GROUND PENETRATING RADAR AND SEDIMENTOLOGICAL ANALYSIS OF AN ANTHROPOGENICALLY BURIED STREAMBED, SOUTH-CENTRAL PENNSYLVANIA: IMPLICATIONS FOR GROUNDWATER FLOW PATHS IN A SPRING-FED FLOOD PLAIN

The original goal of this study was to identify and confirm the location of a buried stream channel in a floodplain utilizing ground penetrating radar (GPR) and comparative sedimentologic analyses for a sedimentology class project. Sediment characteristics reveal information about the flow path and velocity within channel environments and are distinct from flood plain sediments. This study investigated a buried (ditched) stream channel adjacent to the restored Burd Run stream in Shippensburg, (south central) PA. Restorative anthropogenic activities nearly 20 years ago diverted flow from the now buried channel to a restored channel with natural flow characteristics to improve water quality, implement flood control measures, and improve recreational land use. GPR, guided by historic aerial photos, was used to find the approximate location and depth of the buried stream channel in four transects. Sediments were collected from the restored reach of Burd Run and from the buried stream channel via test pits. Sedimentological sieve analyses were completed and Gradistat software was employed to characterize grain size distributions.

Sediments recovered from both the buried and restored stream channels were sieved. The coarse-grained characteristics emanating from the high stream flow velocities were predominant and distinct from the floodplain silts and clays and the fill material. Dug pits allowed easy identification/confirmation of the channel within GPR radargrams. Another significant finding during the recovery of the buried stream channel sediments in test pits was the highly active subsurface groundwater flow in the coarse-sand/gravel bed even after stream/ditch burial. With higher porosity and permeability, the coarser sediments in the buried channel are still noticeably conducting groundwater despite natural soil development within the fill. Future investigations are needed to reveal the source(s) of this groundwater. Is the buried channel flow carrying a portion of the surface water diverted from the restored channel just upstream? Or is the channel carrying spring water discharged from the karst below and adjacent to the floodplain? Or both? This investigation highlights the potential role of buried stream channel gravel beds in the development of subsurface flow networks within floodplains.