Characterization of the Venezuelan Environment and Its Potential for Acidification

Acidification in Tropical Countries Edited by H. Rodhe and R. Herrera @ 1988 SCOPE. Published by John Wiley & Sons Ltd

CHAPTER 7 Characterization of the Venezuelan Environment and its Potential for Acidification

EUGENIO SANHUEZA (CASE STUDY COORDINATOR), GISELA CUENCA, MARIA J. GOMEZ, RAFAEL HERRERA, CHANEL ISHIZAKI, IRIS MARTI AND JORGE PAOLINI

7.1 Introduction...... 198 7.2 Physico-chemical Properties of Venezuelan Soils and Their Sensitivity to Acidification...... 201 7.2.1 Soil Distribution in Venezuela...... 201 7.2.2 Nature of the Acidity of Tropical Soils...... 203 7.2.2.1 Exchangeable Aluminum...... 203 7.2.2.2 Venezuelan Soil-pH...... 205 7.2.3 Sulfate Adsorption Capacity in Tropical Soils...... -. 207 7.2.4 Sensitivity of Soils to 'Acid Rain' ...... 210 7.3 General Characteristics of some Orinoco Basin Rivers...... 210 7.3.1 Hydrology...... 212 7.3.2 Effect of Anthropogenic Inputs on Venezuelan Aquatic Ecosystems - ...... 215 7.3.3 Physico-chemical Characteristics of the Orinoco River and Tributaries...... 215 7.3.4 The Nature of Alkalinity and Related Buffer Capacities. . . . 219 7.4 Environmental Acidity and Natural Vegetation in Venezuela. . . . 221 7.4.1 Effects of Acidification on Plants...... 221 7.4.1.1 Metabolic Effects...... 222 7.4.1.2 Non-metabolic Effects...... - . . - . 222 7.4.2 Environmental Acidity and Natural Vegetation. . . . . 222 7.4.3 Physiological and Developmental Adaptations of Acidophyll Plants to Naturally High A13+ and H+ Levels. . . . 225 7.4.4 Possible Responses of Natural Vegetation to Increased Acidification...... 227 7.5 Atmospheric Cycles of Acidic Compounds...... 228 7.5.1 Emission Inventory...... 229 7.5.1.1 Anthropogenic Sources of SOz and NOx ...... 229 7.5.1.2 VegetationBurning...... 231

197 . - H' . .- u.--

198 Acidification in Tropical Countries

7.5.1.3 Natural Emissions...... 231 7.5.1.4 Anthropogenic/Natural Emissions...... 235 7.5.2 Atmospheric Concentrations...... 236 7.5.3 Transformation Reactions. . . - ...... - 238 7.5.4 Atmospheric Deposition...... 238 7.5.4.1 Rain pH...... - ...... 238 7.5.4.2 Sulfate and Nitrate Depositions...... 238 7.5.4.3 Weak Organic Acids in Rain...... 242 7.5.5 Atmospheric Dispersion...... 243 7.5.5.1 Dispersion Models...... 244 7.5.6 Extent of Current Acidification Damage...... 245 7.5.6.1 Local Scale...... 245 7.5.6.2 Regional Scale...... 245 7.5.7 Potential for Acidification Damage...... - - . . . . 245 7.6 Summary and Recommendations...... 246 7.6.1 Soils...... 246 7.6.2 Aquatic Ecosystems. . - ...... 246 7.6.3 Vegetation...... 247 7.6.4 Atmospheric Cycles of Acidic Compounds...... 247 References...... 248

7.1 INTRODUCTION Venezuela is located in the northern part of the South American continent between 10 and 12° N latitude. It has a territory of 912,050 km2 and a population of 14,602,480 (1980 census). The main economic activity is the oil industry. As shown in Figure 7.1 most of the population lives in a small part of the country, and large areas have very low population densities. The central part of the country consists of low, flat land belonging to the Orinoco River Basins while important hilly areas exist in the northwest (Andean Cordillera) and southeast (Guayana Shield) regions. Venezuela's climate is driven by seasonal movements of the equatorial trough (Riehl, 1979a). The changing 'position' of the trough influences the continental rain pattern in Venezuela. During winter when the trough is in the Southern Hemisphere, a five-month dry season occurs (December to April); a seven-month wet season occurs when the trough is over Venezuela (May to November). The distinction between dry and rainy seasons becomes progressively less marked toward the south. At latitudes lower than 50 average rainfall during winter months is more than 100 mm. This area is always under the influence of the equatorial low pressure cell. Figures 7.2 and 7.3 show the monthly average rainfall throughout Venezuela for dry and wet seasons respectively. A region of trade winds lies between the equatorial trough and the sub- tropical highs. These wind systems are the steadiest of the globe at the surface and are weaker in summer than in winter. In general, regions directly ~ 'V CI'i.RI68Ei':1N SEA j) <2

POPULATION DISTRIBUTION URBAN POPULATION (103 INHABITANTS)

~-500m-;;' 1000 \.iJ)lt~~~~¥~=~§ RURAL POPULATION . 1,000 INHABITANTS

..... \0 \0 Figure 7.1 Population distribution of Venezuela. From Discolar (1983). Reproduced by permission of Editorial America, SA u------

200 Acidification in Tropical Countries

70° 65° 60°

12° Caribbean Sea 12°

9° 9°

6° 0 < 20 mm ~ 20 - 60 mm IlliI60 - 160 mm ~ > 160mm 3° Brazil 0 300 km . ,

70° 65° 60°

Figure 7.2 Dry season mean monthly rainfall (mm) in Venezuela for the period 1951-1970. Adapted from FAV, 1984 affected by the trough present weaker winds (Riehl, 1979b). Wind roses (10 to 20 years average) for stations representative of the wind system on a regional scale (FAV, 1984) show that the northeast and east-northeast (northeast trades) are the most frequent directions for the area north of 5° N latitude, indicating that a large amount of surface air comes from the ocean. Wind speeds generally are lower in southernmost stations and during the wet season (FAV, 1985) due to position changes of the 'equatorial trough over Venezuela. Surface temperature depends on altitude (Figure 7.4). Daily variation in surface temperature is more significant than seasonal variation (2-3°C), especially in the savanna regions where daily temperature changes exceeding 10 °C are recorded. Based on the average temperature and the amount of rainfall, Figure 7.4 shows Venezuela's climatic regions according to Koeppen classification. Tropical savanna (drought during the winter solstice) covers the largest area, followed by wet tropical forest. Smaller areas have climates ranging from glacial to arid. The researchers responsible for the Venezuelan case study identified three main reasons why evaluating the Venezuelan environment and defining n------

Characterization of the Venezuelan Environment 201

70° 65° 60° 12" 12° Caribbean Sea

OG)

9° 9°

6° 6° 0 < 100 mm ~ 100 - 200 mm IffiJ 200 - 300 mm ~ 300 - 400 mm 3° . > 400mm Brazil 0 300 km >-

70° 65° 60°

Figure 7.3 Wet season mean monthly rainfall (mm) in Venezuela for the period 1951-1970. Adapted from FA V, 1984 areas needing more research should be important priorities: it is very unlikely that regional (large-scale) anthropogenic acidification problems exist in Venezuela; most Venezuelan ecosystems are naturally acid; and not enough information exists to estimate how Venezuelan ecosystems will respond to increases in anthropogenic emissions of acidic or acid precursor gases. A second stage of the study should asses potentially harmful effects of increased anthropogenic emissions, based on likely scenarios of future development of the country.

7.2 PHYSICO-CHEMICAL PROPERTIES OF VENEZUELAN SOILS AND THEIR SENSITIVITY TO ACIDIFICATION JORGE PAOLINI 7.2.1 SoH Distribution in Venezuela Climate, parent material, vegetation, and time determine most properties of tropical soils. High isothermic regimes at low and intermediate elevations in the tropics cause higher overall rates of soil formation than in temperate regions, producing more advancedweathering of the original materials. The 60° 70° 65° N 0 N 12° Caribbean Sea 12"

go 9°

Colombia 6° 6°

KOEPPENCLIMATIC CLASSIFICATION Source FA.V (1984)

-.Afi Tropical of forest (ever rainy), f >18°C Ami Tropical monsoonic (transition Afi-Aw;) Awi Tropical of savanna, (drought in winter),f >18°C Tropical of desert (arid), f >18°C Trop,cal of steppe (semi arid), f >18°C CHli Temperate of highlnnd (e.,er rainy), f< 18°C Brazil CHwi Temperoteof highland(droughtin winter),f< 18°C ETHi Cold of highland, f < 10°C EFHi Glacial of highland, f < OOC

goo" I 4 000.000 a '00 200 300 km ,--~ 70° 65° 60°

Figure 7.4 Koeppen climatic classification (FAV, 1984) - _u -- . u------

Characterization of the Venezuelan Environment 203 high temperatures also accelerate the turnover of organic matter. Moreover, during most of the year in the tropics, precipitation exceeds evapotranspira- tion, producing more pronounced leaching of the soil. This leaching grad- ually eliminates soluble salts, less stable minerals, and bases (Na, K, Ca, Mg). Consequently, surface soil layers tend to be somewhat acid, although the subsoii can remain neutral or alkaline. The soil profile will eventually become acid as weathering continues. The only parent materials to persist under this weathering effect are those that are resistant to it, such as iron and aluminum oxides, quartz, and some trace metal oxides. A large proportion of tropical soils have developed on relatively stable and smooth areas where physical erosion has not been strong enough to remove the weathering products, but chemical and biological weathering have been more intense. These soils have low mineral reserves for supplying nutrients to plants. They are dominated by kaolinite in the clay fractions, and have retained many free iron oxides from the parent material. Most contain some free aluminum oxides, have low cation exchange capacities and acid pH values in the solum, and are generally well drained. Another factor affecting tropical soils is the recycling of the nutrients by the forest vegetation. Studies by Herrera et al. (1978) in San Carlos de Rio Negro, T.F. Amazonas, demonstrate that traditional slashing and burning of trees eliminates the internal biogeochemical cycles causing a net leaching of nutrients and impoverished soils. Figure 7.5 shows the principal soil orders found in Venezuela (Comerma, 1971; MARNR, 1983).

7.2.2 Nature of the Acidity of Tropical Soils

7.2.2.1 Exchangeable Aluminum Chernov (1947), and Coleman and Thomas (1967) demonstrated that the weak acid properties of 'hydrogen-saturated' soils and clays actually resulted from partial or complete saturation of the exchange sites with weakly acidic aluminum ions. These studies determined that exchangeable aluminum is the dominant cation associated with soil acidity. The hydrogen ions produced by decomposition of organic material are unstable in mineral soils because they react with the clay minerals, liberating exchangeable aluminum and silica (Coleman and Thomas, 1967). In soils with high organic content, exchangeable hydrogen is associated with carboxylic and phenolic groups of the organic matter. -, Exchangeable aluminum is determined by extraction with KCI 1 Nand titrating with bases (Black, 1965). It precipitates at a pH of about 5.5-6.0. Soils with pH values higher than 6.0 show little or no exchangeable aluminum. Figure 7.6 illustrates this relationship for Venezuelan soils. tV 73° 72° 71° 70° 69° , , " 68° 6 66° 65° 64° 63° 62° 61° 60° 59° 0 " " " " " .j:>.

