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A comparative evaluation of differentapproaches for assessing soil acidificationimpactsSelçuk Soyupak a , Basak Kiliç a , Labeeb Mukhallalati a & Coskun Yurteri aa Environmental Engineering Department , Middle East TechnicalUniversity , Ankara, 06531, TurkeyPublished online: 17 Dec 2008.

To cite this article: Selçuk Soyupak , Basak Kiliç , Labeeb Mukhallalati & Coskun Yurteri (1993) Acomparative evaluation of different approaches for assessing soil acidification impacts, EnvironmentalTechnology, 14:1, 59-70, DOI: 10.1080/09593339309385264

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Environmental Technology, VoL 14. pp 59-70©Publications Division Selper Ltd., 1993

A COMPARATIVE EVALUATION OF DIFFERENTAPPROACHES FOR ASSESSING SOIL

ACIDIFICATION IMPACTS

SELÇUK SOYUPAK, BASAK KILIÇ, LABEEB MUKHALLALATI AND COSKUN YURTERI*

Environmental Engineering Department, Middle East Technical University, 06531, Ankara, Turkey

(Received 30 April 1992; Accepted 29 September 1992)

ABSTRACT

Long-term impacts of acidic depositions on soils can be predicted with coupled applications ofacid deposition models and approaches to assess soil acidification. Soil acidificationassessment can be achieved by using the following approaches :(i) Mechanistic modelling(ii) Experimental acid buffering capacity (ABC) method(iii) Qualitative evaluationsThe mechanistic modelling approach adopted in this study utilises predicted acid deposition ratesand soil properties as well as dominant soil mechanisms including utilisation, immobilization,nitrification, dissociation, association, carbonate weathering, silicate weathering, aluminiumhydroxide disintegration, and cation exchange. The experimental ABC method, on the other hand,was based on titration curves obtained by adding different dilutions of H2SO4 to known quantitiesof soil samples. The sensitivities of the soil samples to acidification were also evaluated using aqualitative approach based on the pH and cation exchange capacity (CEC). A comparativeevaluation of these approaches was made using the results of an environmental impactassessment (EIA) study conducted for a proposed coal-fired thermal power plant to be sited atAliaga near Izmir, Turkey. In this context, the regional soils were first evaluated qualitatively.Then, the results of mechanistic and experimental approaches were compared in terms of thetime required to reach certain critical pH levels. For the case of less sensitive calcareous soils,the mechanistic modelling approach yielded more conservative predictions. In the case of highlysensitive non-calcareous soils, however, the experimental ABC approach resulted in moreconservative predictions.

Keywords: Acid deposition, soil acidification, environmental impact assessment, bufferingcapacity, mechanistic modelling

INTRODUCTION simulates the long-term soil response to thedeposition of acidic precursors in various buffer

Acidification of soils occurs as a result of ranges and can be utilised to estimate the changeslong-term atmospheric deposition of SO2, NOX expected as a result of acid deposition. Forand NH3 to soil and it involves complex chemical instance, given the acid deposition rates and thechanges in soil and soil water. The major soil properties, SMART can predict the time tochanges are expected to occur in pH, base reach certain critical pH values,saturation and molar ratio of aluminium to In estimation of the time required to reachdivalent base cations (Al/BC). The pertinent critical pH values, an alternative to mechanisticliterature offers several mechanistic approaches modelling is the experimental acid bufferto estimate the magnitude of these changes (1-4). capacity (ABC) method. The acid deposition ratesAmong these, the mechanistic model SMART are again the key input for the experimental ABC(Simulation Model for Acidification's Regional approach which is particularly useful inTrends), developed by De Vries et al. (3), environmental impact assessment (EIA) studies

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(5). Another useful alternative for theassessment of sensitivity to soil acidification isthe well-established qualitative approach (6).

The purpose of this study is to compare themechanistic model SMART and the experimentalABC approach as alternative tools in assessingsoil acidification. The results obtained fromthese quantitative approaches were interpreted inlight of the qualitative approach based on physico-chemical soil properties.

METHODOLOGY

Regional Study Area

The comparative evaluation of themethodologies for estimating soil acidificationwas achieved for the regional impact area of aproposed thermal power plant of 2x500 MWcapacity (see Figure 1). In the regional study area(RSA), soil types differ with location andagriculture of a wide variety of plant types isbeing practised. The Menemen Plain, one of themost fertile agricultural plains of Turkey, iswithin the RSA. Also existing in the RSA arePinus brutia forests, olive trees, maquisvegetation and some barren land.

