Soil Pollution and Its Mitigation

Soil Pollution and Its Mitigation - Impact of Land Use Changes on Soil Storage of Pollutants INTRODUCTION

cases, soil pollution is being associated with agricultural pracI tices.mostHowever, the overall picture is far more complex, and soil and its N

pore water can become polluted from a multiplicity of sources. Furthermore, the pollutant inputs into soils have occurred for a long time. Man has always had the tendency to dispose of waste and dead organic matter in soil, and the waste assimilative capacity of soils was considered to be infinite (Novotny and Olem, 1994). Sediments and soils have, indeed, a high storage capacity for contaminants, but in no way is this capacity infinite. As a matter of fact, there are many instances where the storage capacity has been exceeded, resulting in a massive release of contaminants. Such excessive accumulations of pollutants in soils and sediments, which have resulted, or potentially can result, in a sudden release of chemicals mainly into groundwater and surface water bodies, have been termed a "Chemical Time Bomb" (Stigliani et aI., 1991; Salomons, 1993; Stigliani and Salomons, 1993). Besides exceeding the storage (assimilative) capacity of soils by increased inputs of pollutants, the release of soil chemicals into water can also be caused by changing land use, which works both ways. For example, change of prairies into arable lands and drainage of wetlands triggered nitrification of large amounts of organic nitrogen and released large quantities of nitrate into groundwater and then into surface water bodies (Kreitler and Jones, 1975). However, as it will be pointed out in this chapter, conversion of agricultural lands into forest reduces pH and changes the overall chemistry of the soils and, consequently, solubility of some pollutants may increase and the soil-bound pollutants may be released into pore water and then as diffuse pollution into surface waters. Pollutants exist in soil in two phases: (a) adsorbed on soil particles or precipitated (i.e., in particulate form) and (b) dissolved in pore water of 153

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SOIL POLLUTION AND ITS MITIGATION

the soil (sediment). Colloidal fractions can be considered as a transition between the two phases. In most cases, the adsorbed and dissolved fractions are in a dynamic equilibrium expressed by the well-known partition concept (5.1)

where r represents the adsorbed (particulate) fraction of the pollutant expressed in micrograms of the pollutant per gram of soil, Cp is the concentration of the pollutant in the pore water of soil (sediment) in micrograms per liter, and n is the partition coefficient, which due to consistency of the units, has dimension of liters per gram. See Novotny and Olem (1994) for a more detailed discussion on retention and release of pollutants in soils. Over 99% of most pollutants are stored in sediments or soils in particulate fraction. Less than 1 % is dissolved in water, however, the dissolved fraction is of great importance because it represents the fraction, which is bioavailable to organisms. The pollutants associated with particulates can be moved by erosion if they are in soils or by scour-deposition processes if they are contained in sediments. Dissolved pollutants can be leached by diffusion and move with water. The distinction between soils and sediments is only semantic. Both soils and sediments were formed by sedimentation of weathered solids over the geological times. Sediments typically imply at least temporal submergence in water, and the pore volume is saturated with water most of the time. Also, soil contains air in its voids, however, many soils become saturated under certain meteorological and pedological conditions. CLASSIFICATION OF POLLUTED SOIL AND SEDIMENT SITES Hazardous Waste Sites

By definition, these sites can be characterized as point sources. The soil or sediment l contamination is very high, and in most cases, the polluter can be identified and made responsible for remediation. Large Contaminated Areas (Dredge Spoil, Industrial Sites, and Urban Areas

In the case of cities, the polluting activities may date back several hunlIn the United States, the problem at a large portion of so-called "Superfundn sites (i.e., sites with very high contamination) was submerged aquatic sediments. Examples include PCB contamination of sediments in Waukeegan Harbor (IlJinois), Sheboygan River, and Cedar Creek sediments in Wisconsin; severaJ sites in the Hudson River (New York); and others.

