused on the effects of both time and location on beach water quality and if these effects are tangible. We also studied individual beaches and their levels of bacteria.

All of the visualizations seen below were made in R (ggplot2) or ArcGIS Online.

Fecal Coliforms do not appear to distinctly peak in any particular month or season. Instead, Fecal Coliform levels tend to stay low throughout the year, with notable exceptions in December 2022 and July 2023, where more individual beaches had higher average bacteria levels. Lastly, the trend of Total Coliform levels across all beaches very closely followed that of Fecal Coliforms; there were no notable monthly or seasonal peaks but instead remained relatively low throughout the data.

The observed spike in bacteria levels in the winter-spring transition may be due in part to the result of most of the storm water in San Diego county leading directly to the ocean. As this storm water leads to the drains built into neighborhoods and other areas, it collects runoff, which may include dirt or bacteria within manure on farms, and takes this to the ocean. As a result, most winters we may expect to see a rise in bacteria levels in the oceans, especially after particularly rainy periods or seasons. Waiting a few days to swim in the ocean after a particularly rainy day or even avoiding areas that receive runoff altogether can prove beneficial.

The trends we discovered are identical for all four types of bacteria. Over the course of the three years studied, hot spots of bacteria levels cluster to the south of San Diego. In other words, beaches closer to the United States-Mexico border tend to have higher levels of bacteria, regardless of the time of year. More specifically, most of the hot spots cluster around Imperial Beach and the Naval Outlying Landing Field. On the other hand, beaches around the city proper tend to trend lower in terms of bacteria level, again regardless of the time of year. There are three distinct cold spot clusters throughout the county: Mission Bay Park, Cardiff by the Sea, and Oceanside (see below). 

These particular trends may be explained through several reasons. For example, lower water quality levels in South County San Diego and toward northern Mexico can be linked to the sewage issues present within Tijuana. As a result of the lack of sewage treatment infrastructure in the Tijuana region, “Tijuana River annually discharges tens of millions of gallons of untreated sewage into the ocean”.1 Although the U.S. government and the Environmental Protection Agency are actively working to find a permanent solution to this problem and to cleanse the affected regions, the most we can do is advise beachgoers to proceed with caution when enjoying the coastal waters in South County. 

We looked at five of the most popular and most visited beaches in San Diego county: La Jolla Shores, Ocean Beach, Windansea Beach, Coronado City beaches, and Mission Beach. Each of these had at least 360 individual observations within the data, allowing for a more comprehensive look into the underlying bacteria trends.

At La Jolla Shores, E. Coli levels were consistently unsafe over the three years but other than that was relatively safe for the rest of the bacteria types. It saw an E. Coli and Enterococcus spike in the winter, as well as high Fecal Coliform levels in in the summer. Ocean Beach saw high Enterococcus levels throughout 2022 and winter 2023 but experienced relatively good water quality otherwise. Windansea Beach marks as among the safest in our study, with no exceptionally high bacteria levels throughout the three-year time period, though this beach had relatively less data than others. Similarly, Mission Beach did not see any notably high values. Coronado City beaches, on the other hand, saw several instances of extremely high bacteria levels, most notably E. Coli and Enterococcus in 2021, Enterococcus and Fecal Coliforms in winter and summer of 2023, and Total Coliforms in August 2023. 

In addition to these five, we studied the five beaches with the most individual observations in our data: Imperial Beach Municipal, Silver Strand State Beach, South Carlsbad State Beach, Border Field State Park, and Oceanside Municipal. Imperial Beach was generally unsafe in terms of all four types of bacteria, most notably Total Coliform levels throughout 2023. Six months of that year saw an average Total Coliform level of well over 1000 MPN/100 mL, our defined threshold for swimmable water. Silver Strand, on the other hand, saw exceptionally low levels for all four types of bacteria; one exception is worth mentioning, however. In January 2023, Silver Strand saw an average Total Coliform level of over 10,000 MPN/100 mL. South Carlsbad State Beach saw the safest waters out of these five beaches, with the global peaks for all four bacteria types being sub-100 MPN. In contrast, Border Field State Park was among the poorest in water quality; many monthly averages for Enterococcus and both Coliform types were far above our threshold. Finally, Oceanside was most similar to Silver Strand, with the majority of monthly averages being under the threshold, but saw three notable exceptions: January, March, and August of 2023 saw major spikes in Total Coliform levels. 

1. Water Quality at California Beaches Dips in 2023-24
   https://healthebay.org/water-quality-at-california-beaches-dips-in-2023-24/