WATER GEOCHEMISTRY ANALYSIS OF STREAMS AND HYDROTHERMAL WATERS IN DOMINICA, LESSER ANTILLES

The island of Dominica (750 km²) is located in the center of the 850 km long Lesser Antilles island arc in the Caribbean and is dominated by nine young Pleistocene volcanic centers. Dominica has one of the highest mean annual precipitation rates in the world with rates of 10,000 mm yr^-1 in the island interior and has one of the highest stream densities with approximately 365 meteoric streams. Additionally, there are multiple hydrothermal water sources including hot springs, fumaroles, and mud pools found in association with four of the volcanic domes: Valley of Desolation, Watten Waven/Micotrin, Plat Pays, and Morne aux Diables. The Penville cold springs are located at the northern end of the island on Morne aux Diables. This activity could indicate degassing of the underlying magma chamber. Monitoring variations in stream and hydrothermal water geochemistry associated with the magmatic system can help determine the influence of degassing in the magma chamber to track changes in volcanic activity. In June 2014, water data was obtained for 72 streams and hydrothermal waters across Dominica. Water was collected from small streams (22), medium to large rivers (35), thermal streams (7), cold springs (5), hot springs (2), and a mud pool (1). Temperatures range from 20.4 to 97.2˚C, with most meteoric water at ~25˚C, most cold springs at ~24˚C, and the majority of hydrothermal water at ~40˚C. The pH ranges from 2.25 to 8.28 with the majority of the acidic water associated with hydrothermal water, cold springs, and volcanic areas. The hydrothermal waters are chemically unique from the meteoric streams in that they generally have higher TDS with three samples containing TDS above 1000 ppm. Hydrothermal waters also have higher concentrations of SO₄, Mg, and Ca, with the most hydrothermal waters at ~400 ppm SO₄. All hydrothermal waters are immature, indicating they are at equilibrium with the host lithology, based on Na-K-Mg^1/2 ternary plots. Hydrothermal streams include both acid sulphate waters and bicarbonate waters based on Cl-SO₄-HCO₃ ternary plots. Ongoing investigations of carbon, oxygen, and deuterium isotopic analysis can further help categorize Dominica water sources, determine magma outgassing rates, determine the contribution to the total carbon flux, and differentiate between magmatic and biogenic weathering.