by Alex Ferron

San Diego just experienced its first rains after a long, relatively dry Spring/Summer season. While our drought-prone region  benefits greatly from rainfall, it presents a double-edged sword for coastal communities due to the heightened levels of pollution it brings to our coastal waters. This is especially true for people who – like us –  love to surf, swim, and recreate at our local beaches. 

As we move back into Southern California’s rainy season, we want to remind you- or maybe even explain for the first time- why we recommend waiting 72 hours after rainfall before jumping back into the water.

Welcome to the Watershed

No matter where you live, water from rains and snowmelt will ultimately drain to a common body of water like a lake or an ocean. This is called a watershed. Here in coastal San Diego County, the majority of our water flows into the Pacific Ocean. Our rainwater (aka stormwater) flows to our beaches via natural systems, such as rivers and creeks, and engineered systems, like roads and our stormwater drains. More often than not, stormwater is untreated.

San Diego’s topography naturally directs the vast majority of rainwater west towards the ocean

Rainwater alone does not pose a threat to the ocean or our health. However, in highly urbanized areas like San Diego, rainwater picks up and accumulates pollutants from our surfaces and transports them to the ocean. This phenomenon is called “urban runoff,” and unfortunately for us, it’s nasty. Runoff commonly contains motor oil, grease, pesticides, industrial chemicals, litter, synthetic fertilizer, pet waste, human waste, toxins, bacteria, even viruses. 

Urban runoff is the #1 cause of ocean pollution in most coastal cities, and it’s a problem of our own creation. Due to our paved, impermeable surfaces and the stormwater drainage systems built specifically to prevent flooding after a rain, our cities transport untreated runoff to the ocean very efficiently. 

This process would work quite differently in a pristine, natural environment. In the absence of pavement and stormwater drains, the earth could absorb a decent amount of rainwater before it reached the ocean. Meanwhile, the water that did reach the ocean would be relatively clean, while our coastal wetlands – most of which have been replaced by development – would naturally filter many of the pollutants that did accumulate. Healthy wetlands also act as a natural buffer against sea level rise, and are remarkably efficient at sequestering carbon from our atmosphere. 

San Diego stormwater ditch after 2 days of heavy rain.

The 72 Hour Rule

As it stands, urban runoff brings dangerous levels of pollution to our nearshore waters after a rainfall. This is especially true for the first rain after a long dry spell, commonly referred to as the “first flush,” which brings months of accumulated pollution to the coast. 

So why wait 3 days? Simply put, it takes time for polluted runoff to dilute with the rest of our coastal waters. Until then, the heightened levels of bacteria and other pollutants pose a significant and well-documented risk to ocean recreators. This is especially true when fecal matter (aka poop) is present in the water.

The Surfer’s Health Study, conducted in partnership with Surfrider here in San Diego between 2012-2015, showed a strong correlation between fecal indicator bacteria and illness rates during wet weather. The study also validated the 72-hour rule: “illness rates were highest when surfing during rain and the first day after the rain, declining to near baseline levels after 3 days.” 

There are other factors to consider – proximity of your beach to river mouths or other runoff sources, direction of ocean currents, etc. The “72 hours rule” is backed by solid science, but rain or no rain, we always recommend erring on the side of caution if your beach looks or smells funky. 

Crossborder sewage flows regularly foul up our South Bay beaches during dry weather, for example. Untreated sewage flowed continuously across the border since heavy rains in November 2019 damaged critical sewage equipment in Tijuana, and continued to flow throughout the entire summer.

An extreme example of contaminated water, still concentrated at the shore from a TRV sewage spill

Stay Informed!

Here in Southern California, over 1.5 million people get sick each year from polluted beaches and waterways. Surfrider operates the Blue Water Task Force in order to provide science-based, local water quality information to our coastal community. Our results are published weekly, and we regularly hold outreach events and other activations intended to educate the public on the importance of water quality and public health. 

While our South Bay testing is still paused due to the coronavirus pandemic, we will continue to rely on the San Diego County Department of Environmental Health for their official water quality data. If there are any questions or concerns about water quality, the DEH is the regulatory body in charge of opening and closing our beaches. We will continue sharing The Weekend Beach Report with updates from the DEH until further notice.