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Chapter 9 - Contamination of Agricultural Soils

M.D. Webber and S.S. Singh

Highlights

• Pesticide contamination of agricultural soil is not a serious problem in Canada; occasional locally elevated levels of some pesticides are thought to reflect use during the previous growing season or prior to bans of persistent pesticides in the mid 1970s.
• Nonpesticide organic contaminants cause little soil contamination; those detected degrade readily in soils and are not thought to pose a risk to agriculture or the environment.
• Heavy metals are the inorganic soil contaminants of greatest concern; they enter agricultural soils mainly through atmospheric deposition and application of soil amendments (fertilizers, animal manures, and sewage sludge); levels of heavy metals in soils must be controlled, possibly on the basis of the cation exchange capacity of the soil (its ability to retain heavy metal ions).
• Land application of sewage sludge, which contains valuable organic material and plant nutrients, is a beneficial method of sludge management; levels of contaminants, especially heavy metals, in sludge and soil receiving sludge must be regulated.

Introduction

Some soils contain naturally high levels of potentially toxic substances (selenium, lead, and so on) resulting from weathering of minerals. However, much soil contamination is the result of human activity, including the entry of industrial wastes into soil through atmospheric deposition or application of agrochemicals and sewage waste to the land.

Contaminants have the potential to reduce soil quality for agricultural production. They may be divided into two groups: 1) organic contaminants, which contain carbon, and 2) inorganic contaminants, which do not contain carbon. The organic contaminants of greatest concern are industrial in origin and include agricultural pesticides. The most important inorganic contaminants are heavy metals, which derive mainly from industrial processes although they may occur naturally in soil.

Many potential contaminants are necessary for agricultural production but become hazardous when they occur in excess in the soil. For example, small amounts of copper, molybdenum, and selenium are needed for proper plant and animal growth, but large amounts can be toxic. Similarly, pesticides enhance agricultural production but can pose a risk to human and animal health and environmental quality. Most chlorinated organic pesticides (such as DDT, chlordane, aldrin, and dieldrin) have been banned from use in Canadian agriculture because they persist in the environment and negatively affect food chains.

In this chapter we present information about the levels of organic and inorganic contaminants in agricultural soils. We also discuss land application of municipal sewage sludge in relation to soil contamination.

Modern agriculture coasts on the sunlight trapped by floras long extinct; we pump it, process it, transport it over the countryside as chemicals, and inject it into our wasting fields as chemotherapy.

Wes Jackson
Altars of Unhewn Stone

Organic contaminants

Organic contaminants include many industrially derived compounds. Some of these compounds, such as agricultural pesticides, are intentionally applied to soil. Others, such as polynuclear aromatic hydrocarbons, polychlorinated biphenyls, and polychlorinated dibenzo-p-dioxins and furans, enter the soil accidentally. Analysis of organic contaminants requires specialized techniques and equipment and is costly.

Prior to our study, information about the levels of organic contaminants in Canada's agricultural soils was limited (data existed for certain pesticides at experimental sites). To provide more information, we analyzed soils (mainly topsoil) from eight national benchmark sites used to monitor soil quality (sites 5, 9, 12, 13, 18, 19, 20, and 21; see Chapter 4, Fig. 4-1) and from six intensively cropped farms in southern Ontario for 122 organic contaminants (grouped in Table 9-1). This analysis does not provide information about changes in the levels of organic soil contaminants over time, but it does provide baseline data against which to make comparisons in the future.

Pesticides

Production and use of organochlorine pesticides, except endosulfan (an insecticide that does not persist in soil), has been banned in Canada since the mid 1970s. We did not expect to find large concentrations of these pesticides in soils. Concentrations of about 10 parts per billion (ppb) are thought to represent background levels (concentrations in soils with no history of organochlorine pesticide use).

