USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Changes in the organic material in lakes in the blast zone of Mount St. Helens, Washington
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McKnight, D.M., Klein, J.M., and Wissmar, R.C., 1984,
Changes in the organic material in lakes in the blast zone of Mount St. Helens, Washington:
U.S. Geological Survey Circular 850-L, 26p.
Abstract
The productive conifer forest that had surrounded Mount St. Helens, Washington, was destroyed by the cataclysmic eruption of the volcano on May 18.1980. Large quantities of organic material were introduced into the nearby surface waters, either as soil and plants incorporated into pyroclastic, mudflow, or debris-avalanche deposits, or as blowndown timber swept into lakes by the lateral blast of hot volcanic gases. The resulting major increases in the concentration of dissolved organic material were one of the most significant changes in the water chemistry of surface waters of the blast zone. The concentrations of dissolved organic material were greater in lakes directly associated with pyroclastic, mudflow, and debris avalanche deposits, than in lakes receiving only ash deposits and blowndown timber. In these lakes, increases in dissolved organic material were correlated with dense bacterial populations, and large increases in the concentrations of dissolved manganese, iron. and sulfur. The majority of the dissolved-organic material was higher molecular-weight, yellow organic acids similar in their general properties to aquatic fulvic acids found in most surface waters. Many of the specific organic compounds identifiable by gas chromatography-mass spectrometry were derived from the pyrolysis of plant and soil organic material.
The major changes in the organic chemistry of the blast-zone lakes between late summer 1980 and spring 1981 can be attributed to the tremendous inflow of rain and snowmelt. For example. the decrease in the dissolved organic carbon concentration of South Fork Castle Lake from 135 milligrams carbon per liter on September 11, 1980, to 10 milligrams carbon per liter on May 1, 1981, is attributable to the almost 14-fold increase in lake volume. Aquatic fulvic acids continued to be the major fraction of the organic material, and the refractory nature of humic substances is the major explanation for the persistence of the dissolved organic material in these lakes. During the spring and summer of 1981, there were some changes in the chemical nature of the aquatic fulvic acids, such as a decrease in sulfur content. There appeared to be two minor sources of dissolved organic material in these lakes. .In Spirit and South Fork Castle Lakes, the hypolimnion became anaerobic during the summer and the concentrations of dissolved organic carbon increased, indicating that anaerobic decomposition of organic material in the volcanic deposits covering the lake bottoms was a small, but continuing source of dissolved organic material. The other minor source was dissolved organic material leached from volcanic ash and dead plant material. Many of the organic compounds identified in surface waters within the blast zone in the summer of 1980 were not identified in surface waters within the blast zone in the late spring of 1981, probably because of the considerable dilution by rain and snowmelt. The few compounds that were identified in the late spring of 1981 probably were transported into the lakes with rain and snowmelt that had leached volcanic debris.
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03/07/07, Lyn Topinka