An intact soil-core microcosm and static exposure system were used to evaluate the potential ecological effects of two obscurant smokes, red phosphorus/butyl rubber (RP/BR) and white phosphorus/felt (WP/F), used in the field during U.S. Army training exercises. The terrestrial microcosm technique differed from the test protocol prepared for the U.S. Environmental Protection Agency by using a preliminary plant stress-ethylene test to help select the dose range for microcosm testing, and by combining aspects of traditional range-finding and definitive tests into a single microcosm test.
Three plant species (white sweetclover, perennial ryegrass, and wheat) were used for both preliminary and microcosm tests. The stress-ethylene tests indicated that extremely high doses of either smoke would be required to elicit a response in the microcosm test. This information guided selection of microcosm test concentrations. Microcosms were exposed to either RP/BR or WP smoke at target concentrations of 0, 100, 300, 600, and 1500 mg/m3. These concentrations bracket typical field concentrations.
Minor ecosystem-level effects were detected in microcosms only at the highest smoke concentration and included increased nutrient (Ca) loss in leachate, increased (wheat) or decreased (sweetclover) biomass yield, and increased element (Al, As, Pb, and P) uptake in plant tissue. No negative ecological effects of either smoke were detected at smoke concentrations equal to or below 600 mg/m3, even for 16 semiweekly exposures over an 8-week period.
It was concluded that: (1) deployment of these two smokes at typical field concentrations is unlikely to cause significant problems to most terrestrial systems; (2) the terrestrial microcosm system appears to be appropriate for evaluating the static, long-term, particle-deposition effects of other aerosols; and (3) the test design produces input for hazard assessment at considerable savings in cost and time compared to the more traditional range-finding plus definitive test strategy.