With minor exceptions, concentrations of organochlorine pesticides were less than 10 ppb in all soils analyzed. Endrin levels marginally exceeded 10 ppb in topsoil from the benchmark sites in Prince Edward Island and New Brunswick and in subsurface soil from the benchmark site in Quebec. Dieldrin marginally exceeded 10 ppb in the topsoil of four southern Ontario sites. Total endosulfan was found at levels of about 80 ppb in topsoil from two southern Ontario sites and the Prince Edward Island benchmark site. Total DDT exceeded 100 ppb in topsoil from the Prince Edward Island and New Brunswick sites and from three southern Ontario sites. Soil from one Ontario site had total DDT in excess of 70 parts per million (ppm).

The larger concentrations of total endosulfan in three soils probably resulted from use of Thiodan during the growing season in which the samples were taken. Dieldrin concentrations of more than 10 ppb in the intensively cropped soils probably reflect heavy use of this insecticide in past crop production. High total concentrations of DDT probably result from use of this insecticide prior to the mid 1970s. One soil with a total DDT concentration of more than 70 ppm has supported an apple orchard since the 1930s; this level is not unusual for topsoil under this land use (Orchard soils).

Fonofos was the only organophosphorous pesticide observed, and concentrations were similar in all study soils. We measured concentrations of up to 100 ppb, which are consistent with its use in field-crop production, particularly to control corn rootworm species.

We detected no neutral, phenoxy acid, or carbamate herbicides in most of the soils analyzed. Persistence of these herbicides in soil is not a significant agricultural or environmental concern. Metolachlor, used to control grasses in row crops, was detected in two southern Ontario topsoils at 118 ppb. Trifluralin, used to control annual grasses and broadleaf weeds in beans, canola, and other crops, was measured in soils from the Alberta and Manitoba benchmark sites at 145 and 167 ppb. Triallate, used to control wild oat in a variety of crops, was measured in the Alberta soil at 48 ppb. Phenoxy acid herbicides were measured at concentrations that were usually below 25 ppb but sometimes reached 60 ppb.

Nonpesticide compounds

We found little contamination of agricultural soils by nonpesticide compounds (Table 9-1) in either benchmark or southern Ontario soils. It is unlikely that the nonpesticide compounds observed are significant agricultural or environmental concerns. Except for polychlorinated biphenyls, the compounds (naphthalene, phthalate esters, phenol, and pentachlorophenol) that exceeded the method detection limits (MDL; the smallest concentration of a substance that can be measured accurately by a certain analytical method) are known to degrade rapidly in soils.

Total polychlorinated biphenyls exceeded the MDL of 100 ppb in six soils but did not exceed 250 ppb. Napthalene, a polynuclear aromatic hydrocarbon, exceeded the MDL of 50 ppb in 13 soils, and a concentration of 1.2 ppm was found in one soil. Phthalate ester concentrations frequently exceeded the MDLs but only three values for dibutyl phthalate and two values for bis(2-ethylhexyl) phthalate exceeded 1 ppm (the greatest concentration was 3.1 ppm of dibutyl phthalate). Phthalate esters, which make plastics flexible, are commonly present in the environment, and it is possible that these compounds observed resulted from contamination of the soil samples during collection, preparation, and analysis. Phenol concentrations in 21 soils exceeded the 20 ppb MDL and in five soils exceeded 150 ppb, but none exceeded 1 ppm. Pentachlorophenol was reported in eight soils but did not exceed 360 ppb. Phenol is a breakdown product of many materials, including pesticides and organic matter, and pentachlorophenol is a widely used wood preservative. Phthalate ester, naphthalene, phenol, and pentachlorophenol concentrations in the soils appeared to be unrelated to position in the soil profile (topsoil or subsurface) or to sampling position on slopes (upper, middle, or lower).

The chemicals to which life is asked to make its adjustment are...the synthetic creations of man's inventive mind, brewed in his laboratories, and having no counterparts in nature.

Rachel Carson
Silent Spring

Inorganic contaminants

In general, inorganic contaminants enter soil as a result of human activities. The heavy metals arsenic, cadmium, chromium, cobalt, copper, lead, mercury, molybdenum, nickel, selenium, and zinc are the most hazardous of these substances. Although many of these heavy metals are needed in small quantities by plants and animals (Table 9-2), they may enter soils in great enough quantities to pose risks to the health of plants, animals, and humans. Heavy metals are persistent, and their negative effects in soil are long-lasting. For this reason, their levels in soils must be controlled.