Time Frame of Pollutant Accumulation and Clean Up

155

dred years. These areas are characterized by pollution by heavy metals, organic chemicals, and, to a lesser degree, by nutrients. There is no single responsible party, and identification of polluters that would be responsible for remediation is impossible. Contamination may affect several geohydrological units (soil water, groundwater, and receiving surface water bodies). Such sites require management at the regional level. If the contamination affects only one unit (e.g., groundwater zones), remediation may be possible. Smaller Contaminated Sites

Such sites are typically contaminated by emissions from one source and exhibit a decreasing gradient. Examples include soils contaminated by atmospheric pollution from industry and pollution of soils near highways and railroads. Geohydrological control is difficult because the areas comprise several hydrological units. Control measures are limited and rely on increasing natural degradation rates or improving retention properties of the soil and, thus, decrease the impact on biota (decreasing bioavailability) and leaching into groundwater. Pollution of Agricultural Soils

Many regions have contaminated soils caused by intensive agricultural practices. Examples include phosphate saturated soils, nitrate leaching, and leaching of certain organic chemicals such as atrazine. As pointed out, certain agricultural practices may increase or decrease mobility of some pollutants and in this way affect the water contamination. Farming practices increase pH levels in soils, which decreases mobility of metals. Also, higher organic content of farmed soils increases retention of some organic chemicals, keeps them in an oxidized state, and maintains aerobic environment, which may be more favorable for biological degradation.

TIME FRAME OF POLWTANT ACCUMULATION AND CLEAN UP SOILS AS A PART OF THE HYDROLOGICAL CYCLE

The pathway of contaminants in the hydrologic cycle from soils to oceanic sediments is shown in Figure 5.1. Figure 5.l(a) shows the situation in western Europe and the United States before 1980 (little or no control of soil pollution). This situation still exists in central/eastern Europe, some Pacific Rim countries, and increasingly in some developing countries. The pollutants enter the soil from the atmosphere, from losses of stored pollutants on the land, from direct application of chemicals, sewage sludge and

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SOIL POLLUTION AND ITS MITIGATION

Figure 5.1 The wheels transporting pollutants throughout the environment.

fertilizers onto land, and from contaminated surface water bodies, including water used for irrigation. The hydrological cycle is represented in Figure 5.1 as three interlocking wheels. The inner wheel, the atmosphere, is fast running and has a short residence time; the waterwheel is slower and has residence times ranging from weeks (rivers and estuaries) to years (lakes and coastal waters). The outer wheel is the very slow-moving particulates wheel where the residence times exceed those of the waterwheel by an order of magnitude. The ultimate storage place of all materials on the continents, including soils

Time Frame of Pollutant Accumulation and Clean Up

157

and sediments, is the oceanic floor, but it may take millions of years before the pollutants presently occurring in terrestrial systems will reach their final disposal in the oceans. Because of man's cultural heritage and long-lasting extraction of metals from ores, starting with the Bronze Agel (Figure 5.2), heavy metals have already accwnulated in sediments and soils for several centuries. The accwnulation of organic chemicals has occurred for the last fifty years. Also, intensive agricultural practices with associated high application of manmade fertilizers has occurred over the same time frame. Generally, the rate of accwnulation of pollutants in western Europe, as well as in the United States and Canada, has decreased. Pollution control has a fast effect (response) on concentrations in water and in the air because of the short residence time. The contaminants stored in sediments and soils will require much longer cleanup time by natural means. It is anticipated that cleanup will be faster for sediments than for soils. In areas exhibiting higher sedimentation, the polluted sediments will be covered with cleaner, less contaminated ones, which will act as a barrier between the buried contaminated layer and overlying surface waters. For soils, the burial is less likely and most of the cleanup will be by leaching.

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SOIL POLLUTION AND ITS MITIGATION

However, as is manifested in central Europe, the dramatic decrease in application rates of fertilizers has already resulted in a noticeable reduction of nitrate contamination of groundwater. Figure 5.3 shows the nitrate application rates and nitrate levels in base flow, which document the effect of nitrogen application rate reduction after 1990 on the nitrate levels in groundwater and base flow. Differences in the rate of transfer of pollutants from soils to groundwater and surface waters are significant and are caused by the difference in properties of the chemicals and soils. To illustrate the difference, the time needed to remove various chemicals from 1 m of soil colunm by natural leaching was calculated and the results are presented in Figure 5.4. The selected soil was a sandy soil, which has low retention properties. For strong hydrophobic compounds, such as PCBs and DUT or some toxic metals, the time needed for cleanup is on the order of thousands of years. For chemicals that are not strongly adsorbed such as nitrate and potassium the removal time is less than ten years. This long retention time of hydrophobic pollutants puts severe limits on the allowable inputs of contaminants from fertilizers and deposition into soils to prevent further increases of the pollutants in the topsoil . Human activities such as engineering works, drainage of land, and change of land use disturb the equilibrium between the toxic metals in the ----~--------------------------------~--_,

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