Soil Sampling and Characterisation

Soil samples were collected at 31 differentlocations representing 31 distinct soil types asmapped by the related agencies of the TurkishMinistry of Agriculture and Forestry (7). Thelocations of the soil sampling points areexhibited in Figure 1. According to the results oflong-term deposition modelling studies (8), aportion of the sampling points are likely toreceive high amounts of wet and dry sulphurdeposition.

Soil sampling was conducted in February1991 through January 1992 and the samples werecollected by pick and axe. The sampling depthwas 50 cm from the surface for forest areas and25 cm from the surface for agricultural and othersoils. The soil samples were analysed for pH(saturation, 1:2.5 in water and 1:1 in 0.1 MCaCl2); total, soluble and exchangeable basecations (Ca2+, Mg2*, Na+ andK+); base saturation;cation exchange capacity (CEC); CaCO3; sodiumadsorption ratio (SAR); potassium adsorptionratio (PAR); total aluminium; AI2O3; A1(OH)3;percent saturation and bulk density.Conventional techniques for soil sampling andanalysis were adopted for the experimental work(9-15).

Acid Deposition Rates

Under different conditions of variousemission scenarios, the acid deposition rates tothe RSA were predicted using the ADEPT (AlbertaDeposition Model With Terrain) computerprogram (8,16,17). ADEPT can estimate seasonaland annual SO2 concentrations as well as wet anddry sulphur depositions for up to 10 point sourcesand can distinguish between stacks and flares.ADEPT predictions can cover a radius of 100 kmof complex terrain.

In addition to the potential effects of theproposed power station, several other industrialsources already influence the RSA in terms ofacidic depositions. In this regard, a petroleumrefinery, a petrochemical complex and another4x165 MW coal-fired power plant at about 70 kmNNE of the proposed plant are among the mostsignificant point sources. Currently, theemissions of all the existing industries arehigher than the standards set by the Turkish AirQuality Regulation. In the near future, it isexpected that these sources will reduce theiremissions to allowable levels.

As listed in Table 1, various emissionscenarios were devised for the case study,describing the sole and combined effects of the

CfBtHÜ T A *

KALCM/IOOOOO rmijAKIW?« i W I 5 t D 0 "

* Ç «ALTfUO . " " " " " « M t ^ »M61 MS . « V ^ * - =

KEY MAP» «1

SCA

MESITCIIIMNtM HA

M4 » X

Figure 1. Sampling points for soilcharacterisation and soil acidificationstudy.

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Table 1. Emission scenarios.

Scenario Definition

C-l Acid deposition due to the proposed ATPP+ with 100% FGDC-2 Acid deposition due to the proposed ATPP with 63% FGDC-3 Acid deposition due to the proposed ATPP with 0% FGDC-4 Acid deposition due to EllC-5 Acid deposition due to Ell + ATPP with 100% FGDC-6 Acid deposition due to Ell + ATPP with 63% FGDC-7 Acid deposition due to Ell + ATPP with 0% FGDC-8 Acid deposition due to EI2 **C-9 Acid deposition due to EI2 + ATPP with 100% FGDC-10 Acid deposition due to EI2 + ATPP with 63% FGDC-ll Acid deposition due to EI2 + ATPP with 0% FGD

ATPP+ = Aliaga Thermal Power PlantEll* = Existing industries in compliance with national emission standards.EI2** = Existing industries with current air pollution control practices.

proposed power plant and the existing industries. calculate total deposition at each location, theThree separate options were tested for the estimated background deposition values shouldproposed power plant, corresponding to 0, 63 and be added to the values predicted by ADEPT. The100% flue gas desulphurization (FGD). With a estimates for background deposition due to long-design efficiency of 90%, desulphurization of range SO2 transport were obtained from the63% of the flue gases will satisfy the current results of two recent studies (18,19).Turkish regulatory limit of 1,000 mg.m"3 forSQj. Acid deposition rates were also predicted Quantitative Methods for Assessing Soilfor the future case of compliance when the Acidificationexisting industries comply with the currentlyeffective emission standards. The details of the Mechanistic modelling:long-term deposition studies are presented The dynamic soil acidification modelelsewhere (8,17). SMART was adopted to predict the expected

For each scenario, annual average total changes in soil due to total acid deposition undersulphur deposition rates were obtained as equal- the influence of local factors such asdeposition contours. The deposition rates meteorology and agricultural practices. The totalpredicted under the worst-case scenario (C-ll) list of mechanisms involved in SMART areare presented in Figure 2. Similar figures are summarized in Table 2. The basic equationsavailable for the other scenarios (17). To describing the mechanisms and their solution

Table 2. Mechanisms for calculating the expected changes in soils* (3,4).