Caribbean Sea :r J ) -,G.;;:" -

'O :'2'1Q)\i-VCO-/5 5 2 -.. - 2" - 1 }2 100 l:

9° 9°

8° so

7°

6° Soils 6°

Ultisols 5°1-rn Entisols rn

" ".- 4'1-[]] lnceptisols []] Oxisols fi (ld0 ,: \--",.--" r4°

' \\'::-'\. - 3° 3°1- rn Vertisols III Aridisols c .Q E 0 0 100 200 .00 2°1- '0 'L! ,-') \) 7 J--,.: 2° []] Mollisols rn Histosols u

10l '.1:);;;,,;:; i10 rn Alfisols ooL ' " , , , , , " oo 74° 73° 72° 71° 70° 6 68° 67° 66° 65° 64° 63° 62° 61° 60° 59° 58°

Fi,,,,rp 7 'i r.p.nfT;!li7,ed Venezuelan soil maD (adaDted from MARNR. 1983) Characterization of the Venezuelan Environment 205

'0 0 2 0 "-

0- 0 '<0

3 4 5 6 7 pH KCl

Figure 7.6 Exchangeable aluminum at different pH values in Venezuelan soils (Espinoza, 1976; Lavieri, 1982; LOpez, 1980; Palacios, 1982; Shargel, 1977)

Another way to describe the acidity of soils is the percentage of exchangeable AI within the 'effective' cation extracted with NH4 OAc IN and of the exchangeable acidity extracted by KCI 1 N. The 'effective' CEC is a more realistic and useful parameter to describe the exchangeable properties of highly weathered tropical soils dominated by variable-charge colloids (Juo, 1977; Uehara and Gillman, 1981).

7.2.2.2 Venezuelan Soil-pH When we talk about pH we refer to the activity of hydrogen ions in a solution; this concept cannot be directly applied to a polyphase system of soil, so the soil pH can be referred to as either the soil-water pH (outer solution), or the total activity of hydrogen ions in the soil system. The latter value is considerably higher than that of soil water. Generally, the soil pH is determined by adding distilled water or a neutral salt solution (KCI, CaCl2 at a concentration between 0.1 and 1.0 N) to a soil sample, usually in proportions ranging from 1: 1to 4 : 1. The pH is measured after shaking the suspension. The ion exchange reaction with soil colloids gives salt solutions slightly lower pH values than those for pure water. The pH values measured in water from selected Venezuelan topsoils are given in Figure 7.7.Most values are lower than 6.0, reflecting their acid nature. Figures 7.5 and 7.7 indicate that Oxisols, Ultisols and Alfisols are acid, while the Entisols and Inceptisols are highly variable. The Aridisols presentthe e)(pectedbasic character. 70° 65° 60° N 0 0\ Caribbean Sea

~ 7.2 4.g~ 7 0 10° 74 . 4J 6.4 10° 6.2 62 . 5.6 5.6 6.9 7.1 5.6' 5.3 5.6 7.8 4.4 5.1 7 7.3 6.27.8 5.4 6.1 5.5 4.9 5.1 7. 4.5 4.9 5.6.5 6.3 .6 .5 5.2 6.6 4.5 5.5 5.9 5.3 4.8 5.4 4.7 .27.6 4.6 5.14.8 4.9 5.6 5.0 5.2

4.4 5.3 6.0 5° 5°

4.2 c :c 3.9 4.3 E 0 Brazil 0 U

0 300 km

0° 0° 70° 65° 60°

Figure 7.7 Average pH of water in Venezuelan topsoils - d- n- - -- _n---

Characterization of the Venezuelan Environment 207

Table 7.1 is an overview of representative acid soils in Venezuela. The surface pH, exchangeable base cation concentration, and organic matter are all low, and aluminum saturation is high, especially in Ultisols and Oxisols.

7.2.3 Sulfate Adsorption Capacity in Tropical Soils Recent literature concerning anion adsorption by soil components has drawn a number of important conclusions: (i) surface AI.OH and Fe.OH groups are the important sites for the adsorption of anions; (ii) anion adsorption involves an electrostatic interaction as well as chemical interactions between the surface and the ion; (iii) the order of adsorption by a soil is probably phosphate> sulfate> chloride> nitrate; (iv) anion adsorption reactions in soils are complicated by competition by the presence of cations such as calcium and aluminum (Parfitt, 1978). Harward and Reisenauer (1966) summarized the factors involved in sulfate retention: many soils adsorb sulfate; capacity for sulfate adsorption varies with soil properties; sulfate adsorption increases with decreasing pH in the range 6.5-4.0 for a given soil; and sulfate is adsorbed less strongly than phosphate. The most likely mechanism for adsorption is the ligand exchange, M.OHi or M.OH groups on the soils are replaced by sulfate ions (Hingston et at., 1967). Parfitt and Smart (1977) proposed the formation of a binuclear bridging complex FE.OS(OO)O.Fe for adsorbed sulfate on goethite. The amount of sulfate adsorbed by tropical soils varies greatly, as shown in Table 7.2. Hasan et at. (1970) found that the adsorption capacity of sulfate in soil derived from volcanic ash in Hawaii is related to precipitation. Highly weathered and leached soils might contain 7,000 p,g/g of sulfate S in the subsoil. This sulfate can be displaced with phosphate, but is not readily displaced with water. Barrow et al. (1969) in their studies of four localities in Australia demonstrated that sulfate adsorption increases with increasing rainfall above 30 inches and that adsorption was great in soils derived from basic rather than acidic rocks. Haque and Walmsley (1973) demonstrated that sulfate adsorption in soils of the West Indies is correlated with extractable aluminum rather than extractable iron. Moreover, they indicate that sulfate is adsorbed by these soils less strongly than phosphate. Aylmore et at. (1967) indicated that both iron oxide and hydrous aluminum surfaces adsorb sulfate strongly. Sulfate adsorption on soils is generally described by its adsorption isotherm. At low solution concentrations the isotherm follows the Langmuir or Freundlich equations (Bornemisza and Llanos, 1967; Couto et at., 1979; Hasan et at., 1970; Harward and Reisenauer, 1966; Singh, 1984). These models fail when adsorption conditions change or increase in range (e.g. changes in pH or salt concentration, or the introduction of competing or comptexing N Table 7.1 0 Selected topsoil chemical properties of representative acid soils of Venezuela (Source: Espinoza, 1976; Lopez, 1980; 00 Segnini, 1971; Shargel, 1972, 1977)

Org Acidity Effective Aluminum Base Soil Location pH C N Clay Ca Mg K Na Al M CEC saturation saturation - (%) --> , (m eq/IOO g) I (%) (%) Alfisols Oxic Paleustalf Barinas 5.7 1.01 0.08 17.1 2.1 1.4 0.4 0.1 t 0.3 4.3 n.d. 93 Oxic Haplustalf Barinas 5.4 1.13 0.09 16.7 1.5 0.8 0.1 0.04 0.06 0.3 2.8 2.3 87 Aqualtic Haplustalf Barinas 5.8 1.61 0.08 22.4 6.2 1.4 0.2 0.8 - 0.09- 8.7 n.d. 99 Vertic Tropaqualf Bruzual 5.1 1.98 0.18 42.0 4.2 1.7 0.3 0.16 - 0.60 - 6.8 n.d. 91 Ultic Haplustalf Machiques 6.0 0.75 0.08 7.2 0.3 1.4 0.0 0.01 0.1 2.1 3.7 2.7 44 Ultisols Oxic Paleustult Barinas 5.6 1.91 0.17 32.6 6.6 0.9 0.5 0.1 t 0.3 8.4 n.d. 96 Typic Paleustult Maturin 4.8 1.09 0.05 11.8 0.6 0.4 0.1 0.1 1.2 0.3 2.7 44.4 44 Oxic Paleustult EI Pao 4.4 0.82 0.05 18.7 0.4 0.4 0.2 0.1 1.3 0.3 2.7 48.1 41 Plinthic Paleudult Manapiare 5.2 2.81 0.23 33.8 4.4 1.6 0.3 0.1 t 0.5 6.9 n.c. 93 Oxic Plinthustult Jusepin 4.7 0.87 0.04 27.7 0.3 0.3 0.02 0.00 0.64 0.4 1.7 38 36 Typic Paleudult Machiques 4.1 1.3 13.3 0.6 0.2 0.09 0.03 1.9 0.3 3.1 61 30 Oxisols Haplic Acrustox Caicara 4.7 0.59 0.04 24.4 0.4 0.3 0.1 0.1 0.3 0.4 1.6 19 56 Typic Acrorthox S.F. Atabapo 3.8 1.40 0.18 5.3 0.3 0.1 0.1 0.1 1.4 0.8 2.8 50 21 Tropeptic Haplustox P. Ayacucho 4.4 1.25 0.10 40.1 0.4 0.1 0.2 0.1 1.0 0.5 2.3 43.5 35 Haplic Acrustox P. Avacucho 4.8 0.66 0.02 23.5 0.2 t 0.1 0.1 0.4 0.3 1.1 36 36 Typic Acrorthox Trapichote 4.0 2.77 0.19 35.8 0.2 0.1 0.2 0.1 1.9 0.8 3.3 58 18 Tropeptic Haplorthox Tama-Tama 3.9 3.2 0.22 19.4 0.1 0.1 0.3 0.1 2.5 1.0 4.1 61 15 Psammentic Haplustox Guanipa 5.2 0.41 1.2 0.2 0.1 0.01 0.03 0.11 0.3 0.7 16 41 Characterization of the Venezuelan Environment 209

Table 7.2 Sulfate adsorption capacity of tropical soils

Country Soil Sulfate adsorption References capacity (J,LeqIlOOg) Australia - 0.4-2 Barrow, 1970

Costa Rica Latosols and 2 Bornemeza and Andepts Llanos, 1967

Costa Rica Andepts >6 Johnson et al., 1979 Costa Rica - 2.1-7.5 Ramirez and Delsligle, 1978

Brazil Alfisols and 0.4-1.1 (surface) Couto et al., 1979 Oxisols 0.2-0.4 (subsoil)

Mexico, Colombia Andepts 120 (surface) Gebhardt and and Hawaii 15-60 (subsoil) Coleman, 1974

Hawaii Andepts >10 Mekaru and Uehara, 1972

New Zealand Andepts 28.2 Rajan, 1979

species as occurs with soil testing extractants) (Bowden et aI., 1977). Numerous reports demonstrate that the sulfate ion is the main component of rain in industrialized countries (Cogbill and Likens, 1974; Granat, 1972; Johnson et aI., 1972; Semb, 1976). The detrimental effects of atmospheric deposition of strong acids can be seen in aquatic ecosystems before they can be observed in terrestrial ecosystems. The chemical reactions between acidic substances in atmospheric deposition and the catchment soils control surface water quality. These reactions are: anion retention (e.g. sulfate adsorption); weathering of minerals as source of base cations (Ca2+, Mg2+, Na+, K+) and aluminum (AI3+); adsorption and exchange of base cations and aluminum; and, alkalinity generation by dissociation of carbonic acid with a subsequent exchange of hydrogen ions for base cations (Cosby et aI., 1985; Reuss and Johnson, 1985). In this context the sulfate adsorption capacity of catchment soils is important in regulating the sulfate concentration of the soil solution (Johnson et aI., 1981; Johnson and Henderson, 1979; Singh, 1984). Cosby el al. (1986) developed a dynamic model that indicates the implied long-term control that mineral soil adsorption exerts on drainage water sulfate concentration. The authors found that the response time of simulated drainage water sulfate concentration is a function of the hydrological re------n_-

210 Acidification in Tropical Countries tention time and the amount of sulfate adsorbedon the soil. Therefore, soils with small adsorption capacities respond at essentially the hydrological response time (months to a year), while soils with large adsorption capacities respond more slowly (decades). Response time decreases dramatically as adsorbed sulfate accumulates.

