- Lake Forest Association (LFA) - Caretakers of Eastwood Lake in Chapel Hill, NC.
I collected our first pre-season water samples beginning at 11:50am on Wednesday May 2, 2018. The following results indicate very low levels of normal bacterial contamination.
#1 Dock 3 cfu
#2 Cedar Fork Creek 1 cfu
#3 Booker Creek 2 cfu
Caution level is individual reading above 400 cfu, or average above 200 cfu. CFU = colony forming units per 100ml.
Lake water temperature was 71F/22C at all sample sites The SECCI clarity reading was 34 inches.
We test for the same bacterial indicator organism (fecal coliform) that is present in all open bodies of water, using the same methodology that is used in public water recreation areas and drinking water supply reservoirs. The health department requires this testing if we allow swimming in the water. We are fortunate that our water quality has remained good to excellent for several years. It was more variable before OWASA built the Countyside wastewater lift station in our watershed but just outside the boundaries of our neighborhood, on Kenmore road.
Our water is warming up quickly. There were swimmers in the lake the evening before the test and on the morning of the test, although 71 seems a bit cool for me now. Where I grew up, they did not open the beaches for swimming until the water reached 72.
The water is also rapidly clarifying. One week ago the Secci reading was 21 inches, and the temperature was 64F. The SECCI turbidity test can be affected both by suspended particulates (mud, erosion) and algae or bacteria. There is a “sweet spot” that indicates a healthy ecosystem. If it is too turbid, we may be having an algae bloom or extraordinary erosion.
We have built the two forebays to manage silt from erosion which is a natural phenomenon in this part of the world. This process, called eutrophication, is the reason why there is only one naturally occurring lake in North Carolina; White Lake. At this time we are getting additional erosion from deteriorating water mains that are in our watershed but outside our neighborhood.
The other source of turbidity is algae. Typically, at this time of year, when the water warms up and everything comes back to life, the algae overdoes it then dies off. During this process the water gets cloudy first with green algae, then black with dead and dying algae as it slowly sinks. This is a normal “boom and bust” algae cycle. It usually peaks in early June, but it did not happen for the last two years. Humans contribute to this by over-fertilizing or fertilizing with the wrong formulation for “lake margins”. Algae is “nutrient sensitive” and grows rapidly when there is any additional nutrient in the warming late spring lake water.
As I said, there is a sweet spot for turbidity. Even though our lake has several springs in it as well as the two main tributary streams (Booker Creek and Cedar Fork Creek), if the clarity of the water is too great, that is not an indication of health….In this part of the world there should be some “stuff” in the water as an indication of normal biological activity. If the SECCI reading goes above 65”, I think we should be concerned…..That has happened only once since 1994, in the year before our most recent restoration. If the lake ever becomes persistently turbid (very cloudy) or very clear, we should start measuring dissolved oxygen to assess its health.
We normally test the water 4 times per year, three during the “official” swimming season: Memorial Day, Independence Day, and Labor Day, and one additional test before Memorial Day as soon as the water is warm enough to be used regularly for wading or swimming. If we have an unusually warm fall, we may also take one more test after Labor Day.
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