GROUND PENETRATING RADAR SURVEYS OF LARGE POINT BARS ALONG THE SANDY RIVER, WESTERN MAINE

The Sandy River originates in the western mountains of Maine and is characterized by numerous large sand and gravel bars along its banks from its middle reaches to its outlet into the Kennebec River. Historically, sand and gravel were mined from several point bars along this river, but permits were suspended for this activity about ten years ago. The possibility of renewed approval for sand and gravel mining indicated that additional information, especially sand and gravel volume estimates, would better inform this process. Using GPR data, this project will quantify sand and gravel volume in these point bars and can estimate the percent removed in different mining scenarios.

Using the MALA GPR system, 100 and 500 MHz profiles were collected along the longitudinal axis of each bar as well as several cross-lines. Topographic data for each line were collected with a laser theodolite. These data were used to correct the surface elevation of the line, allowing for more careful interpretation of subsurface features. Historic aerial photo imagery (1951 – 2002) and repeat high-resolution topographic surveys 2002-2006 show patterns of erosion and deposition at the study sites. The data were processed using RadExplorer software. The primary targets were the water table, basal sand contact, and bedrock presence (acting as a pinning point in bar migration). The water table for all four sites is identified by a high amplitude reflection. Both the basal sand contact and reflectors showing internal structure of fluvial sand deposits can be identified on profiles. At Farmington Falls, an application has been submitted requesting a permit for sand and gravel removal beginning in winter 2011-12. The sand bar here has a basal sand contact at a depth of about 2.5 m. Because the submitted mining application specifies removing material to approximately 20 cm above the water table, the percent of gravel that might be removed during this round of mining can be estimated. There will be future GPR surveys done at the Pillsbury bars to see the changes of the meander after the gravel removal job is complete.