Background levels

To assess the quantities of heavy metals that enter soil as a result of human activity, we need to know background levels of these substances in soil. In the natural soil environment, the main source of metals is the parent material from which soils are derived. Occasionally, heavy metals become locally concentrated by natural processes. For example, high levels of boron, cadmium, and selenium may occur naturally in discharge and saline seep areas in the prairies. The range of metal concentrations in Canadian topsoils is considerable (Fig. 9-1).

Human sources of heavy metals

Heavy metals enter soil as a result of mining, metallurgy, use of fossil fuels, and application of soil amendments. Heavy metal concentrations for various soil amendments are shown in Table 9-3. Higher concentrations of heavy metals in phosphate fertilizers are related to their aquatic biological origin; high levels in animal manures may be related to feed additives; high levels in sewage sludge reflect the composition of domestic and industrial discharge. There is concern that heavy metals may be absorbed by crops and subsequently enter the human food chain. (Cadmium in soil)

Limiting heavy metals in soils

Recent studies in Quebec have shown that concentrations of heavy metals in soils vary with soil texture. They are greatest in clay, followed by clay loam, loam, and sand. For example, average concentrations of zinc and copper in soils of these textures are shown in Figure 9-2.

Heavy metal concentrations in soils are also related to soil order. Gleysols and Luvisols have the highest concentrations, followed by Brunisols and Podzols. Average concentrations of zinc and copper in these soil orders are shown in Figure 9-3. However, this observation may also be related to soil texture because Gleysols and Luvisols generally have the highest clay content, followed by Brunisols and Podzols.

Quebec researchers suggest that the maximum heavy metal concentration in soils be based on the cation exchange capacity of the soil, which is a measure of the soil's ability to retain heavy metal ions. The cation exchange capacity increases with increasing clay content of the soil. Thus, they recommend higher maximum heavy metal concentrations for fine-textured soils, such as clay and clay loam, than for coarse-textured soils, such as sand (Table 9-4).

Land application of sewage sludge

Sludge is the solid waste resulting from the treatment of domestic and industrial effluents (liquid wastes) that are released into sewers. It may contain metals and organic contaminants at widely varying concentrations, depending on the nature and amount of industry and the enforcement of guidelines for effluent quality. Municipal sludge is applied widely to agricultural land in Canada. This process uses about one-third (120 000 tonnes, dry weight annually) of all the sludge produced in Canada and about one-fifth (60 000 tonnes, dry weight annually) of that produced in Ontario. This method of sludge disposal is often the most environmentally acceptable and economic. It is also desirable because it recycles organic matter and valuable nutrients, particularly nitrogen and phosphorus, back to the soil. However, there is concern that heavy metals and other contaminants in sludge will accumulate in the soil, reducing its quality for agriculture. (Sludge application in Ontario)

Metal contaminants

Applying sludge to land increases metal concentrations in soil (Table 9-5). Considerable information exists concerning the fate and effects of metals in soil, and criteria have been developed to protect soil quality. For example, in Ontario, metal limits have been defined for sludge and soil, and maximum permissible sludge loadings (the maximum amount of sludge that can be applied safely to soil) are regulated depending upon the metal concentrations in both materials.

Organic contaminants

Organic contaminants are difficult and expensive to measure in municipal sludges, so less information is available for them than for metal contaminants. However, a recent analysis of Canadian municipal sludges showed that organic contaminants are found in low concentrations (Table 9-6). Under current restrictions of sludge application (in Ontario, about 40 tonnes of dry sludge may be applied per hectare over 15 years), it is unlikely that applying sludge will result in significant buildup of organic contaminants in agricultural soils. The limited information on levels of organic contaminants in sludge-treated soils (Table 9-7) supports this expectation.

