The Impact of Acid Deposition on Groundwater: A Review

W O R K I I V G P A P E R

THE IMPACT OF ACID DEPOSITION ON GROrnWATER: A m E [ q

Maria Holmberg

J u l y 1986 W-86-931

I n t e r n a t i o n a l I n s t i t u t e for Applied Systems Analysis

NOT FOR QUOTATION WITHOUT PERMISSION OF THE AUTHOR

THE IMPACT OF ACID DEPOSITION ON GROUNDWATER: A RENIEW

Maria Holmberg

July 1986 WP-86-31

Working Papers a r e interim r e p o r t s on work of t h e International Institute f o r Applied Systems Analysis and have received only lim- ited review. Views o r opinions e x p r e s s e d h e r e i n do not neces- s a r i l y r e p r e s e n t those of t h e Institute o r of i t s National Member Organizations.

INTERNATIONAL INSTITUTE FOR APPLIED SYSTEMS ANALYSIS 2361 Laxenburg, Austria

PREFACE

IIASABs Acid Rain p r o j e c t i s developing a set of computer models link- ing e n e r g y pathways, s u l f u r emissions, long r a n g e t r a n s p o r t and deposition of sulfur compounds with environmental e f f e c t s on f o r e s t soils, l a k e water and f o r e s t s . The main p u r p o s e of t h e p r o j e c t i s t h a t this model, known as RAINS, can b e used in international deliberations on reducing emissions of SOZ.

During t h e 1985 Young Scientists Summer Program (YSSP), Maria Holm- b e r g worked on modeling t h e impact of acid deposition on groundwater. S h e not only succeeded in producing a n excellent overview of these e f f e c t s ( t h e r e s u l t s of which are described in t h e Working P a p e r ) but a l s o s t a r t e d t h e development of a n additional submodel of RAINS. Due t o h e r excellent job, Maria Holmberg w a s awarded IIASA's P e c c e i Scholarship which enabled h e r t o continue h e r work on groundwater modeling in 1986. The r e s u l t s of t h e modeling will b e presented in a s e p a r a t e working p a p e r .

Leen Hordijk

Leader, Acid Rain P r o j e c t

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ⁱⁱⁱ

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ACKNOWLEDGEMENTS

A f i r s t draft of this report w a s read and commented upon by Stefan Kaden, Juha Kamari and Michael Starr. Pertti Hari, Lena Maxe and Annikki Makela reviewed the final version. The author wishes to thank the reviewers f o r valuable comments and fruitful discussions on the subject of the study.

ABSTRACT

Theoretical mechanisms and empirical evidence of t h e potential acidifi- cation of groundwater under f o r e s t e d soils in E u r o p e h a v e been reviewed in o r d e r t o provide a s t a r t i n g point f o r a n assessment of t h e acidification of groundwater in t h e s c o p e of t h e Acid Rain study c a r r i e d out at IIASA. Basic c h a r a c t e r i s t i c s of f o r e s t e d soil-water systems are p r e s e n t e d as a back- ground t o a discussion of t h e t r a n s p o r t and t h e r e l e a s e and r e t e n t i o n of ele- ments. The r e p o r t r e l a t e s t h e impact of acid deposition of groundwater t o t h e pedology, t h e climatic regime and t h e geohydrology of t h e region. Some a p p r o a c h e s of modeling t h e problem are reviewed, including dynamic models and sensitivity analysis. A p r o c e d u r e f o r assessing t h e impact of various e n e r g y pathways on t h e potential f o r acidification of groundwater i s out- lined. The method proposed combines a regional ranking of t h e s t a t i c sensi- tivity with a dynamic simulation of t h e impact of acid deposition on t h e chemistry of top soil.

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^vii

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TABLE OF CONTENTS

1. INTRODUCTION

2. STRUCTURE OF THE SOIL-WATER SYSTEM 2.1 Physical S t r u c t u r e

2.2 P r o p e r t i e s of t h e P h a s e s and t h e I n t e r f a c e 2.3 Cation Exchange Capacity and Base S a t u r a t i o n

3. TRANSPORT AND TRANSFORMATION IN THE SOIL-WATER SYSTEM 3.1 T r a n s p o r t

3.2 P r o t o n Production and Consumption 3.3 Impact of Acid Deposition o n Groundwater

4. MODELING THE IMPACT OF ACID DEPOSITION ON SOIL-WATER SYSTEMS 4.1 S t r u c t u r a l Models

4.2 Sensitivity Analysis

5. SUMMARY

APPENDIX : Mathematical Formulation of T r a n s p o r t REFERENCES

THE JMPACT OF ACID DEPOSITION ON GROUNDWATER- A

REXIEW

Maria Holmberg

1. INTRODUCTION

Groundwater under f o r e s t e d soils in Europe may b e a f f e c t e d by t h e deposition of sulfuric and n i t r i c a c i d s on vegetation and soil. T h e r e i s now increasing evidence t h a t acid deposition causes leaching of b a s e cations, aluminum, s u l f a t e and hydrogen ions t o groundwater. Recent changes in t h e chemical p r o p e r t i e s of soils and s u r f a c e waters in s m a l l watersheds in N o r t h e r n E u r o p e and in t h e N o r t h e a s t e r n United S t a t e s have been r e p o r t e d (e.g. Likens et al., 1977; Overrein et al., 1980; Wright et al., 1980;

Matzner, 1983; Goldstein et al., 1985; Nihlgard et al., 1985). Nutrient cycling in soil a p p e a r s t o b e disturbed and mobilization of metals from mineral s t r u c t u r e seems t o a c c e l e r a t e in c e r t a i n a r e a s d u e t o acidic p r e - cipitation (e.g. Norton et al., 1980; Hanson et al., 1982; Ulrich, 1983). The chemical composition of groundwater which i s r e c h a r g e d by infiltration

t h r o u g h t o p soil, i s likely t o change as a r e s u l t of c h a n g e s in t h e p r o p e r t i e s of t h e overlying soil strata. The o b s e r v e d r e c e n t t r e n d s in t h e concentra- tions of s u r f a c e waters t h a t are fed p a r t l y by groundwater, indicates i n c r e a s e d c o n c e n t r a t i o n s of aluminum and hydrogen ions in groundwater.

The t e r m acidification will in t h i s c o n t e x t b e used to d e n o t e increasing t r e n d s of hydrogen ion c o n c e n t r a t i o n s in soil solution, in t h e pool of exchangeable cations on soil p a r t i c l e s , or in groundwater. The i n c r e a s e in t h e f r a c t i o n of exchangeable hydrogen ions c a u s e s a n i n c r e a s e d mobiliza- tion of aluminum, i r o n , calcium, magnesium, potassium etc. from t h e minerals. Thus t h e acidification of soil may not n e c e s s a r i l y b e r e f l e c t e d as a d e c r e a s e of t h e pH of groundwater, but may a l s o lead t o a n i n c r e a s e in t h e alkalinity.

The consequences of acidification of soils and groundwater are t h r e e - fold. Firstly, hydrogen ions originating from p r e c i p i t a t i o n r e p l a c e calcium, magnesium, potassium and sodium ions on t h e e x c h a n g e s i t e s on soil p a r t i - cles. Under c e r t a i n hydrologic conditions t h e s e ions are leached o u t of t h e rooting zone of t h e plants. If t h e y r e a c h groundwater, t h e y c o n t r i b u t e to t h e h a r d n e s s of groundwater. Secondly, high concentrations of hydrogen ions, aluminum and i r o n a f f e c t soil micro-organsisms (e.g. ~ & g t h et a l . , 1980; Bewley and Parkinson, 1986). S u r f a c e w a t e r s r e c h a r g e d through soil and groundwater are likely t o r e f l e c t increasing c o n c e n t r a t i o n s of hydrogen and aluminum (e.g., Hultberg and Johansson, 1981). Thirdly. acid groundwater may c o r r o d e supply pipes (Levlin, 1978). Drinking water e x t r a c t e d from p r i v a t e wells, contaminated by aluminum, i r o n , c o p p e r , zinc and a s b e s t o s f i b r e s originating from supply pipes may violate human h e a l t h s t a n d a r d s (McDonald, 1985).

