Communities and Ecosystems
The Hubbard Brook Ecosystem Studies
An In-Depth Analysis of the Small Watershed Technique
Summary of Biogeochemical Patterns
Acidic Deposition - Amounts and Trends at Hubbard Brook
Much of the following summary of background information comes from the book by Likens and Bormann (1995).  See reference list at the end of this lecture.
 
Introduction
    Although acidic deposition was discovered in the late 1950s by Eville Gorham and xx, the phenomenon of "acid rain" really gained the public's attention with the publication of research results on precipitation chemistry by Likens and Bormann in the early 1970s (Likens et al. 1972).  Based on their continuous sampling of the precipitation coming into Hubbard Brook (HB), they found that the annual weighted pH from 1964-1965 through 1973-1974 ranged from 4.03-4.21.  The lowest value recorded was pH 2.85, and the highest was 5.95 pH.  Between 1969 and 1974, no pH reading exceeded 5.0.  "Clean" rain should have a pH value somewhere between 5.3 and 5.6 pH.  The difference between 5.6 pH and 7.0 pH is due to the presence of CO2, which dissolves to form carbonic acid.  Based on the amount of CO2 in the air, the predicted pH of unpolluted rain should be 5.6 or so (Barrett and Brodin 1955).

    Thus, the precipitation at HB was 50-100X more acid than it should have been.  The main constituents of this acid rain are nitric acid, and sulfuric acid.  These are derived from anthropogenic pollution in the form of NOx and SO2.  This is important when considering the sources of input of these materials, since HB is > 100 km from the nearest major industrial sources.  The record of acid rain at HB is the longest continuous record in the U.S.  From 1964 to 1971, there was a slight upward trend in H+ ion concentrations, while from 1971 to 1974, the trend reversed.  Over the entire interval, there is no trend.  Some of the trend in H+ ions was due to more rainfall in the earlier period, but they do not explain all the trend.  Concentrations of both SO4- and NH4+ show no trend during this time either, but nitrates (NO3-) show an upward trend (annual weighted concentrations are 2.3 fold greater in 1974 than in the 1950s or 60s.

    Most of the acidity is due to sulfuric acid (65% of the acidity).  Nitric acid contributes about 30%, while HCl makes up the remaining, plus carbonic acid and some other minor organic acids.  The rise in H+ ion concentration is not correlated with sulfate levels, but is strongly correlated with nitrate concentrations.  Thus, the increase in nitrates may be the prime cause of the increase in H+ ion concentrations in that period where it rose.  The increases in nitrate are roughly correlated with the increase in use of fossil fuels, and possibly with the increased use of nitrogenous fertilizers.

    Snow melt is initially (the first 20%) more acidic than rain.  During snowmelt this large influx of very acidic water can be damaging to fish and invertebrates in streams and lakes.  Fish kills caused by snowmelt have been reported around the world.

    Streamwater pH does not change much after snowmelt, which shows the large capacity of the terrestrial component at HB to buffer the acidity, and to protect aquatic ecosystems from large fluctuations in pH.  But acidic deposition may have significant effects on ecosystem processes.  For example, in laboratory studies, acid rain induces leaching of nutrients from leaves; Eaton et al. (1973) found that 90% of the H+ ions striking the canopy were absorbed there, while presumably at the same time, releasing an equivalent amount of cations.  The soils at HB are already acidic, so the acid rain is probably not having much of an effect on current soil processes.  But over the long haul, there could be detrimental effects.  Nitrification rates might be affected, decomposition rates might be slowed, and soil fertility may decline further with continued inputs of acid rain.  It has been calculated that 1 m of rain at pH 4.0 could leach 50 Kg/ha of CaCO2 from the system.

