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Learn about naturally occurring contaminants in soils, risk assessment, pollutant pathways, and proper waste disposal regulations to protect environmental quality and human health.
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Chapter 14 Soils for Environmental Quality and Waste Disposal
What is a Contaminant? • Nearly all materials we consider contaminants are naturally-occurring elements or compounds, but they become a hazard when present in soils or waters at elevated concentrations. • Some compounds such as pesticides are synthesized by humans for their useful toxicity to target organisms, but become contaminants when they are found in food or water supplies. • Most contaminants are similarly useful materials that just get in the wrong place at too high a concentration, due to human carelessness or mishap.
Risk Assessment • An attempt to quantify the risks posed by a certain level of some pollutant (contaminant) in the environment, particularly to human health. • There are a number of pathways by which contaminants might directly affect humans
Pollutant movement and risk pathways for soil contaminants. A: Direct Soil Ingestion - Some metals, such as lead and arsenic B: Leaching to Ground Water - Many organic contaminants C: Runoff to Surface Water - Many organic contaminants D: Plant Uptake - Some metals, such as cadmium, and radionuclides E: Animal Uptake - Some metals, such as lead/mercury, organics/PCBs, radionuclides
Food chain • When contaminants enter plants or animals which constitute our food supply, thereby transferring contaminants to humans. • Water supplies • Either surface or ground water, are often contaminated when pollutants leach through soil or run off the surface, as well as direct discharge into streams (often the case with industrial discharges). • Direct ingestion • Refers to direct eating of contaminated soil or waste materials— mostly referring to children who, by constantly putting their hands in their mouths, may eat up to 10 g of soil per day!; if this soil is contaminated (as many urban soils are), the children are potentially exposed to significant amounts of pollutants.
For any given contaminant, risk tends to be greater for certain pathways rather than others, due simply to the chemical nature of the contaminant. • Organic contaminants are seldom are taken up by plants, but may readily leach to ground water because they are not adsorbed by soils. • Some heavy metals (such as Cu and Zn) may be taken up by plants, but often kill the plants at high concentrations, and thus do not often enter the food chain. • Other metals, as well as some radionuclides, can accumulate in plants to high enough levels to threaten human health; radioactive cesium (Cs) and strontium (Sr) may contaminate milk grown on lands contaminated with radioactive fallout (for instance, around the Chernobyl nuclear plant in Russia), while metals like cadmium (Cd) can reach dangerous levels in plants grown on contaminated soils.
Waste Disposal • Many cases of soil and water contamination result from improper waste disposal • Prior to the 1970s, there were few regulations concerning waste disposal, and highly toxic wastes were simply dumped anywhere - into rivers and lakes, or buried in shallow trenches. • In the late 1970s a subdivision in New York at Love Canal was found to have been built directly on an older toxic waste dump containing hundreds of drums of industrial solvents. • During the same era Lake Erie, a huge water body and major fishery, was essentially dead due to eutrophication resulting from discharge of raw sewage into it. • Today the EPA enforces federal laws that regulate how wastes can be discharged or disposed of in order to protect human health. • These laws are constantly changing as scientists better understand the risks of various contaminants in the environment.
Landfills • Most solid wastes are disposed of in landfills. • Prior to 1980, most wastes were buried in unlined trenches. • Nearly all such landfills have plumes of contaminated groundwater below them containing both solvents and metals. • Current regulations (described in the Resource Conservation and Recovery Act, RCRA of 1980) specify that non-hazardous wastes like municipal garbage and some industrial wastes must be buried in landfills that have plastic and clay liners on the base to prevent leaching of any contaminants to ground water, and must also have a cap to reduce rainwater infiltration into the landfill.
Because even household garbage may contain small amounts of both heavy metals and organic contaminants, these Class D landfills must prevent leaching of any contaminants to aquifers below. • Hazardous wastes, which contain high levels of contaminants, must be buried in Class C landfills, which must have even greater protection against possible leaching. • While Subtitle C landfills reduce leaching potential, they are costly to build and have caused dramatic increases in waste disposal costs for both cities and many industries. • All landfills must obviously be built on deep, well-drained soils, preferably with a clayey Bt that can be excavated and later compacted into the bottom liner and cap. • The liner and cap must have a low hydraulic conductivity (K < 10-6 cm/sec), and are usually made of highly compacted clay.
Wastewater • From our sinks, toilets, bathtubs, as well as industry - is typically treated one of two ways: • in larger towns and cities, a central waterwater treatment facility receives this water through a sewer system, which is then treated and discharged, • in the country wastewater drains into a series of perforated pipes buried in the yard of each house — referred to as a septic disposal system.
Central Waterwater Treatment Facilities • Various microbial digestion processes are used to remove organic matter from the water, as well as nutrients such as N (mostly through denitrification). • Pathogenic organisms are also killed during this process. However, some nutrients and dissolved organic materials remain, which can lead to eutrophication of rivers receiving this treated wastewater. • Rather than using expensive methods to reduce these pollutants, some cities spray this wastewater on land, allowing it to percolate through soils where microbial action will degrade the organics, and plants will utilize the nutrients.
