E.Coli levels low in most of Lake
May 11, 2018
LAKE COMMITTEE REPORT May 10, 2018
Our lake monitoring program for this season started on Monday, April 30, and the lake E. coli data are now being posted on the Association website.
In last week's e-Bits we described which data would be reported and briefly described the format.
Please refer back to that if you missed it. You can now access the test data on the https://www.lwwa.org. From the Home page, select Lake then Lake News.
This week we will discuss our data reporting in more detail.
Before we do that, we can report that the April 30 E. coli test results were very low in most of the Lake, except for the area around Hideaway Park. Two samples on the southwestern side of Hideaway exceeded the recreational limits. It is not possible to draw any conclusions with only one set of results.
We will be watching the trends and keeping everyone informed.
The next test results will be posted Thursday afternoon.
We are using the 2012 USEPA Recreational Water Criteria as the guideline for assessing the suitability of the Lake water for primary body contact recreation, including swimming, water skiing, tubing, water play by children, and other activities where full body immersion and water ingestion are likely to occur.
The EPA Recreational Criteria do not apply to activities such as boating or fishing when body immersion in the water is unlikely to occur.
The Recreation Criteria list two indicator organisms that can be used for monitoring, Enterococci and E. coli.
Experience has shown that the enterococci are better for marine water beaches and E. coli is the preferred indictor for fresh water recreation.
The EPA criteria are complex and include values that relate to both the most recent test results and the average of results from the previous 30-days.
The specific parameters include the Statistical Threshold Value (STV), the 30-day Geometric Mean and the Beach Action Value (BAV). The STV value provides additional flexibility for very large sampling programs.
For our purposes, the STV will be viewed as a maximum limit. The Geometric Mean is a special way of computing an average that is commonly used for data that do not follow a normal distribution.
The 30-day limit helps distinguish between normal day-to-day microbial variability versus chronic contamination. The BAV value provides guidance for when public notification is required.
For fresh water recreation using E. coli per 100 mL as the indicator, the STV value is 320, the 30-day geometric mean is 100 and the BAV value is 190.
We appreciate that the EPA Criteria are complicated; to assist we will be providing a simple color code in our website reports to help interpret the reported data.
Any daily test value that exceeds the maximum limit will be coded red. Cautionary readings will be coded yellow. Test results in the lower portion of the acceptable range will not have any color.
We have elected to take a more conservative approach toward the Cautionary Warning category than that afforded by the EPA BAV value.
We will designate all test values from 100 E. coli per 100 mL to the 320 per 100 mL maximum as being in the Cautionary zone.
This represents a very conservative interpretation of the EPA Criteria, but we feel this is appropriate until we better understand the factors that led to last year's outbreak.
As noted at the beginning, our lake bacterial data are now available to view on the Association website at https://www.lwwa.org.
We also expect to have advisory signs posted at the Beaches soon that will contain a QR Code that will point a cell phone directly to the most current test data.
We expect to finalize the sign wording with the County shortly. We will get the signs made and posted as soon as approved.
When viewing our lake data, it is important to understand that the EPA standards are not absolute.
When the test results are below the limits, the probability of becoming ill when swimming is considered to be low, but it is not zero.
There is always some risk associated with recreational water activities.
People vary in their susceptibility to microbial infections. Infants, toddlers, the elderly and those with other medical conditions are the highest risk groups for microbial infections.
These factors should be considered when evaluating the bacterial test results.
Given the uncertainty that exists, the Nevada County Public Health Department is continuing their advisory, which recommends not engaging in any recreational activities at Lake Wildwood that could result in the ingestion of lake water (e.g., swimming and water skiing).
If someone does engage in recreational activities, it is recommended that they wash their hands thoroughly and/or shower after recreating in order to prevent the unintentional ingestion of potentially harmful bacteria.
The April 12 e-Bits Lake Update noted that there were simplified test procedures for E. coli O157:H7 that had been developed for use in the food processing industry, and we were looking at adapting those for use in our lake monitoring program.
We will provide an update on the status of that effort next week.
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