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Undergraduate and Faculty Research
Examining Sedimentary Environments in Andover Lake
Hyatt JA, Environmental Earth Science, Eastern Connecticut State University,
83 Windham Street, Willimantic, CT 02662, USA, hyattj@easternct.edu)

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Preliminary Findings

Research thus far has mapped the bathymetry of Andover Lake, identified differing bottom types, collected sediment core from all depositional zones in the lake, and evaluated bulk sediment physical and geochemical properties of sediment sub samples.

Detailed reports on the findings from this work are provided within student final reports and a variety of other reports and maps. The following summarizes the most important findings thus far:

Bathymetry
Digital elevation model (DEM) topography for Andover Lake, constructed from 17.1 line-km’s of acoustic profiling data, provides more precise estimates of lake area and volume than has been previously available. Lake bed topographic elements include a 400 m long submerged stream channel at the north end of the lake, a broad undulating basin that is subdivided lengthwise by a discontinuous rocky and/or sandy ridge in the central part of the lake, and a shallow sandy basin at the south end of the lake. This survey indicates a mean lake depth of depth 2.9 m (9.6 ft), a lake surface area of 643,430 m2 (159 acres), and a lake volume of 1,879,090 m3 (1,523.4 acre-feet).

Bottom Conditions
Acoustic profiling identifies 4 different bottom types within the lake. Acoustically hard sandy and/or rocky bottom conditions predominate around the periphery of the lake and account for 212,400 m2 (52.5 acres) or 33.0% of the lake bed. Weedy bottom conditions are restricted to the south end of the lake near the swimming area and account for 43,100 m2 (10.7 acres) or 6.7% of the lake bed. Soft organic-rich "gyttja" sediments occur in deeper portions of the lake and adjacent to steep side-sloping basin walls occupying 252,100 m2 (62.3 acres) or 39.2% of the lake bed. Finally, a centrally located zone of brush-cover occupies 135,900 m2 (33.6 acres) or 21.1% of the lake bed.

Surface Sediment Phosphorous and Organic/Inorganic Carbon Concentrations
Analysis of fifteen surface sediment samples provides insight into variations in the concentration of carbon (organic and inorganic) and total phosphorous. Mean organic carbon and inorganic carbon (% dry weight) for all samples are 16.1% and 2.5% respectively. Mean total phosphorous is 104 ppm, more than 5000 times greater than average total phosphorous concentrations (0.02 ppm) for overlying lake water.

Spatial trends in geochemistry with lake depth, bottom type, and location in the lake are well defined for organic and inorganic carbon but are statistically insignificant for total phosphorous concentrations However, these results should be treated with caution as they are based on a small small number of chemical analyses (n=15).

Sample carbon concentrations increase with increasing water depth (organic carbon r2=0.62; inorganic carbon r2=0.72) and display weakly to strongly significant spatial trends with bottom type, lake location, and bay vs. open water sampling sites. Mean carbon concentrations: (1) are higher for samples collected from brush & soft organic bottom sites than they were for hard bottom sites; (2) differ for samples collected in the southern third of the lake as compared with samples collected from the central third of the lake; and (3) are lower for samples from near-shore "bay" sites than samples collected in open water sites.

Mean total phosphorous concentrations do not vary systematically with water depth, nor do they display significant differences when comparing mean concentrations grouped by bottom type, basin location, or bay vs. open water class.

Sediment Cores
A total of 19 sediment cores, to lengths of 1.5 meters, have been collected from Andover Lake. One of these cores is currently being analyzed to establish a Pb-210 chronology of deposition at the lake. Most cores display three stratigraphic units that include (1) pre-lake, (2) transitional, and (3) modern lake deposits.

The basal pre-lake unit consists of variably coarse sand to silty sand sediments that reflect depositional conditions prior to the formation of the lake in 1927. The character of these sediments varies with location in the lake but often contains evidence of soil forming processes and other indicators of terrestrial processes (e.g. abundant intact vascular root fragments).

The transitional is also well represented in lake cores including a mix of sandy sediments from below, the introduction of finer grained lake sediments, and an abundance of woody debris, and is thought to be detritus introduced to and redistributed throughout the lake during initial flooding.

All cores are capped with a fine-grained, generally massive accumulation of organic-rich lacustrine mud. These sediments vary in thickness with location in the lake, and are much thicker and sandier in shallow water locations than they are in deepwater settings.

Analysis of physical and bulk sediment geochemical characteristics by Brian McCann at the south end of the lake reveals that deeper pre-lake and transitional sediments are coarser grained and have lower organic carbon and raw elemental concentrations. However, the uppermost modern lake sediments, which are finer and have higher organic concentrations, are enriched in all elements including phosphorous. Nonetheless, iron-to-phosphorous ratios suggest that sufficient iron is present to retain phosphorous within sediments within the present oxygenated bottom-waters at these sites. Should dredging be undertaken, however, the sediments with the highest bulk elemental abundances will certainly be mixed within the water column.

Constrained cluster analysis of bulk sediment geochemistry results are highly correlated with the physical stratigraphy described above. Ms. Jennifer Vinci is presently analyzing all cores to develop a lake-wide depositional model and to identify spatial trends in the bulk sediment geochemistry.

(All work conducted at Andover Lake was approved by the
ALPOA and ALMA)