Lay Monitoring Water Clarity Report to Groton Lake Association, 2008

For a number of years we have been measuring water clarity of the pond during the summer season. We do this as volunteers to the Department of Environmental Conservation, which collects similar data from all over Vermont. Since 2005, we have also been taking samples for water quality testing by the State of Vermont.

Using a SECCHI disk, we take clarity readings in designated spots in the southern and northern parts of the pond. The disk is a heavy steel plate painted in black and white quadrants that we lower into the water. We measure how deep the disk can be lowered and still be seen.

At the northern location, which is at or near the deepest part of the pond (about 45 feet), we collect samples from twice as deep as SECCHI visibility, usually 6 to 8 meters or so. We send the samples to Montpelier for testing for phosphorus, and we also send filter samples for chlorophyll content.

We take readings and samples about once a week from June through August, at least 9 times a season. The following are this year’s readings (in meters [1M = about 3 1/2 ft.]):

Date Station 1 (north) Station 2 (south)

6/15 3.4 3.4
6/20 3.6 3.8
6/27 3.8 4.1
7/5 3.4 3.4
7/15 3.5 3.1
7/22 3.0 3.6
8/1 2.6 2.6
8/17 2.7 2.4
8/23 2.8 2.7
8/30 2.8 3.0

Average water clarity in 2008 declined to 3.19 meters, compared with 2007 (3.46 meters), 2006 (3.17 meters), 2005 (3.56 meters), and 2004 (3.58 meters). This year’s heavy rains and runoff are probable culprits, with the mid 3’s and 4’s readings of the first half of the summer dropping to the mid 2’s in late July. Water temperature can also be a factor, enhancing the growth of algae (chlorophyll content).

On the next page is a table showing readings since 1999 (also 1979-1988 for comparison), when sampling began at Groton Pond. We will update this chart each year as we receive the data from the State of Vermont. We are also posting these results on the Groton Pond website.

We’re looking forward to continuing this project in the coming year.

David & Cathy Donath
August 30, 2008

Annual Average Water Quality Results – Groton Pond

Year Days Secchi Chloro-a Summer TP Spring TP
Sampled (m) (ug/l) (ug/l) (ug/l)

1979 6 4.0
1980 10 3.6 8.0
1981 9 3.7 7.0
1982 12 4.0 9.0
1983 4 10.0
1984 13 3.0 3.6 6.0
1985 13 3.9 3.9 7.0
1986 14 3.5 2.6 8.0
1987 13 3.9 3.2 16.0
1988 21 4.3 2.5

1999 11 3.7 6.8
2000 10 3.4
2001 11 4.3
2002 8 3.2
2003 8 3.7
2004 9 3.6
2005 9 3.6 3.4 12.0 6.5
2006 9 3.2 4.1 12.0 9.8
2007 10 3.4 6.7 12.0
2008 10 3.2

Key: Secchi refers to visible water clarity (depth in meters).
Cloro-a refers to the sampled amount of chlorophyll.
Summer TP refers to the level of phosphorus in the summer samples.
Spring TP refers to the level of phosphorus in the spring samples.

[Amy Picotte, 2007 Vermont Lay Monitoring Report (Waterbury: ANR, 2007), p. 69]