7.2.4 Sensitivity of Soils to 'Acid Rain' Soils with a low capacity for neutralizing acidic inputs are sensitive to acidic deposition. It is generally assumed that an increase of H + in acid rain results in the leaching of cations from soils on a charge-equivalent basis. However, the ability of H + to remove cations from soils with pH values less than 5 is low (Wiklander, 1975). Under these conditions some nutrients in rainwater are more efficiently retained by the soils. Wiklander (1979) also indicated that soils with pH values greater than 6 and with high calcium content are less sensitive to acidic precipitation than soils with pH less than 5 and low calcium content. Another criterion for establishing the sensitivity of soils to acidic deposition is the cation exchange capacity (CEC). McFee (1980) established that soils with an average CEC of less than 6.2 meq/l 00 g in the top 25 cm layer are sensitive to 100 cm annual precipitation at pH 3.7, while soils with a CEC between 6.2 and 16.4 meq/lOO g are only slightly sensitive. Thus, Ul- tisols, Inceptisols and Spodosols found in the western USA are sensitive to acidic deposition (Roth el al., 1985). Using a method based on the buffering capacity of soils, Lau and Mainwaring (1985) elaborated a regional sensitivity soil map for the Latrobe Valley of Victoria, Australia, and indicated that soil texture is a factor in soil sensitivity to acidification. Using the criterion mentioned above, and a pH value and cation exchange capacity both below 6, we can prepare a preliminary map of Venezuela indicating areas where soils might be sensitive to acid rain (Figure 7.8). We consider regions where Oxisols, Ultisols, Inceptisols, and soils developed on sandy substrates (Quartzipsamments) predominate as sensitive to acidity. The map in Figure 7.8 could be improved with data from studies related to sulfate adsorption capacity; aluminum, calcium, and organic matter content; and buffer capacity.

7.3 GENERAL CHARACTERISTICS OF SOME ORINOCO BASIN RIVERS CHANELISHIZAKI,IRISMARTIANDMARIAJ. GOMEZ

In order to assess the possible impact of acid deposition on aquatic ecosystems it is first necessary to understand the natural processes that take place 70° 65° 60° T

Caribbean Sea

IDO- -10° I

Columbia

5° 5°

[ZJ High sensitive

Brazil D Potentially sensitive

0 Non-sensitive . km ~ Presumably non-sensitive

0° 0° t-J 70° 65° 60° - Figure 7.8 Sensitivity of Venezuelan soil to acid rain (preliminary map) ~ - --~ - - H_-- -~------

212 Acidification in Tropical Countries in these ecosystems and how equilibrium is reached. Such factors have been widely studied for aquatic ecosystems in temperate climates, but little is known about tropical aquatic ecosystems. The climatic and biogeochemical characteristics of each environment determine the final acid-base equilibrium in aquatic ecosystems as well as the predominant species responsible for acidity. Several characteristics of the acid-base equilibria in nonpolluted Venezuelan black water rivers are discussed here. Since the Orinoco River basin occupies most of Venezuela, the emphasis is on the Orinoco River watershed.

7.3.1 Hydrology The Atlantic Ocean and the Caribbean Sea watershed receive Venezuelan river flow. The Atlantic Ocean receives the waters of the Orinoco River, and the Caribbean Sea receives the water of the Maracaibo Lake and littoral rivers. The Valencia Lake watershed is a closed system with no drainage to the sea. The Orinoco River system drains a basin of approximately one million square kilometers through its course of 2,100 km. Approximately 70% of the basin area lies within Venezuelan territory. The river originates in the Guayana highlands, near the Brazilian border at approximately 1,000 meters altitude (Paolini et al., 1983). As shown in Figure 7.9 the basin is divided into three main regions: Upper Orinoco, Middle Orinoco and Lower Orinoco. The Middle Orinoco region is subdivided into four subregions: the western, central, and eastern plains, and the southern subregion. The Upper Orinoco might be subdivided based on the origin of the tributaries in either Venezuela or Colombia. The Orinoco River and tributaries for which water quality data are available are shown in Figure 7.10. The Orinoco River course changes toward the northeast from a point approximately 6° N latitude and 67°20'W longitude. Using this as a reference, the 'northern tributaries' are located above the main river course, from the reference point to the Orinoco River outlet, and the 'southern tributaries' are below the main course. The last group includes all Orinoco River tributaries that flow before the reference point. The northern tributaries drain the 'Llanos' or plains, which lie on an east-northeast axis extending from the Orinoco River to the foothills of the Venezuelan Andes and the Coastal Range. They are characterized by low relief, approximately 150 meters on the average with annual precipitation between 600 and 1,800 mm (Figures 7.2 and 7.3). These areas contain accu- mulated sediments of Quaternary origin, with a predominance of Entisols and Ultisols, and savanna vegetation. The southern tributaries drain the physiographic provinces of the Guayana 74' 73' 7Z' 71' 70' 69' 68' 67' 66' 65' 64' , , , , , , 63', 6Z', 61', 60', 59' 58' Caribbean Sea IZ' r ) /'\. '. 0 , .-. " D -1'2' c;? eo, c:;£)

J , 10' , ;\ r i' ', ' - ===::::ll!ll'- - - - -r / / / / / / j / /1.i 8'9'[ I \------// / // //,.,/...._r: L ' -- 1/" / / /_/ -I----J r8'

6'7'[ - / '1:: Orinoco River Basin Upper Orinoco 5' Venezuelan tributaries 1,'1' IV///////h I -15'

IillI Colombian tributaries -, - 4' ; I : , I 1).// / / / / / / / /':' - ,- . 4' Middle Orinoco

3' § Southern plains tributaries V-- ,::///////////>. 13' Western plains tributaries Z' Central plainstributaries -12' a 100 ZOO 300 Eastern plains tributaries km " 7-r -1,' Lower Orinoco

ITI1I Delta region 0'1 , , , , , ' , , , ' , , I 74' 73' 7Z' 71' 70' 69' 68' 67' 66' 65' 64' 6'3' 6Z' 61' 6'0' 59' 58' -to.) Figure 7.9 Hydrological subdivisions of the Orinoco River basin (MARNR, ] 983) 74' 73' 72' 71' 61' 60' 59' 58 70' 69' 68' 67 - -OJ;' 65' 64' 63' 62' N ...... j:o. 12'f- I r \ 11 " D J'2' ' .. 0 J

--J\ ?' -111'

J .J)

9'. r9'

8'. '\ 8' jJ " , 7' 7'

6' 6'

5' 5' - 4' '--- 4' ,, '..' 3' 3' -- -- 2' , ,_: 2' i u ,, ". -- -" I' --- ,-,, 0'1 , , , , , , , , ' , , , 74' 73' 72' 7" 70' 69' 68' 67' 66' 6' 64' 63' 62' 6" 60' 59' 58'

Figure 7.10 Rivers of the Orinoco River basin for which water quality data have been analyzed in the Venezuelan case study presented in this chapter - H - - U -- H- - UH - ----

Characterization of the Venezuelan Environment 215

Shield, which is one of the oldest formations on earth (Precambrian). Al- titudes range between 100 and 2,000 meters. This region is characterized by humid climates with annual precipitation between 1,200 and 2,400 mm or more (Figures 7.2 and 7.3). The soils are acid and constitute mainly Oxisols stabilized by a dense vegetation and high accumulation of organic matter. The discharge contribution from the southern tributaries is 84%. The northern tributaries contribute only 16% to the total discharge (Paolini etal.,1983).

7.3.2 Effect of Anthropogenic Inputs on Venezuelan Aquatic Ecosystems Figure 7.1 shows that the bulk of Venezuela's population is located north of the Orinoco River, and a large portion of it outside the basin limits. With the exception of the Guayana industrial complex at the Orinoco River margin, Venezuelan industrial activity is situated near lakes Valencia and Maracaibo and littoral areas. The industrial activity distribution presents a pattern quite similar to the population distribution. The population density of the littoral area without sewage treatment makes the littoral rivers extremely polluted. In addition, domestic and industrial wastewater discharges to lakes Valencia and Maracaibo render these two water bodies extremely contaminated, and therefore out of the scope of this study. Although the northern tributaries drain areas of intense agricultural activities and receive untreated domestic water, they are included in this study. It seems unlikely that industrial atmospheric pollutants will be dispersed over the southern tributaries. Hence, the ecosystems south of the Orinoco River may be regarded as in their natural states; the acidic characteristics of the area (e.g. water bodies) are probably a product of natural processes occurring along the Guayana Shield geological history (approximately 600 million years).

7.3.3 Physico-chemical Characteristics of the Orinoco River and Tributaries

Reliable data on physico-chemical parameters for rivers of the Orinoco basin (Bone, 1986; Colmenares, 1984; Edwards and Thornes, 1970; Ishizaki et at., 1977; Ishizaki and Gomez, 1985; Ishizaki and Marti, 1985; Lasi, 1984; Lewis and Weibezahn, 1976; Marti, 1986; Paolini et aI., 1983; Paolini, 1984, 1985; Portillo, 1985; Sanchez, J.e. el aI., 1983, 1985; and Sanchez, P., 1987) have been analyzed. Unfortunately, only a few parameters were measured in most of the studies and most of them were not done systematically. The parameters most commonly measured were conductivity, pH, dissolved Na+, K+, CaH , MgH , and to a lesser extent, alkalinity. Most of the data corresponds IV ...... 0\ Table 7.3 Range and average values for total major cations (TZ+) and conductivity in rivers of the Orinoco basin

TZ + = Ca2+ + Mg2+ + K+ + Na+ (j.leq/liter) Conductivity (JLmhos/cm) Range Average Number n* Range Average Number n* rivers rivers

Upper Orinoco Venezuelan Trib.c, f,g, I 31-144 91 7 8 6-24 13 15 24 Colombian Trib.c, I 17-161 81 3 4 3-22 13 3 5 Orinoco Riverc, I 68-293 136 1 5 9-33 16 1 13

Middle Orinoco Southern subregion 38-273 130 8 13 5-20 12 9 16 Trib.d, h, i, j, k, ° Western Plains Trib.a, e, h, ° 17-823 232 7 7 6-125 36 9 13 Central Plains Trib. ------Eastern Plains Trib.o 366-428 397 2 2 25-65 40 4 4 Orinoco Riverd, j, k, ° 158-393 285 1 II 11-39 26 1 19

