ACID DEPOSITION & ITS IMPACTS ON TERRESRIAL & AQUATIC ECOSYSTEMS

Why this topic?

• Nonpoint source – atmosphere; widespread dispersal

• Great example of how pollution problems created far away (distal source) can effect local ecosystems; global extent

• Cascading effects of pollution; connection and feedbacks between pollutants mechanisms

• A success story for environmental science, management and regulation.

NOTES from:

• Acid Rain Revisited – Hubbard Brook Ecosystem Forest, NH.

• Driscoll et al. 2001 – Acidic deposition in the Northeastern US. BioScience 51(3): 180-198.

• Acid Rain in the Adirondacks: An Environmental History. J. Jenkins, K. Roy, C. • Driscoll, and C. Buerkett. Cornell University Press. 2007.

• NAPAP 2011 Report (updated trends and results)

Organization of lecture:

• Background, what is acid rain or deposition?• History of acid rain• Sources of acid deposition• Spatial pattern of acid deposition across US• Trends in deposition• Consequences of Acid Rain

a. Soilsb. Forestsc. Streams & lakes

• Recovery

BACKGROUND; WHAT IS ACID DEPOSITION?

How do we define acidity?

pH definition -pH measure of acidity = - log10 [H+]

• neutral pH = 7.0 (H+ = 10-7)• less than 7.0 – acidic• greater than 7.0 – basic

when H+ ion increases pH decreases;

1 unit of pH change = an order of magnitude change in H+ concentration

Natural rain water is NOT Neutral (pH not equal to 7)

Why?

Because rainwater dissolves CO2 and thus forms Carbonic acid – which is a weak acid

H2O + CO2 = H2CO3

H2CO3 = HCO-3 + H+

HCO- = CO32- + H+

Thus because of these H+ ions, natural rainwater will typically be around pH = 5.6 to 5.7

* Thus Acid Rain is when pH < 5.6

Acid deposition - Transfer of strong acids and acid forming substances from the atmosphere to the surface of the earth

Agents of transfer:

• Wet deposition (with rainfall)• Dry deposition (with particulate matter)

3 main Chemicals involved:

• Sulfur oxides – SOx• Nitrogen oxides - NOx• Ammonia – NH3

An excess of these chemicals has been created in the air because of anthropogenic activities – power plants, waste incinerators, vehicle emissions, etc.

Acids generated by these oxides are MUCH STRONGER than Carbonic acid, and thus small amounts of the acids can depress the rain pH significantly

For Sulfur (S) only - the reactions proceed faster in humid temperatures with high sunlight

Thus acid rain is typically more elevated during summer

BREIF HISTORY OF ACID RAIN

• Term coined by Roger Angus Smith in England in 1872

• First emerged as an issue of concern in the late 1960s and early 1970s – with reports of surface water acidification in Sweden and around Scandinavia. .

• First reports – Sweden – pH of lakes dropped 2 units from 1930s to 1960s. By 1960s – 50% of the lakes < pH 6.0. fish populations were affected.

• First reports of impacts of acid rain in the US: 1960-70 studies of high elevation (greater than 600 m) Adirondack Lakes - pH of less than 5 and 90% of the lakes had no fish at all!

• Mean pH of Adirondack lakes during 1930-38 = 6.5; but the mean pH between 1969 and 75 dropped to 4.8!

• Likens visited Sweden in 1969. 1974 Science paper by Likens and Bormann -http://science.sciencemag.org/content/184/4142/1176

• 2 factors accelerated the problem of acid rain – increased use of fossil fuels & increased height of smoke stacks!

• Acid Rain became a regional problem (from a local one) – when smoke stack heights were increased!

• Fish affected by acid rain – smallmouth bass, walleye, northern pike, lake trout, lake herring, perch and rock bass.

• Controls on sulfate emissions were first implemented through amendments to the 1970 Clean Air Act (CAA) – Acid Rain Program (ARP).

• In 1980, the U.S. Congress passed an Acid Deposition Act. -- established a 10-year research program under the direction of the National Acidic Precipitation Assessment Program (NAPAP). NAPAP looked at the entire problem.

In 1991, NAPAP provided its first assessment of acid rain in the United States:•5% of New England Lakes were acidic, with sulfates being the most common problem. •2% of the lakes could no longer support Brook Trout•6% of the lakes were unsuitable for the survival of many species of minnow.

In 1990 via Title IV of the Acid deposition control program US Congress implemented further controls http://www.epa.gov/oar/oaqps/peg_caa/pegcaain.html

Spatial pattern of rain pH in 1999.

So where are the inputs of SOx and NOx coming from??????

SOURCES OF ACID RAIN

• SO2 are derived from burning of S in coal; higher the S content – the greater the emissions. Eastern US coal is typically higher in S content.