Process H+ Al3+ BC2* NH4+ NO3" SO4

2" HCO3-

Deposition + - + + + +Growth uptake + - + + + -Nitrogen immobilization + + +Nitrification + + + -Denitrification + - - - + -Dissociation/Association + . . . +Carbonate weathering + - + - - -Silicate weathering + + + . . .A1(OH)3 weathering + + - . . .Cation exchange + + + . . .Sulphate adsorption + - - - - +

*(+) means that the ion is included in the respective process; a (-) means that the ion is not included.

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Figure 2. Annual average calculated total sulphur deposition for scenario C-ll (contour intervals: 2,3,4, 5, 7,10, 20, 50,100,1000 kg ha'1).

methodology are given elsewhere (3,4). Theeffects of the current irrigation practices withinthe study area and the quality of irrigation waterwere taken into account for the case ofagricultural soils.

Experimental ABC method:The acid neutralizing curves were obtained

for each soil. The buffering capacities weredetermined by the addition of different dilutionsof H2SO4 to pre-determined quantities of soil.The soils were allowed to equilibrate for 10 daysand pH measurements were taken for eachaddition at the end of 10 days. The results wereplotted as pH versus meq H+ added per 100 g ofsoil.

Basis of comparison:The results obtained from mechanistic

modelling and acid buffering methodologies

were compared in terms of the calculated yearsrequired to reach preset pH values which havepractical significance. The critical pH valuesand their practical importance are summarizedin Table 3.

In the case of mechanistic modelling withSMART, pH versus time curves were obtained foreach soil type and scenario condition. The timefor each pH level was directly read from thesecurves. In the case of the experimental ABCmethod, the moles of acid required to reach apreset pH value (meq H+/100 g of soil) wasobtained from the associated experimentaltitration curve of the given soil. The amount ofacid required for an hectare of land (kg H+ ha"1 )was then calculated by using the bulk density andthe depth of the soil column (50 cm for forestsoils and 25 cm for agricultural and other soils).Under each deposition scenario, the number ofyears that must pass to reach a critical pH value

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Table 3. Critical values for soil pH.

Class pH Remarks

No significant limitations for production of most crops.Slight limitations that may restrict the range of crops. Somemodifications of the agricultural practices may be necessary.These soils have moderate limitations that restrict the rangeof crops that can be grown, and require special managementpractices.These soils have severe limitations and require specialmanagement practices.

ClassClass

Class

12

3

6.0 - 7.06.0>pH>5

5.5>pH>4

.5

.0

Class 4 pH<4.0

was then calculated by dividing the necessaryamount of acid(kg H+ ha"1) by the predicted totalsulphur deposition rate (kg ha'1 y r 1 ) assumingthat one mole of SO2 will produce 2 H+ ions.

Qualitative Approach for Assessing SoilAcidification

The sensitivity of soils to H+ addition canalso be evaluated by using a qualitative approachoriginally suggested by Holowaychuck andFessenden (6). In particular, the qualitativeanalysis is based on the CEC and pH levels of thesoil before receiving any acidic load. Thegeneral criteria for rating the sensitivity ofsoils to acidic inputs is given in Table 4.

HESULTS AND DISCUSSION

Qualitative Evaluation

The sensitivities of the regional soils toacidification were first evaluated by means of thequalitative approach. Table 5 presents theimportant characteristics of the regional soils.The sensitivity of the regional soils to base loss(Sx), acidification (S2 ), and aluminiumsolubilization (S3) were evaluated based on pH,CEC and CaCO3 content. Then, an overallsensitivity rating (S4) was assigned to each soil,based on the individual sensitivities. Thefollowing conclusions were derived as a result ofthe qualitative evaluation:

Table 4. Criteria for sensitivity of soils to acid additions (6).

