August 9, 2003
To: OWL Water Quality Monitors
From: Dick Coalson

Subject: Coliform and Turbidity Numbers in Perspective

Recently one of our monitors asked what the numbers that we were generating really meant as far as "Normal Values", "Suitable for Swimming", "Drinkable Water", and what "Turbidity" really meant. To answer these questions suitably I consulted Eileen O'Connor, the Cayuga County Director of Environmental Health.

As far as "Normal Values" are concerned, the Coliform numbers are site specific. Each of you probably has a good feel for the typical numbers that your site generates each week. By way of example, OWL-161, Dutch Hollow Brook at Route 38A Bridge, usually gives numbers in the hundreds for both Total and Fecal Coliform whereas OWL-109 at Wide Waters generally gives numbers of <10 for both Total and Fecal Coliform. Occasionally we may get a big deviation from these numbers such as happened on July 23rd at Wide Waters where the Total Coliform was TNTC (too numerous to count) and the Fecal Coliform was over 700. On the 30th of July the numbers were back to <10. This swing might have been a spot contamination (Goose, seagull, etc.) or even a lab error. If the numbers are exceedingly high, a second sample might be taken. In the case cited, the data were not available from the lab until the 30th of July so that the next week's samples had already been taken. I alerted the lab to let me know the results ASAP on the new sample and got the results on the 31st showing that the values were back to normal so no alert was issued or resampling done. On two other occasions this year specific sites were notified about high readings and close follow-up was done.

The issue with "Drinkable Water" is much clearer. The U.S. rule is that the Total Coliform must be less than 1. Thus, Lake water would be deemed unfit to drink without some sort of treatment to eliminate bacteria. The City of Auburn and the Town of Owasco use chlorine treatment coupled with filtration to accomplish this.

"Suitable for Swimming" requires that the average Fecal Coliform number be 200 or below. The County uses a "Moving Average" technique to establish the state of the water at Emerson Park and to decide whether the Park should be open or closed to swimming. For our purposes, numbers below 200 will be fine for swimming. A high number should be cause for concern until a number below 200 is again found for any specific site. Continued high numbers indicate a problem in the immediate area which must be addressed.

"Turbidity" is a measurement of the clarity of the water. Low numbers mean that the water is clear and high numbers mean it is more cloudy or opaque. The turbidity is usually caused by silt or suspended clay particles in the water. Coliform are too small to contribute to turbidity directly but may be associated with silt or sand particles that do cause turbidity. As a result, higher turbidity numbers may or may not be associated with higher Coliform values.

If you would like to discuss any of this information in more detail, give me a call at 784-5412.