Groundwater i s a n important s o u r c e f o r t h e water supply in Europe.

In Denmark a l l drinking water is groundwater, in Austria, Belgium, t h e Federal Republic of Germany, Finland, France, G r e a t Britain, Italy, t h e Netherlands and Switzerland, 30-70% of t h e drinking water i s e x t r a c t e d from groundwater. Norway i s a n exception, where almost only s u r f a c e water i s used (IWSA, 1985). In t h e United S t a t e s , 50% of t h e population are s e r v e d by groundwater (Russell, 1978). Problems r e l a t e d t o t h e quantity and quality of groundwater h a v e been largely studied.

Groundwater pollution h a s s o f a r been a c a u s e of concern mainly in connection with local pollutants and point s o u r c e s (Young, 1981).

Reported pollutant s o u r c e s are a g r i c u l t u r e (Csaki and Endredi, 1981;

Oakes et al., 1981; Vasak et al., 1981), mining a c t i v i t i e s (Henton, 1981;

Luckner et al., 1985), urbanization and industrial a c t i v i t i e s (Kondrates, 1981). The pollution of groundwater d u e t o long r a n g e t r a n s p o r t of atmospheric pollutants h a v e not been extensively discussed o r r e p o r t e d .

Groundwater in regions with g r a n i t i c bedrock, shallow soils and humid climate may show comparatively high concentrations of hydrogen ions, independently of t h e deposition of anthropogenically derived acidic sub- stances. In t h e 1970's, Swedish groundwater w a s r e p o r t e d t o acidify (e.g. Hultberg and Johansson, 1981). Later, Aastrup and P e r s s o n (1984) a r g u e d t h a t t h e o b s e r v e d acidification of superficial groundwater in Sweden probably w a s d u e t o climatically induced extremely low groundwater levels, which led t o t h e oxidation of sulfur-containing minerals t o form sul- f a t e . No increasing t r e n d s of groundwater acidity have been o b s e r v e d in t h e Nordic countries, although changes in bicarbonate, calcium, magnesium and s u l f a t e concentrations h a v e been noticed, according t o a r e p o r t

e d i t e d by S o v e r i (1982).

In a g r i c u l t u r a l r e g i o n s in E u r o p e , intensive f e r t i l i z a t i o n c o n t r i b u t e s p r o b a b l y more t o g r o u n d w a t e r acidification t h a n long r a n g e t r a n s p o r t of a t m o s p h e r i c p o l l u t a n t s originating from e n e r g y production. The acidi- fying impact o n s o i l s a n d g r o u n d w a t e r c a u s e d by t h e a p p l i c a t i o n of nitrogen-based f e r t i l i z e r s i s discussed by Appelo (1982). Hoei jmakers

(1985) a n d P a c e s (1985).

This r e p o r t i s a review of t h e p o t e n t i a l impact of long r a n g e t r a n s p o r t of s u l f u r i c a n d n i t r i c compounds o n g r o u n d w a t e r quality. I t i s based o n l i t e r a t u r e c o n c e r n i n g o b s e r v e d a n d assumed influence of a c i d deposition o n soil-water systems. In t h e Acid Rain P r o j e c t of IIASA, s o i l a n d l a k e acidifi- c a t i o n following t h e deposition of s u l f u r h a v e previously b e e n modeled b y Kauppi et al. (1985) a n d Kamari et al. (1985). The f o c u s of t h i s s t u d y i s o n f o r e s t e d ecosystems, s i n c e a g r i c u l t u r a l s o i l s are p r i m a r i l y influenced by intensive management. The h y p o t h e s e s and e v i d e n c e p r e s e n t e d in t h e l i t e r a t u r e are compiled t o p r o v i d e a s t a r t i n g point f o r a n a t t e m p t t o assess t h e r i s k of g r o u n d w a t e r acidification o n a r e g i o n a l scale in E u r o p e . F u t u r e emission s c e n a r i o s r e s u l t i n g in v a r i o u s s u l f u r deposition p a t t e r n s are discussed b y Alcamo et a l . (1985). The question t o which e x t e n t a n y p o t e n t i a l c h a n g e i n g r o u n d w a t e r quality may a f f e c t water supply and r e q u i r e new technology f o r adjusting t h e quality t o human h e a l t h s t a n - d a r d s , i s beyond t h e s c o p e of t h i s study.

The r e p o r t i s divided into f o u r c h a p t e r s . C h a p t e r 2 summarizes b r i e f l y t h e physical s t r u c t u r e of t h e soil-water system a n d d i s c u s s e s t h e p r o p e r - t i e s r e l e v a n t f o r t h e t r a n s p o r t , r e t e n t i o n and r e l e a s e of elements. The p o t e n t i a l impact of a c i d deposition o n g r o u n d w a t e r i s discussed in

Chapter 3, a f t e r a presentation of t h e dynamics of t h e t r a n s p o r t of elements in t h e soil-water system and t h e production and consumption of hydrogen ions in t h e soil. Chapter 4 gives examples of two dif- f e r e n t a p p r o a c h e s on modeling t h e acidification p r o c e s s e s , dynamic models and sensitivity analysis.

2. STRUCTURE OF THE SOIL-WATER SYSTEX

2.1. Physical Structure

The uppermost l a y e r of t h e e a r t h ' s c r u s t i s a multiphase system with abiotic and biotic components. The dead f r a c t i o n of t h e solid phase consists of r o c k s and o t h e r a g g r e g a t e s of t h e minerals as well as organic material in various s t a g e s of decay. The liquid phase i s a solu- tion of w a t e r and elements entering with infiltration o r mobilized from t h e solid p h a s e o r t r a p p e d from t h e gaseous phase. The gaseous phase consists of a i r , with p a r t i a l p r e s s u r e s different from t h o s e of t h e ambient a i r . The t e r m soil will sometimes b e used t o denote t h e e n t i r e system of solid, liquid and gaseous phase, while t h e term soil solution will b e used f o r t h e liquid phase in t h e u p p e r , unsaturated zone and t h e term groundwater f o r t h e liquid phase in t h e d e e p e r , s a t u r a t e d zone. With t h e objective of studying t h e impact of acid deposition on soil and groundwater, t h e environment of t h e soil system may b e defined as t h e atmosphere, t h e vegetation and t h e s u r f a c e waters. The primary o b j e c t s of i n t e r e s t in connection with acidification are t h e temporal developments of t h e b a s e saturation and t h e concentrations of elements in t h e groundwater.

The long-term development of t h e concentration of elements in t h e d i f f e r e n t p h a s e s i s determined by t h e t r a n s p o r t of elements in t h e liquid and t h e gaseous p h a s e as well as by t h e t r a n s f e r of elements between t h e solid, liquid and gaseous phase. The dynamics of t h e t r a n s p o r t p r o c e s s e s and t h e mass t r a n s f e r s between t h e p h a s e s connected with t h e impact of acid deposition will b e discussed in C h a p t e r 3, p a r t i c u l a r l y t h o s e involving t h e liquid and t h e solid phase. A s a background to this, t h e physical s t r u c - t u r e of t h e soil-water system i s p r e s e n t e d h e r e . Generalizing slightly, a p i c t u r e of t h e v e r t i c a l p r o f i l e of f o r e s t soil c a n b e drawn as in Figure 1.

I. b

soil unsaturated zone :

-

v ₁

^-

soil solution subsoil

-

^{soil air}

fcapillary fringe

A

rater table ₁₁ ^{1 r}

saturated zone : -free groundwater w

Figure 1. The coarse s t r u c t u r e of a soil-water system ( a f t e r Lee, 1980)

Beneath t h e f o r e s t f l o o r vegetation t h e r e i s a l a y e r of t o p soil with a h i g h c o n t e n t of o r g a n i c matter. The t o p soil, or t h e rooting zone, i s

followed by t h e subsoil, a l a y e r of predominantly mineral soil which con- tains l e s s or no o r g a n i c matter. The subsoil may end with a l a y e r of h a r d r o c k , impermeable clay or some o t h e r formation which p r e v e n t s t h e t r a n - s p o r t of water. Soil and subsoil form a system of p a r t i c l e s and p o r e s show- ing d i a m e t e r s of varying dimensions. Colloidal p a r t i c l e s have d i a m e t e r s of t h e dimension of

<

1 nm, w h e r e a s g r a v e l and s t o n e s are much l a r g e r . Root channels a n d m a c r o p o r e s with diameters of t h e dimension

>

1 c m c o n t r i b u t e substantially to t h e h e t e r o g e n i t y of soil a n d subsoil.

