Studies of TFA at Hubbard Brook are a collaborative effort among:
* Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545, USA
CHLOROFLUROCARBONS (CFCs) catalyze the breakdown of stratospheric ozone, and future manufacture of these materials is banned by international agreements. In the USA, manufacture of CFCs ceased on 1 January 1996. Degradation of CFC replacements, HFCs and HCFCs (such as HCFC-123, HCFC-124, and HFC-134a), in the atmosphere forms trifluoroacetate (TFA), which is highly soluble in water (>10 g ml-1) and thus is transported rapidly back to the Earth's surface in atmospheric deposition. Manufacturers have suggested that TFA is inert and conservatively transported in water, without significant long-term accumulation in ecosystem compartments. We are testing these assumptions by study of the transport and fate of TFA in natural ecosystems within the Hubbard Brook Experimental Forest.
Experimental studies in upland forest and wetlands
Trifluoroacetate (TFA) was applied to irrigated plots within an upland forest and to a small wetland ecosystem within the Hubbard Brook Experimental Forest. We analyzed TFA in surface water, stream water, and soil water by ion chromatography. Selected water, soil and vegetation samples were analyzed by ion-selective mass spectrometry to detect trace concentrations of TFA. All samples were stored at approximately 4EC until analysis.
Upland forest plots
Twice during the summer of 1994, sodium trifluoroacetate (molecular wt.=137) was added to plots within an upland beech (Fagus grandifolia Ehrh.) forest, in the Norris Brook watershed. One of the six, 1-m H 2-m plots received 274 mg/m2 TFA and two of the plots received 68 mg/m2 TFA. No TFA was added to three, adjacent reference plots. The plots were irrigated once (40 R/m2) or twice (80 R/m2) per week from July to September (total 1140 R/plot; approximately twice the mean ambient precipitation input for the experimental period) with water from nearby Mirror Lake (total dissolved solids = 21 mg/R). In each plot, soil water was collected from a pair of zero-tension lysimeters, installed in 1988, beneath the following horizons: Oa (at ~8 cm), Bh (at ~28 cm) and Bs2 (at ~48 cm). We estimated the amount of TFA transported through the soil profile by multiplying the concentration of TFA in each sample of soil water by the water output from the related soil horizon and summing these values during the study period. We used a modified version of the Brook2 hydrology simulation model, developed and parameterized for the HBEF, to estimate the water transport through the soil profile of the upland, forest plots. This modification calculated flow through each soil horizon based upon the assumption that water removed from the soil profile by transpiration was proportional to the distribution of roots in each soil horizon. We collected vegetation (September) and soil (November) samples from each plot.
The small (407 m2) forest wetland was located at 701-m elevation within the Bear Brook drainage. It contained mosses (Sphagnum spp.) and sedges (Carex spp.), and was surrounded by an 80-yr-old northern hardwood forest dominated by American beech (Fagus grandifolia Ehrh.), yellow birch (Betula alleghaniensis Britt.) and sugar maple (Acer saccharum Marsh.). This small wetland has organic soil extending to a maximum depth of 1 m, probably experiences both oxic and anoxic conditions, and is a net source of methane. Groundwater wells and zero-tension lysimeters were installed in 1988-89. We collected pre-treatment samples of precipitation and stream water in 1993. Three times during the summer of 1994, sodium trifluoroacetate dissolved in previously collected stream water from just below the weir on nearby Bear Brook, and stemflow from the surrounding forest, was sprayed manually onto the wetland at ground level. The three sections of the wetland received a total of 112, 6 or 3 mg/m2 of TFA.
Rainfall data were collected from a standard rain gauge, located approximately 236 m from the wetland. Water outflow was measured at a temporary gauging weir located on the stream draining the wetland. Samples of surface water, stream water, and soil water were collected at least weekly from June to November. We collected vegetation (September) and soil (August, November) samples after all the treatments were applied.