CEC(cmol kg'1)

<6

6-15

>15

pH

<4.64.6 - 5.05.1 - 5.55.6 - 6.06.1 - 6.5

>6.5<4.6

4.6 - 5.05.1 - 5.55.6 - 6.0

>6.0<4.6

4.6 - 5.05.1 - 5.55.6 - 6.0

>6.0

Sensitivity tobase cation

loss

HHHHHLHMMMLHMMLL

Sensitivity toacidification

LLMHHLLL

LtoMLtoM

LLLL

LtoML

Sensitivity toAl

dissolution

HHHMLLHHM

LtoMLHHM

LtoML

Overallsensitivity

HHHHHLHMMMLHMMLL

H : High sensitivityM : Medium sensitivityL : Low sensitivity

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Table 5. Soil properties and results of qualitative evaluation.

SamplePoints

MlM2M3M4M5M6M7M8M9MIOM i lF lF2F3F4PIP2P3K3K4K5N4N5N6AÏN6B1N7N8N9N10N i lN12

CECcmol kg^1

4.0646.7786.3492.5175.5903.1393.7303.7643.8592.8463.5415.7134.7435.6563.7613.0453.4562.5942.4382.4503.1492.4253.5283.5253.7184.4722.5474.0714.7265.6102.410

PH*

5.457.907.707.517.957.707.797.967.908.107.907.907.907.956.906.507.006.105.005.806.205.505.525.905.007.106.006.157.205.555.80

% basesaturation

44.380.179.784.666.269.863.374.762.485.075.149.766.059.163.852.595.253.267.370.661.663.161.568.969.993.566.449.479.358.673.0

% CaCO3

0.2924.528

12.1615.2594.5282.1554.1890.7306.8825.4625.097

21.91024.03113.3080.1460.0005.9700.0000.0000.0000.0000.2880.2880.1800.2881.3690.0000.0005.8710.0000.000

TEC

3.115.435.062.133.702.192.362.812.412.422.662.843.133.342.401.603.291.381.641.731.941.532.172.432.604.181.692.013.753.291.76

SI

HLLLLLLLLLLLLLHHLHHHHHHHHLHHLHH

S2

MLLLLLLLLLLLLLHHLHMHHMHHMLHHLHH

S3

HLLLLLLLLLLLLLHLLLHMLHMMHLMLLMM

S4

HLLLLLLLLLLLLLHHLHHHHHHHHLHHLHH

* :(l:lin0.01MCaCl2)TEC: Total Exchangeable Cations (Ca + Mg + K + Na)SI : Sensitivity to Base LossS3 : Sensitivity to Aluminium SolubilizationH : High Sensitivity M : Medium Sensitivity

S2 : Sensitivity to AcidificationS4 : Overall SensitivityL : Low Sensitivity

N6A and N6B were collected at the same location, N6

Out of a total of 31 soils, 15 had acidic pHvalues below 7. The CEC of the regional soilsranged in between 2.410 and 6.778 cmol kg"1. TheCaCC>3 content, on the other hand, ranged from0.000 to 24.031%.

The agricultural soils of the MenemenPlain (Ml through Mi l ) had pH values rangingfrom 5.45 to 8.10. Except Ml, pH values are allabove neutrality. The CaCO3 content, which is themajor p a r a m e t e r governing the acidneutralization capacity, was relatively high forthe majority of the Menemen soils (except Ml

and M8).In overall, agricultural Menemen soils had

low sensitivities to base loss, acidification, andaluminium solubilization, except Ml . Thus,their overall sensitivity ratings are also low.The relatively high sensitivity of Ml soil was dueto the lack of free carbonates (CaCO3 ) in the soilsolution as well as a comparatively low pH.

Forest soils, F l , F2 and F3, were found tohave alkaline pH values larger than 7.9.Compared to other soils, the CEC and CaCO3contents of these soils were also high. Therefore,

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forest soils PI, F2 and F3 have exhibited lowoverall sensitivities. Forest soil F4, on the otherhand, was found to have a high overall sensitivityrating due to the comparatively low values of pH,CECandCaCO3 content.

For similar reasons, agricultural soils (Piand P3) collected outside the Menemen Plain hadhigh sensitivities to base loss and acidification.Mainly due to a high CaCO3 content and a neutralpH value, P2 had low sensitivities to base loss,acidification and aluminium solubilization.

With the exception of N7 and N10, theremaining soils of the K and N series, whichhave relatively lower agricultural values,exhibited high overall sensitivity.