Typically t h e p o r e s in t h e d e e p e r l a y e r s of t h e subsoil are s a t u r a t e d with water. This water i s r e f e r r e d to as f r e e groundwater ( a s opposed to confined groundwater, which may b e found beneath a n impermeable l a y e r of r o c k or clay). Above t h e groundwater t a b l e c a p i l l a r y f o r c e s c a u s e t h e water to move upwards, producing a diffusive boundary, called t h e capil- l a r y f r i n g e , between t h e s a t u r a t e d a n d t h e u n s a t u r a t e d zone.

2.2. Properties of the Phases and the Interface

The solid, liquid and gaseous p h a s e s of t h e soil-water system are c h a r a c t e r i z e d by v e r y d i f f e r e n t volumetric densities. The densities of t h e solid p h a s e are 1 0 0 to 1000 times l a r g e r t h a n t h o s e of t h e liquid p h a s e and t h e gaseous p h a s e s ( s e e Table 1). Consequently t h e volumetric c o n c e n t r a - tions of elements in t h e solid p h a s e a r e much l a r g e r than t h o s e in t h e o t h e r phases. The c o n c e n t r a t i o n s of elements in t h e d i f f e r e n t p h a s e s , as well as t h e i n t e r f a c i a l area and t h e electrochemical p r o p e r t i e s of t h e p a r t i - c l e s u r f a c e s , determine t h e rates at which t h e elements migrate from o n e p h a s e to a n o t h e r .

The area of t h e c o n t a c t s u r f a c e between t h e solid soil matrix and t h e soil solution depends on t h e geometry of t h e c r y s t a l l a t t i c e and t h e o r g a n i c matter content. The size of t h e soil p a r t i c l e s v a r i e s , giving r i s e t o interfacial a r e a s of dimensions in t h e r a n g e of

l o 3

t o

lo9

m m -3 ( s e e Table 2).

Table 1. Densities of t h e p h a s e s in t h e soil-water system.

P h a s e Specific Volumetric Volumetric

density f r a c t i o n density

g m - 3 m 3m -3 g m - 3

Solid 0 . 1 - 1 . 8 ~ 1 0 ~ 0.4

-

0.7 4 . 0 ~ 1 0 ~ - 1 . 3 ~ 1 0 ~ Liquid 1 . 0 ~ 1 0 ~ 0.03

-

^0.54 3 . 0 - 5 4 . 0 ~ 1 0 ~

Gaseous I . O X I O ~ 0.06

-

^0.27 0.6-2.7x102

i

Sources: Kuntze et al. 1983. Westman et al. 1985

1

Table 2. P r o p e r t i e s of t h e solid

-

solution i n t e r f a c e

Solid component S u r f a c e area S u r f a c e c h a r g e

m ' m -3 meq m 3

Sand

2-layer clay 3-layer clay Organic matter

S o u r c e s : Talibudeen, 1981; Kuntze et a l . ,1983.

The n e t c h a r g e of t h e solid soil components i s negative o r positive, depending on t h e concentration of hydrogen ions in soil solution. The dis- sociation of hydroxyls at t h e s u r f a c e s of t h e minerals, t h e dissociation of carboxyl g r o u p s and phenolic hydroxyls in soil o r g a n i c m a t t e r , c a u s e a negative c h a r g e which i n c r e a s e s with a decreasing concentration of hydrogen ions in soil solution (Talibudeen. 1981). The permanent negative

c h a r g e i s a r e s u l t of isomorphous substitution of elements of smaller positive c h a r g e ( ~ i + , ~ g ' + , ~ e ' + , A13+) f o r t h o s e of h i g h e r c h a r g e ( M ~ ~ + , A13+, s i 4 + ) in t h e c r y s t a l l a t t i c e . Polymeric aluminosilicate anions a l s o give r i s e t o c o n s t a n t negative c h a r g e . The permanent negative c h a r g e of t h e s u r f a c e of t h e soil components i s of dimensions in t h e r a n g e of

l o 4

t o 106meq m -3 ( s e e Table 2).

2.3. Cation Exchange Capacity and Base Saturation

The negative c h a r g e of t h e soil p a r t i c l e s c a u s e s them t o attract cations from soil solution, generating a c h a r g e distribution in t h e liquid film surrounding t h e colloids, i.e. t h e diffusive l a y e r . The c h a r g e distribution i s s u c h t h a t t h e concentration of cations in t h e diffusive l a y e r i s g r e a t e s t n e x t t o t h e colloid s u r f a c e and d r o p s away exponentially t o t h a t in t h e bulk solution. The e l e c t r i c double l a y e r model ( s e e e.g.Talibudeen, 1981), defines t h e cation e x c h a n g e c a p a c i t y as t h e e x c e s s of cations in t h e diffusive l a y e r o v e r t h a t in bulk solution. These cations are r e a d i l y exchangeable t o o t h e r s in bulk solution. The mechan- ism of a t t r a c t i n g c a t i o n s from soil solution gives t h e soil solid p h a s e t h e p r o p e r t y of a dynamic r e s e r v o i r of cations. The size of t h i s r e s e r v o i r , o r t h e cation e x c h a n g e c a p a c i t y , i s t o a c e r t a i n d e g r e e a function of t h e concentration of hydrogen ions in soil solution. This dependency i s s t r o n g e r when t h e c o n t e n t of o r g a n i c m a t t e r in t h e soil is h i g h e r . The f r a c t i o n of t h e cation e x c h a n g e c a p a c i t y which i s occupied by b a s e c a t i o n s (potassium, sodium, calcium, magnesium) i s r e f e r r e d to as t h e b a s e s a t u r a t i o n . The b a s e s a t u r a t i o n v a r i e s with t h e c o n c e n t r a t i o n s of ions in soil solution.

3. TRANSPORT AND TRANSFORMATION

IN

THE SOIL-WATER SYSTEM The temporal development of t h e concentrations of elements in t h e solid, liquid and gaseous phases of t h e soil is determined by t h e interaction with t h e environment, by t h e r a t e s of t h e t r a n s p o r t p r o c e s s e s in t h e liquid and t h e gaseous phase, by chemical r e a c t i o n s within t h e p h a s e s and by t h e rates of t h e transition p r o c e s s e s between t h e t h r e e phases. The long t e r m changes in t h e b a s e s a t u r a t i o n of t h e soil and t h e concentrations of hydrogen, aluminum, b i c a r b o n a t e and b a s e cations in t h e groundwater a r e r e s u l t s of t h e i n t e r a c t i o n of t h e various p r o c e s s e s o c c u r r i n g in t h e soil, in combination with t h e influence of t h e environment on t h e soil system.

Figure 2 is a schematic p i c t u r e of t h e interaction of t h e soil system with i t s environment and of t h e r o u t e s of element flux within t h e system.

Input of elements from t h e atmosphere o c c u r s through w e t and d r y deposition on soil s u r f a c e and vegetation ( l a , c , d ) . The soil system a c q u i r e s elements also from decaying o r g a n i c matter (2a.b). Removal of ele- ments from t h e soil system o c c u r s through t h e u p t a k e of n u t r i e n t s and water by t h e vegetation (2b) and through t h e leaching of elements t o s u r - f a c e water (6d,c). The t r a n s p o r t p r o c e s s e s within t h e soil system are dif- fusive and convective flow of solutes (6a,b) and diffusive flow of g a s (7).

