THE ARCHITECTURE OF DEEP WEATHERING AT TWO CZOS: GROUNDWATER, REGOLITH, AND SOLUTES AT SUSQUEHANNA SHALE HILLS AND LUQUILLO EXPERIMENTAL FOREST

To study processes in the deep critical zone, we have undertaken drilling projects to return samples of deep regolith and weathering fluids. Samples from the deep critical zone at both the Susquehanna Shale Hills (PA) and Luquillo (P.R.) Critical Zone Observatories (CZO) confirm the complex architecture of weathering environments and underscore their relationship with groundwater hydrology. Lithology is a major control on these processes: fracture density, orientation, and mineralogy are important components of this lithological control. At both CZOs, we hypothesize that a relatively deep oxidation front is the leading reaction that is capable of initiating deep weathering by increasing bedrock porosity and lowering pH of the weathering fluids. In the Luquillo CZO, these efforts are focused on the Río Icacos watershed where quartz diorite is weathered in fractures at depth, transforming to concentrically weathering corestones and eventually saprolite at decreasing depths. Water chemistry from three groundwater wells in the corestone zone (~15 m depth) indicates different extents and types of water/rock reactions taking place in close proximity. To constrain groundwater residence times we have begun to study anthropogenic tracers (CFCs, SF₆, ³H), which indicate that the sampled groundwater has spent <20 years in the subsurface. The groundwater characterized here is similar in chemistry to that of baseflow of the Río Icacos, but concentrations of Mg and Si in groundwater are not completely consistent with baseflow concentrations. We interpret this to indicate that a pool of groundwater with low water/rock ratios has not been sampled, and must contribute to solute flux in Río Icacos baseflow. This might result from unsampled groundwater environments or from longer groundwater residence times in environments similar to those sampled. At Shale Hills CZO, groundwater sampled in the valley floor at ~15 m depth has significantly higher solute concentration than the adjacent stream. Here, groundwater in fractured shale has a flow weighted mean age of ~25 years whereas water in the stream has a residence time of less than one year. If the efflux of this groundwater in the subsurface is similar to the stream, weathering and solute export may be dominated by groundwater at Shale Hills.