Structural, Functional, and Recovery Responses of Stream Invertebrates to Fly Ash Effluent

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Abstract
Small fly ash settling ponds (e.g., 2 yr life time) should not be used after they are .apprx. 87-90% full when effluents are released into lotic systems. When a pond was > 90% full, mayflies (Stenonema, Heptagenia, Paraleptophlebia), stoneflies (Acroneuria) and caddisflies (Cheumatopsyche) were significantly (.alpha. = 0.05) reduced in density, number of taxa and diversity in the stream receiving system. Highly resistant coleopterans (e.g. Psephenus herricki) became the dominant species. Two to 10 mo. were required for the insect community to recover after cessation of ash effluent. Potential lethal effects of fly ash may have resulted from ash particles, heavy metal and other elements (especially As and Se) and alkaline pH increases > 9.0. Which fraction of fly ash was most limiting to each insect group or species needed more examination. Ash particle concentrations (total suspended solids .ltoreq. 4000 mg/L) were not acutely toxic in 96 h laboratory bioassays to sensitive mayflies (S. pudicum) or resistant coleopterans (P. herricki), nor were heavy metals (Cd, Cu, Zn) at concentrations similar to those found in the fly ash effluent, although data were lacking to evaluate long-term (e.g. .gtoreq. 28 days) effects. Alkaline pH exposures need more research, since the 96 h LC50 of 9.5 for S. pudicum was similar to pH values observed in the receiving stream during summer low-flow, high-temperature conditions when the ash pond was > 95% full.