The major p h a s e transition p r o c e s s e s within t h e soil system are t h e exchange of ions between t h e soil solution and t h e solid p h a s e (4a,b,c), t h e weathering of mineral s u r f a c e s (5) and reduction and oxidation p r o c e s s e s (3).

atmosphere

Figure 2. Major fluxes in t h e soil system and i t s environment. The boxes r e p r e s e n t element r e s e r v o i r s of t h e system. The environment i s marked by b r o k e n contours. The arrows r e p r e s e n t element fluxes : deposition l a , c , d ; gaseous e x c h a n g e l b ; o r g a n i c cy- cling 2 a , b ; reduction/oxidation 3; ion e x c h a n g e 4 a , b , c ; weathering 5; s o l u t e t r a n s p o r t 6a,b,c,d; gaseous t r a n s p o r t 7.

The f l u x e s may b e e i t h e r in t h e direction of t h e arrows or in t h e opposite direction.

L - -

- - - , - - - , - - - I

3.1. Transport

l b

T r a n s p o r t of elements within t h e soil system o c c u r s in t h e liquid and t h e gaseous p h a s e . Soil i s not a homogenous medium. The p o r e s i z e and

1 c

h e n c e t h e permeability v a r i e s with d e p t h and t h e c o n t e n t of o r g a n i c matter.

Id

V

Adhesive f o r c e s between soil p a r t i c l e s and water c a u s e small p o r e s to b e r

-

I

'

^2a

I vegetation

1 ^>

L,

^C

1

waterfilled b e f o r e t h e l a r g e r p o r e s . In d r y soil, t h e r e f o r e , w a t e r flows soil air

predominantly in t h e small p o r e s , w h e r e a s a h i g h e r water c o n t e n t may induce m a c r o p o r e flow. P u r e t r a n s p o r t n e v e r o c c u r s alone u n d e r field con-

2b

d e exchangeable

ions

h

ditions, b u t t r a n s f e r of elements between t h e p h a s e s , i.e. cation exchange,

5 6b

r - - ¹

mineral 4c surface I

3 groundwater

4

structure ^I water 1

L - - - - J

n u t r i e n t uptake, reduction and oxidation, always t a k e p l a c e simul- taneously t o diffusion and m a s s flow.

The maximum flow rates a r e determined by t h e values of t h e dif- fusivity p a r a m e t e r s and t h e hydraulic conductivity which r e f l e c t s t h e permeability of t h e soil. Theoretical values of diffusion coefficients are of dimensions in t h e r a n g e of 1 0 - ~ c r n ~ s in a i r and 1 0 - ~ c r n ~ s - ~ in solution ( s e e Table 1 A in Appendix). The t h e o r e t i c a l values of flow rate are n o t observed under field conditions, s i n c e t h e solutes are affected by vari- ous adsorption mechanisms as t h e y move in t h e soil. The contribution of t h e diffusion in t h e liquid phase i s usually small in comparison t o gravita- tional m a s s flow.

The rate of percolation, o r t h e rate of t r a n s p o r t of t h e precipitation through t h e u n s a t u r a t e d zone t o t h e s a t u r a t e d groundwater zone, depends on t h e rates of precipitation, evapotranspiration and t h e soil t e x t u r e . The slower t h e t r a n s p o r t , t h e longer t h e time available f o r t h e chemical r e a c t i o n s between t h e mobile phases and t h e solid p h a s e and f o r t h e migration of elements from one p h a s e t o a n o t h e r . The h i g h e r t h e rate of downward t r a n s p o r t of water, t h e l a r g e r t h e f r a c t i o n of elements in t h e r e c h a r g e which o r i g i n a t e d i r e c t l y from t h e precipitation. This f r a c - tion d e c r e a s e s with t h e depth of t h e unsaturated zone and with t h e i n c r e a s e of t h e rates of t h e transformation p r o c e s s e s in t h e unsa- t u r a t e d zone. A slow percolation rate allows f o r a long r e a c t i o n time with t h e soil and t h e b e d r o c k , which may c a u s e changes in t h e b a s e s a t u r a t i o n of t h e soil and a r e c h a r g e of low acidity but high concentrations of o t h e r elements. A f a s t percolation rate, on t h e o t h e r hand, may c a u s e acid r e c h a r g e with low concentrations of o t h e r ions.

3.2. Proton Production and Consumption

The transformation p r o c e s s e s o c c u r along t h e way as elements are t r a n s p o r t e d in solute o r gaseous form in t h e soil and subsoil horizons.

They include mineralization and assimilation of elements, ion exchange between solution and t h e solid phase, weathering of t h e mineral s t r u c t u r e , reduction/oxidation p r o c e s s e s and complexation processes.

The rates of t h e s e processes depend t o a c e r t a i n e x t e n t on t h e con- c e n t r a t i o n s of cations and anions in t h e soil solution, which in t u r n are determined by t h e rates of t h e transformation processes. This feedback system i s influenced by t h e environment through t h e input of atmospheric deposition of various concentration and through t h e uptake of solutes by t h e vegetation. The evapotranspiration of water from t h e soil causes a n i n c r e a s e in t h e solute concentrations.

The potential impact of t h e deposition of hydrogen ions originating from anthropogenically derived sulfuric and n i t r i c compounds on t h e rates of soil p r o c e s s e s h a s been discussed by Abrahamsen et al. (1975), Reuss (1975), M a l m e r (1976), Tamm (1976), Likens et al. (1977), Mayer and Ulrich (1977), Norton (1977), Wiklander (1979), Abrahamsen (1980), Cowling (1980). Overrein et a1 (1980), McFee (1980), van Breemen et al. (1982). van Breemen et al. (1984), Matzner (1983), Ulrich (1983), P a c e s (1985) among o t h e r s .

With t h e objective of studying t h e acidification of soil and w a t e r , t h e soil p r o c e s s e s may b e divided into proton producing and p r o t o n consum- ing processes. The following presentation i s based on t h e thorough dis- cussion of t h e p r o t o n balance in soil published by van Breemen et al.

(1984). They compare t h e i n t e r n a l and e x t e r n a l s o u r c e s of p r o t o n s in ecosystems. The r e s u l t of t h e quantitative analysis i s t h a t significant amounts of hydrogen ion and aluminum are leached from acidic soils with low i n t e r n a l rates of p r o t o n mobilization due t o a n e x t e r n a l input of anthro- pogenically d e r i v e d acidic substances.

Following van Breemen et al. (1984) t h e p r o t o n producing and con- suming p r o c e s s e s in soil-water systems have been compiled into Table 4.

Table 4. P r o t o n production and consumption in soil-water systems (van Breemen et al. 1984)

Production Consump tion

Atmospheric input Drainage

Assimilation of cations Mineralization of cations Mineralization of anions Assimilation of anions Dissociation of a c i d s Protonation of a c i d s

Oxidations Reductions

Cation adsorption Cation weathering

Anion weathering Anion adsorption

These p r o c e s s e s are in t h e following formulated as r e a c t i o n equations, using a notation where t h e s u b s c r i p t s stands f o r t h e solid phase, aq f o r soil solution and o r g f o r o r g a n i c matter. In r o o t cation uptake, i.e. assimila- tion of cations by vegetation o r g a n i c a c i d s dissociate and o r g a n i c s a l t s are formed in t h e plant, whereby equivalent amounts of hydrogen ions are pro- duced corresponding t o t h e amounts of calcium, magnesium, potassium, sodium and ammonium ions assimilated. In mineralization of cations from

decomposing o r g a n i c matter, o r g a n i c a c i d s are formed whereby hydrogen ions are consumed and cations are mobilized. van Breemen et a l . (1984) give t h e following r e a c t i o n equation:

where M n + comprises c a 2 + , M ~ ' + , N a + ,

K +

and hH4+. In t h e mineralization of anions from decomposing o r g a n i c matter, anions are r e l e a s e d f r o m o r g a n i c s a l t s , whereby o r g a n i c a c i d s are formed. Equivalent amounts of hydrogen ions are r e l e a s e d , corresponding to t h e amounts of phosphate, n i t r a t e and s u l f a t e mobilized. In assimilation of anions, o r g a n i c salts are formed and hydrogen ions are consumed. Mineralization and assimilation of s u l f a t e and n i t r a t e involves oxidation and reduction p r o c e s s e s , but t h e n e t r e a c t i o n s are equivalent to equation given below, a c c o r d i n g to van Breemen et a l . (1984).

w h e r e An - comprises H S 0 6 , NO< and SO:-. Weathering of cationic com- ponents from mineral s t r u c t u r e c a n p a r t l y b e s e e n as cation exchange between soil solution and soil p a r t i c l e s in which equivalent amounts of hydrogen ions are bound when sodium, potassium, calcium, magnesium and aluminum ions are r e l e a s e d from t h e p a r t i c l e s u r f a c e s and mineral s t r u c - t u r e . In adsorption of cations on mineral s u r f a c e s , hydrogen ions are mobil- ized and cations are immobilized:

where M n ⁺ comprises c a 2 + ,

M ~ ~ + ,

Nu +,

K +

and A [ ~ + .

