SUBTERRANEAN ESTUARIES IN KARST SYSTEMS: BIOGEOCHEMICAL CYCLING OF TRACE METALS AND NUTRIENTS

Coastal hydrogeology influences the flux of trace elements and nutrients into and out of subterranean estuaries. In many regions characterized by karst lithology, highly permeable soils result in fast infiltration of rainfall, such that there are no surface streams or rivers. As a result, there is significant submarine groundwater discharge at the coast and subterranean estuaries are present where this freshwater interacts with seawater either through induction at the boundary of submarine springs at the coast or within the regional groundwater-seawater mixing zone. This talk will focus on early results from process studies at two karst settings: the Yucatan Peninsula, Mexico and Guam in the western Pacific Mariana Island chain. A feature common to both locations was minimal non-conservative behavior of trace metals and nutrients within subsurface mixing zones at the coast and substantial fluxes of trace metals associated with dissolution of biogenic carbonates within the aquifer. The levels of trace metal enrichment appear correlated with subsurface path length, with groundwater endmember concentrations at the Yucatan site being ~2-5 times higher than at Guam. Nutrient concentrations are controlled by local inputs to the aquifer and biogeochemical processing during transport to the coast appears limited, most likely due to rapid transport times coupled with high water-rock ratios (which limit bacterial abundances for lack of suitable attachment sites).