As explained further in our article on Epidemiological Studies, certain "indicator bacteria" (total coliform, fecal coliform, E. coli and enterococcus) are measured in recreational waters and compared against federal and state standards to determine whether the water is safe to swim in. At many locations where bacteria levels frequently exceed standards, investigations have been conducted to try to identify and eliminate the pollution source(s) (see article Bacterial Pollution, Tracking the Sources). All too often, however, it has been difficult to locate a source of the pollution. At some beaches, scientists have noticed a correlation between bacteria levels in the water and extreme ("spring") tides. In particular, the highest bacteria levels are often associated with ebb (outgoing) tides under spring tide conditions. Could the sand be acting as a "reservoir" for bacteria and therefore a source of bacteria found in the ocean? Trying to answer this question has led to studies that measure indicator bacteria in beach sand.
Conventional wisdom has been that fecal indicator bacteria such as enterococcus and E. coli do not survive for very long in the environment once they are disassociated from a human or animal host and their waste material. It was also believed that these bacteria could not replicate and maintain colonies in the environment. So, it was somewhat of a surprise when beach sand was tested for indicator bacteria and the bacteria were pretty much found – everywhere. In Hawaii, California, Florida (1, 2), Great Lakes Beaches (1, 2) - seemingly everywhere researchers collected sand samples - bacteria such as enterococcus or E. coli (enterococcus is generally measured at salt water beaches while E. coli is generally measured at fresh water beaches) were detected in the beach sand.
So far, the results of sand testing have raised more questions than they have provided answers. The pattern of bacteria distribution appears to be different at different beaches. In general, researchers in California found the highest levels in wet sand and/or near storm drain outlets. They also found higher levels at "enclosed" beaches than at open ocean beaches. Researchers in Florida found the highest levels in dry sand. Some researchers noted higher concentrations at beaches with the highest human usage, while other researchers at different locations did not observe this pattern.
This has led to a lot of confusion and differing theories. Do the bacteria in sand indicate a human health threat? If so, what is a safe level? Are the bacteria in sand a significant source of the bacteria that are detected in water samples? If beach sand is a significant source of bacteria in the water, do these bacteria detected in water indicate a real health threat from a recent pollution event or do they represent a resident population of sand bacteria that pose no harm?
The answer to all these questions is the same – no one knows.
There are several challenges to making sense of all this. First, a standard method does not yet exist to measure bacteria in sand. Sampling procedures vary amongst researchers, but, in general, a sand sample is collected and weighed. Then the sample is mixed with a specified volume of either distilled water or phosphate buffered water. After allowing the mixture to settle, the water is poured (decanted) off and the water is then measured for bacteria using either a membrane filtration method or a "defined substrate" (IDEXX) method. The many potential variables (relative amounts of sand and water, type of water, mixing method, settling time, analytical method) need to be standardized to allow comparison between test results from different researchers. Scientists at Southern California Coastal Water Research Project recently completed a method comparison study which developed a recommended standard method.
Even after there is a standard method, the results (for the water extracted from sand-water mixing) can not be directly compared to any existing health standard, because no such standard exists. To develop such a standard, sand testing would have to be combined with health surveys of thousands of beach-goers to evaluate if any sand exposure-related illnesses are occurring and if these correlate with levels of measured bacteria in the beach sand. An obvious difference between water and sand exposure is that people are less likely to ingest sand, with the notable exception of small children. Some recent epidemiological studies have started to incorporate sand testing, but consensus results and a health standard, if warranted, are probably years away.
There continues to be a lot of research in this area, including several presentations at technical conferences (1, 2) devoted to this subject, so stay tuned for further developments.
In the meantime, an article on the discovery of bacteria in the sand in South Florida recommends:
"If the beach is closed due to contaminated water, stay off the sand. Keep in mind the "swell area" where crashing waves meet the beach can potentially carry the highest risk exposure. Always use a towel. Rigorously wash your hands and the hands of your children before eating. Report any illness following a visit to the beach to your local department of health."
Here's a post from Surfrider's Coastal Blog on this subject.