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