GPR PROFILES OF GYTTJA, TILL AND BEDROCK BENEATH MIRROR LAKE, NH

The ecology, sedimentology and subbottom hydrology of Mirror Lake, in Thornton, NH, has been studied since the 1950s with coring in and around the lake, but without geophysical reflection profiling. Ground-penetrating radar profiles I acquired in 2013 delineate subbottom deltaic and basin formations comprised of organic-rich gyttja sedimentation, glacial till, and some bedrock, and provide permittivity values that allow estimated thicknesses. At least 26 m of till, based on a maximum possible relative permittivity of 33, was recorded because of the low water conductivity of 0.002–0.003 S m⁻¹, likely low sediment conductivity and scattering losses, and the increased antenna gain caused by the high permittivity of water. Such thicknesses agree with those in the surrounding terrain, and verify earlier seepage studies that concluded till is much thicker than the few meters previously assumed to justify bedrock fracture research. Bulk gyttja permittivity was generally 39, within which maximum penetration was possibly 16 m. A relatively large delta originates from the west side stream. Its clinoforms are draped on till, and indicate a complex history of accumulation and erosion. The delta separates and feeds subbottom accumulation basins on the NW and SE sides of the lake. The mostly unstratified and thicker NW gyttja supports earlier core-based work that concluded the gyttja was well-mixed and focused to deeper water by resuspension and transport. In contrast, the SE gyttja is continuously stratified from shore to shore, suggesting simultaneous deposition around the lake, likely caused by storms. The general sediment thickness, particularly the till, implies a far greater groundwater capacity and subbottom hydraulic conductivity than previously assumed.