Reservoir siltation and quaternary stratigraphy beneath the Mactaquac headpond as revealed by acoustic and ground penetrating radar sub-bottom imaging

Abstract

The Mactaquac Hydroelectric Generating Station, located on the Saint John River in New Brunswick, Canada, is approaching the end of its life due to deterioration of the concrete structures. As part of an aquatic ecosystem study, designed to support a decision on the future of the dam, physical and chemical characteristics of sediments beneath the headpond, extending 80 km upriver, are being examined. The focus of this sub-study lies in (i) mapping the thickness of sediments that have accumulated since inundation in 1968, and (ii) imaging the deeper glacial and post-glacial stratigraphy. Acoustic sub-bottom profiling surveys were completed during 2014 and 2015. An initial 3.5 kHz chirp sonar survey proved ineffective, lacking in both resolution and depth of the penetration. A follow-up survey employing a boomer-based "Seistec" sediment profiler provided better results, resolving sediment layers as thin as 12 cm beneath up to 40 m of water, and yielding coherent reflections from the deeper Quaternary sediments. Post-inundation sediments in the lowermost 25 km of the headpond, between the dam and Bear Island, are interpreted to average 20 cm in thickness, corresponding to an average reservoir sediment accumulation rate of 4.3 mm/year. The thickest deposits (up to 60 cm, corresponding to a sedimentation rate of 13 mm/year) are found in deep water areas overlying the pre-inundation riverbed west of Snowshoe Island, and south and east of Bear Island. A coring program confirmed the presence of silty sediment and showed good correlation with the Seistec thickness estimates. In the ~15 km stretch upriver of Bear Island to Nackawic, the presence of gas in the uppermost sediments severely limits sub-bottom penetration and our ability to interpret sediment thicknesses. Profiles acquired in the uppermost ~40 km reach of the headpond, extending from a few km upriver of Nackawic to Woodstock, show a strong, positive water bottom reflection and little to no sub-bottom penetration, indicating the soft post-inundation sediment in this region is either absent, or thinner than the 12 cm resolution of the Seistec profiler. Deeper reflections observed within 5 km of the dam reveal a buried channel cut into glacial till, extending up to 20 m below the water bottom. Channel fill includes a finely laminated unit interpreted to be glaciolacustrine clay-silt and a possible esker – similar to stratigraphy found 20 - 30 km downriver at Fredericton. A small scale survey was conducted near Nackawic to evaluate the suitability of waterborne ground penetrating radar (GPR) profiling as an alternative to acoustic profiling in areas of gas-charged sediment. The GPR was able to image thin sediments in the area, showing sediment thicknesses of less than a meter. However due to rapid attenuation of the GPR signal in the water column, this survey method was only viable in the relatively shallow areas near the shoreline, less than 15 meters water depth.