Control of Acidification External Measures

External measures include the (1) reduction of emissions, (2) neutralization measures in the catchment, and (3) the treatment of inflows.

The most important way to abate atmogenic acidifica tion is the 'reduction of emissions' of sulfur and nitrogen into the atmosphere. International agreements aim at redu cing sulfur emissions by reducing combustion of fossils fuels and using modern technology to minimize emissions. For example, many European countries have agreed on a reduction target for sulfur emissions of 70-80% by the year 2010 relative to 1980. The geogenic acidification due to mining activities can be influenced by reducing the exposure of sulfur containing minerals (pyrite, marcasite)

to atmospheric oxygen. Groundwater for filling the mining lake should be gained from regions where the soil is not, or is only minimally, oxidized. In regions without mining activity, the desiccation of wetlands and changing groundwater levels should be prevented.

'Neutralization in the catchment', achieved, for exam ple, by liming with calcium carbonate, magnesium, or alkali carbonate (e.g., sodium carbonate), is one way to counteract the acidification. An alternative way is to stimulate alkalinity producing processes such as micro bial sulfate reduction and microbial denitrification in the soils of the catchment, provided that sufficient supply of organic substance and N fertilizers can be guaranteed. This can be realized by adding these substances to the recultivated mining waste heaps (e.g., as liquid manure) or by establishing reactive systems with increased decom position of organic matter (e.g., fish ponds with feeding, constructed wetlands). For mining lakes, a number of measures aim at minimizing the groundwater influx. These include the installation of underground bulkheads, the draining of acidic water from the mining waste heaps, or afforestation, whereby water bound transport of acid is lowered by evaporation. Another possibility is to fill the mining lake with well buffered river water to avoid the influx of groundwater. In mining lake areas, the input of acids can also be decreased by the addition of basic materials to the heaps. This measure also introduces P which induces positive feedbacks for alkalinization by increased primary production (see the section titled 'Biological neutralization').

The acid waters can also be neutralized by 'treatment of inflows' in anaerobic systems such as ditches filled with straw bales, constructed wetlands, and anoxic limestone drains.

In Situ Technologies

In lake measures against acidification include (1) chemi cal and (2) biological neutralization.

Chemical neutralization

Chemical neutralization can be achieved by liming or the addition of other bases. The aims are (1) to detoxify the water to allow the survival or reestablishment of natural flora and fauna and (2) to raise the pH above 6 for several water uses (drinking water, fishery, recreation).

Scientific background. Basic chemicals react with H+ ions to form H2O. Raising pH values reduces toxic Al species and dissolved heavy metals. The success and duration of the effects depend on the subsequent delivery of acids.

Techniques. Many different deacidifying bases, such as carbonates, oxides, hydroxides, and alkaline industrial waters, have been used to neutralize acid waters. Common agents include dolomite (CaMg(CO3)2), sodium carbonate (Na2CO3), olivine (Mg2SiO4), and

Table 1 External and internal measures for decreasing the primary production and for abatement of negative symptoms of eutrophication in lakes and reservoirs

Influenced parameters/processes

Possible measures

No. Potential problems/adverse effects

Example

A. Parameters controlling phytoplankton growth

Table 1 External and internal measures for decreasing the primary production and for abatement of negative symptoms of eutrophication in lakes and reservoirs

A. Parameters controlling phytoplankton growth

External phosphorus load

#

Elimination of P emissions

1

Identification of nonpoint sources, delay of improvements

Lake Constance (D) Lake Washington (USA)

Increase of P retention in the landscape

2

Conflicts with land users

Lake Okeechobee/ Everglades (USA)

Purification of inlet water

3

High costs

Wahnbach Reservoir (D)

Phosphorus export

Î

Hypolimnetic withdrawal

4

Negative impacts on downstream waters,

Lake Piburger (A)

Phosphorus sedimentation

Phosphorus release

External P elimination Dredging

Destratification

Biomass harvesting Dilution and flushing

P inactivation

Food web manipulation

Promotion of macrophytes

Aeration and oxygenation Nitrate addition

Sediment capping

P inactivation destruction of stratification, drawdown of water level

5 Low efficiency

6 Increase of turbidity, release of harmful substances, deposition of toxic sediments, space required for dewatering and treatment, high costs

