GEOPHYSICAL ANALYSIS OF A POORLY DRAINED GLACIAL DELTA PLAIN IN ACWORTH, NEW HAMPSHIRE

The Cold River, NH watershed contains glacial outwash sediments deposited during the retreat of the Laurentide Ice Sheet. Abundant glacial deltaic deposits are expected to drain well due to high permeability of sands and gravels, but one particular location poor drainage is observed. Geophysical analysis was implemented to understand the local subsurface stratigraphy in light of this local drainage anomaly. Ground penetrating radar (GPR) was used to collect data at 200MHz and 400MHz frequencies along a grid with line spacing of 15m. A SYSCALKID resistivity meter hooked to 24 electric nodes and 2 cable wires was used to quantify electrical resistivity along two of the GPR lines. To groundtruth geophysical data a square-rod piston corer was used to take a sediment sample (length = 0.53m) at the intersection of two GPR lines. GPR data has 3 different facies separated by distinct reflectors. The basal reflector, presumably radar basement, varies from depths of 8.5m to 2.5m, west to east. Radar facies 3 drapes over this surface and is approximately 5m thick and is consistently shown throughout the area, peaking at the surface through the wet area. Radar facies 2 is about 2.5m thick and is only present in the dry region. Radar facies 1 is sandy at the surface and is only visible at the west edge of the GPR line, with a max depth of .5m. The resistivity data reveal relatively low values ~ 240 Ωm throughout the area with little variation. The sediment core reveals a top layer (4cm) of organic root and grass presumably due to plant growth in the wet spot. Sequentially, a very fine-grained micaceous sandy mud is present from 4-17cm into the core. The rest of the core alternates between tan sand and grey soil. Interbedded clay and sand deposits found close to the surface in thin beds may be the reason the water is not percolating through the soils and may be the source of the reflector identified near the surface. Additionally there may be a thicker widespread clay deposit just below core refusal depth where a reflector is observed. The subsurface stratigraphy is presumably the factor contributing to the poor water drainage since some facies seen in the GPR data are pinched out where the east wet areas begin. Clay beds may have been deposited due to glacial lake level fluctuations, due melt water drainage shifts, or as discreet floodplain deposits.