• Sulfur content of Midwestern coal = 2 to 4%

• Sulfur dioxides – electric utilities account for the largest proportion [60-70%] (point sources)

• NOx is generated during the combustion process – oxidation of N2 in the atmosphere

• Nitrogen oxides – transportation/auto emissions

• Ammonia – land application of animal wastes; feedlots; agricultural operations

Seven states in the Ohio River Valley – contributed towards 41% of the total national emissions – the regional nature of the problem!

SOx

NOx

SPATIAL PATTERN OF ACID DEPOSITION

Total deposition = Wet + Dry

• Wet deposition = concentration x amount of rainfall• Wet deposition - easy to measure and quantify• Dry deposition – greater variability and considerable uncertainty

Lovett (1994) estimated –

• Dry deposition of S = 9 – 59% of total S deposition• Dry deposition of NO3

- = 25 to 70 % of total NO3- deposition

• Dry position of NH4+ = 2 - 33% of total NH4

+

Wet deposition is being monitored via National Atmospheric Deposition Program (NADP) sites –http://nadp.sws.uiuc.edu/

Dry deposition is being monitored by:

EPA –CASTNEThttp://www.epa.gov/castnet/index.html

NOAA-AirMONhttp://www.arl.noaa.gov/research/programs/airmon.html

TRENDS – EMISSIONS AND DEPOSITION THROUGH TIME

SOx emissions

• 1970 and 1990 Clean Air Act Amendments (CAAA) made an impact on the Sulfur emissions. Required reductions in SO2 emissions from electric utilities.

• SO2 emissions were 5.7 million tons in 2009 - 64% lower than 1990 emissions and 67% lower than 1980 emissions.

• Below the 2010 Title IV statutory cap of 8.95 million tons of SO2 emissions.

• SO2 emissions from all sources including those sources not covered by the ARP, have decreased by 59% since 1990

How were the reductions achieved?• Low-sulfur coal• Scrubbers on smoke stacks• Increased combustion efficiencies and new technology• Cap and trade policies

Emission Control Policies –

• Phase I began in 1995, and limited sulfur dioxide emissions from 263 of the largest (> 100 MW) power plants to a combined total of 8.7 million tons of sulfur dioxide: about 10 million tons less than those plants had emitted annually during 1985-1987.

• Phase II began in 2000, and included all power plants in the country > 25 MW and all new fossil-fuel plants.

NOx emissions:

• Lesser changes than SO2

• NOx emissions were 2 million Tons in 2009, 67% lower than 1995 emissions, exceeding the Title IV goal of a 2-million-ton reduction in NOx emissions from projected 2000 levels without the ARP, as required by the 1990 CAAA.

• Emissions of NOx from all sources have decreased by 40% since 1990.

• NH3 – contribute to 30% of the total N – trends suggest no significant decreases?

• Ammonia – increasing opportunities sought for land application of animal wastes; increasing number of hog farms in the Southeast – long term region-wide implications?

Deposition trends –

• Wet sulfate deposition has decreased since 1990.

• Average annual sulfate deposition in the Northeast and Southeast in 2007–2009 was 43% lower than in 1989–1991, deposition in the Mid-Atlantic was 42% lower, and the Midwest was 44% lower.

• Wet inorganic nitrogen deposition has decreased regionally. Decreases were less than those of wet sulfate deposition - continuing large contribution from other sources of NOx such as on-road vehicles and non-road vehicles.

• Average annual wet inorganic nitrogen deposition in 2007–2009 was 16% to 27% lower than in 1989–1991 in the Midwest and eastern United States.

CONSEQUENCES OF ACID RAIN

Impact on soils

• Depletion of base cations• Mobilization of Aluminum • Increase in S and N

(Ca2+, Mg2+, K+, Na+) – referred to as base cations – increase pH

H+ and Al3+ - acid cations – decrease pH

What type of soil is this?What is the white layer?

SO42- and NO3

- anions associated with acid rain displace base cations off exchange sites

-- Removal of cationsfaster than replenishment from weathering of rocks and minerals

Sorption strength dictated by charge and concentration

• If the soil has a large store of base cations (Ca2+, Mg2+, K+, Na+) – it will neutralize the acidity greater buffering capacity

• Landscapes with thicker soils, greater amount of clays, large amounts of adsorbed cations will be more resistant to acid rain.

• This ability to neutralize acidity is measured by ANC – acid neutralizing capacity

• The greater the amount of base cations – the greater will be the ANC.

ANC can be estimated by (when expressed in moles) = (Σ strong base cations - Σ strong acid anions)

Strong base cations = Ca2+, Mg2+, K+, Na+

Strong acid anions = SO42-, Cl-, NO3

-,

Where the soil and bedrock have low base cations to start with – the soil and water can become deficient in base cations quickly.

Acid deposition > rate of base cation supply by weathering

This can lead to Alteration of cycles of Ca2+, Mg2+….

