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 and the degassing of the magma chamber affects the surrounding hydrologic system. Joseph et al. (2011) stated that continued chemical monitoring of the thermal discharges could provide valuable information in understanding the dynamic processes occurring in hydrothermal systems, and help determine if the compositional changes of water chemistry can be used as a reliable sign of volcanic unrest. We are continue 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 located northeast of Roseau, featuring the Boiling Lake (~50 m wide and ~15 m deep), the second largest volcanic crater-lake in the world. The Valley experienced a phreatic eruption in 1997 and experienced a lake draining event in early 2004 and early November, 2016. During lake draining events, the normally acidic (pH 5-3) and hot water (>80˚C) recedes and the lake eventually refills with meteoric water, taking several months to show the influence of heat and gasses from the underlying system. Unlike in 2004, the lake draining event in 2016 was not preceded by any earthquake activity. Field and water samples for chemical and isotopic analysis were collected on June 18^th, 2016 from 12 locations. Samples sites included flowing streams, fumaroles, and Boiling Lake. Fumaroles are characterized by their acidity (pH <2.6), high temperatures (>88^OC), enrichment in TDS (>2000 ppm), and heavier isotopic values (¹⁸O: 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 (¹⁸O: -1.5 to -0.5 ‰ and deuterium: -1.4 to 1.1‰). Time series analysis of the Boiling Lake suggests that lake draining events may be preceded by chemical changes, including a decrease in pH (<3.8), and an increase in brine character ( Cl: >3500ppm , Br: >10 ppm, Na: >1500 ppm). In 2004, SO₄ values were high (4204 ppm), but since 2009 have not exceeded 56 ppm. 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.