IMPACTS OF WILDFIRE ON VOLCANIC (LAVA TUBE) CAVE WATER CHEMISTRY

The destructive effects of high magnitude wildfires on surface vegetation and the alteration of soil properties are known and well-studied. However, their impacts on subsurface environments such as caves, shallow aquifers and vadose zone are, by comparison, understudied. Lava Beds National Monument (Lava Beds, N. California) hosts volcanic (lava tube) caves that provide a habitat for diverse groups of organic life including mammals, invertebrates and microorganisms. Recently, 97% of the surface area of Lava Beds was burned by the Caldwell (2020) and Antelope (2021) wildfires. In this study, we aim to understand the impacts of high magnitude wildfires on the chemistry of cave water by analyzing water samples from caves at burned areas at Lava Beds (collected in December 2021, May and September 2022), control caves, and comparing these data sets with pre-fire data (August 2017-2019) from these caves. Pre-fire water chemistry was characterized by pH 7.76±0.25, specific conductance 76±16 µS/cm, concentrations of Na+, K+, Ca2+, Mg2+, Cl-, NO3-, and SO42- as 7±2, 1.2±0.2, 1.7±0.8, 1±0.4, 4±0.4, 4±2.5, and 2±0.3 mg/L respectively. Post-fire water chemistry in the same cave showed pH 6.55±1.17, specific conductance 82±21 µS/cm, and concentrations of the same ions as 3±1.7, 1.1±1.2, 5.7±3.3, 2.1±0.8, 0.6±0.3, 10±6.5, and 1.1±1 mg/L respectively. These results suggested that in post-fire samples there was a decrease in pH, concentrations of monovalent ions (Na+ and Cl-), and an increase in Ca2+ and NO3- concentrations. Analyses of dissolved organic matter revealed the presence of highly humic-like material (abs254 of 2.05±0.51, fluorescence peak C intensity of 0.42±0.3 and humification index of 18 ± 5) which changed to 0.18 ± 0.18, 0.83 ± 0.73, and 11 ± 19 respectively. Drastic surficial disturbances due to wildfires alter in-cave water chemistry. We hypothesize these changes may also influence the cave ecosystem. Additional analyses of these samples to determine dissolved concentrations of trace elements, water isotopic signatures, characteristics of dissolved organic matter, surface soil geochemistry, and geochemical modeling are underway. This work was supported by NSF EAR Award # 2203517 and National Park Service CESU agreement P21AC12062-00.