Mobilization of Aluminum:

Base saturation - % of cationexchange sites occupied by base cations

• If base saturation is below 20% -mobilization and leaching of Al3+ will occur

• Release of inorganic Al3+ is extremely toxic to terrestrial and aquatic biota (organic Al is not!)

• Al3+ is also tied up with organic complexes – but removal of base cations and the addition of anions leads to the removal of Al3+ from these organic complexes – and its eventual release into stream waters

Accumulation of Sulfur and Nitrogen in the soils

• Current exports of sulfate from the watershed exceed the inputs from precipitation – this pattern suggests that S has accumulated over decades and is now being released?

• Similar trends for N – N saturation hypotheses.

Question - What are the long-term implications of S & N accumulation in soils?

Impacts on vegetation

Two species that have especially been impacted in northeastern US and Scandinavian forests –

• Red Spruce

• Sugar Maple

• Mortality of high elevation spruce observed since the 1960’s

• Dieback of sugar maple stands in the 1980s

• Acidic water reduces the cold tolerance of current year Red Spruce needles by 3-10 deg C – because of leaching of calcium from the needles – susceptibility to freezing

• Mobile Al in the soils can block the plant uptake of Ca. Ca is an important macro-nutrient for the trees.

Sugar Maple – impaired health due to lack of Ca and Mg – makes them susceptible to other stresses such as insect defoliations – Multiple stress syndrome

Question - Sugar Maple trees on ridgetops and on upper slopes are especially vulnerable.

Why?

Effects on surface waters (streams, lakes)

Depression of pH

Decrease in ANC

Increase in solubility of inorganic Al

• the ANC value is an indicator of the likely health of the aquatic ecosystem

ANC thresholds:

• < 0 μeq/L – chronically acidic (throughout the year)

• 0-50 μeq/L – suffer from episodic acidification(seasonal or for short periods)

• 50 μeq/L – relatively insensitive to acidic deposition

What is Episodic Acidification?

Associated with:

• Spring snowmelts – high nitrate release!• large storms in summer and the fall• winter snowmelts

Why these periods?

Spatial watershed-scale patterns in episodic acidification – interest in identifying streams that are acidic and the likely influences of –

• Topography• geology • Soils• Hydrologic flow paths

Deeper flow paths,Greater contact timeMore cationsHigher pH

Shallow flow pathsLess contact timeLow cationsLow pH – closer to rainfall

Hydrologic flow paths and chemistry

Question - Acidification has a greater impact on streams than lakes. Why?

Fishes that are affected by acidity – smallmouth bass, walleye, northern pike, lake trout, lake herring, perch and rock bass.

Aquatic biota can be affected by –

• Acidity – affects the fish reproductive cycle

• Ca levels in fish may be lowered –o eggs fail to pass from ovarieso if fertilized, the eggs or larvae develop abnormally

• failure of fish to osmoregulate – imbalance of salts in the body – Ca, Na. – leads to heart attack!

Effects on aquatic biota

• mobilization of metals like Al that are toxic to the fish

•solubility of Al increases when pH is lowered from 7 to 5•Acutely toxic results – severe necrosis of the fish’s gill epithelium

oAcidity may affect frogs and salamanders – acids in the meltwater pools may prevent hatching of 80% of the eggs

• Lake acidification – leads to declines in algal species – survival of the acid tolerant species only -- also impacts macroinvertebrate and zooplankton communities

• Decrease in decomposer organisms like bacteria and fungi – buildup of water products and organic matter – reduction in cycling of N and P.

•Adirondack lake survey performed in 1984 and 87 of 1469 lakes – showed that 346 lakes (24%) did not support any fish

•Radio-tagged brook trout emigrated downstream during acidic episodes in streams

•Lakes experiencing high concentrations of Al – display a crystal-clear appearance (suggesting clean water suitable for habitat) – the clarity is due to the flocculating impact of the Al ions!

ECOSYSTEM RECOVERY FROM ACID DEPOSITION

So, what are the stages of recovery????Characterized by –Chemical recoveryBiological recovery

Chemical recovery thresholds:

• Ca:Al > 1.0• Base saturation > 20%• pH > 6.0• ANC > 50 μeq/L• Al < 2.0 μmol/L

Chemical recovery will depend:

1. Magnitude of decreases in deposition2. Depletion of base cations from the soil3. Local rate of weathering and inputs of base cations4. Accumulated S and N and its release

• Acidic sites with low base saturation will recover slowly.• At most sites it appears that recovery will require DECADES

Biological recovery

• Sustained biological recovery will occur only after chemical soil water conditions have improved.• Considerable uncertainty• Stream populations of macroinvertebrates ~ 3 years• Lake populations of zooplankton ~ 10 yrs• Fish populations ~ 5-10 yrs after zooplankton recovery