Chemical Changes of Lakes Within the Mount St. Helens Blast Zone

Abstract

Differences in the dissolved chemistry of lakes devastated by the 18 May 1980 eruption of Mount St. Helens are attributable to location relative to the lateral blast trajectory of the eruption and to the emplacement of mineral deposits. Elemental enrichment ratios of pre- and posteruption measurements for Spirit Lake and comparisons of the chemical concentrations and elemental ratios for lakes inside and outside the blast zone reflect the influences of the dissolution of magmatic and lithic deposits. The pH changes were minor because of buffering by carbonic acid and reactions involving mineral alteration, dissolved organics, and biological processes.

Limnology of two new lakes, Mount St. Helens, WA

“…In terms of cations, concentrations of calcium, magnesium, and sodium were increased in all blast-zone lakes up to forty times the levels found in the lakes unaffected by the blast (Wissmar et al 1982a); potassium was elevated as well, although most markedly in Castle Lake. Levels of manganese and iron were also increased, generally by one to two orders of magnitude, although iron in Castle Lake was increased an additional order of magnitude higher than in the other two blast zone lakes for which data is available.…”

“…Initial studies of the lakes on Mount St. Helens were begun late in June, 1980, and assessed the impact of the eruption by comparison of lakes located within the blast zone, defined as the area contained within a radius of 16-28 km north of the crater, to lakes outside of the blast zone, considered to have been unaffected by the eruption (Wissmar et al 1982a). Both Coldwater Lake and Castle Lake (referred to as North Coldwater Lake and West Castle Lake in the above paper) are located within the blast zone.…”

Changes in the organic material in lakes in the blast zone of Mount St. Helens, Washington

“…The inorganic chemistry of blast-zone lakes was altered by dissolution of magmatic and lithic deposits (Wissmar, Devol, Nevissi, and Sedell, 1982). Increases in major cations, anions, and trace metals generally were greatest in the first group of lakes associated with mudflow, debris avalanche, and pyroclastic deposits, as illustrated by selected pre-and post-eruption data for Spirit Lake (table 1) obtained by Dion and Embry (1981).…”

“…The increase in the concentration of dissolved organic carbon (DOC) in blast-zone lakes has been discussed by Dion and Embrey (1981), Wissmar, Devol, Nevissi, and Sedell, (1982), Pereira and others (1982), and McKnight and others (1982). During the summer of 1980, DOC concentrations were greatest in South Fork Castle, Coldwater and Spirit Lakes.…”

“…The volume of the lake increased by about 14 times during this interval, which indicates that dissolution of volcanic deposits in the watershed continued to be a source of dissolved inorganic constituents. The changes in the inorganic chemistry of lakes in the second group varied with the position of the lake relative to different blast deposits (Dion and Embrey, 1981;Wissmar, Devol, Nevissi, and Sedell, 1982). Lake, March 28, 1980, andJune 30, 1980 [Date from Dion and Embry.…”