Why did the SLA have a ‘core sediment analysis' done?
The Core Sediment Analysis, i.e., the Paleolimnology report, was done to meet the first objective and first action item of the Watershed Management Plan, which were respectively to "investigate the cause of low dissolved oxygen in Spofford Lake" and “determine the cause and extent of dissolved oxygen impairment and inform any adjustments to the water quality objectives." Taken together, the Watershed Management Plan and the Core Sediment Analysis form foundational pillars for maintaining and improving Spofford Lake’s water quality and informing any remedial actions.
What is the Report's goal?
"To identify probable causes for deteriorating water quality conditions as described in the Spofford Lake watershed management plan."
Who authored the Report and when?
The Report was joint effort of Lisa Doner PhD and William Tifft and Plymouth State University's Sedimentology Lab. Samples were taken in October 2018 and the report issued in April 2020. The Report was paid for by donations from Spofford Lake Association members.
What are the Report's conclusions?
The report concludes that decaying organic material eroded from the shoreline is the cause of the Lake becoming ‘impaired’ for aquatic life. See pages 15 and 16 for these conclusions and the preceding pages for the underlying science.
The report goes on to note that after 1910 and after 1980, there has been more organic material in the accumulating sediments, and the amount of organic material reaching the lake bottom exceeds that of any other period since 1700.
Since 1980, sediment characteristics have changed more than any other time in the past several hundred years and that the increased organic deposits are associated with increases in oxygen impairment in the (Lake's) deep basin.
It goes on to note that these time frames coincide with periods of shoreline and watershed development, higher damming of the Lake, and the advent of more powerful watercraft.
Why is this important to me, a homeowner or a Spofford Lake lover?
Oxygen impairment is an important by-product of a nutrient rich plant environment which fosters, among other plants, algae and cyanobacteria blooms. Oxygen impaired environments are harmful to some fish species, particularly game fish such as trout and salmon. As these fish seek colder, deeper water in summer, there is too little oxygen to reproduce and survive. A robust fish population is one factor in a healthy lake, which itself is an underpinning of strong property values and a measure of the Lake’s overall recreational value.
What's the connection between shoreline erosion, increased sedimentation and oxygen impairment?
The increased sedimentation carries more organic matter into the deeper areas of the Lake. Even decay in shallower areas eventually finds its way to the “deep”spot.”
Bacterial respiration of this organic material and other chemical reactions consume oxygen. As dissolved oxygen is depleted, further reactions occur that liberate phosphorus, manganese, and other substances from the sediment. Too much phosphorus in the water column often leads to a nutrient rich environment for plants, accelerating algal growth, more oxygen impairment, blooms, poor water clarity and further declines in fish and other aquatic species.
This cycle, called “internal loading' occurs and continues when sufficient phosphorus and other minerals mix in the water column each spring and fall during lake 'turnover' to stimulate the plant growth cycle.
Lakes are essentially traps for sediment and thus reflect the activities of its watershed. Internal phosphorus loading is, in essence, the lake recycling phosphorus derived from the watershed. More sedimentation will yield more phosphorus which in turn exacerbates oxygen impairment and stimulates conditions conducive to plant growth…a virtuous circle.
Core Sediments from 4 locations were analyzed to find the causes of the Lake's low oxygen levels
Sediment Lead Levels: The Core Sediment Analysis also found an increase in lead levels in the lake sediment from 1980 through 1998. (See pages 14, 15 & 17 in the above report.) The lead source, according the analysis, is most likely leaded gasoline leaking from an underground tank. Lead concentrations in Spofford Lake reach a peak in 1996 and show a much different pattern than those of other lakes. Spofford Lake's lead levels then fall in subsequent years, mirroring the profile of these other lakes.
A toxicologist with the NH DES Environmental Health Program subsequently reviewed these findings. His conclusion is that "Based on the available data there doesn't appear to be a health risk due to sediment contact..."
Page updated April 19, 2021