Monitoring geochemistry in the Valley of Desolation/Boiling Lake, an active hydrothermal system in Dominica

Dominica is a volcanically active island in the Lesser Antilles. The degassing of the magma chamber affects the surrounding hydrologic system. We are continuing the work of Joseph et al. (2011), Metzger et al. (2015, 2016) and DeFranco et al. (2016), focusing specifically on the Valley of Desolation, a hydrothermally active valley, featuring the Boiling Lake. Water samples for chemical and isotopic analysis were collected on June 18th, 2016 from 12 locations. Samples sites included flowing streams, fumaroles, and the Boiling Lake. Fumaroles are characterized by their acidity (pH <2.6), high temperatures (>88OC), enrichment in TDS (>2000 ppm), and heavier isotopic values (δ18O: 3.7-7.6 ‰ and deuterium: 10.9-20.5 ‰). Hydrothermal streams, despite being only meters from these fumaroles, are characterized by their intermediate acidity (pH ~4), milder temperatures (~40 ˚C), lower TDS (<500 ppm), and lighter isotopic values (δ18O: -1.5 to -0.5 ‰ and deuterium: -1.4 to 1.1‰). Water from fumaroles is classified as acid sulfate, whereas Boiling Lake is classified as natural chloride.  Hydrothermal stream samples are both acid sulfate and bicarbonate. Year 1-1 comparisons of geochemistry data show that there is no appreciable difference from 2015 to 2016. DIC values show little variation (2‰ to -4‰) whereas  δ18 O (-0.5 ‰ to 7.6 ‰) and deuterium (-1.4‰ to 20.5 ‰) show greater variability. The carbon isotopic values are derived from degassing of CO2 magma to water whereas the deuterium and oxygen isotopes are derived from evaporation of meteoric water. Therefore degassing appears to be relatively similar throughout the valley whereas evaporation is extremely variable. Continued monitoring of the Valley of Desolation is needed to understand the complex hydrothermal activity, and why chemical signatures may be associated with lake draining.