7 Increase of nutrient availability in the euphotic zone, negative downstream effects, increase of oxygen demand at greater depths

8 Low effectiveness

9 Water consumption, negative downstream impacts

10 Toxicity of Al to fish and benthos at low pH, phosphorus release under anoxia or extreme pH

11 Shifts in algal composition to less desired species

12 Interference of excessive macrophyte growth with recreational use

13 No effects on P cycle, N2 oversaturation leading to gas bubble disease in fish

14 Only short-term effects on internal P cycle

15 Low effectiveness due to low temporary P in sediment

16 See 10

Lake Apopka (USA) Lake Finjasjon (S) Lake Trummen (S)

Lake Nieuwe Meer (NL) Biesbosch Reservoirs (NL)

Chemung Lake (CA) Moses Lake (USA) Lake Veluwe (NL)

Lake Groß-Glienicker (D) Lake Sonderby (DK)

Lake Haussee (D)

Alte Donau (A)

Lake Sempach (CH)

Lake Lyng (DK) Lake Arendsee (D)

Dredging

Phytoplankton mortality | Food web manipulation

Dest ratification Compartmentation

B. Undesired eutrophication symptoms

Excessive development of macrophytes J. Sediment capping

Manual and mechanical harvesting Herbicides

Biological control

Water-level drawdown

Fish kills J. Destratification

Accumulation of reduced and toxic substances

Aeration and oxygenation

J. Oxidation measures (nitrate additions, destratification, oxygenation, and aeration)

17 See 6

18 See 11 Bautzen Reservoir (D)

Lake Zwemlust (NL)

19 See 7

20 Interferences with water uses Blelham Tarn (GB)

21 Mechanical instability, mortality of Long Lake (USA)

invertebrates

22 Destruction of habitats Halverson Lake (USA)

23 Prohibited by law in many countries due to Mason Lake (USA)

negative effects on other biota

24 Complete eradication and switch to turbid Lake Baldwin (USA)

conditions

25 Algal blooms after reflooding Blue Lake (USA)

26 See 7, warming of deep water endangers coldwater fish species

27 See 13

hydrated lime (Ca(OH)2). Calcium carbonate (CaCO3) as dry, finely grained powder is most widely used and is dispersed from boats, pontoon vessels, or helicopters, or distributed on the ice cover in winter. The extent of the impact depends on the retention time and mean depth of the lake and the acidity of the inlet stream water. The liming of large lakes with continuous input of acids and of strongly acidified mining lakes is very expensive.

Biological neutralization

Biological neutralization aims at an increase of biological processes, such as denitrification, sulfate reduction, and primary production, which can lead to neutralization.

Scientific background. When all molecular oxygen in the water has been consumed, oxygen bound in NO3, SO42 , or other electron acceptors is used for respiration pro cesses. During these processes the acid anions are eliminated and basic cations (e.g., NH4, Fe2+) are formed. Alkalinity production by these processes demands large amounts of biodegradable organic matter as electron donator.

Techniques. An increase in reductive processes can be achieved by saprobization. Ethanol, methanol, and glu cose, as well as straw, potato peels, and cow dung have been tested as organic sources. Alternatively, the required organic material may also be formed by a stimulation of primary production within the lake by the addition of P. The increase in alkalinity is compen sated when reduced compounds such as H2S or Fe2+ are oxidized. Therefore, only a permanent deposition of iron sulfide under strong reductive conditions can lead to long term deacidification. The stratification of the water body should be stable to ensure oxygen free con ditions in the hypolimnion for at least some months each year or, in case of meromictic lakes, in the monimolim nion. Planting trees on the shore, deploying floating barriers on the water, and installing of bulkheads in the lake are measures against wind and wave action that can help prevent full circulation. The combination of che mical and biological neutralization could prove to be an efficient solution. After establishing neutral pH values by liming, the primary production is no longer limited by bicarbonate and the microbial neutralization works more efficiently. Biological measures as new ecotechnologies are still in the development stage, so that only a few examples of whole lake application exist.

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