Table 9-6 Average conecntrations of organic contaminants in Canadian municipal sludges, expressed on a dry weight basis

• Volatile organic contaminants: many ND; most others <2 ppm; a few petroleum derivatives >2 but <10 ppm; toluene in three sludges
  <45 ppm
• Polynuclear aromatic hydrocarbons; most <1 ppm; a few <5 ppm; elevated but <20 ppm in one sludge with large contribution from
  steel mill
• Haloethers: ND
• Chlorinated benzenes: ND except 1,2-diclorobenzene <0.5 ppm and 1,4-diclorobenzene <3 ppm
• Nitrosamines: ND
• Phthalate esters: <15 ppm except bis(2-ethylhexyl) phthalate
  <250 ppm
• Phenols and cresols: many ND; others <1 ppm except phenol
  <10 ppm
• Heterocyclic nitrogenous and other compounds: many ND; nitrobenzene <5 ppm
• Organochlorine pesticides: ND except <0.1 ppm gamma BHC, hexachlorobenzene, methoxychlor and DDT in a few sludges
• Total polychlorinated biphenils: <250 ppb
• Polychlorinated dibenzo-p-dioxins and furans: <36 ppb total dioxins and furans; <120 ppt total TEQ

Guidelines for organic contaminant levels in sludge have not been developed in Canada, with one exception. Quebec defines a maximum permissible total polychlorinated biphenyl concentration in sludge of 10 ppm for use on agricultural land; sludges with more than 3 ppm polychlorinated biphenyls must be injected into the soil to avoid direct ingestion by animals. In Ontario, guidelines have been proposed to clean up industrially contaminated soil for agricultural and other uses. Using these guidelines along with information about organic contaminant levels in Canadian sludges, the maximum amount of sludge that may be applied to agricultural land can be calculated. The main concern when developing guidelines for sludge loading is the potential for contaminants in the sludge to build up in soil and enter the food chain.

The following contaminants are of particular interest:

• Benzo-a-pyrene, a polynuclear aromatic hydrocarbon, is carcinogenic (causes cancer) and resists degradation; the maximum permissible loading to agricultural soil must be low (in Ontario, about 2 kilograms per hectare).
• Fluorene, a polynuclear aromatic hydrocarbon, is not carcinogenic and is much less resistant to decomposition than benzo-a-pyrene; its maximum permissible loading may be much larger than for benzo-a-pyrene.
• 2-Methyl naphthalene, a polynuclear aromatic hydrocarbon, is neither carcinogenic nor resistant to degradation but imparts a foul odour to water; it may not be added to soil overlying groundwater for human use.
• Dioxins and furans are highly toxic, carcinogenic, and very resistant to degradation; they are much more limiting for sludge use in agriculture than benzo-a-pyrene.
• Polychlorinated biphenyls are very resistant to degradation and are thought to be carcinogenic; they occur in smaller concentrations in sludge and are less limiting for land application than benzo-a-pyrene.
• Bis(2-ethylhexyl) phthalate and toluene; concentrations in sludge may be large, but because these compounds degrade rapidly in soil, they are less limiting for land application than benzo-a-pyrene.
• Endosulfan, an organochlorine pesticide, is used widely in crop production; small amounts in sludge are unlikely to limit land application.

Suitability for agriculture

Sewage sludge contains organic matter and plant nutrients that are beneficial to soil health and crop production. Applying sewage sludge to agricultural land fits well with the current concepts of resource recycling and sustainable agriculture. However, sewage sludge contains contaminants that may decrease the quality of agricultural soils. Even though the quality of sewage sludge has improved in recent years (because of better measures for pollution control), careful control of contaminant levels in soils receiving sludge is needed.

Canadian guidelines for sewage sludge regulate the amount of heavy metals added to agricultural soils. Work is currently under way to assess the need to regulate the amount of organic contaminants that can safely be added to soil.

Conclusions

Contamination of Canadian agricultural soils with pesticide and nonpesticide contaminants is not a serious problem. Currently used pesticides are not persistent in soil, and insignificant amounts are observed. But contamination of soil by heavy metals is a concern because metals are persistent and may affect plant, animal, and human health.

Application to agricultural soils is a beneficial method of managing sewage sludge, but adding contaminants to soil in this, and other, waste materials must be controlled. Currently, heavy metals are regulated and organic contaminant regulation is under review.