It has been more than a month since roughly 48 million gallons of raw sewage was released into the Ala Wai Canal to prevent even more catastrophic problems as repairs were made to a major sewer line in Waikiki.

The discharge lasted six days and visitors and residents are still afraid to go back into the water.

Reports of "flesh-eating bacteria" and other life-threatening infections have scared people out of the water in many areas around the state; the beach at Ala Moana Park was practically deserted when I went for a swim last week.

It's not surprising that people avoided beaches and sewage-contaminated water during and immediately after the discharge was reported by the city. What is surprising is how long people are staying out of the water, including surfers, swimmers and others who are not usually averse to taking risks. Perhaps most disturbing is how little is known about such risks to public health or the environment.

Social psychologists generally agree that people are most afraid of things they cannot see or do not understand that may hurt them. In this situation, we have an unseen, misunderstood and, to some extent, unknown health threat.

Recent monitoring results indicate that bacterial counts have returned to "pre-spill" levels in the Ala Wai Canal, according to Watson Okubo of the state Department of Health's Clean Water Branch. And water quality has returned to normal in most areas around Waikiki based on the department's monitoring. It helps to be in the middle of the Pacific Ocean.

And it's true that most bacteria and viruses die within 48 hours of exposure to saltwater.

Nevertheless, concerns linger about residual, longer-term threats to public health and the environment.

At a recent Surfrider Foundation meeting, concerns were raised about the survival of bacteria and viruses in sediments and sand. More specifically, it was suggested that sediments in the Ala Wai Canal may contain disease-causing organisms that may be re-suspended after a heavy rain, washed out into areas where surfers and others may be exposed, and, possibly, present a recurring health threat.

Concerns also have been raised that the sand along shorelines affected by the sewage spill may present a risk to those walking on the sand.

And what about Hawai'i's pristine and precious ocean environment?

The bottom line here is that the true environmental impacts associated with large sewage spills have yet to be defined or studied.

Raw sewage contains nutrients that can have adverse effects on marine life. A large phytoplankton bloom can be seen offshore from the Ala Wai, indicating that dissolved nutrients from the spill and polluted runoff are supporting the proliferation of algae in this area.

Algae can smother and kill coral, causing a troubling imbalance in our ocean environment.

The distribution of fish and other marine life will shift with changes in the nature of the biomass anywhere.

Longtime residents may recall conditions in Kane'ohe Bay, when treated sewage was routinely discharged into the bay. I remember diving in the bay and seeing bubble algae so thick that it was hard to see the sand or underlying coral.

If you could see the coral, you would learn it was dead.

In fact, there was little sea life except algae and other organisms that are capable of living in oxygen-deprived sediments anywhere near the sewage outfall. It took decades for marine life there to recover and for coral to start growing again after a new sewage outfall was constructed off Mokapu Point.

Wastes typically removed at sewage treatment plants also were discharged into the Ala Wai Canal. Chemicals bound to solid matter will remain in the canal until they are removed or, eventually, wash out to sea. The Ala Wai has been a sink for polluted runoff since it was constructed in the 1920s, and chemical pollution in the canal is well-documented.

Heavy metals and pesticide residues in the bottom sediments accumulate up the food chain. Tilapia and other fish in the canal already have high levels of dieldrin and other pesticides. It is not clear what impacts the spill has had on the composition of sediments in the canal, but the problems there could only be exacerbated by the spill.

Advisories warning people not to eat fish from the canal will undoubtedly remain posted for many years to come.

Unfortunately, health officials, biologists and other scientists who we would normally turn to do not have clear answers to these questions nor can they quantify the risks.

There are no state or federal health standards for biological contaminants in sand or sediments, and the length of time viruses and bacteria can survive in sand or sediments and their ability to cause infection has not been studied.

Since the 1980s, the University of Hawai'i's Water Resources Research Center has been studying these issues.

The director of the center, Dr. Roger Fujioka, analyzed sand for bacteria at Hanauma Bay, Ala Moana Park and other beaches. He found that fecal bacteria in sand on beaches can contribute to counts in the water. He attributed the high counts to pigeons and other birds, contaminated soil that mixes with the sand, septic tanks, cesspools and numerous other possible sources of bacteria.

No attempt was made to determine human health risks.

There is no conclusive evidence to link bacteria in sand to human health. Although finding high levels of indicator bacteria in sand may suggest a significant health risk, it is not clear whether individuals walking or sitting on contaminated sand may actually be at increased risk of infection.

Bacteria and viruses that cause disease are found practically everywhere and we are exposed to them every day.

To assess health risks from exposure to contaminated sand or water, it is also necessary to show a correlation between the levels of bacteria or viruses in sand or water to cases of human illness. This is easier said than done.

Illnesses that would be expected, primarily skin infections or gastrointestinal illnesses, are very common and it is practically impossible to determine the source of exposure.

Scientists at the UH center did find, however, that bacteria associated with sewage spills can last for months in moist sand or soil. Certainly, more research is needed on the relationship between bacteria in sand and their risk to human health.

Tests for live disease-causing bacteria and viruses are difficult, expensive and only a limited number can be done. Molecular methods of testing for the DNA of pathogens are being used today because these tests are more feasible than trying to culture the live organisms. Unfortunately, molecular tests detect the DNA in live and dead organisms.

For risk assessment purposes, this is a problem because only live organisms are a concern. If we want to accurately assess health risks, it will take time and lots of money.

Clearly, effective preventive measures are key: Avoid contact with potentially contaminated water or sand if you have open cuts or wounds, and rinse off after swimming or wading, preferably washing with soap. If these measures are taken, the risk of infection is minimal, and everyone should be able to enjoy swimming, surfing and walking on our beautiful beaches without concern.

The recent sewage spill is a cause of concern for many Hawai'i residents, particularly those who frequently enjoy water recreation activities. Their concerns will not go away without better understanding the health risks.

We have a great team of concerned scientists at the University of Hawai'i and the Oceanic Institute who are eager to find answers to these questions. The ocean is important to all of us and we need to be sure it remains clean and safe.