Surface water quality in the Sunapee watershed, as in other parts of the northeast, has been impacted heavily by unusually high levels of precipitation for three of the last four years.
Normal annual precipitation levels in the Sunapee area are about 42" to 44". 2003 and 2005 were well above normal with 56" and 66" respectively, and levels this year after seven months were already at 43". A factor in these high annual amounts has been a number of extreme storms which have caused more than normal amounts of erosion and sediment and other pollutant transport. As one might expect, water clarity as measured by Secchi disc visibility and from personal experience has decreased somewhat over this period. Volunteer Weed Watchers, particularly this past summer, have been reporting difficulty in weed watching due to very low visibility.
LSPA’s water quality data from lake samples has shown long-term trends of increasing concentrations of phosphorus, the primary growth nutrient in fresh water, and specific conductivity, a measure of salts, metals and other ions. Most of the conductivity measured here consists of dissolved chlorides from road salting. A look at tributary data from the watershed for the past ten years, not surprisingly, shows the same trends. Average concentrations of phosphorus and conductivity are higher in the tributaries than in the lake. (In-lake average phosphorus is approximately 7 micrograms per liter (µg/L) while in-lake conductivity concentrations are about 104 microSiemans (µS). The tributaries average about 15 µg/L phosphorus and well over 150 µS for conductivity.) These numbers are important to consider because well over 50% of the water supply into the lake is from tributary streams.
A look at total precipitation levels for the last decade adds another layer to the water quality picture. Aside from the drought period in 2001-2002, total annual precipitation (based on 1995-2005 state data recorded at Mt. Sunapee) has been steadily increasing. This means that not only are average concentrations of phosphorus and conductivity increasing, but the volume of water (containing these higher concentrations) is increasing. Of course, the water and nutrient budgets of a lake are not that simple. More flow into the lake means more flow out of the lake. But there is no doubt that higher precipitation levels and the number of significant storms in the last several years have impacted water quality.
Though the long-term trend of increasing conductivity continues, a preliminary look at data suggests that conductivity concentrations did decrease a little in 2006. This is very likely due to the mild and low-snowfall winter of '05-'06. With less snow and ice, less salt is applied to our roads.
Date published on web site:
12-01-2006
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