Chemical weathering fluxes from volcanic islands and the importance of groundwater: The Hawaiian example

We investigated the products and rates of chemical weathering on the Hawaiian Islands, sampling streams on Kaua’i and both streams and groundwater wells on the island of Hawai’i. Dissolved silica was used to investigate the flowpaths of water drained into streams. We found that flowpaths exert a major control on the observed chemical weathering rates. A strong link exists between the degree of landscape dissection and flowpaths of water through the landscape, with streams in undissected landscapes receiving water mainly from surface runoff and streams in highly dissected landscapes receiving a considerable fraction of their water from groundwater (springs and/or seepage). Total alkalinity in Hawaiian streams and groundwater is produced exclusively by silicate chemical weathering. We find that fluxes of total alkalinity (often called “CO2 consumption rate” in the geochemical literature), from the islands are lower than those observed in basaltic regions elsewhere. Groundwater is, overall, the major transport vector for products of chemical weathering from the Hawaiian Islands. On the youngest and largest island, submarine groundwater discharge (SGD) transports more than an order of magnitude more solutes to the ocean than surface water and on the youngest part of the youngest island, SGD is the only link between the terrestrial weathering system and the ocean. These results suggest that groundwater, and particularly SGD, needs to be included in geochemical weathering budgets of volcanic islands.