How Is Our Lakes’ Water Quality Classified by State of Vermont?
Over 50 years ago, when the US Clean Water Act was passed, states were required to classify lakes and streams according to their water quality and to protect those with good water quality for normal lake uses. Those with higher water quality were afforded more protection.
Numerical thresholds for several water quality parameters were established as criteria for these classifications. By default, all Vermont lakes at an elevation of less than 2500’ were designated Class B lakes. Lakes with a 5-year history of numerical values warranting a higher classification can petition the state to be reclassified – providing more protection.
Three of the parameters used in classification are routinely monitored through the Vermont Lay Monitoring Program. These are TP (total phosphorus concentration, more TP means more muck, more weeds, and increased risk of algae blooms), Secchi depth (a measure of water clarity with high values being good) and Chla (Chlorophyll-a indicates the concentration of algae in the lake). Trends in these parameters for Lake Dunmore are shown below, along with the numerical threshold between A and B classifications.
https://vermont-lakes-and-ponds.shinyapps.io/vt-lake-data/
There is considerable year-to-year scatter in these data, but if the recent 2-year trend in TP and Chla hold for more years, these are signs of water quality improvement that should also impact water clarity. In this scenario, a petition for reclassification may be considered.
Notes:
1. Values for Lake Dunmore and Fern Lake represent averages of 6 summer samples for secchi, phosphorus, and chlorophyll taken in the deepest section of the lakes. E-coli samples were taken regularly at Branbury State Park and occasionally at various locations on Fern Lake.
2. High values for phosphorus and chl-a for Fern Lake are due to a release of legacy phosphorus during the summer stratification. This concentrates through the summer in a relatively small fraction (<10%) of the lake volume and feeds algae in this deep region. This fully mixes with the rest of the lake volume during the fall turnover.