Soil adsorption studies
Batch equilibrium studies of soil adsorption were conducted on each of the upland forest soil horizons and the wetland soil. Adsorbate solutions of 226, 452, 791, 1130, 2260, 3390, and 4520 Fg/R TFA were equilibrated with air-dried, sieved (2 mm) soil for 24 hr. A 1:10 soil solution ratio was used for organic soils and a 1:5 soil to solution ratio for mineral soils. After equilibration, the suspension was centrifuged and the centrate was filtered and analyzed for TFA by ion chromatography. Adsorption isotherms were plotted to evaluate the extent of TFA retention on all of the soils.
Hypothetical calculations of the fate of atmospheric inputs of TFA on concentrations of TFA in drainage water and soil were conducted using a chemical equilibrium model.
Inputs of TFA were not transported conservatively through these ecosystems, instead significant amounts of TFA were retained within the vegetation and soil compartments. More TFA was retained by the wetland ecosystem than by the upland forest ecosystem. Using simulation modeling, TFA concentrations were predicted for soil and drainage water until the year 2040.*
*Much of the above is taken from:
Modified Abstract: Berger, T. W., S. L. Tartowski and G. E. Likens. 1997. Trifluoroacetate retention in a northern hardwood forest soil. Environ. Sci. Technol.31(7):1916-1921.
In this experimental study, TFA and bromide (Br), widely used as a hydrologic tracer, were added to upland northern hardwood forest plots within the HBEF. Analysis of soil solutions, collected by lysimetry, showed TFA and Br concentrations to be highly correlated with similar temporal patterns. Application of a simple hydrologic model was done to estimate solute fluxes. Due to the extremely dry summer in 1995, significant amounts of TFA and Br probably were taken up by plants or retained within the soil. Drainage losses did not support the hypothesis for conservative transport through surface organic layers; retention in the lower mineral soil horizons was minimal. Transport of TFA and Br in this upland soil is controlled primarily by hydrologic processes.
Modified Abstract: Richey, D. G., C. T. Driscoll and G. E. Likens. 1997. Soil retention of trifluoroacetate. Environ. Sci. Technol. 31:1723-1727.
Soil retention studies were conducted to evaluate the potential accumulation of TFA in soil of terrestrial ecosystems. Batch equilibrium studies showed that detectable TFA was retained by 34 of 54 soils collected from diverse locations. Retention ranged from a high of 260 to 25 F mol kg-1 (0-60% of added TFA). Most soils (43 of 54) did not retain TFA strongly (>25%), but soils high in organic matter and some mineral soils with high iron and aluminum content exhibited strong retention. Trifluoroacetate retention was correlated (r = 0.60) with % organic matter content. Organic soils from wetlands, peatlands, and a boreal forest showed the greatest retention. The retention of TFA increased with decreasing pH. The magnitude of TFA retention was similar to Cl- and Br-. Trifluoroacetate retention decreased with increasing concentrations of F-, Cl-, and SO42-. The ultimate fate of TFA retained in soils is uncertain.
Publications on TFA:
Berger, T. W., S. L. Tartowski and G. E. Likens. 1997. Trifluoroacetate retention in a northern hardwood forest soil. Environ. Sci. Technol.31(7):1916-1921.
Driscoll, C. T. and G. E. Likens. 1994. Transport and fate of trifluoroacetate in the terrestrial environment. In: Alternative Fluorocarbons Environmental Acceptability Workshop on Decomposition of TFA in the Environment. Washington, DC. 13 pp.
Likens, G. E., S. L. Tartowski, T.W. Berger, D. G. Richey, C. T. Driscoll, H.G. Frank and A. Klein. 1997. Transport and fate of trifluoroacetate in upland forest and wetland ecosystems. Proc. National Acad. Sci. USA 94:4499-4503.
Richey, D. G., C. T. Driscoll and G. E. Likens. 1997. Soil retention of trifluoroacetate. Environ. Sci. Technol. 31:1723-1727.