Lower Orinoco Delta Regionb, m,n, p 177-646 352 3 9 20-74 36 3 17 Orinoco Rivero - - - - 22-24 23 1 3 n* represents number of arithmetic means either of different authors or sites during the rainy season. The only exception are the values of the Upper Orinoco, which correspond to several authors who each made only one independent determination. (a) Bone, 1986,(b) Colmenares, 1984,(c) Edwards and Thornes, 1970,(d) Ishizaki et ai., 1977,(e) Ishizaki and Gomez, 1985,(f) Ishizaki and Marti, 1985,(g) Lasi, 1984,(h) Lewis and Weibezahn, 1976,(i) Marti, 1986,G>Paolini et ai., 1983,(k) Paolini, 1984,(I) Paolini, 1985,(m) Portillo, 1985,(n) Sanchez, J. C. et at., 1983,(0) Sanchez, J. C. et at., 1985,(p) Sanchez, P., 1987. Table 7.4 pH and alkalinity measured values in rivers of the Orinoco basin

pH Alkalinity (JLCq/liter) Range Average Number n* Range Average Number n* rivers rivers

Upper Orinoco Venezuelan Trib.c, f, g, I 4.1-6.3 5.2 12 18 10-193 92 12 10 Colombian Trib.c, I - 5.5 1 1 36-210 141 3 4 Orinoco Riverc, 1 5.6-6.5 5.8 1 8 68-309 153 1 5

Middle Orinoco Southern subregion 4.6-6.3 5.8 8 IS 30-214 78 9 11 Trib.d, h, i, j, k, 0 Western Plains Trib.a, e, h, 0 6.3-7.7 7.2 5 9 23-667 180 9 13 Central Plains Trib. ------Eastern Plains Trib.o 6.0-7.1 6.5 4 4 120-488 287 4 4 Orinoco Riverd, j, k, 0 5.4-6.8 6.2 1 19 92-250 154 1 17

Lower Orinoco Delta Regionh, m,n, p 5.1-6.9 5.9 3 17 53-172 107 3 15 Orinoco Rivero 6.3-6.5 6.4 1 3 120-172 151 1 3 n* represents number of arithmetic means either of different authors or sites during the rainy season. The only exception are the values of the Upper Orinoco, which correspond to several authors that made each of them only one independent determination. (a) Bone, 1986,(b) Colmenares, 1984,(c) Edwards and Thomes, 197O,(d) Ishizaki el aI., 1977,(e) Ishizaki and Gomez, 1985,(f) Ishizaki and Marti, 1985,(g) Lasi, 1984,(h) Lewis and Weibezahn, 1976,(i) Marti, 1986,(j) Paolini el aI., 1983,(k) Paolini, 1984,(I) Paolini, 1985,(m) Portillo, 1985,(n) Sanchez, J. C. el aI., 1983,(0) Sanchez, J. C. el aI., 1985.(p) Sanchez, P., 1987...... to..) -...J 218 Acidification in Tropical Countries

to the rainy season. The data (Tables 7.3 and 7.4) have been divided according to the subregions described earlier (Figure 7.9) and include 44 rivers and 114 independent sets of data values. All authors did not measure all the parameters, so the number of independent data values for each parameter may not be 114. All rivers show acidic pH values, the most acidic being the Upper Orinoco and its tributaries. The second important feature is the extremely low dissolved solids content, which is reflected in the measured low conductivities, and total major cations (TZ +). The lowest dissolved solids contents and pH values are in the southern tributaries (Upper Orinoco-Venezuelan and Colombian tributaries, and Middle Orinoco-southern tributaries). The northern tributaries show a wider range of pH and dissolved solids content than the southern tributaries with the western plains tributaries pre- senting the highest dissolved solids content, corresponding to the Apure River. The differences can be explained by geological, edaphological, and climatic differences. The southern tributaries drain old (Precambrian), highly weathered terrain comprising very stable minerals. The minerals predominantly found in tropical soils are quartz, micas, and feldspars (Pasquali et at., 1972; LOpez and Bisque, 1975). The northern tributaries drain through young ter- rains (Cenozoic) with a predominance of Entisols. The climatic regimes, altitudes, and vegetation of the right and left margins of the Orinoco River are also different. The data were analyzed in terms of climatic regimes and compared with estimated values for morphoclimatic types presented by Meybeck (1979). As shown in Table 7.5, the potassium values approach the estimates made by

Table 7.5 Comparison of the composition of dissolved materials from the Orinoco River tributaries with estimated values corresponding to different morphoclimatic types (concentrations in jleq/liter)

Alk CaH Mg2+ Na+ K+

Tropical very humid* 184 157 82 78 15

Tropical very humidt 116 36 17 36 19

Tropical humid* 744 412 288 213 42 Tropical humidt 233 137 96 110 39 Semihumid* 492 314 214 217 46 Semihumidt 107 41 30 246 36

* Estimations made by Meybeck, ]979. t Orinoco River tributaries. Upper Orinoco River is included. Characterization of the Venezuelan Environment 219

Table 7.6 Comparison of the composition of dissolved materials from the Orinoco River tributaries with estimated values for different Continents and World average (concentrations in J.&eqlliter)

Alk Ca2+ Mg2+ Na+ K+

World * 852 669 276 224 33

Oceania* 1067 749 313 304 27 Europe* 1313 1208 428 137 27 Asia* 1085 828 354 287 40 North America* 1170 1003 403 280 38 South America* 400 314 115 143 26 Africa * 438 262 177 165 36

Upper Orinocot 153 42 20 43 26 Middle Orinocot 154 132 72 40 24 Lower Orinocot 151 171 63

* Estimations corrected for anthropogenic inputs made by Meybeck, 1979.Includes only exoreic run-off. t This work.

Meybeck, followed by sodium. The more disparate values are for calcium, magnesium, and alkalinity for the three climates considered. The average values for the Orinoco River are shown in Table 7.6 in relation to the continental river averages estimated by Meybeck. The average values for dissolved matter and for alkalinity of the Orinoco River are well below the averages reported for South America and Africa, which them- selves are much lower than the averages for other continents and the world average. Most of the rivers have acid pHs and the alkalinity in most cases is below 200 J,teq/liter.

7.3.4 The Nature of Alkalinity and Related Buffer Capacities Alkalinity is the acid-neutralizing capacity of an aqueous system. According to Meybeck (1979), the weathering of calcium and magnesium carbonates, and to a lesser extent the alteration of silicates, are the main sources of inorganic carbon. For these waters, alkalinity is defined as:

Alk = [HC03] + 2[CO~-] + [OH-] - [H+] (1)

Since the bicarbonate ion is the main species present in these waters, the alkalinity of most natural waters is considered to be equal to the bicarbonate. 220 Acidification in Tropical Countries ion. It has been recognized however, that the organic anion is also an important contributor in the measured total alkalinity of highly colored waters. Colmenares (1984) specifically showed this for rivers of the delta region. Dickson (1981) defined the total alkalinity of a natural water as the number of moles of hydrogen ion equivalent to the excess of proton acceptors over proton donors, defining the zero level of protons with reference to the strength (pK value) of each acid-base system. If 4.5 pH is used as a reference, the proton acceptors are the bases from weak acids with dissociation constants K ~ 10-4.5 and the proton donors acids with K > 10-4.5. A major difficulty in applying this concept comes from a disagreement about the pK values of the acidic functional groups and from controversy about the extent of acidic functional groups present on the organic molecule. Eberle and Fenerstein (1979) proposed a pK spectrum for the acidic functional groups, and Oliver et al. (1983) presented an operational equation of the form

pK = 0.96 + 0.90 pH - 0.039 (pH)2 (2) to calculate the average pK of the acidic groups as a function of pH. Oliver et al. (1983) found an average value of 10 JLeq/mgorganic carbon for aquatic humic substances of colored waters from several locations in the United States and Canada. Based on this knowledge, and using Dickson's definition, a modified alkalinity equation for natural waters with dissolved humic substances may be written as:

Alk = [HC03] + 2[CO~-] + [A-] + [OH-] - [H+] - [HA] (3)

Where A-and HA are the dissociated and undissociated species of the organic acids, respectively. The question of how Orinoco basin rivers would react to acidic or basic inputs is answered by the buffer capacity «(3),which is defined as:

(3 = dCB = - dCA (4) dpH dpH or the amount of acid or base (in moles) necessary to change the pH of the water one unit (Stumm and Morgan, 1981), where dCB and dCA are the amounts of bases or acids added respectively. The buffer capacity «(3)as a function of pH was calculated for typical rivers of the different subregions considered in this study. The results are shown in Figure 7.11. The rivers used were: the Caiio Negro (1shizaki and Marti, 1985) for Upper Orinoco-Venezuelan tributaries, which have high organic matter content; the Aponwao (Maldonado, 1988; Marti, 1986) for --- _d.

Characterization of the Venezuelan Environment 221 the southern tributaries; the Apure (Sanchez, J.e. et al., 1985) for the western plains tributaries, which have the highest alkalinity; and the Morichal Largo (Sanchez, P., 1987; Villani, 1986) for the delta region. As expected, the only river with a reasonable buffer capacity is the Apure, although this value is smaller than for a typical river from the temperate zone. Other rivers in the basin lack buffer capacity.

8.0

7-0

6-0

~5.0 c ~ :a. 4.0 "' 0 ~3.0 v 0 )( co..2.0

1.0

0'~.5 4.0 5.5 8.5 pH Figure 7.11 Buffer capacity (/3)versus pH for the following rivers: (6) Apure, (0) Cano Negro, (0) Morichal Largo, and (x) Aponwao

7.4 ENVIRONMENTAL ACIDITY AND NATURAL VEGETATION IN VENEZUELA

GISELA CUENCA AND RAFAEL HERRERA

7.4.1 Effects of Acidification on Plants Acid soils produce in non-tolerant plants a wide spectrum of toxicity symp- toms (Foy, 1974). In most acid soils AI3+ and MnH are probably more toxic than H + ions as they limit the growth of higher plants, particularly non-legumes (Foy, 1984). Therefore, within the normal range of soil pH, variations in acidity do not directly influence the growth of natural vegetation. On the other hand, aluminum is an important ecological factor in acid soils. Soluble forms of aluminum, especially AI3+ and intermediate hydro- lysis products, are responsible for negative effects to plants. -- -- - n__- - - n_- -u-

222

7.4.1.1 Metabolic Effects Growth inhibition. in roots is a direct consequence of interference of ionic AI in cell division. Matsumoto and Morimura (1980) propose a model in which AI binds phosphate groups in the genetic material (DNA), hindering its duplication. As a result of the greatly reduced root growth in non-tolerant plants, roots do not penetrate deeply into the soil and the plants die as a result of water deficit. Clarkson (1969) demonstrated that a 1 millimolar (mM) solution of Al(S04h can produce this effect in onion plants. Aluminum interferes with phosphorylation of glucose and fructuose, key processes in metabolism; Le. carbohydrate input to the respiratory process (Clarkson, 1966). It has also been shown to cause degeneration of chloroplast membranes (Hampp and Schnabl, 1975).

