THE EFFECT OF AN EXPERIMENTAL SLURRY WALL ON GROUNDWATER FLOW BY MEANS OF SEISMIC REFRACTION AND ELECTRICAL RESISTIVITY METHODS

摘要

The placement of a 100 m long experimental slurry wall in Montandon Marsh, West Chillisquaque Township, PA has triggered a series of geophysical explorations due to its proximity to a wetland. This has also offered an opportunity in this study to investigate how it may affect the local hydrology, which includes the wetland. Several geophysical techniques have been performed to explore the local shallow alluvial unconfined aquifer. Fifteen electrical resistivity with 4 m electrode spacing taken in the N-S direction parallel to the slurry wall over a length of 216m, Because the aquifer is shallow, three parallel short orthogonal electrical resistivity surveys were also conducted with 0.25 m spacing. All resistivity surveys were implemented using Dipole-Dipole array, and performed with an automated data acquisition and inversion similar to that discussed in Dahlin (2001). In Addition, a seismic refraction was conducted with 4 m spacing over 96 m along survey in the same direction as the slurry wall to investigate the thickness of the aquifer. The combination of electrical and seismic methods, in addition to field observations, has shown where the water table stands and how thick the alluvial aquifer. The fifteen-resistivity surveys performed prior to the construction of the slurry wall have shown that the aquifer is extremely heterogeneous and a depth to the bedrock varying between 5 m and 15 m. An additional resistivity survey taken five months after the construction of the slurry wall shows similar areas of saturation, but with higher resistivity values while low resistivity area has shifted down by 2 m. The slurry wall obstructing the flow has caused the saturation zone to displace south from the previous location. The combination of all electrical resistivity profiles are showing an alteration of the saturation zones and a clear impact on the groundwater flow pattern.