Comparison of Quantitative Approaches forLong-Term Predictions

By using the mechanistic modelling

approach, separate pH versus time curves wereobtained for each soil under different scenarioconditions. As examples, the predictedresponses of soils Ml and Mil are illustrated inFigures 3 and 4 for the worst-case scenario of C-11 (see Table 1). As discussed previously, Ml issensitive to acidification and model predictionsindicate that its pH value will drop below 5.5 inabout 100 years. Whereas, Mil has a low overallsensitivity rating, and it is predicted that it willtake more than 500 years for the pH of this soil todrop to a critical value of less than 5.5 under thegiven scenario.

Further, for each soil and scenariocondition, acid buffering curves were obtainedexperimentally. Figures 5 and 6 depict the acidbuffering curves for soils Ml and Mi l asexamples. Using these curves and the predictedacid deposition rates for C-ll, it can be estimatedthat it will take about a year for the pH of Ml to

7.0

6.5 :

6.0 -

9.9

LSO

4.9

4.0

3.5ioo ioo

Time (year)400 500

Figure 3. Long term pH values predicted by SMART for soil Ml.

7.0

6.5

e.o

£»5.0

4.5

4.00 100 200 300 400 500 «00 700 800

Time (yr)

Figure 4. Long term pH values predicted by SMART for soil M i l .

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6.00 -a

5.50 -J

S.00

«..SO

4..00

3.50

3.00O.OO 1.00 2.00 3.00 4..0O

Meq./100g H+ added

Figure 5. Acid buffering curve for soil Ml.

5.00

9.00 -a

6 . 0 0 ••

7.00 -.

6.00 •:

5 . 0 0 ••

4 . 0 0 •:

3.000.00 SO.00 100.00 190.00

Meq./100g H+ added200.00

Figure 6. Acid buffering curve for soil Mil.

drop below 5.5. For the case of insensitive Mil,experimental ABC predictions imply that, underthe conditions of scenario C-ll, the pH of thissoil will not drop below 5.5 in a practical frameof time.

From these results, it is apparent that the twoquantitative approaches produce quite differentpredictions in terms of number of years requiredto reach certain pH values. In particular, forsoils having a low overall sensitivity themechanistic modelling approach predicts lessnumber of years as compared to the experimentalABC method. An opposite trend was observed forthe soils that are sensitive to acidification.

Under the conditions of other emissionscenarios, the comparative evaluation lead tosimilar results. In this regard, Figures 7, 8, 9and 10 compare the predictions of the two

approaches to reach a pH value of 5.5 forinsensitive calcareous soils under the scenarioconditions of C-8, C-9, C-10, and C-ll. The samecomparisons for sensitive non-calcareous soilsare presented in Figures 11, 12, 13 and 14.

The main reason for the observeddiscrepancy is believed to be the inherentdifferences between the two methodologies. Themechanistic modelling approach takes intoaccount a number of processes including silicateweathering, carbonate weathering, nitrogenuptake by plants, nitrogen immobilization, andprecipitation surplus or leaching (precipitation +irrigation - evaporation -interception by plants).The experimental approach is based on shortterm titration, and cannot incorporate naturalprocesses which can provide long term bufferingcapacity. On the other hand, since leaching is not

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10000

Figure 7.

100 1000SMART Results

10000

Calculated number of years to reachpH 5.5 for calcareous soils under thescenario conditions of C-8.

1000n4>

OH

O

Sc

ua

100:

100 1000SMART Results

10000

Figure 8. Calculated number of years to reachpH 5.5 for calcareous soils under thescenario conditions of C-9.

to SMART ° f lResults1000 10000

Figure 9. Calculated number of years to reachpH 5.5 for calcareous soils under thescenario conditions of C-10.

10000

3 looo

100

ctl

'Cua.

100 1000SMART Results

10000

Figure 10. Calculated number of years to reachpH 5.5 for calcareous soils under thescenario conditions of C-ll.

permitted during the experimental ABC approach,the dissolved carbonates of calcareous soilsremain within the system providing a buffercapacity which do not appear in the results ofmechanistic modelling. As a result of theseeffects, the ABC approach yields a higher numberof years as compared to the mechanisticmodelling approach to reach a pH level of 5.5 forall calcareous soils (see Figures 7, 8, 9 and 10).

For all sensitive soils, the number of yearscalculated by the mechanistic approach to reach apH level of 5.5 was higher than the calculatednumber of years to reach the same level by theABC approach (see Figures 11, 12, 13 and 14).

This difference might be due to the fact that whilemechanistic modelling approach takes a numberof processes that create long-term bufferingeffect into account, the effects of these processesdo not appear in the ABC method. Furthermore,sensitive soils do not have an additionalbuffering capacity due to the dissolvedcarbonates, since they do not contain CaCC>3initially.

