Using Hydrogeophysical Methods to Understand the Spatial Distribution of the Bedrock-regolith Interface in the Rio Icacos Watershed (Luquillo Critical Zone Observatory, Puerto Rico).

Abstract:

The Rio Icacos watershed in the Luquillo Mountains (Puerto Rico) is a unique location due to the rapid weathering rates and formation of altered materials (i.e. regolith) in the critical zone. The watershed is characterized by a quartz diorite bedrock with a system of heterogeneous fractures that apparently drive the formation of corestones and associated spheroidal fracturing and rindlets. Although spatially limited, direct observations along drilled boreholes from previous studies have conclusively shown that regolith thickness increases with topography, from 20-30 m at the hornfels-facies ridges to several m in the quartz diorite-dominated valleys to tens of centimeters on ridges near the major river knickpoint. In this study we used a suite of hydrogeophysical methods (mainly ground penetrating radar, GPR, and terrain conductivity measurements), to characterize regolith thickness and the lateral distribution of fracturing along km-long transects across topographic gradients. GPR common offset profiles detected vertical areas of concentrated chaotic reflections and diffraction hyperbolas (in contrast to reflection-free areas) associated with fracturing and corestone-rich zones. Terrain conductivity measurements also detected contrasts in electrical conductivity associated with both the presence of fractures/corestones and regolith thickness variability. This research demonstrates the potential of hydrogeophysical measurements for understanding variability of bedrock-regolith interface in the Icacos watershed at large (i.e. km) scales.