7.4.1.2 Non-metabolic Effects The most general mechanism of Al in growth inhibition has been described by Clarkson (1966) in his hypothesis of precipitation-adsorption of Al in root surfaces at cation exchange capacity sites. Monomeric forms of AI ions hydrolyze toward a solid phase formed by Al hydroxy-polymers on the root surface attaining even the neutral crystalline gibbsite form that restricts growth (Matsumoto et al., 1977) Phosphorus deficiency in acid soils has long been interpreted as a primary toxicity factor (Wright, 1953). Phosphorus immobilization occurs when the last hydroxyl group in Al (OH)i can be substituted by H2P04 in solution. This leads to severe depletion of available P for non-tolerant plants and even restricts growth in tolerant plants. Exchangeable Ca in roots' free space decreases due to its precipitation with Al (Clarkson and Sanderson, 1971) making Ca2+ less available to roots. For non-tolerant plants this effect can be more severe if the N source is predominantly NO] (Kotze, 1979), which is the case in most neutral or slightly alkaline soils (Raven and Smith, 1976). In addition, acidity can alter the functioning of some biological mechanisms that enhance nutrient absorption, such as mycorrhiza, which is operative only within a strict pH range (Hayman and Tavares, 1985).

7.4.2 Environmental Acidity and Natural Vegetation Edaphic, topographic, geologic, and climatic considerations are important for each biome. Using the most recent vegetation maps of the country (MARNR, 1982), the coverage of each biome was assessed region by region. In classic literature relating to influence of calcium on vegetation, the term 'acidophylI' is used when referring to vegetation that grows in acid soils, im- plying that are completely adapted to acidity. In Venezuela, arid zones, the Andean and Central Cordillera foothills, and deciduous and semi-deciduous 12. 70. 68...... 2" .0.

C A R I B B E A N

II. ATLANTIC

7. ]

C 0 :e> L 0 M : (' C S. B '.

'j -< I ....» A J.,

/zl " ;' » WFO"EST ON A.S. z "'.' ,. DSAVANNA ONA.S. L . TEPUY H'GHLAND B R N Figure 7.12 N Environmental acidity and vegetation in Venezuela W 224 Acidification in Tropical Countries forests are considered to have non-acidophyll vegetation types. Figure 7.12 shows the area of tropical rainforest in the lowlands south of the Orinoco River, and savannas on acid soils. The highlands in the southern half of Venezuela are among the most acid systems in the country. The montane forest (1,500 to 2,500 meters above sea level) is considered acidophyll, but occupies only about 5% of the acidophyll forest, an area too small to represent in a map of this scale. Fifty-nine percent of the total land area of Venezuela is forested soils. Of this, 75% grows on very acid soils. Ninety-four percent of this forest area is located south of the Orinoco River where old geologic surfaces predominate and climate is the hottest and wettest in the country. Two-thirds of total land area is occupied by acidophyll vegetation, of which forests represent 44% and different types of acid savannas occupy 24% (Table 7.7). The possible effects of the annual savanna burnings on Sand NOx emissions are important (see section 7.5.1). The regional distribution of acidophyll forests shows that those south of the Orinoco River are most extensive.

Table 7.7 Approximate distribution of the Venezuelan 'acidophyll' vegetation. The areas were calculated from the regional data regarding the actual vegetation of the country in ESIUdiosAmbientales Venezolanos, Proyecto VEN/79/001, MARNR, 1982

Ecosystem Surface covered Percentage of the country (million hectares) Forest 34.74 44.09

Open savanna 11.88 13.18

Savanna with 'chaparros' 3.75 4.16

Woody park savanna 5.13 5.69

Woody savanna with palms 0.52 0.57

Woody savanna with 'chaparros' 0.23 0.25

Total 67.94

Seasonally flooded savannas occupy important areas in the central plains. The Middle Orinoco floodplain, including numerous islands, occupy about 40,000 hectares. Wetlands can be important sources of gaseous forms of S and N that could contribute to natural emissions. The above estimate of acidophyll vegetation coverage in Venezuela can be considered conservative as it does not include many small-scale geographic areas. It is noteworthy that most of the systems grow at around pH 5.0 and a ~------

Characterization of the Venezuelan Environment 225

large fraction (mostly wet forest) has its root system in soils with pH ranging from 3.5 to 4.5. The deforestation rate, while generally lower in Venezuela than in other countries of the region, should be considered in relation to acidification. Burning, which produces considerable amounts of NOx and S02, often ac- companies deforestation, and the resulting bare soil is susceptible to the leaching of bases in the exchange complex. This in turn causes an increase in Al saturation. In the short term this source of acidification could have more pervasive effects than large-scale air pollution.

7.4.3 Physiological and Developmental Adaptations of Acidophyll Plants to Naturally High A13+ and H+ Levels Plants that are tolerant to high AI, low pH, and low P availability manage to produce organic matter through several mechanisms. First, if rhizosphere pH is increased, Al is prevented by precipitation from entering the root. Differences in the rate of absorption of NH: and N03 have been interpreted by Taylor and Foy (1985a, b) as a causal mechanism for the pH increases observed in wheat grown in nutrient solutions. Second, adsorption of Al in exchange sites on cell walls impedes its penetration into the cell (Wagatsuma, 1984). Third, these plants can convert Al into an innocuous form once it has penetrated into the cell, thus preventing its toxic effects on mitochondria and nuclei. Chelated Al was found to be innocuous to sensitive plants (Foy and Brown, 1964). This mechanism has been documented for Lycopodium in which Al accumulates in the vacuole (Krupitz, 1969). Cam- braia et al. (1983) found increases of AI chelates with increasing amounts of Al in tolerant varieties of sorghum. Despite the fact that the majority of the vegetation in Venezuela thrives under relatively high natural Al conditions, little is known about the mechanisms that prevent its toxicity. We have measured increments in the pH of nutrient solutions used to grow seedlings of Guapira olfersiana to which increasing amounts of AI were added. The highest Al concentrations were approximately 1,000 times greater than those causing toxicity in crops (Table 7.8). In savannas dominated by a grass stratum, and in most tree species, one could expect the first of the above-mentioned mechanisms to be operative. Other plants can accumulate large amounts of Al in their roots but its pas- sage to the rest of the plant is restricted. Al is deposited in dead tissues, such as cell walls, and does not cross the endoderm is (Table 7.9). The plas- malemma of the endodermal cells imposes a strict barrier to Al (Wagatsuma, 1984). In recent work on morichales (Maurilia flexuosa forests in savanna environments), Mazorra (1985) reports high Al values in roots of grasses and palms. 226 Acidification in Tropical Countries

Table 7.8 Changes of pH measured in solution culture with different concentrations of AI3+, in which seedlings of Guapira olfersiana were grown. This cloud-forest tree, in the field, has only traces of AI in its leaves, although it grows in a soil with pH 4.5

Treatment Initial pH pH (24 hours) pH (48 hours) 0 Al 4.19 + 1.60 +1.72

0.1 mM Al 3.47 + 1.42 + 1.54

1 mM Al 3.17 +0.15 +0.12

10 mM AI 2.84 0.00 +0.12

Some plants are capable of accumulating more than 1,000 ppm AI in their shoots (Table 7.9) which is normally associated with primitive characters (Cheneryand Sprone, 1976). Aluminum reaches the leaves and is immobi- lized, increasing its concentration with the age of the leaves (Table 7.10). In Vochysia venezuetana, a tree of the park savannas in central Venezuela, Cuenca (1976) found that AI concentration is higher than N concentration at all stages of leaf development. While N concentration decreases toward senescence, that of AI increases either because AI continues to move with the transpiratory current or because of relative accumulation due to retranslo- cation of other elements. We do not know the chemical form of AI in these leaves but it has been found in cell walls, inside epidermal cells, constituting up to 2% of leaf dry weight. Sclerophylly is usually associated with acidophyll vegetation types and low Nand P concentrations in leaves (Medina, 1984; Marin and Medina, 1981; Sobrado and Medina, 1980). The cycles of AI, N, and P are interrelated in oligotrophic environments. High AI concentration renders soil P insoluble, and low P availability limits N use by plants. This is common in cloud forests (Fassbender and Grimm, 1981; Cuenca, 1976) and in lowland oligotrophic forests (Sobrado and Medina, 1980; Sprick, 1979; Herrera, 1979). AI tolerance mechanisms represent a high ecologic cost, which in many cases is expressed in low growth rates. Figure 7.13 shows the development of leaves in Graffenrieda tatifolia. These leaves are long-lived and reach their full extension in about five months. Another cost is the high root renewal rate in those species that accumulate AI in their below-ground structures. AI tolerance demands that part of a plant's productivity be invested in chelat- ing substances. Most plants tolerant to naturally high AI and acidity have narrow ecological limits (Webb, 1954), and despite their adaptation to acid conditions, can reach a threshold to intolerance if acidity were to increase. Characterization of the Venezuelan Environment 227 I Table 7.9 Aluminum concentrations in leaves and roots of species of the cloud forest (1747 m.a.s.I.) at IVIC, Edo. Miranda, Venezuela

Aluminum (ppm) Species Leaves Roots

Richerias grandis 11582 6222 Graffenreidia latifolia 7028 5457 Palicourea fendleri 3953 3729 Palicourea angustifolia 7085 1371

Vismia ferruginea 199 3650 Protium tovarense 351 2664 Podocarpus pittieri 199 2503 Myrcia faZZax 305 2000 Caracasia tremadena 175 1902 Baccharis sp. 210 1670 Tetrorchidium rubrivenium 199 1502 Tapirira dunsterviZZeorum 85 1230 Aspidosperma fendleri 351 1096 Clusia alata 199 1086 Didymopanax glabratus 300 605

Table 7.10 Changes with age of the AI and N concentration in leaves of Vochysia venezuelana, a tree of the savanna 'matas'. From Cuenca, 1976

Development stage AI N (mg/g) (mg/g)

Young 9.72 9.21 Mature 12.09 6.52

Old 12.90 5.12

Leaf litter 15.98 3.83

7.4.4 Possible Responses of Natural Vegetation to Increased Acidification Virtually no experimental evidence on the responses of major tropical vegetation types to increased acidity exists. To the best of our knowledge there is no evidence of direct effects of increased NOx and S02 concentrations. Various savanna types occupy about a quarter of the land area in Venezuela. The majority of these ecosystems thrive in soil pH around 5.0. One could expect that lowering the pH values to around 4.5 could happen easi.l,Ydue to the soils' low buffering capacity (Herrera, 1967). At this point 228 Acidification in Tropical Countries

200 b C'I E c..> . c( w a: c( LL c c( 100 W ..J d

e,f g,h

J 'F'MA M J'JA'SO NDJ MONTHS

Figure 7.13 Seasonal changes in leaf growth of Graffenrieda latifolia, a common shade tree of the Venezuelan cloud forests. a,b =leaves produced previous year; c,d = leaves produced previous year that expanded totally the following year; e,f,g,h = leaves produced during the year. This plant produces a pair of opposite leaves in each node (from Cuenca, 1976)

AI3+ becomes more soluble and the system might reach a threshold of intolerance from the higher AI3+ concentrations and from P immobilization. Montane forests, lowland forests south of the Orinoco, and morichales make up 44% of the national territory. These vegetation types live on soils with very high AI3+ concentrations, with pH between 3.5 and 4.7. These systems seem to be more tolerant of increased acidity, but the efficiency of AI tolerance mechanisms probably decreases with pH. The high ecologic costs of these mechanisms could exceed compensation levels.

7.5 ATMOSPHERIC CYCLES OF ACIDIC COMPOUNDS EUGENIO SANHUEZA

The typical formation of acid rain in industrial areas begins with the emission of S02 and N02 followed by the conversion of these oxides to nitric and Characterization of the Venezuelan Environment 229 sulfuric acids during dispersion. The acids are deposited on the surface by rain. Dry deposition of S02 and NOx is important at short (local) distances, but acidic compounds can be transported thousands of kilometers to 'clean' regions. This section presents a physical and chemical characterization of the Venezuelan environment relative to atmospheric acid depositions. No attempt to evaluate local situations is made.

