Acid Deposition

(Acid Rain; pH<5.6)


I. Sources
    A. Natural and burning of fossil fuels
            (man made emissions well over 10X natural emissions in most regions)
    B. Strong acids of N (NOx) and S
        1. wet deposition (H2SO4, HNO3)
        2. dry deposition – aerosols, gases, deposited on surfaces
        3. current controls on S emissions; N from automobiles is harder to control
                (NOx has increased 12-20X in Eastern U.S. since 1900)
    C. History
        1. first noticed in 1660’s
        2. by the 1700’s scientists realized that S was in coal/fossil fuel emissions
        3. 1850 - sulfuric acid first discovered
        4. 1900-1980 -- S emissions doubled in N. America and Europe
        5. 1920’s began using S in crop fertilizer
        6. 1920-1960 – found that precipitation inputs can affect bogs
        7. 1950’s – 2000 people died in one episode of acid fumes in a city; solved with higher smokestacks --
                                made this a regional issue
        8. 1960 – fish kills observed in Norway and attributed to acidic deposition
        9. 1967 – concept of regional precipitation – link between emissions in England and pollution in Scandinavia –
            became a political issue
       10. 1972 (US) Clean air act
       11. early 1970’s Gene Likens – regional precipitation in the US – Hubbard Brook Forest –
            record of precipitation chemistry since 1967; coined the term 'acid rain'
    D. Average pH of precipitation in central NY in early 1990’s: 4.3

II. Catchment effects (what happens when acid hits the ground)
    A. Weathering reactions
        1. CO2 in water weathers rock minerals to produce of HCO3- to neutralize acids (alkalinity; acid neutralizing capacity)
        2. Ca2+ and Mg2+ are also weathered out
        3. loss of H+ in cation exchange; (Al3+) is also released
        4. sulfate is not well absorbed and runs through to lake in ground water
        5. some nitrate is taken up by vegetation
    B. Cation exchange for H+
        1. H+ comes in and displaces other cations in soil
        2. eventually the soil runs out of cations to exchange, and then there is no more buffering
            (decreasing pH decreases cation exchange capacity, decreases base saturation, decreases the
                pH of groundwater runoff)
        3. extent of reaction related to base saturation of soils and to time
        4. most lakes get much of their water from the catchment
        5. Cation exchange effects on drinking water – acidic water can leach copper and lead from pipes.
            Aluminum can be leached from soil into drinking water
    C. Base saturation of soils related to rock type and to elevation
    D. pH and alkalinity of lakes and streams decreases with increasing elevation
    E. Sensitive areas found in crystalline bedrock (granite, gneiss), old soils

III. Biological effects
    A. Strongly related to pH
        1. Not necessarily H+, but can be related to low pH
        2. Increased aluminum concentrations (increased solubility of aluminum at low pH);
            increases gill mucus production and clogs gills; affects respiration efficiency
        3. Increased concentration of other heavy metals (lead, cadmium, iron, copper, zinc, nickel); solubility
            increases with decreasing pH (vanadium and mercury become less soluble with low pH)
        4. Increased water transparency (changes thermal regime and UV penetration)

    B. Effects are species dependent
        1. Initial effects of acidification – encroachment by benthic algae (clearing and loss of benthic invertebrates);
            invasion by sphagnum
        2. 6.0 lose some mollusks (calcium carbonate shells)
        3. 5.5 lose some fish, amphipods, crayfish
        4. Between 5-6, algal species diversity decreases considerably – blue green algae and diatoms are the most
            susceptible; become dominated by dinoflagellates, chrysophytes
        5. some fish (perch, pike) and some mayflies can remain until pH slightly below 5 (although reproduction may be impaired)
        6. Daphnids are severely affected, while Bosmina are not
        7. 4.0 lose all fish; often left with large calanoid copepods, some rotifers (Keratella and Polyarthra),
            some insect larvae (Chaoborus and Corixids)
    C. Effects are dependent on life stage – embryos and fry of trout (both rainbow and brook) are less resistant to
            pH change; adults can sometimes survive to pH 4.5, but their eggs can not develop

IV. In-lake biogeochemical reactions
    A. Hard rocks – crystalline, igneous – slow weathering
        1. tend to find oligotrophic lakes in these geological substrates
        2. effect of catchment is small -- little neutralization of acid
        3. acid brought into surface waters consumes alkalinity
    B. Soft rocks – sedimentary or carbonates
        1. tend to find mesotrophic and eutrophic lakes in these geological substrates
        2. effect of catchment is large
            a. neutralizes acid
            b. NO3- and SO42- brought into surface waters, accompanied by major cations
        3. fate of NO3- and SO42-
            a. NO3- uptake (assimilatory reduction) consumes acid
            b. SO42- reduction consumes acid – confined to lakes with sufficient organic matter and low Eh (redox)
                1) One idea for natural remediation – will sulfate redution produce alkalinity and
                    negate the H+ coming in?
                2) Problem: high potential at lake turnover for oxidation of sulfides and production of acid
                3) Must bury solid phase sulfide or lose sulfur gas to make the gain in alkalinity permanent
        4. Water renewal time important – then in lake reactions are less important and catchment reactions are more important
        5. More exposure to hypolimnetic sediments (redox reactions), then more importance of in lake processes
    C. Effects on DOC – breaks down DOC and reduces color in water (also precipitates with aluminum)
    D. Effects on production – aluminum causes precipiation of phosphorus

V. Remediation efforts
    A. Regulation – slow natural recovery
    B. Liming – add calcium carbonate
        1. expensive
        2. rapid increases in pH and alkalinity
        3. reduction in transparency
        4. reduction in metal concentration
        5. increase in species diversity and biomass – not as good a recovery as hoped;
                >10 years for zooplankton; more for fish if not re-stocked
        6. only a temporary solution if acid rain continues
    C. Sometimes add NaOH
    D. Regulation of emissions
        1. Decreased loading
        2. Recovery of pH is slow
        3. Recovery of communities is slow even when pH recovers
 

 Return to Limnology Lecture homepage

 Return to K.L. Schulz's homepage