    Early on, researchers at HB postulated that acid rain might be one factor responsible for reduced growth of forests in the northeastern United States (Bormann 1974).  Over the two decades from the 1950s to the 1970s, forests at HB slowed down in their growth (Whittaker et al. 1974, Cogbill 1976).  However, in recent years (1990s) enough data have been collected to show that there has been no trend in H+ ion concentrations over this period.  Shorter periods, randomly picked from the record (3-5 years long) can show any trend, which emphasizes the point that long-term data are needed before you can make definitive statements about trends in atmospheric pollutants.

    Based partly on the work at HB, the U.S. Congress in 1990 amended the Clean Air Act of 1977 to include provisions for regulating acid rain.  These amendments called for the reduction of 10 million tons (9.1 million metric tons) of SO2 emissions per year below the 1980 levels, to be achieved by the year 2000.  Prior to this amendment, projections showed the emissions of SO2 continually increasing up to the year 2000 and beyond (Streets and Veselka 1987).  Thus, all told, the effective emission reductions are now above the required ones.  However, even with the reductions that are taking place, researchers at HB theorize that current inputs are still 3X higher than required to protect sensitive forests and aquatic systems.  This, coupled with the long-term trend in declining base cation inputs, is making the forest ecosystem even more sensitive to continued inputs of acidic deposition.  The recent and dramatic decline in forest growth may be related to lower Ca in the soil, acidic deposition, disease, climate changes, and nutrient limitations.  Research is currently underway to try and understand the controls on forest growth and why it has stagnated these recent years.

References

Barrett, E. and G. Brodin. 1955. The acidity of Scandinavian precipitation. Tellus 7:251-257.

Bormann, F.H. 1982. The effects of air pollution on the New England landscape. Part 1. Ambio 11:188-194.

Bormann, F.H. 1982. The effects of air pollution on the New England landscape. Part 2. Ambio 11:338-346.

Butler, T.J. and G.E. Likens. 1991. The impact of changing regional emissions on precipitation chemistry in the eastern
        United States. Atmos. Environ. 25A:305-315.

Cogbill, C.V. 1976. The effect of acid precipitation on tree growth in eastern North America. pp. 1027-1032.
        In: Proceedings of the First International Symposium on Acid Precipitation and the Forest Ecosystem, L.S.
        Dochinger and T.A. Seliga, eds., USDA Forest Service Gen. Tech. Rpt. NE-23.

Cogbill, C.V. and G.E. Likens. 1974. Acid precipitation in the northeastern United States. Water Resources Research
        10:1133-1137.

Gorham, E. 1958. Atmospheric pollution by hydrochloric acid. Quart. J. Royal of the Meteorological Society 84:274-276.

Granat, L. 1972. On the relation between pH and the chemical composition in atmospheric precipitation. Tellus 24:550-560.

Likens, G.E. and F.H. Bormann. 1974. Linkages between terrestrial and aquatic ecosystems. Bioscience 24:447-456.

Likens, G.E. and F.H. Bormann. 1974. Acid rain: a serious regional environmental problem. Science 184:1176-1179.

Likens, G.E., N.M. Johnson, J.N. Galloway, and F.H. Bormann. 1976. Acid precipitation: strong and weak acids.
        Science 194:643-645.

Likens, G.E. 1989. Some aspects of air pollution effects on terrestrial ecosystems and prospects for the future. Ambio
        18:172-178.

Munn, R.E., G.E. Likens, B. Weisman, J.W. Hornbeck, C.W. Martin, and F.H. Bormann. 1984. A meteorological
        analysis of the precipitation chemistry event samples at Hubbard Brook, New Hampshire. Atmos. Environ.
        18:2775-2779.

Streets, D.G. and T.D. Veselka. 1987. Future emissions. In: Interim Assessment of the National Acid Precipitation
        Assessment Program, pp. 3-1 to 3-33, Washington, DC.

Weathers, K.C. et al. (lots and lots of authors!). 1988. Cloudwater chemistry from ten sites in North America. Environ.
        Sci. & Technol. 22:1018-1026.


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