NOx -N 100

Others

Natural gas

c o~------~ < -;;; 100

c 1 SOx-S 0

"'8

Residual ail

0 j I 1970 1975 1980 1985 Year Figure 7.14 NOx-N and S02-S anthropogenic emissions from fuel combustion in Venezuela (MEM, 1986; US EPA, 1977)

7.5.1 Emission Inventory

7.5.1.1 Anthropogenic Sources of S02 and NOx Using fuel consumption levels to evaluate anthropogenic emissions in Venezuela gives a lower limit for emissions. However, estimates show that fuel combustion produces more than 80% of total emissions in Venezuela. Figures 7.14 and 7.15 show NOx-N and S02-S emissions as functions of 230 Acidification in TropicaL Countries

NOx -N

100 Others

Transport 50

Industrial 0 ------~ °t"------j 1°°, SOx-S '" (2

° iIndustrial 1970 1975 1980 1985 Year Figure 7.15 NOx-N and S02-S anthropogenic emissions from various sources in Venezuela (MEM, 1986; US EPA 1977) the fuel used and the type of source, respectively (MEM, 1986). Estimates are based on emission factors given by the US EPA (1977). Emissions of NOx have increased 131% between 1970 and 1984. Motor vehicles are the most significant source and natural gas the most important fuel. Including other anthropogenic sources of NOx we estimate total emissions in the range of 118 x 103 to 142 X 103 tons NOx-N/year. The variations of SOrS emissions given in Figures 7.14 and 7.15 result from changes in policies related to the use of natural gas or residual oil for electric power generation. The rapid increase observed between 1977 and 1980 resulted from the preservation of natural gas resources, used in the petrochemical industry. However, since 1980, the discovery of large amounts of gas and the need to reserve oil for export, power plants are changing back to gas, decreasing significantly S02 emissions to the atmosphere. Estimated emissions range from 85 x 103 to 102 x 103 tons SOrS/year. Characterization of the Venezuelan Environment 231

Figure 7.16 summarizes the geographic distribution of S02-S and NOx-N emissions in part of Venezuela. The values serve as a preliminary emissions inventory; no data exist for the entire country. Most of the emissions are produced in the northern part of the country, a low level occurs in the savanna, and practically none in the forest regions. Table 7.11 shows a comparison between anthropogenic emission densities in Venezuela and several industrialized areas. Emissions in Venezuela are much lower than in developed countries.

7.5.1.2 Vegetation Burning Important emissions of Sand N to the atmosphere are produced from vegetation burning, which can be natural (fires initiated by lightning) or anthropogenic (agricultural practices). Most of the burning occurs during the second half of the dry season (March to May) forming a clear seasonal pattern. Complete data regarding the areas or types of vegetation burned in Venezuela do not exist; however, we offer an estimate of the potential contribution of this source to total acid precursor emissions. The savanna region is burned most intensively. Table 7.12 summarizes the Nand S content of the predominant vegetation and gives an estimate of the savanna area burned. Using the values given in Table 7.12 and 12% as the efficiency of conversion of nitrogen in biomass into nitrogen oxides for the Brazilian Cerrados (Galbally, 1985), a total annual emission of 18,300 tons NOx-N is estimated for savanna vegetation burning in Venezuela. This figure is arbitrarily mul- tiplied by 1.5, to give 27,000 tons NOx-N per year in order to account for other types of vegetation burned in Venezuela. Estimates for the conversion efficiency of sulfur in biomass to S02 during vegetation burning are not available, therefore the same value used for N- conversion is assumed. Following the same steps as in the NOx case, an emission of 3.7 x 103 tons SOrS/year is estimated.

7.5.1.3 Natural Emissions

With the exception of NO emissions from savanna soils (Johansson and San- hueza, 1987) no other determinations of S- and N-compound emissions from natural sources have been performed in Venezuela. However, an estimate based on values from the literature can be made, albeit with uncertainty (Galbally et at., 1985). Venezuela has no volcanoes, and most natural sulfur gaseous emissions are biogenic emissions from soil and plant systems. The only measurements available for tropical soils were made in Africa. Delmas et al. (1980) reported emissions of 1.27 x 10-9 and 47.7 x 10-9 g H2S-S/m2/s for savanna and rainforest regions of the Ivory Coast, respectively. These values agree with CARIBBEAN SEA I to..) I Vol °" I to..) I I I I I I t------I 0.94 I 0.24 I I 0.20 0.03

I .

I I ! I

I '0 1 '0 I '0 ! '0 ! '0 I 5------+------I I '0 I COLOMBIA I '0 I I I '0 I

I '0 I I I I 70

Figure 7.16 Anthropogenic NOx-N (top values) and 802-8 (bottom values) anthropogenic emissions (103 tons/year) in regions east of the Andean Cordillera of Venezuela Table 7.11 Anthropogenic emissions density in several sites of the world

Site Area (km2) SOx-S NOx-N Reference (tons/km2/year)

Venezuela 912,050 0.09-0.11 0.13-0.16 This work

Texas (state) 685,500 0.56 3.36 NRC, 1983

USA 7,977,000 0.21 0.73 Roth et al., 1985

Sweden 450,000 0.61 Swedish Ministry of Agriculture, 1982

France 551,600 3.2 Swedish Ministry of Agriculture, 1982

FRG 245,289 7.4 Swedish Ministry of Agriculture, 1982

DRG 107,173 18.7 Swedish Ministry of Agriculture, 1982

Europe 10,000,000 2.88 Swedish Ministry of Agriculture, 1982

IV W W ------u - ----

234 Acidification in Tropical Countries

Table 7.12 Nand S content of the Venezuelan savanna vegetation and estimated area burned

Vegetation Vegetation Estimated Savanna type N-content S-content burnt area (kg N1km2) (kg S/km2) (km2/year)

Trachypogon 850* 200* 75,000

P. fasciculatum 15300t (200)§ 30,000

Others (850)§ (200)§ 50,000

* Medina, 1982;t Escobar, 1978;:j:Medina et aI., 1978;§ assumed the emissions predicted by the model proposed by Adams et al. (1981). Using these emission factors and the areas of Figure 7.4 an emission of 530 x 103 tons S/year is estimated for Venezuela. Since no other sulfur compounds (e.g. MeSH, COS) were considered, this value should be considered as a lower limit. Large areas of the savanna become flooded (wetlands) part of the year, during which time emissions of sulfur compounds could increase notably. Lightning and biological action in soils are the most likely source of oxidized nitrogen compounds in the tropics. In Nigeria, Ette and Udoimuk (1984) found good correlation between lightning activity and the nitrate concentration in rain. They concluded that electrical discharges are the major sources of NO;- in tropical rains. Following the idea of Ayers and Gillet (1988) and assuming that the relationship between the number of lightning flashes and the amount of rainfall obtained for tropical Australia is also valid for Venezuela, a total emission of 15,600 tons NOrN/year can be estimated for Venezuela (Table 7.13). Two unpublished values for emission of NO from tropical forest soils are available. Servant et al. (1986) found an emission of 11 x 10-9 g NOx- N/m2/sec in the rainforest of the Ivory Coast, and Kaplan et al. (1986) obtained a similar value of 12 x 1O-9g NO-N/m2/sec in the Amazon forest of Manaus, Brazil. Using the last value, an emission of 129 x 103 tons NOx-N could be estimated for the Amazon forest of Venezuela. During February 1987 (dry season), measurements of NO emissions were made in a savanna site of Venezuela (Johansson and Sanhueza, 1987). Emis- sions ranged from 2.5 x 10-9 g NO-N/m2/sec to 14 x 10-9 g NO-N/m2/sec for dry savanna soils. Watered soils, simulating rainfall, produce NO emissions ranging from 50 to 100 X 10-9 g NO-N/m2/sec. No values are available for an actual rainy season. Servant et al. (1986) give the only other value for a savanna site reporting an emission of 2.2 x 10-9 g NOx-N/m2/sec for soils of the Ivory Coast during April 1983. Characterizationof the VenezuelanEnvironment 235 Table 7.13 Estimation of the amount of lightning and atmospheric N-fixationover Venezuela during the rainy season

Rainfall Lightning Area NOx-N (mm) flasheslkm2* (km2) (tons)t

> 2000 34 256,600 7.1 x 103

1000-2000 21 413,500 7.1 x 103

600-1000 11 153,400 1.3 x 103

200-600 6 25,000 0.12 x 103 Total 15.6 x 103

* Calculated from the equation: flasheslyear/km2 =0.0134Imm] + 0.575 (Ayers and Gillett, ]988). t Calculated from the factor: 8.2 x 10-4 tons N/flash (Ayers and Gillett, ]988).

Estimating annual NO emissions from the savanna data is difficult. How- ever, the conservative value of 6.4 x 10-9 g NO-N/m2/sec gives an idea of the significance of natural NO emissions from soils. A total annual emission of 116 x 103 tons NO-N is estimated for the savanna region.

7.5.1.4 Anthropogenic! Natural Emissions Table 7.14 summarizes the total estimated anthropogenic and natural S and N emissions. Both estimates are uncertain and conclusions are tentative.

Table 7.14 Summary of estimated Nand S emissions

Tons N/year Tons S/year x 103 x 103 Soils 245 530 Lightning 15.6 Vegetation burning 27 3.7 Total (Natural and vegetation burning) 287.6 533.7

Motor vehicles* 61.7 8.4 Power plants* 27.4 54.9 Other fuel consumption * 29.5 22.5 Other emissionst - (24) (16) Total (Anthropogenic) 118-142 86-102

* From ]981 fuel consumption (MEM, 1986). t Assumed. 236 Acidification in Tropical Countries

Nationally, natural sulfur emissions seem larger than the anthropogenic S-source. Since major anthropogenic sources are near Caracas, the NOx- SOx emission density from these sources could cause 'local' to mesoscale air pollution problems, but this situation has not been evaluated. The natural sources of NOx-N emissions seem to be only double the anthropogenic ones. Motor vehicles are the more important anthropogenic source, distributing emissions over a large area. However, the emissions in the Caracas-Maracay-Valencia area are large and significant downwind NO)" depositions can be expected.

