HYDROGEOCHEMISTRY OF CAVE DRIPWATERS FROM A TROPICAL OCEAN ISLAND KARST

Niue Island is a large uplifted former atoll in the tropical South Pacific Ocean where karst is pervasive in the carbonate cap. The elemental chemistry and stable isotopes of dripwaters from which speleothems are actively forming in two adjacent coastal caves were determined in order to identify their sources. This study also examined the differences and similarities between vadose (cave dripwaters) and phreatic groundwaters on the island. Water chemistry data indicate that cave dripwaters are a Na-Ca-HCO₃ type with Na, Ca and Mg as the dominant cations and HCO₃ as the dominant anion. Relatively elevated concentrations of Na (up to 327 ppm) suggest mixing of sea-spray with rainwater at the points of recharge. Differences in Mg/Ca molar ratios of dripwaters from adjacent caves suggest variability intrinsic to the specific infiltration pathways. Cave dripwaters exhibit elevated DIC values (6 ±1 mM, n=7) and are ¹³C-depleted (d¹³C=-11 ±2 per mil PDB, n=7) attesting to contribution of soil CO₂. On the basis of material balance it is estimated that about 40% of the DIC is derived from soil CO₂ and the remainder 60% from dissolution of the carbonate cap. d¹⁸O values of dripwaters (-4.6 ±0.5 per mil SMOW, n=7) are statistically indistinguishable from those of the phreatic groundwaters (-4.2 ±0.8 per mil SMOW, n=19) and are compatible with the d¹⁸O values of winter monsoon rainfall. Phreatic groundwaters sampled from 19 wells around the island are clustered into two distinct chemical groups: (1) Cl-HCO₃ water with Na and Ca as the dominant cations within 1 km of the coast and (2) Ca-Mg-HCO₃ water over most of the island interior. Carbonate speciation data indicate that phreatic groundwaters are slightly undersaturated with respect to calcium carbonate whereas cave dripwaters are all supersaturated thus explaining the active formation of speleothems in the Niuean caves.