The interaction of t h e s e processes is complex because of t h e l a r g e variations in t h e rates of t h e reactions and because of t h e coupling with t h e t r a n s p o r t of solutes. The reaction r a t e s depend on t h e concentrations of cations and anions and consequently also on t h e water content. The concen- t r a t i o n s are in t h e i r t u r n influenced by t h e t r a n s p o r t , which contributes t o t h e dilution o r distillation of t h e solutes.

In o r d e r t o analyze quantitatively t h e interactions of t h e processes above, t h e reaction equations would b e written in t h e form of differential equations. To formulate t h e differential equations giving a quantitative approximation of t h e change p e r unit time in t h e volumetric concentrations of cations and anions, detailed knowledge of t h e reaction rates is required.

For t h e time being, approximations of t h e rate of ion exchange, weathering and t h e cycling of elements in biomass a r e available f o r c e r t a i n specific minerals and soil t y p e s (e.g. Likens et al. 1977; Matzner 1983). Given approximations of t h e rates of t h e soil p r o c e s s e s and weathering in t h e s a t u r a t e d zone, dynamic models combining t h e transformation processes with t h e t r a n s p o r t may b e written. Some attempts in this direction are r e f e r r e d t o in c h a p t e r 4.1.

3.3. I m p a c t o f A c i d D e p o s i t i o n on G r o u n d w a t e r

The concentrations of elements in groundwater depend on t h e chemical composition of t h e r e c h a r g e from t h e unsaturated zone, on t h e mineral com- position and t h e weathering rate in t h e s a t u r a t e d zone, and on t h e residence time of groundwater.

The c o m p o s i t i o n of t h e r e c h a r g e t o g r o u n d w a t e r i s influenced by t h e r e s i d e n c e time of t h e w a t e r in t h e u n s a t u r a t e d zone. The ion e x c h a n g e e q u a t i o n s are almost instantaneous, b u t t h e l o n g e r t h e time a v a i l a b l e f o r c o n t a c t between t h e s o i l solution a n d t h e m i n e r a l s u r f a c e s , t h e h i g h e r t h e c o n t e n t of w e a t h e r e d c a t i o n s in t h e r e c h a r g e to g r o u n d w a t e r . The r e s i d e n c e time in t h e u n s a t u r a t e d zone i n c r e a s e s with t h e d e p t h a n d d e c r e a s e s with t h e p e r m e a b i l i t y of t h e soil. F u r t h e r m o r e , t h e m i n e r a l com- position of t h e s o i l a n d t h e o r g a n i c m a t t e r c o n t e n t influence t h e r e c h a r g e composition, t h r o u g h t h e i r impact o n t h e rates of ion e x c h a n g e a n d weath- e r i n g .

The w e a t h e r i n g r a t e a n d t h e m i n e r a l composition of t h e b e d r o c k in t h e s a t u r a t e d zone d e t e r m i n e t h e alkalinity p r o d u c t i o n rate in t h e g r o u n d w a t e r . C a l c a r e o u s b e d r o c k i s e a s i l y w e a t h e r e d , w h e r e a s s i l i c a t e b e d r o c k w e a t h e r s slowly. Examples of t y p i c a l weathering r e a c t i o n s , as given by Stumm a n d Morgan ( 1 9 8 1 ) , show t h e weathering p r o d u c t s of some minerals.

Calcite :

C a C 0 3

+

H 2 0

= ca2+ +

HCOQ

+

OH-

A n o r t h i t e :

CaA120,

+

3 H 2 0

= ca2+ +

2 0 H -

+

k a o l i n i t e Kaolinite :

A1 .#205(0H)4

+

5 H 2 0

=

2H$i04

+

g i b b s i t e

Gibbsite :

A1,03x3H,0

+

2H,O

=

2AL (OH),

+

2 ~ '

Thus t h e weathering of c a l c i t e and a n o r t h i t e produces calcium and hydroxyls. If kaolinite weathers f u r t h e r t o form gibbsite, t h e production of p r o t o n s cancel t h e production of hydroxyls connected with t h e weathering of a n o r t h i t e . The dissolution of 2Al(OH)4 f u r t h e r consumes p r o t o n s and p r o d u c e s ~ 1and ~ water ' under specific values of pH.

The residence time of t h e groundwater r e s e r v o i r s determine t h e time available f o r t h e weathering reactions. The r e s i d e n c e time of groundwater i s determined by t h e rates of r e c h a r g e , d i s c h a r g e and e x t r a c t i o n , as well as by t h e size of t h e aquifer. The climatic regime influences t h e rate of r e c h a r g e through t h e v a r i a b l e s t e m p e r a t u r e and vegetation, which t o g e t h e r a f f e c t t h e rates of precipitation and evapotranspiration. If t h e r e i s no s u r f a c e runoff, t h e rate of r e c h a r g e may b e calculated as t h e differ- e n c e between t h e rates of precipitation and evapotranspiration. The rate of r e c h a r g e i n c r e a s e s with t h e hydraulic conductivity of t h e soil, which depends on soil t e x t u r e and water content. The rates of r e c h a r g e and d i s c h a r g e depends on t h e relief of t h e region, on t h e physical location of t h e a q u i f e r , and on whether i t i s confined or unconfined.

The r e s i d e n c e time of groundwater i s a l s o a measure of t h e reversibil- ity of changes t h a t o c c u r in t h e chemical composition. The s h o r t e r t h e r e s i d e n c e time, t h e f a s t e r t h e chemistry of groundwater reacts t o t h e com- position of t h e r e c h a r g e and t h e more r e v e r s i b l e are t h e changes in t h e chemical composition of groundwater.

On t h e b a s i s of t h e discussion above concerning t h e t r a n s p o r t a n d t h e transformation p r o c e s s e s , i t may b e concluded t h a t t h e deposition of acidi- fying s u b s t a n c e s originating from s u l f u r i c and n i t r i c compounds may contri- bute significantly to changes in t h e c o n c e n t r a t i o n s in groundwater, if t h e e x t e r n a l load is high in comparison to t h e i n t e r n a l production of protons.

F u r t h e r m o r e , t h e most p r o b a b l e impact o n t h e c o n c e n t r a t i o n s in groundwa- ter may b e classified as follows.

1 . S u r f i c i a l groundwater t a b l e , h i g h l y permeable o v e r b u r d e n , s l o w l y w e a t h e r a b l e b e d r o c k a n d s h o r t r e s i d e n c e time of groundwater may imply increasing t r e n d s of h y d r o g e n ion concentration.

2 . S u r f i c i a l groundwater t a b l e , h i g h l y permeable o v e r b u r d e n , h i g h l y w e a t h e r a b l e b e d r o c k a n d s h o r t r e s i d e n c e time of groundwater may imply increasing t r e n d s of cations originating from t h e mineral s t r u c - t u r e ( c a l c i u m , m a g n e s i u m , s o d i u m , p o t a s s i u m ) .

3. S u r f i c i a l groundwater t a b l e and g r a n i t i c b e d r o c k may imply i n c r e a s - ing t r e n d s of a l u m i n u m .

4. Deep groundwater t a b l e and Long r e s i d e n c e times of groundwater may imply t h a t t h e deposition of acidic s u b s t a n c e s will h a v e n o i m p a c t of t h e c o n c e n t r a t i o n s in groundwater.