7.5.2 Atmospheric Concentrations Measurements of NOx, S02, and H2S were performed in four air quality stations in the savanna regions (states of Monagas and Anzoategui). How- ever, during the one-year monitoring period, atmospheric concentrations were below detection limits (4 ppbv) of the analyzers used (Arrocha et al., 1983). No measurements of these gases exist for the rainforest region. Airborne SO~- and NO)" particulates have been measured in the savanna region, Calabozo (Edo. Guarico) and La Paragua (Edo. Bolivar) (Sanhueza et al., 1986b). The locations are given in Figure 7.17. Concentrations of NO)" range from 0.04 to 3.6 p,g/m3, with the higher values observed at the end of the dry season in both locations. The SO~- levels range from 0.1 to 2.4 p,g/m3;no significant differences were observed between dry and wet seasons in Calabozo, and concentrations at La Paragua were significantly higher at the end of the dry season than during the rainy period. No measurements of SO~- and NO)" exist for the rainforest region. Preliminary data (Table 7.15) show that the atmospheric concentrations Tab]e 7.15 Trace meta] atmospheric concentration in Ca]abow (Sanhueza et al., 1986a)

Atmospheric concentrations (ng m -3) Element n Range Average

Pb 12 0.9-7.5 4.3 :t 1.8*

Zn 11 3.5-]0.6 6.7 :t 2.8

Ni 12 1.1-7.5 4.8 :t 2.4

Cd 12 0.04-0.22 0.]8:t 0.07

V 6 1.03-2.46 1.8] :t 0.47 * Standard deviation. I I I I I' ICARIBBEAN SEA I I I I I I I Caracas. I I I . I I I . A.Plp I - - -+ ------_V..Q.lenc.UL_- ---1------I .Camburito I I I I I . I I . I Calabozo I I San Eusebiol [

: - I I , I I I La paragua I . I I I I .Guaquinina [ I I I I 5 -- --, ------I -- I I I I I Marahuaca' I . , I I COLOMBIA I I I .San Carlos I

: de Rio Negro: I I I I I I I 65 N 70 v,) I

Figure 7.17 Locations of monitoring sites 238 Acidification in Tropical Countries of typical anthropogenic metals Pb, Zn, Ni, Cd and V in Calabozo (located in the middle of the savanna region) are very low, and similar to the levels reported for semi-remote sites of the world (Sanhueza et aI., 1986a). There- fore, rural savanna areas of Venezuela are little affected by upwind sources of industrial or vehicular air pollution. No data exist for the rainforest region.

7.5.3 Transformation Reactions This author knows of no chemical transformation experiment conducted in the tropics. However, the atmospheric oxidation of NOx and SOx to HN03 and H2SO4 seems to be a well-known process. In principle, the major pathway for the physical and chemical transformation given in the literature for temperate regions (National Research Council, 1983; Charlson et aI., 1985) should apply in the tropics. The concentration of weak organic acids in rainwater is important, but their source is unknown. They might result from chemical reactions in the atmosphere. Studies are needed to elucidate the hydrocarbon oxidation mechanism that produces these organic acids.

7.5.4 Atmospheric Deposition

7.5.4.1 Rain pH Table 7.16 summarizes the rain pH at several sites. The map of Figure 7.17 shows monitoring site locations. In these studies, rainwater samples were collected by event and the pH measured immediately after the rain ended. With the exception of Caracas, Altos de Pipe and Valencia, the sites are not affected by local anthropogenic air pollution:

- San Carlos de Rio Negro and Marahuaca are located in the Amazon Jungle. - San Eusebio is a cloud forest in the Venezuelan Andes. - La Paragua, Joaquin del Tigre, Parupa, and Camburito belong to the savanna region.

Acid rains, with pH around or below 5.0, appear to be produced in all kinds of ecosystems.

7.5.4.2 Sulfate and Nitrate Depositions Tables 7.17 and 7.18 summarize the available deposition values of sulfate and nitrate, respectively. Due to the low sensitivity of the turbidimetric Table 7.16 Rainfall pH in Venezuela

Site Years of Number of pH References collection events Range Average * Rurai San Eusebio (Edo. Merida) 1973-1974 3.83-6.21 4.6 Steinhardt and Fassbender, 1979 Guaiquinina Plate (Edo. Bolivar) ]977 ] 3.5 Urbani, 1982 San Carlos de Rio Negro (T.F. Amazonas) ]978-]980 70 4.0-6.7 4.7 Haines et ai., 1983t San Carlos de Rio Negro (T.F. Amazonas) 1980-1981 19 4.0-5.4 4.8 Galloway et al., 1982 Camburito (Edo. Portuguesa) 1983-1984 ]7 3.0-5.5 4.4 Sanhueza et ai., 1987 La Paragua (Edo. Bolivar) 1985 14 4.03-5.6 4.8 Sanhueza et al., 1987 Marahuaca Tepuy (T.F. Amazonas) 1985 3 4.5-4.7 4.6 Ishizaki and Marti, ]985 Joaquin del Tigre (Edo. Monagas) 1985 17 4.2-5.8 s5.1 Sanhueza et ai., 1987 Parupa (Edo. Bolivar) 1985 9 4.53-5.55 5.0 Marti, 1986 Urban Caracas (D. Federal) 1981-1982 5 4.16-5.34 4.7 Sanhueza et ai., 1986b Altos de Pipe (Edo. Miranda) 1981,1982,1984 12 4.21-5.80 4.7 Sanhueza et al., 1986b Valencia City (Edo. Carabobo) 1984-1985 24 4.0-6.6 4.9 Escalona, 1986

* Volume-weighted average t See also Keene and Galloway (1984) to.) \JJ \0 240 Acidification in Tropical Countries

Table 7.17 Deposition of SO- in Venezuela. Rain concentration values are volume-weighted averages; number of events shown in parentheses

Site SO- in rain Rainfallc Annual deposition (mg/liter) (mm/year) (mg/m2) Rural San Carlos de Rio Negro 0.14(14)a,d 3900 >546 (T.F. Amazonas)

Camburito 0.64(18)b,e 1600 1726e (Edo. Portuguesa)

La Paragua 0.36( 12)a,f 1600 >576 (Edo. Bolivar) Joaquin del Tigre 0.16( l1)a,f 1000 >160 (Edo. Monagas) Paru pa 0.08(9)b,g 2800 >224 (Edo. Bolivar) Marahuaca 0.84(3)b,h - - (T.F. Amazonas) Valencia Lake - 1000 1630b,i (Edo. Carabobo) Calabozo - 1300 116-385b,j (Edo. Guarico) San Eusebio - - 1180k (Edo. Merida) Urban Altos de Pipe 1.7(15)b,f 1100 > 1870 (Edo. Miranda) Caracas 1.23(7)b,f 900 > 1107 (D. Federal) Valencia City 0.59(24)a,1 1200 8171 (Edo. Carabobo)

(a) Ion chromatography, (b) turbidimetry, (c) FA V, 1984, (d) Galloway el ai., 1982, (e) Graterol, 1984, (f) Sanhueza el al., I986b, (g) Marti, 1986, (h) Ishizaki and Marti, 1985, (i) Lewis. 1981, G) Montes and San Jose, 1986, (k) Steinhardt and Fassbender, 1979, (I) Escalona, 1980. Characterization of the Venezuelan Environment 241

method at low So~- concentrations, some of the So~- data presented in Table 7.17 may be highly uncertain; concentrations might be overestimated up to a factor of 5 (Sanhueza et aI., 1986b). However, Table 7.19 shows that rainwater So~- concentrations in rural sites of Venezuela are significantly lower than concentrations found in industrialized areas of the world. The sulfate at urban sites generally is larger than at the rural sites. The NO;- data are reliable. Table 7.18 shows that a significantly higher NO;- concentration was found at Camburito, compared to the other rural sites. The relatively large NOx emissions in the Caracas-Maracay-Valencia area upwind from Camburito could produce this. With the exception of one event in the Guaiquinina Plate, the volume-weighted pH in Camburito is the lowest value in Table 7.16. Table 7.18 also shows that the NO;- concentration in urban rainwater is higher than in the rural areas. The comparison in Table 7.19 shows that the NO;- concentrations in Venezuelan rural rains are lower than the concentrations observed in the industrialized areas. Using surrogate surface, Plexiglas bulk collectors, dry deposition rates of Sand N measured as SO~- and NO;- have been obtained during the dry season in several savanna sites (Sanhueza et ai., 1986b). During this period, howev~r, large ash particles from vegetation fires get in the collector, producing high deposition rates that are not representative of the whole year. No data exist for the rainforest. Furthermore, with the exception of the atmospheric concentrations of SO~- and NO;- in the savanna regions, no data are available for the atmospheric concentrations of acidic gases or particles. Therefore, it is not possible to estimate the dry depositions of S and N compounds in Venezuela. Two different types of values are given in the last columns of Tables 7.17 and 7.18: (i) for San Carlos, Joaquin del Tigre, Parupa, La Paragua, Altos de Pipe, and Caracas, only the rain concentrations are available and a lower limit of the total depositions was estimated using the annual rainfall of the site; and (ii) in Camburito, Valencia Lake, Calabozo, San Eusebio, and Valencia city, bulk depositions were determined for long periods and the annual depositions reported by the authors are given directly. Using values from Tables 7.17 and 7.18 it is possible to estimate annual deposition rates of 0.43 to 0.86 g SO~- -S/km2/year and 0.11 to 0.22 g NO;-- N/km2/year in rural Venezuela. These values, especially that for SO~-, are lower than the deposition observed in industrial areas of the world. The estimated annual total depositions in Venezuela are 400-800 and 100-200 103 tons/year for S04-S and N03-N respectively. The data also show that the rate of nitrate deposition increases significantly during the first rains after the dry period (Graterol, 1984; Lewis, 1981; Sanhueza et ai., 1986b). Vegetation burning at the end of the dry season is the most likely seasonal source of nitrogen oxides, which are oxidized to nitricacidand nitratein the atmosphere. 242 Acidification in Tropical Countries

Table 7.18 Deposition of NO; in Venezuela. Rain concentration values are volume-weightedaverages;number of events shown in parentheses

Site NO:;- in rain Rainfallc Annual deposition (mg/liter) (m m/year) (mg/m2) Rural San Carlos de Rio Negro 0.16( 14)a,d 3,900 >628 (LF. Amazonas) Camburito 0.84(18)b,e 1,600 1120e (Edo. Portuguesa) La Paragua 0.21(12)a,f 1,600 >336 (Edo. Bolivar) Joaquin del Tigre 0.25(17)b,f 1,000 >250 (Edo. Monagas)

Parupa 0.16(9)b,g 2,800 >448 (Edo. Bolivar) Marahuaca 0.05(3)b,h - - (T.F. Amawnas) Valencia Lake - 1,000 5561 (Edo. Carabobo) Calabow - 1,300 7-29b,j (Edo. Guarico) Urban Altos de Pipe 0.8( 15)b,f 1,100 >880 (Edo. Miranda)

Caracas 0.68(7)b,f 900 >612 (D. Federal) Valencia City 0.64(24)a,k 1,200 1247k (Edo. Carabobo)

(a) Ion chromatOgraphy,(b) colorimetry-Griess reaction, (c) FAY, 1984,(d) Galloway el aI., ]982, (e) Gratero], ]984, (f) Sanhueza el aI., ]986b, (g) Marti, ]986, (h) ]shizaki and Marti, ]985, (i) Lewis, ]98], G) Montes and San Jose, ]986, (k) Esca]ona ]986.

