USING HANDS-ON GEOLIMNOLOGY AS AN EXERCISE FOR TEACHING ENVIRONMENTAL GEOCHEMISTRY

We have been incorporating a combined field and lab-based exercise as part of an upper-division environmental geochemistry class. Each Fall, following lectures on water geochemistry, water pollution, and geolimnology, we take our students to Louisa Pond for hands-on investigation. Louisa Pond is a shallow, polymictic, eutrophic lake with a small natural catchment situated atop Shaupeneak Ridge in the town of Esopus, New York. Our students examine the watershed surrounding the lake making notes on the bedrock (Ordovician greywacke) and vegetative cover (mixed forest), and then sample the water chemistry. Probes are used to measure temperature, dissolved oxygen, pH, nitrate concentration, and electrical conductivity. We then use a Russian Peat borer to sample the organic sediments down to about 3 m. As a contrast we provide the students with a box core taken in about 60 m of water from the northern end of Seneca Lake; a large, deep, monomictic Finger Lake in central New York. The sediment samples from both lakes are analyzed in the laboratory for organic carbon/carbonate using LOI, and Pb concentrations using flame AA.

While there are some small variations in the student data from year to year, the overall results have been generally consistent. The measured pH of Louisa Pond averages around 6, as the local bedrock provides very little buffer for the precipitation. Seneca Lake, on the other hand has a pH closer to 8, as there are Devonian carbonate rocks in the watershed. The upper few meters of Louisa Pond sediments are organic rich (35-45%) and carbonate poor (2-5%), while Seneca Lake sediments are the opposite (3-6% organic carbon and 4-20% carbonate). Both coring sites are relatively close to the New York State Thruway, and have very similar lead concentration curves showing a rapid increase to a peak, followed by a rapid decline. Our students realize that this is the signal for increased automobile use in the region followed by the disuse of leaded gasoline in the mid-1970’s. With this insight our students are then able to estimate sedimentation rates. They come up with about 0.8 cm/year of organic rich sedimentation in Louisa Pond and about 0.3 cm/year of mostly inorganic sedimentation in Seneca Lake. Their calculated sedimentation rate in Seneca Lake is the same as that calculated using radiometric methods.