TFA and Seasonal Wetlands

AFEAS has sponsored studies to gauge the potential for trifluoroacetic acid (TFA) to accumulate in seasonally transient wetlands, in response to speculation that appreciable concentrations of TFA - one of the decomposition products of some fluorocarbon alternatives - could occur in seasonally transient wetlands. The events or circumstances that might contribute to enhanced levels of TFA are described below.

Factors that might cause local enhancement of TFA concentration in rain:

High sustained local emission rates of precursors, such as HFC-134a.

High local atmospheric concentrations of OH•, the naturally occurring oxidizing agent that converts these precursors into TFA.

Extended length of time that the HFC emissions spend trapped in polluted air.

Low levels of rainfall coupled with elevated atmospheric TFA (high rainfall would dilute the TFA).

Factors that might cause enhancement of TFA concentration in seasonal wetlands:

Locally elevated levels of TFA in rain that enters the seasonal wetlands.

Rainwater that contains these elevated levels of TFA collecting in the wetland pools continually throughout at least a decade.

Evaporation being the dominant mode of water removal from these pools rather than discharge to streams, mixing with groundwater or seepage.

Little or no removal of TFA from the pools either by biological activity or physical processes.

Without quantitative information about individual emission sources or specific seasonal wetlands, a range of values was used for model parameters based on information in various references. A sensitivity analysis was then performed for the full range of responses ⁽¹⁾. For example, using a scenario where the concentration of TFA in rainwater was about six times the calculated global average of 0.14 micrograms per liter for the year 2010 ⁽²⁾, the TFA was assumed to accumulate in enclosed seasonal wetland basins (no overflow or seepage) and concentrate five-fold by evaporation annually. After 20 and 60 years of continual accumulation, the concentration of TFA was calculated to reach levels of 100 and 300 micrograms per liter, respectively.
Species of algae that are known to inhabit seasonal wetlands - Anabaena flos-aquae and Scenedesmus subspicatus - are not affected by TFA at concentrations of 600,000 micrograms per liter. Of the range of species studied, one freshwater algal species, Selenastrum capricornutum, was found to be sensitive to TFA. Even with this sensitive species, growth inhibition of less than 20% was observed at 300 micrograms per liter of TFA, a concentration which could possibly be reached but only after 60 years of constant annual deposition and accumulation of TFA. Another species known to reside in seasonal wetlands, Daphnia magna (crustacean), has been shown to be unaffected by TFA at concentrations up to 1,000,000 micrograms per liter; Daphnia magna is well known to be highly susceptible to pollutants. In mixed diatom communities which were subjected to long-term TFA exposure, photosynthesis was not affected even at the concentration of 200,000 micrograms per liter of TFA. The growth of terrestrial plants - such as wheat and soya - are not affected by prolonged exposure to concentrations of 1,000 micrograms per liter.

AFEAS Research on Trifluoroacetic Acid

The Alternative Fluorocarbons Environmental Acceptability Study (AFEAS) has committed more than 1 million U.S. dollars to study the environmental fate and potential effects of trifluoroacetic acid, which may be formed when certain fluorocarbons (HFC-134a, HCFC-123 or HCFC-124) decompose in the atmosphere. Based on current knowledge, TFA derived from these compounds is expected to occur in rain and snow in very low concentrations. At these concentrations, TFA is not likely to have an impact on any of the wide range of organisms studied to date, which includes humans, animals, plants and microorganisms. Because TFA has very low affinity for lipids (fatty materials), there is no potential for passive accumulation in fatty tissues, even after long exposure at low levels, thus precluding bioconcentration in the food chain.
The results of the AFEAS program on TFA have been reviewed with academic experts at two scientific workshops and the proceedings have been published by AFEAS ⁽³⁾⁽⁴⁾. A final risk assessment has been conducted and published in Human and Ecological Risk Assessment. Independent experts were involved in the review process.

References

(1)

Tromp, T.K., M.K.W. Ko, J.M. Rodriguez and N.D. Sze, Potential accumulation of a CFC-replacement degradation product in seasonal wetlands, Nature, 376, 327-330, 1995.

(2)

Rodriguez, J.M. in Proceedings of a Workshop on the Environmental Fate of Trifluoroacetic Acid, 3-4 March 1994, published by AFEAS (Alternative Fluorocarbons Environmental Acceptability Study), 1333 H Street NW, Washington DC 20005, USA.

(3)

Ibid.

(4)

Proceedings of a Workshop on the Decomposition of TFA in the Environment, 8-9 February 1994, published by AFEAS (Alternative Fluorocarbons Environmental Acceptability Study), 1333 H Street NW, Washington DC 20005, USA.