5 . Short r e s i d e n c e times of groundwater may imply t h a t changes t h a t o c c u r in t h e chemical composition of groundwater are well r e v e r s i b l e .

4. MODELING THE IMPACT OF ACID

DEPOSrrION ON SOIL-WATER SYSTEMS

The need f o r predicting t h e development of groundwater and soil under t h e influence of acidic deposition h a s initiated a number of modeling e f f o r t s . S t r u c t u r a l models, based on physical and geochemical principles, h a v e been developed f o r locally defined soil s i t e s and catchments (Reuss, 1980; Arp, 1983;Booty and Kramer, 1984, Christophersen et al., 1984;

Bergstrom et al., 1985; Cosby et al., 1985a,b; Gherini et al., 1985; Holmberg et a l . , 1985). Few models of regional acidification e x i s t , exceptions being t h o s e developed by Kauppi et at. (1985) and Kamari et al. (1985).

Another a p p r o a c h t o assessing t h e impact of acid deposition on soil- water systems, besides simulating a c t u a l concentrations of elements in t h e soil and water, i s t o e v a l u a t e t h e regional potential f o r acidification on t h e basis of a sensitivity analysis. Work on mapping t h e sensitivity of soils and groundwater t o acidification h a s been r e p o r t e d by s e v e r a l a u t h o r s (Bache, 1980; McFee, 1980; Norton, 1980; P e t e r s o n , 1980; J a c k s and Knutsson, 1982;

Axelson and Karlqvist, 1984).

4.1. Structural Models

S t r u c t u r a l simulation models of acidification d e s c r i b e geochemical and physical p r o c e s s e s which c a u s e some c r u c i a l v a r i a b l e s t o change with time.

The mechanisms of soil p r o c e s s e s are often w e l l known and t h e main problem r e l a t e d t o modeling l i e s in t h e quantification of t h e rates of t h e p r o c e s s e s . The s t r u c t u r a l models a r e e x p r e s s e d as differential equations, describing t h e change p e r unit time in t h e state v a r i a b l e s o r as a l g e b r a i c equations, describing equilibrium r e a c t i o n s . Their mathematical r e p r e s e n t a t i o n s show various d e g r e e s of synthesizing, o r lumping, depending on t h e need t o

d i f f e r e n t i a t e t h e dynamics of s e p a r a t e p r o c e s s e s and v a r i a b l e s within t h e system.

With t h e o b j e c t i v e of keeping a low level of lumping, i.e. obtaining a high level of differentiating, o n e may a r g u e t h a t information on t h e t h r e e - dimensional distributions of t h e following v a r i a b l e s i s needed in o r d e r t o simulate t h e impact of acid deposition on groundwater : 1 ) cation e x c h a n g e c a p a c i t y , 2) weathering rate 3) permeability and 4) depth of groundwater t a b l e . This d a t a i s p r e s e n t l y not available o n a regional scale.

A conceivable synthetizing s t r a t e g y would b e t o lump t h e v e r t i c a l dis- t r i b u t i o n s of t h e cation e x c h a n g e c a p a c i t y and t h e weathering rate. In anal- ogy with Kauppi et al. (1985), t h e s e p a r a m e t e r values could b e considered s t a t i c in c e r t a i n compartments of t h e v e r t i c a l soil and subsoil profile. The problem of simulating t h e mixing and horizontal flow in t h e s a t u r a t e d zone might b e a p p r o a c h e d t h r o u g h a system of linked r e s e r v o i r s , in analogy with C h r i s t o p h e r s e n and Wright (1981) and Gherini et al. (1985). Such detailed calculations on t h e E u r o p e a n s c a l e would, however, r e q u i r e c o n s i d e r a b l e computer r e s o u r c e s .

In t h e following, r e f e r e n c e s are made t o work o n modeling t h e impact of acid deposition o n s p e c i f i c soil s i t e s and small catchments. Reuss (1980) developed a model f o r predicting t h e most likely effect of acid p r e c i p i t a - tion on t h e leaching of c a t i o n s from noncalcareous soils. The model i s based on equilibrium between solution ions and s o r b e d ions and d e s c r i b e s s u l f a t e adsorption and r e a c t i o n s involving hydrogen and calcium, b i c a r - bonate and aluminum.

Christophersen and Wright (1981) developed a model f o r analyzing t h e sulfate budget of t h e Birkenes-catchment in s o u t h e r n Norway. The hydrolog- ical p a r t of t h e model i s a lumped two-reservoir a p p r o a c h , which simulates infiltration t o t o p soil, s u b s u r f a c e flow t o d e e p e r l a y e r s of soil and d i s c h a r g e t o t h e stream. L a t e r , t h i s model w a s extended t o include t h e dynamics of hydrogen ions, calcium, magnesium and aluminum in t h e catch- ment (Christophersen et al. 1982). These models, as well as a f u r t h e r extended model, which includes sulfate chemistry in snowpack and soil, applied t o t h e catchment of Storgama (Christophersen et a l . 1984), a p p e a r v e r y well suited f o r simulating s u b s u r f a c e flow in a small catchment and f o r i n t e r p r e t i n g t h e extensive d a t a associated with t h e catchment budgets.

Arp (1983) modeled t h e losses of hydrogen, aluminum, iron, sodium, potassium, magnesium and calcium from soils under a constant inflow of hydrogen ions. The calculations a r e based on t h e assumption t h a t cations are r e t a i n e d in t h e soil following a n adsorption isotherm. The simulations were verified t o a g r e e r a t h e r well with experimental r e s u l t s (Arp and Ram- n a r i n a , 1983).

Booty and Kramer (1984) developed a model f o r t h e simulation of water and hydrogen ion flow through a f o r e s t e d catchment. They used a l i n e a r dif- fusion equation t o d e s c r i b e groundwater flow and included evaporation, percolation, cation exchange and mineral weathering in t h e soil. They con- cluded t h a t t h e key p a r a m e t e r s t h a t c o n t r o l t h e rate of acidification of a catchment, besides t h e a c t u a l acid loading r a t e s , are t h e r a t i o of infiltra- tion t o percolation, soil depth and t h e acid neutralizing capacity of t h e soil horizons (Booty and Kramer, 1984).

A mathematical model describing quantitatively anion r e t e n t i o n , cation exchange, primary mineral weathering, aluminum dissolution and C02 solu- bility h a s been developed by Cosby e t al. (1985a, 1985b). Equilibrium equa- tions are used t o account f o r t h e cation exchange r e a c t i o n s and t h e inor- ganic aluminum and c a r b o n reactions. These equations are linked t o dynamic equations based on input-output mass balances of t h e major ions in deposi- tion, t o provide f o r t h e long-term temporal development of t h e r e s p o n s e of t h e catchment t o changes in atmospheric deposition. Cosby et al. (1985b) have applied t h e model t o a small f o r e s t e d catchment in Virginia, recon- s t r u c t i n g historical changes in s u r f a c e water quality f o r t h e last 1 4 0 y e a r s , and forecasting f u t u r e water quality under t h r e e d i f f e r e n t deposition scenarios.

Bergstrom et al. (1985) developed models f o r t h e simulation of n a t u r a l short-term variations in alkalinity and pH in running waters. The hydro- chemical p r o c e s s e s a r e modeled in a semiempirical way without t h e assump- tion of a complete hydrochemical mass balance. The dynamics of t h e models are governed by simple hydrochemical subroutines with a few coefficients which are found by calibration f o r a given basin. The hydrological p a r t u s e s daily t o t a l s of precipitation and daily mean t e m p e r a t u r e f o r t h e calculation of t h e groundwater level. The models c a n b e used t o d e t e c t long t e r m changes in t h e acid s t a t u s of s u r f a c e waters from s h o r t t e r m n a t u r a l varia- tions, but t h e y are not intended t o simulate t h e impact of changes in deposi- tion.

