SCRATCHING THE SUBSURFACE: GPR CHARACTERIZATION OF BIOTURBATION STRUCTURES WITHIN A DELAWARE RIVER BLUFF, PENNSYLVANIA

Burrowing and trampling by animals produces a diverse suite of near-surface disturbances, which affect sediment properties, water flow and infiltration, ground vegetation patterns, and other critical-zone processes. At Washington Crossing Historic Park, eastern Pennsylvania, the distribution and extent of biogenic structures within fluvial deposits were imaged using 500 MHz and 800 MHz ground-penetrating radar (GPR) antennas. Field surveys extended to the edge of a high bluff along the Delaware River and were conducted both before and following a moderate snowfall. The latter helped reduce minor elevation changes and signal noise induced by ground vegetation, with repeat surveys confirming the persistence of near-surface anomalies. Parallel (pseudo-3D) grids captured subsurface expressions of two prominent bioturbation modes: 1) functioning burrows near the top of bluff face (>1 m long, 15-30 cm in diameter) and 2) surficially expressed (interface) mole tunnel-mound systems (>2 m long, 10-15 cm wide). Whereas areas dominated by compact fine-grained material partially attenuated the electromagnetic GPR signal, the dielectric contrast between sand-rich matrix and air-filled tunnels was sufficient for discriminating between subsurface targets. In addition to animal-produced structures, tree roots ranging from 2 to >10 cm in diameter (some trees >150 years in age) generated clear point-source diffraction hyperbolas. Although spatial relationships are often sufficient for differentiating burrow tunnels and large roots in pseudo-3D depth slices, reflection amplitude analysis may be necessary for their accurate discrimination in 2D sections. Individual burrow complexes, phytoturbation, and an occasional tree fall account for rapid removal or mixing of 0.2-0.7 m³ of bluff sediments. In addition to drawing attention to the geomorphological impact of biogenic activity on riverbank stability, our findings have implications to the geological record of bioturbation in fluvial settings and to conservation efforts at an important historical burial ground.