Catastrophic and persistent environmental consequences of an acid crater-lake discharge, Chiginagak Volcano, Alaska

In early summer 2005, acidic, metal-laden water and clay-rich debris flooded down the flanks of the Chiginagak Volcano, Alaska. The estimated 3-million-m^3 discharge occurred during a rapid partial draining of a newly formed summit crater lake. Twenty-seven kilometers downstream from the source, Mother Goose Lake, a 500-million-m3 lake, was acidified from surface to bottom （pH ranged from 2.9 to 3.06）. All aquatic life was killed and salmon spawning runs were eliminated. The flood was accompanied by a simultaneous release of caustic volcanic gas and aerosols that followed the flood flow-path, and caused defoliation and necrotic leaf damage to vegetation in a 30-km^2 area along and above affected streams. In September 2005, approximately 1.4 million m^3 of acidic water remained and continued to drain from the summit crater lake. Water sampled 8 km downstream after considerable dilution from glacial meltwater was a weak sulfuric acid solution （pH-3, SO4^2- ～500 mg/L, and SO4^2-/Cl～10）. Cation proportions decreased in the order of Ca〉Fe〉Al〉Mg〉Na〉K. The eightfold predominance of Ca^2＋ over Mg^2＋ may reflect some dissolution of anhydrite, but only about 30% of the SO4^2- can be balanced by Ca^2＋. The anomalously high concentrations of Fe （-25 mg/L） and A1 （-18 mg/L） are derived from rock dissolution under the acidic conditions within the crater lake, and appear to be the most serious environmental threat in terms of metals. Further downstream, at stream and lake sites ranging from 8 to 40 km from the source, concentrations of cations remained high and hazardous metals such as As, Cr, and Cu were elevated well above background levels. Copper concentrations exceeded the U.S. Environmental Protection Agency＇s chronic exposure limit for aquatic life at most sampled sites and several sites exceeded the acute exposure limit.

Catastrophic and persistent environmental consequences of an acid crater-lake discharge, Chiginagak Volcano, Alaska