A s t r u c t u r a l model of t r a n s p o r t and local p r o c e s s e s in f o r e s t soil t h a t influence t h e temporal development of t h e v e r t i c a l distribution of water and solutes in t h e soil, h a s been developed by t h e a u t h o r (Holmberg et al.,

1985). The model uses experimentally determined values f o r t h e v e r t i c a l distribution of t h e cation exchange capacity a t d i f f e r e n t soil s i t e s . A lumped version of t h i s model h a s been used in estimating t h e impact of acid deposi- tion on t h e productivity of Finnish f o r e s t s up t o t h e y e a r 2040 (Hari and Raunemaa, 1985). The simulation r e s u l t s have not y e t been checked against historical r e c o r d s .

In t h e s c o p e of t h e Integrated Lake-Watershed Acidification Study, Gherini et a l . (1985) developed a model t o p r e d i c t changes in s u r f a c e water acidity as a r e s p o n s e t o changes in t h e deposition. The model w a s applied t o two geologically d i f f e r e n t basins in t h e Adirondack Mountains. The model calculated flow through canopy, soil, streams and l a k e s on t h e basis of hydraulic gradients. Physical and chemical p r o c e s s e s were simulated by dynamic and equilibrium expressions accounting f o r t h e mass t r a n s f e r s between gaseous, liquid and solid phases.

Few dynamic models e x i s t on a regional scale. Exceptions are t h e soil acidification model by Kauppi et a l . (1985) and t h e l a k e acidification model by Kamari et al. (1985). The soil acidification model by Kauppi et al. (1985) i s based on t h e assumption t h a t t h e incoming acid flow dimin- ishes t h e b a s e s a t u r a t i o n and a f f e c t s t h e aluminum solubility.

Kamari et al. (1.905) h a v e developed a dynamic model f o r t h e simulation of regional acidification of s u r f a c e waters. The model i s a submodule in t h e IIASA Acid Rain Model. The IIASA soil acidity model (Kauppi et al., 1985) t a k e s account of soil solution chemistry. The convective flows of ions from t o p soil t o d e e p e r zones of soil and t o s u r f a c e waters are estimated with a f o u r - r e s e r v o i r lumped hydrological submodule, based on t h e Birkenes model by Christophersen and Wright (1981). The f r e s h w a t e r acidification model

h a s b e e n applied t o individual catchments in s o u t h e r n Finland (Kamari e t a l . 1985). I t a p p e a r s well suited f o r t h e description of regional changes in s u r - face water chemistry.

4.2. Sensitivity Analysis

The sensitivity of a n ecological system t o variations in c e r t a i n driving functions is a measure of t h e system's r e s p o n s e to t h o s e variations. The r e s p o n s e of a n ecological system i s t h e change in t h e values of t h e state v a r i a b l e s , incited by changes in t h e driving functions. A p a r t from t h e driv- ing functions, c e r t a i n physical system c h a r a c t e r i s t i c s , or sensitivity indica- t o r s , determine t h e amplitude of t h e r e s p o n s e and t h e r e s p o n s e time of t h e system. The sensitivity of a n ecological system may b e defined t o i n c r e a s e with t h e amplitude of t h e r e s p o n s e . If t h e sensitivity indicators are chosen s u c h t h a t t h e y are independent of time and of t h e driving functions, i t i s possible to d i f f e r e n t i a t e between t h e impact of physical c h a r a c t e r i s t i c s on t h e o n e hand and t h e temporal development of t h e driving functions o n t h e o t h e r hand.

The r e s p o n s e time of a system with r e s p e c t t o a c e r t a i n p r o c e s s i s a n i n d i c a t o r of t h e r e v e r s i b i l i t y of t h a t p r o c e s s . If a system reacts v e r y slowly to t h e driving functions, i.e. if t h e system i s highly i n e r t , changes t h a t o c c u r in t h e state v a r i a b l e s are h a r d l y r e v e r s i b l e . In assessing t h e a n t h r o - pogenic impact on ecological systems, both t h e sensitivity and t h e i n e r t i a of t h e systems are t a k e n i n t o account. In t h e long r u n , a highly sensitive sys- t e m i s worse off if i t i s a l s o v e r y i n e r t , whereas a quickly r e a c t i n g system may r e p r e s e n t t h e worst case in t h e s h o r t r u n . The b e s t case with r e s p e c t to environmental damage, i s a n i n e r t system with l o w sensitivity t o changes

in t h e driving functions. An assessment on t h e basis of sensitivity and iner- t i a , i s necessarily qualitative in n a t u r e . This i s p a r t l y a n advantage, as t h e i n h e r e n t u n c e r t a i n t i e s a r e not concealed in quantitative calculations.

With t h e objective of assessing t h e impact of acid deposition on ground- water on a regional s c a l e , a n a l t e r n a t i v e t o simulating a c t u a l chemical con- c e n t r a t i o n s in t h e groundwater would b e t o evaluate t h e sensitivity t o aci- dification and t h e reversibility of t h e acidification of groundwater. A set of regional indicators may include t h e t y p e of bedrock (mineral composition, weathering r a t e ) , t h e t y p e of o v e r b u r d e n (depth, permeability, cation exchange capacity), t h e t y p e of a q u i f e r (residence time, activity) and climatic conditions (precipitation, evapotranspiration). A temporal evalua- tion of t h e impact of acid deposition may t h e n b e performed by combining t h e s t a t i c sensitivity map with simulations of t h e dynamics of t o p soil.

In t h e following, some work on t h e sensitivity of soils and groundwater i s cited from t h e l i t e r a t u r e . In most c a s e s , no distinction h a s been made between t h e s t a t i c sensitivity and t h e p r o p e r t i e s which change with time due t o acid deposition. This i s quite a p p r o p r i a t e , if t h e objective i s t o provide a n analysis of t h e situation at a c e r t a i n fixed time, and not t o assess t h e impact of various f u t u r e deposition p a t t e r n s .

The sensitivity of soils t o acidification h a s been studied by s e v e r a l a u t h o r s (Bache, 1980; McFee, 1980; P e t e r s o n , 1980). Indicators of sensi- tivity in t h e s e studies include t h e t o t a l cation exchange c a p a c i t y of t h e soil, t h e b a s e s a t u r a t i o n , t h e c a r b o n a t e content of t h e mineral f r a c t i o n and t h e hydrological conditions of t h e soil.

The sensitivity of groundwater h a s been studied both in t h e c a s e of local pollutants (Aust, 1983; H a e r t l e 1983; Schenk, 1983; Suais- Parascandola and Albinet, 1983) and in t h e c a s e of long r a n g e t r a n s p o r t of acidifying substances (Norton, 1980; Axelsson and Karlqvist, 1984).

Aust (1983) h a s developed sensitivity maps f o r t h e groundwater r e s o u r c e s of t h e Federal Republic of Germany. The sensitivity i s classified on t h e basis of t h e a q u i f e r type, t h e kind of mineral s t r u c t u r e t h e percolat- ing water i s in c o n t a c t with and t h e depth of t h e overburden. Schenk (1983) discusses t h e problems of relating t h e German hydrochemical maps t o time and points out t h e importance of complementing knowledge of t h e anthropogenical a n d geological impact on groundwater quality with knowledge of t h e a c t u a l development of t h e quality with time. The French a p p r o a c h of t h e mapping of groundwater sensitivity is commented by Suais-Parascandola and Albinet (1983). I t is a qualitative evaluation of t h e r i s k of groundwater pollution in d i f f e r e n t a r e a s , based on t h e a p t i t u d e of local pollutants t o b e t r a n s p o r t e d t o t h e a q u i f e r s , coupled with assess- ments of t h e consequences of polluted groundwater.

Norton (1980) identified meteorology, pedology and geology as t h e major c h a r a c t e r i s t i c s of t h e landscape which r e n d e r a n area susceptible t o t h e impact of acid precipitation. He classified t h e geology of t h e bedrock in f o u r c a t e g o r i e s on t h e basis of t h e buffering capacity and produced maps of some states in E a s t e r n

U.S.,

showing geologic boundaries according t o t h i s classification. In some instances, areas of b e d r o c k s in t h e two c l a s s e s with low buffering capacity have been found t o show aci- dification of s u r f a c e waters (Norton, 1980).

