During 1999-2000, Comprehensive Environmental Inc. (CEI) completed a sediment impact study for the Pennichuck chain pond system. An evaluation was conducted of the quantity and quality of sediments in the chain ponds to determine the loss of capacity and how these sediments were contributing to water quality degradation. Long-term measurement of sediment accumulation rates was recommended.
During 2010-2011, CEI completed an update of this study to determine sediment accumulation rates in Holts Pond, Bowers Pond, and Harris Pond using data from the 2000 study for comparison. The results indicate that sediment is accumulating in the water supply pond system which over time will reduce supply capacity. Specific locations of sediment accumulation, and options to evaluate long-term sediment accumulation rates were identified.
In 2013, CEI completed a sediment investigation study to estimate sediment loadings over time using a combination of desktop calculations and the collection and analysis of sediment cores collected from bottom sediments at select locations throughout the water supply ponds. Desktop calculations were used to estimate sediment loads from the surrounding watershed and their potential sources, including roadway sanding, surrounding land uses, channel deposition, and algal die-off. Laboratory gradation and Gamma Spectroscopy analysis for radionuclides was performed on select cores to help define the approximate dates and rates of accumulation for the various sediment layers. Permanent sediment sampling gages were installed at the deep sediment core locations for long-term monitoring as provided in recommendations.