The Hydrological and Geochemical Role of the C Horizon in a Glacial Till Mantled Headwater Catchment

The C horizon is the deepest soil layer that is technically unweathered, similar to the rest of the regolith beneath the bottom of the pedon. In New England, the C horizon formed from the retreat of the glaciers eroding bedrock and depositing an unsorted and unstratified glacial till on the surface. This research evaluated the hydrologic and chemical role of the C horizon, inclusive of underlying regolith, in a forested watershed at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire. Results suggest that the C horizon at HBEF is extensive and heterogeneous. Traditional concepts of basal till offer a limited role for the C horizon in watershed hydrology, but the C horizon appears to make a significant contribution to the hydrologic cycle in this watershed system. Hydraulic conductivity across B/C horizon boundaries does not decrease with depth indicating a hydrologic contribution of the C horizon to watershed hydrology. There was no evidence of marked differences in mineralogy among study sites based on weathering studies, although no conclusions could be drawn about the chemical contribution of the C horizon without further analysis and information on mineralogy. However, the presence of water flow paths through the C horizon means that this zone in the regolith contributes to watershed hydrology, and causes us to consider the effects of mineral weathering in the C horizon on deep water and its relative contribution to surface water chemistries. Future studies should be conducted with the aim of refining our understanding of the mineral composition of the C horizon and its relative importance in ground water chemistry