It may be noted that neither the model nor theacid buffering approach considers roottranslocation of ions from deeper to shallowhorizons. In calcitic soils, this couldsubstantially slow the acidification process.

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10OOO

C«I

1000:

100:

10000

3 1000u

OS

cV

sa.

10O 1000SMART Results

tooooSMART™esults

1000 10000

Figure 11. Calculated number of years to reachpH 5.5 for sensitive soils under thescenario conditions of C-8.

Figure 12. Calculated number of years to reachpH 5.5 for sensitive soils under thescenario conditions of C-9.

IC

ilts

MVKo

S•3•5V

1

woo ̂

n/V) -aWW ;

100-

10-

1-

/

/

/

/ '' •

/

100003

10SMART °R.lesults

1000 10000 10SMART °Riesults

1000 10000

Figure 13. Calculated number of years to reachpH 5.5 for sensitive soils under thescenario conditions of C-10.

Figure 14. Calculated number of years to reachpH 5.5 for sensitive soils under thescenario conditions of C-ll .

SUMMARY AND CONCLUDINGREMARKS

Atmospher i c emiss ions of acidicprecursors and their subsequent deposition on tosoil media is an important part of impactassessment studies. One can use differentapproaches in order to be able to predict the long-term impacts on various soil types. In addition tothe so-called qualitative evaluation, the mostcommonly used methodologies are themechanistic modelling and experimental ABCmethods. The following conclusions were

obtained from a comparative evaluation of the twoquantitative techniques:( i ) Even though the results of both techniques

agree with the conclusions of the qualitativeevaluation, there is considerablediscrepancy between the results of the twoquantitative methodologies.

(ii) For sensitive soils, utilization of the acidbuffering capacity approach to estimate thenumber of years to reach some presetcritical pH values gives more conservativeestimates (less number of years) ascompared to the mechanistic modelling

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approach. ACKNOWLEDGEMENT(i i i ) For rather insensitive calcareous soils,

utilization of the mechanistic modelling The authors are thankful to the Electricapproach to estimate the number of years to Power Development Company Ltd. of Japan andreach some preset critical pH values gives PARMAS of the Middle East Technicalmore conservative estimates (less number University for their help in conducting thisof years) as compared to the acid buffering research. The authors are grateful to Drs. W. Decapacity approach. Vries, M. Posch, J . Kämäri , and G.A.

It is, therefore, recommended that, in areas Oosterbaan for providing their valuable reportswhere different types of soils exist, both and computer software. Dr. Ertugrul Alp ofmechanist ic and acid buffering capacity Concord Environmental Corporation of Canada,approaches should be utilized in parallel. Dr. Gürdal Tuncel and Mr. Bora Arpacioglu ofThe results of these quantitative approaches the Middle East Technical University arecan then be judged in light of a qualitative appreciated for their deposition modellingevaluation. efforts.

REFERENCES

1. Alcomo J., Amann M., Hettelingh J., Holmberg P., Hordlijh L., Kämäri J., Kauppi L., Kauppi P.,Kornai G. and Mâkelä A., Acidification in Europe: A simulation model for evaluating controlstrategies. Ambio, 16, 232-245 (1987).

2 . Kauppi P., Kämäri J., Posch M., Kauppi L. and Matzner E., Acidification of forest soils: Modeldevelopment and application for analyzing impacts of acid deposition in Europe. EcologicalModeling, 33, 231-253 (1986).

3 . De Vries W., Posch M. and Kämäri J., Simulation of the long-term soil response to aciddeposition in various buffer ranges. Water, Air and Soil Pollut., 48, 349-390 (1989).

4 . De Vries W., Methodologies for the Assessment and Mapping of Critical Loads and of the ImpactAbatement Strategies on Forest Soils, The Winand Staring Centre for Integrated Land, Soil andWater Sources, Report No.46, Wageningen, Netherlands (1991).

5. Potential Soil Acidification Impacts of Oslo Redwater Upgrades, Concord EnvironmentalCorporation and Graecam Enterprises, Calgary, Alberta, Canada (1991).

6. Holowaychuck N. and Fessenden R.J., Soil sensitivity to acid deposition and the potential of soilsand hydrology in Alberta to the acidity of acidic inputs. Alberta Research Council, Earth SciencesReport No.87-1, Alberta, Canada (1987).

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