7.5.4.3 Weak Organic Acids in Rain Detailed analysis of the chemical composition of rain samples collected in the Venezuelan rainforest at San Carlos de Rio Negro (Galloway et aI., 1982) shows that the acidity is produced mainly by weak organic acids. Galloway et al. (1982) found that H2SO4 and HN03 contribute a maximum of 35% of -- n--

Characterization of the Venezuelan Environment 243

Table 7.19 Comparison of SO;- and NO;- concentration in rain between Venezuela and industrial areas of the world

sO;- NO;- Reference (mg/liter) (mg/liter) Venezuela 0.14-1.67 0.1(H).84 This work

East USA 1.92-4.8 1.24-3.1 Barrie and Hales, 1984

West USA 0.3-1.9 0.18-1.24 Barrie and Hales, 1984

Canada 0.6-4.8 0.6--3.1 Barrie and Hales, 1984

Rural Europe 3-15 1-15 Rodhe, 1986

Southern Scandinavia* 4.08 2.48 Swedish Ministry of Agriculture, 1982

* ]975-]979.

the free acidity, and that formic and acetic acids contribute approximately 50%. Similar results were found in Camburito, located in the Venezuelan savanna (Graterol, 1984). Chemical analysis shows that only 20 to 40% of free acidity is produced by sulfuric and nitric acids; unmeasured proton donors, likely weak acids, probably account for the difference. Previous studies might have failed to reach this conclusion because organic acids disappear from rainwater if the samples are not properly preserved with a biocide. The pH increases very quickly in unpreserved samples (even when refrigerated at 4 DC)and the anion-to-cation ratio changes significantly. The atmospheric source of organic acids responsible for the acidity of Venezuelan rain is not known. The acids might be emitted directly by vegetation, or produced via the photooxidation of other compounds. Gu et ai. (1985) showed that formic and acetic acids are produced in the oxidation of isoprene, and isoprene is emitted to the atmosphere in large quantities by vegetation. Greenberg and Zimmerman (1985) found that isoprene is the most abundant hydrocarbon in the atmosphere in the Cerrados (Brazilian savanna) and in the rainforest of Brazil.

7.5.5 Atmospheric Dispersion The wind roses (FAV, 1984) show that in Venezuela the wind blows toward the northeast, east-northeast and east (northeast trades) nearly 75% of the time, and no major variations in the prevailing wind direction occur during the year. Therefore, on a regional scale, pollutants are usually transported 244 Acidification in Tropical Countries in the same direction. This produces serious local air pollution problems (Sanhueza et al., 1982) and increases the potential for a regional impact if emissions increase. Most of Venezuela's anthropogenic emissions occur in the northern part of the country (Figure 7.16). 'Clean' air from the ocean dilutes pollutants over the savanna area, where anthropogenic emissions and pollutant levels are low (Sanhueza el at., 1986a). A large area downwind Caracas-Maracay- Valencia might already be affected by anthropogenic acid deposition (mainly HN03), as evidenced by the concentration of nitrate in rainwater collected in Camburito. Since no wet deposition occurs during the dry season, and wind speed is greater than in the rainy season, the long-distance transport of pollutants is different in both periods. This is an important factor to consider when evaluating long-distance transport of water-soluble compounds toward the rainforest. Meteorological conditions, as a function of height, have been determined in the Venezuelan savanna (Barnett et at., 1981). The results show that practically every night of the year a temperature inversion occurs in the lower layers of the atmosphere (Octavio et al., 1987). The inversion breaks up between 6 a.m. and 7 a.m. and the atmosphere is usually very unstable during the rest of the day. The contrast between day and night in the savanna is very strong all year long, another factor affecting local or regional atmospheric dispersion in Venezuela.

7.5.5.1 Dispersion Models Theoretical models of regional air quality have been widely used to understand and evaluate acid depositions. These models require extensive information, including complete emission inventory (anthropogenic and natural); detailed meteorological data of the study region to account for transport and mixing of pollutants (e.g. S02, NOx, SO~-, N03); rate of atmospheric chemical transformations (e.g. oxidation of S02 and NOx to sulfuric and nitric acids); and reliable values for dry and wet deposition processes. Most of the data needed to feed the model are poorly known or unknown for Venezuela. In most cases the use of parameters established for temperate areas is not appropriate for tropical regions (Le. natural emissions, dry deposition velocities). In addition the models do not include weak organic acids or H2S, which contributes significantly to the SO~- deposition rate, particularly in rainforests. In Venezuela, and probably throughout the tropics, the primary contri- butions to the chemical composition of atmospheric depositions seem to be from natural processes. Models for evaluating tropical savannas and rainforests should emphasize this. . n - . - -.. ..--

Characterization of the Venezuelan Environment 245

7.5.6 Extent of Current Acidification Damage

7.5.6.1 Local Scale Several areas in Venezuela have relatively high S02 and/or NOx emission densities. The largestisthe Caracas-Maracay-Valenciaarea (~ 160 km long), where high SO;- and NO) deposition values have been observed in urban (Caracas and Valencia) and suburban (Altos de Pipe) monitoring sites. Rela- tively high concentrations of NO) in rainwater were observed in Camburito, 150 km downwind of Valencia. Figures 7.14 and 7.15 show that S02 emissions increased more than two- fold in the 1977-1981 period. Most of these emissions correspond to a large power plant on the coast, 150 km west of Caracas. Due to the topography of the region, the plume disperses in a different direction from the emissions of the Caracas-Maracay-Valencia source area. The SO;- deposition downwind of the power plant must be increased, although no deposition chemistry measurements have been made to evaluate the real impact. Clearly, atmospheric depositions of SO;- and NO) have increased on a local scale. However, due to insufficient data, it is unknown whether increased deposition has affected aquatic or terrestrial ecosystems.

7.5.6.2 Regional Scale Based on the deposition values, especially rain concentrations, atmospheric deposition of SO~- and NO) in rural areas east to the Andes (savanna and rainforest regions of the Orinoco River basin) do not seem to have increased. Urban and industrial emissions are relatively low, dispersed, and diluted with 'clean' air from the ocean. In the Maracaibo Lake basin, in the western part of the country, large agricultural and natural vegetation areas are downwind of potentially important S02 and NOx sources: Cardon and Amuay refineries, EI Tablazo petrochemical complex and the city of Maracaibo (1.5 x 106 inhabitants). Neither detailed emissions nor deposition data are available. An evaluation of the Maracaibo Lake basin is urgently needed.

7.5.7 Potential for Acidification Damage Earlier discussions indicate that terrestrial and aquatic ecosystems of the Orinoco River basin (approximately 640,000 km2 within Venezuelan territory) are sensitive to acid depositions. Most of the soils are acidic, with high exchangeable AI contents. Surface waters have low alkalinities and therefore present extremely low buffer capacities. Furthermore, rainwater is naturally acid, and the total acidity of rain would increase quickly if anthropogenic 246 Acidification in Tropical Countries emissions of acidic gases increased. Venezuelan ecosystems are thus vul- nerable to acidification, even with a relatively low increase in 502 or NOx emissions. However, the soil sensitivity and surface water alkalinity maps reflect incomplete knowledge and must be improved. Detailed long-term development plans are not well defined, which makes estimating future increases of acidic gaseous emissions or their geographical distribution difficult. What seems certain is an increase in the use of natural gas; even power plants that now burn high 5 residual oil will shift to natural gas. This could produce a decrease in future 502 emissions. The rate of increase of the NOx emissions in the near future will depend on the rate of exploitation of the Orinoco Heavy Oil Belt (FPO). With slow development NOx emissions will probably increase at a rate similar to that of the past decade (Figure 7.14) with emissions doubling in approximately ten years. If the FPO is exploited rapidly NOx emissions, from natural gas combustion to produce water vapor for steam injection, would increase enor- mously. Therefore, in the case of Venezuela, NOx emissions, atmospheric formation of nitric acid, and nitrate depositions should be evaluated.

7.6 SUMMARY AND RECOMMENDATIONS

This preliminary evaluation reviews the major characteristics of the Venezuelan environment with respect to potential for acidification, as discussed in the case study.

7.6.1 Soils

Most Venezuelan soils are acidic. While no studies directly related to the effect of anthropogenic acidification of Venezuelan soils exist, natural conditions such as acid pH, low CEC, and high exchangeable aluminum content indicate that the soils are sensitive to acidification. Experiments to determine sulfate adsorption capacity, sulfate mobility, leaching and losses of major nutrients, mobilization of toxic metals, and the alteration of microbial activity in tropical soils should be undertaken. The Venezuelan soil sensitivity map is incomplete and must be improved.

7.6.2 Aquatic Ecosystems The rivers of the Orinoco River basin, with the exception of a few northern tributaries, have low pH and very low alkalinities. They therefore have very low buffer capacities. As in the case for soils, the surface water alkalinity and pH maps reflect incomplete knowledge and need improvement. The low alkalinities in most of the rivers reflect poorly dissolved carbonate content. The humic acids are an important component of the black water Characterization of the Venezuelan Environment 247 rivers and may be considered the most important source of acidity. Little is known about the aquatic biota and nothing is known about the physiological aspects related to their life in the acidic water. These subjects should be investigated.

7.6.3 Vegetation More than two-thirds of Venezuela is covered with vegetation that grows under naturally acidic conditions. Tropical forests south of the Orinoco River, savanna, and montane forests tolerate natural acidity, but with a high ecologic cost reflected in low growth rates and low net productivity. Although these systems evolved under acidic conditions, they might not tolerate increased acidity, as they already survive near the threshold at which Al could become more soluble. A lack of knowledge precludes assessment of how these biomes would respond to increased acidity. Integrated studies, involving plant physiologists, ecologists, and soil scien- tists, should examine AI tolerance mechanisms, K leaching from live canopy, P immobilization in soils, root exudates, vulnerability of cloud forests to acid fog, and effects of increased acidity on nitrogen fixation and mycorrhiza. Since no regional acidification effects have been detected, a useful approach could be to study the impact of local sources on potentially sensitive vegetation types.

7.6.4. Atmospheric Cycles of Acidic Compounds Anthropogenic S02 and NOx emissions and rural SO~- and NO;- depositions are relatively low compared to values for industrial countries. Venezue- lan rural areas are almost unaffected by regional air pollution. 'Natural' acid rain occurs throughout Venezuela, and preliminary results show that the acidity is due largely to acid compounds other than sulfuric or nitric acids; the most likely source is organic acids. Future studies should evaluate the potential for the total acidity of rain to increase with increased anthropogenic S02 and NOx emissions, and determine the source and fate of weak organic acids found in rainwater. Increased SO~- and NO;- depositions are observed on a local scale, but any concomitant effects on aquatic and terrestrial ecosystems remain unknown due to a lack of data. Regionally, increased deposition of Sand N acidic compounds from anthropogenic activities has not been detected. Hence, existing acidification problems are considered negligible. Research is also needed on the natural processes that determine rural atmospheric NO;- and SO~- depositions, and field evaluation to characterize dry- and wet-deposition parameters in the tropics. The potential for acidification of Venezuelan ecosystems is large. Detailed 248 Acidification in Tropical Countries long-t~rm development plans are ill-defined, and estimating future increases of acidic gaseous emissions is difficult. The development of regional dispersion models is an important step, but such models must be based on a precise understanding of the ecological phenomena involved. Given existing knowledge about tropical conditions, modeling efforts should follow progress from field studies. If aquatic or terrestial ecosystems are acidified, biological effects might occur, but insufficient knowledge about the composition of tropical ecosystems and how they function precludes accurate assessment of damage from acidification. Finally, research to increase understanding of carbon, sulfur, and nitrogen biogeochemical cycles in several well-characterized tropical environments is needed. Such work should be carried out by multidisciplinary research teams that collaborate throughout the project, rather than by specialized groups that attempt to integrate results after the project is completed.

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Characterization of the V~nezuelan Environment 249

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