The Missouri Department of Natural Resources has issued a swimming advisory for one of two public beaches in Lake of the Ozarks State Park.

The Missouri Department of Natural Resources has issued a swimming advisory for one of two public beaches in Lake of the Ozarks State Park.

The advisory was issued late this week for Grand Glaize Beach. The advisory was issued after DNR detected elevated levels of E.coli in the most recent round of beach testing. The beach is not officially closed, but DNR officials are recommending swimmers not use Grand Glaize but, instead, use PB1. The marinas and all other park facilities are not impacted by the swimming advisory.

Water samples are analyzed for E. coli, a common indicator species for bacteria. It is normal for E. coli and other bacteria to be found at naturally-occurring levels in ponds, streams, rivers, lakes and reservoirs. Elevated bacteria levels are frequently associated with heavy rains; however, there are a number of other sources that may contribute to elevated bacteria levels, which may pose a health risk.

Water sample results can help visitors decide whether a particular beach is suitable for swimming, based on the bacteria levels.

In accordance with state law, DNR posts signs notifying visitors that swimming is not recommended if the geometric mean of the weekly water quality sample results exceeds the equivalent of 190 E. coli colonies per 100 milliliters of water (190 mpn/100 ml).

At the Lake of the Ozarks State Park, Grand Glaize Beach had a geometric mean of 185.5 on June 9 while Public Beach 1 had a geometric mean of 5.7. On June 16, Grand Glaize Beach had a geometric mean of 613. That's up from the geometric mean on June 2 of 74.4.

Beaches are no longer posted as being closed if the limit is exceeded. If levels are over 190, the state posts that swimming is not recommended along with the geometric mean result. The state posts the most recent test results whether the level exceeds the limit or not.

DNR can still close the beach if a documented health risk is known - such as flooding, debris or some kind of spill. State beaches can also be closed if E.coli levels are "extremely high".

Last year, the Missouri General Assembly passed legislation to more closely define procedures related to e. coli testing of recreational waters at state park beaches by the DNR.

Before August 2013, the single sample maximum was 235 colonies per 100 milliliters of water or a geometric mean of 126 mpn/100 ml from five rolling samples taken during the recreational season. Anything higher than that resulted in temporary closure of a public beach.

Based on EPA guidelines, the limit is a now a geometric mean of 190 mpn/100 ml.

Unlike a standard mean that is basically taking the average of a set of numbers, a geometric mean is a product rather than a sum. In this case, it is found by multiplying the results and taking the fifth root.

The water is tested weekly on Monday or Tuesday in order to have the results posted for visitors as they make plans for the weekend. Sampling is only done once a week.

The status of the beaches is posted both on location with a sign as well as online.

Part of the problem with e. coli monitoring is that results cannot be known for 24 hours due to the science involved in testing, so it cannot be known what the level actually is on any given day. At most, officials can only know what the level was yesterday.

The recommendation with the level of bacteria helps beach-goers make their own decision about how safe the water is for swimming.

According to DNR, there a lot of factors in addition to e. coli that should be considered. These include recent rainfall events that may have caused runoff and subsequent contamination, personal health issues that lower your immune system and potential pollutant sources nearby.

While high results one day can be a predictor that the levels will be elevated the next, there can also be a runoff event or other incident that causes the bacteria level to spike after the weekly testing shows low levels.

One general safety rule when swimming under any conditions is to try to keep your mouth shut to avoid swallowing water. In natural recreational waters, some level of bacteria is always to be expected, and even swimming pools can be hazardous due to the time it takes for chlorine to kill bacteria.