Axelsson and Karlqvist (1984) developed models f o r analyzing t h e sen- sitivity of s u p e r f i c i a l groundwater t o acidification. The sensitivity indica- t o r s used were weathering ability of t h e bedrock, soil t e x t u r e , soil type, lime content of t h e soil, t h e amount of runoff and t h e t y p e of relief of t h e area. The t o t a l sensitivity of a n area w a s calculated as t h e weighted sum of t h e values of t h e indicators. The weighting coefficients w e r e chosen t o r e f l e c t t h e o b s e r v e d acidification in d i f f e r e n t sites.

Their work w a s p a r t of a n extensive study of t h e sensitivity of Swedish groundwater t o acidification, c a r r i e d o u t at t h e Royal Institute of Technol- ogy, as r e p o r t e d by J a c k s and Knutsson (1982). L a r g e d a t a b a s e s of groundwater composition in Sweden w e r e compiled.

A brief s u r v e y of t h e l i t e r a t u r e concerning mechanisms and observa- tions of t h e acidification of soil-water systems indicates t h a t increasing t r e n d s of deposition of n i t r i c and sulfuric compounds would have a n impact on t h e concentrations of elements in groundwater in regions c h a r a c t e r i z e d by s u r f i c i a l groundwater t a b l e s and s h o r t r e s i d e n c e times. If t h e o v e r b u r - den i s highly permeable and t h e bedrock slowly weatherable, t h e groundwa- ter i s likely t o show increasing t r e n d s of hydrogen ions. If, on t h e o t h e r hand, t h e o v e r b u r d e n is poorly permeable o r t h e bedrock easily weathered, t h e groundwater i s more likely t o show increasing t r e n d s of cations o r i - ginating from t h e mineral s t r u c t u r e (calcium, magnesium, potassium, alumi- num). Areas with long groundwater r e s i d e n c e times r e a c t more slowly t o changes in t h e deposition. Damage, o n c e o c c u r r e d i s a l s o not s o easily r e v e r s i b l e in such areas.

The probability of f u t u r e e n e r g y pathways and corresponding p a t t e r n s of deposition of acidifying substances i s not a d d r e s s e d in t h i s r e p o r t . S e v e r a l models of t h e impact of acid deposition on soil a r e documented in t h e l i t e r a t u r e ; some of which are r e f e r r e d to in t h i s r e p o r t . Such models could b e extended t o simulate weathering and t r a n s p o r t in t h e s a t u r a t e d zone as w e l l . In o r d e r to p r e d i c t quantitatively t h e impact on groundwater quality on a regional s c a l e , v e r y detailed s p a t i a l distributions of geological, pedological and hydrological c h a r a c t e r i s t i c s are r e q u i r e d . These are to d a t e not available. Furthermore, a simulation of t h e t h r e e dimensional flows in t h e s a t u r a t e d zone i s not feasible on a regional scale. An a p p r o a c h , based on t h e sensitivity of groundwater a q u i f e r s , i s proposed as a n a l t e r n a t i v e t o dynamic modeling. The regional sensitivity of groundwater to acidification could b e combined with a regional simulation of t h e impact of acid deposi- tion on t h e leaching of elements from t o p soil.

APPENDIX-

MATHEMATICAL FOEWULATION OF TRANSPORT

Assuming t h a t t h e solid soil matrix i s a physically homogenous sys- tem of p o r e s and p a r t i c l e s , diffusion equations based on Fick's law and mass conservation c a n b e used t o d e s c r i b e diffusive flow of elements in t h e soil. The equations h a v e been compiled and modified t o a uniform notation from Hillel (1971) and Greenland and Hayes (1981).

The following notation i s used : x,y,z f o r t h e s p a t i a l coordinates, t f o r time, 8 (x,y,z,t) (m3m -3) f o r t h e volumetric water content, @, (x.y,z,t) (m3m -3) f o r t h e volumetric air content, M(x,y,z,t) (mol m -3) f o r t h e volumetric element content in t h e liquid phase. Mg (x.y,z,t) (mol m -3) f o r t h e volumetric element c o n t e n t in t h e gaseous phase, q(x,y,z, t) (m3m -'s -I) f o r t h e water flow, q, (x,y,z,t) (mol m -'s -I) f o r t h e flow of solutes, qa (x.y.z.t) (m3m-'s -') f o r t h e a i r flow, pg (x,y,z,t) (mol m-'s-') f o r t h e flow of g a s ,

D(O)

( m 2 s - l ) f o r t h e hydraulic diffusivity, D,(M, 8) (m2s-')

f o r t h e solute diffusivity. Dg (Mg , a a . T.P) ( m Z s -I) for t h e g a s diffusivity and K(O)(ms -I) f o r t h e hydraulic conductivity. The notation V f(.) s t a n d s for t h e t o t a l s p a t i a l d e r i v a t i v e or t h e g r a d i e n t of t h e function in question, Vf(.)

=

b f ( . ) / b x

+

b f ( . ) / b y

+

b f ( . ) / b z .

In t h e equations t h e functions f(O), s(M), g(Oa), h(Mg) are s o u r c e s and sinks t h a t r e p r e s e n t t h e p h a s e transition p r o c e s s e s and t h e e x c h a n g e of solute and g a s between t h e system and i t s environment

-

atmosphere, vege- tation. The flow of elements in t h e liquid p h a s e depends on t h e flow of water and t h e equations t a k e t h e following form:

Water flow :

Solute flow :

bM/bt

=

-Vq, + s (M) q,

=

-D,VM + q M / O

The flow of elements in t h e gaseous p h a s e depends on t h e flow of a i r as follows:

Air flow

Gas flow :

In the following table theoretical and measured values of the diffusion c o e f - ficients a r e compiled.

Table 1A. Values of transport coefficients

Diffusion cm 2s In a i r (O°C):

O2 0.18

C02 0.13

In water:

0 2 co2 0.1 x104

K +' 1.35

x ~ o - ~

ca2+

^0.78

xlo5

NO ^-I 1.92 ~ 1 0 "

SO: - 1.08 ~ 1 0 "

In soil solution

K + 1.71-3.68

x ~ o - ~ ca2+

3.28 X I O - ~

S o u r c e s : Wild (1981) and Kuntze e t a l . (1983)

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Abrahamsen, G., R. Horntvedt and B. Tveite, 1975. Impacts of acid precipi- tation on coniferous f o r e s t ecosystems. In: F.H. B r a e k e (ed.) Impact of acid p r e c i p i t a t i o n on f o r e s t and f r e s h w a t e r ecosystems in Norway.

Summary R e p o r t . SNSF P r o j e c t .

Abrahamsen G., 1990. Impact of atmospheric s u l f u r deposition o n f o r e s t ecosystems. In : S h r i n e r , D.S., C.R. Richmond and S.E. Lindberg (eds.) Atmospheric s u l f u r deposition

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environmental impact and health e f f e c t s , Ann A r b o r Science, Ann A r b o r , Michigan. pp. 397-416.

Alcamo, J., L. Hordijk, J. Kamari, P.Kauppi, M. Posch and E. Runca. 1985.

I n t e g r a t e d Analysis of Acidification in Europe. Journal of Environmen- tal Management, 21:47-61.

Appelo, C.A.J, 1982. Verzuring van h e t grondwater o p d e Veluwe, H20, 15(18):464-468.

Arp, P.A., 1983. Modeling t h e e f f e c t s of acid precipitation on soil l e a c h a t e s

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a simple a p p r o a c h . Ecol. Model. 19(2):105-117.

Arp, P.A., and S. Ramnarina, 1983. Verifying t h e e f f e c t of acid p r e c i p i t a t i o n o n soil l e a c h a t e s : a comparison between published r e c o r d s and model predictions. Ecol. Model. 19(2): 119-138.

Aust, H., 1983. The Groundwater Resources of t h e F e d e r a l Republic of Ger- many. In: Groundwater in w a t e r r e s o u r c e s planning, UNESCO. 1:15-33.

Axelsson, C.-L. and L. Karlqvist, 1984. D a t o r b a s e r a d bedomning a v forsurningskansligheten i grundvattnet. NHP 5:173-184.