AN INVESTIGATION INTO THE COMPOSITION OF SEDIMENT DEPOSITED IN TWO VERMONT PONDS DURING THE SUMMER OF 2023 AND ITS IMPACTS ON WATER QUALITY

The summer of 2023 was the wettest summer central Vermont has experienced in 75 years. To assess the impacts of increased runoff on receiving water bodies, sediment cores and water quality measurements were taken from Twin Ponds and Rood Pond, located in Brookfield, Vermont. Water quality data included dissolved oxygen (DO%), chlorophyll a (chla RFU) and total phosphorus (P ppb). Twin Ponds was more productive with higher P and chla concentrations than Rood Pond. When compared to averaged data from previous years, P values were higher in both ponds during 2023. Rood Pond showed a larger increase in P compared to previous years, matching a trend documented in VT oligotrophic lakes. Sediment cores were taken adjacent to the pond inlets, using a gravity corer. The top 4 cm of each core was sampled at 0.5-1.0 cm intervals and analyzed for loss on ignition, grain size and elemental concentrations using an ICP-OES. Two additional samples at 9.5 and 21.5 cm were used to confirm downcore trends. Sediment from Twin Ponds was higher in organic carbon (32-41%) and had less sand (38-79%) versus Rood Pond which had 5-36% organic carbon and 80-96% sand. Elemental analyses indicate that both ponds have experienced an increase in the deposition of elements derived from the weathering of rocks (Fe, Al, K, Ti, Mg) and show matching peaks within the top 4 cm of the core. Rood Pond had consistently higher concentrations of these erosional elements with the exception of Fe, which was equivalent in both ponds at the sediment water interface (SWI). Phosphorus sediment profiles were used to assess the mobility of P between sediment and the water column. Twin Ponds P concentrations (ppm) at the SWI were twice as high (5.1 ppm) as those at Rood Pond (2.3 ppm). Phosphorus values decline sharply at Twin Ponds between the SWI and 3 cm, then continue to decline more gradually to the base of the core (2.2 ppm). Rood Pond P values are nearly constant for the entire core (2.3-2.6 ppm). These results suggest that sediment P at Twin Ponds is more mobile and is a source of P to the water column. Whereas in Rood Pond, a constant P profile indicates burial of stable P minerals. These differences are attributable to higher organic content at Twin Ponds and the decomposition of organic P. Higher concentrations of Al in Rood Pond may also provide a greater rate of P mineral precipitation.