Clayton County (Atlanta) has such a treatment system where up to 50" of wastewater is applied to several hundred acres of pines and pasture. This system works well in the summer, but in winter when plant water and nutrient use is low, the soils stay too wet and there may be leaching of nitrates through the soil profile. • In central water treatment plants, the solids remaining after digestion are collected and de-watered, and then must be dealt with. This sewage sludge is organic- and nutrient-rich, and can be used as a fertilizer. With septic systems the solids must occasionally be pumped out of a holding tank underground.
Septic Disposal Systems • The soil is used as a bio-filter system to purify the wastewater as it percolates through the solum. • Water flow through the soil must be maintained at a sufficient, but not too high rate all through the year. • Soils with a seasonal water table (MWD or SPD soils) will have a saturated zone within 3-4' of the surface part of the year, which is the zone at which the drain pipes are buried. • Septic systems on these soils may not drain during the winter wet season, and sewage will back up into your house or seep out in your yard! • On the other hand, if the soil is very deep sand with a high hydraulic conductivity, water may percolate too quickly with little chance for organic degradation or nutrient removal. Ground water contamination may result in this case.
Recycling Solid Wastes • Many millions of tons of solids wastes are currently buried in landfills every year in this country, and most of those landfills will eventually leak more-or-less toxic materials into the underlying ground water. • The reason is that clay liners and plastic sheeting can only last so long before they crack and degrade. • EPA realizes this, and is encouraging cities and industries to begin looking for alternative waste disposal methods. One of those is to recycle wastes containing organic materials and nutrients back to the soil. • The major types of wastes are from municipal sources and from certain industries.
Biosolids • include the sewage solids from wastewater treatment plants (a partially decomposed organic sludge with 20-60% water remaining in it). • Municipal Gargage • Some cities also make a solid waste compost from the organic components of municipal garbage (the stuff you leave out at the curb every week). • This material is currently being made at plants in Marietta and Crisp County from organics (food scraps, paper, etc.) remaining after plastic, glass, metal, etc. have been removed from the garbage. • Both contain a lot of organic carbon and some macronutrients (1-3% each of N, P, and K, similar to animal manures), and have been used on cultivated and forest soils to add humus and nutrients.
Industrial Wastes • Many industrial wastes are not suitable for land application because they contain toxic materials, and under RCRA they must be disposed of in approved landfills, or recycled or incinerated. • Some industrial wastes can be land-applied with a benefit for agriculture. Pulp and paper mills, very common in Georgia, produce waste pulp (essentially partially digested wood) as well as wood ash and waste alkali (mixed Ca(OH)2/CaCO 3). These materials are low in heavy metals, although some have organic contaminants - including dioxins - that must be measured to be a very low levels before land application can be done.
The ash and alkali wastes can be used on farm fields as lime substitutes, and the waste pulp sludge is a good soil C source (although it is pretty low in nutrients, which would have to be added as fertilizer). Fly ash and gypsum are mostly made by the electric power industry, and are low in C and nutrients, but contain a lot of Ca and other nutrients. • Gypsum is very high in Ca, and useful for adding soluble Ca to subsoils, and fly ash is probably less useful; in addition, most coal ash contains appreciable contaminants such as As, and is still under study as a soil amendment. Food processing generates a range of liquid and solid wastes, most of which are high in C and nutrients, and can be readily land-applied.
EPA has a set of rules that must be followed in order to land-apply these kinds of wastes. The major potential hazards are: • pathogens in the waste (especially sewage sludge); • runoff or leaching of contaminants or nutrients; • excess loading of the soil with heavy metals. • Wastes generally need to be incorporated into the soil directly after application to prevent disease organisms from spreading, and grazing animals have to be kept off treated pastures for certain time periods.
The key factor for nutrients and metals is to calculate the correct application rate so as to not overload the soil's capacity to handle either nutrients or metals. • For N and P, which can cause environmental problems, you should treat them like any other fertilizer – add what the crop needs, and no more. N is usually tied up in organic forms in organic- based fertilizers, and extra needs to be added to account for mineralization. • Usually only about 20-30\% of the N in these wastes will mineralize the first year. Thus, continued applications will build up nutrients (and humus) in the soil, which is good– but too much long-term application could build up soil levels too much, resulting in water contamination. • Probably no more than 200-300 kg/ha of N should be applied in any one year; for P, if soil tests show P levels in the very high range, applications should be stopped.
A real controversy surrounds heavy metals in wastes right now. • EPA has established total loadings (in kg/ha) of different metals that can be added to soils in wastes – lifetime loadings that are cumulative over time. • But many people (scientists and otherwise) think these levels are too high, and eventually public health will be endangered by food chain or water contamination. • Sewage sludges - also called biosolids - from industrial cities (Atlanta, Birmingham, Augusta) can have significant heavy metal levels, and long-term application can build up metals in soils – and they can't be removed once in there . . . • Maintaining pH levels in the 6.0-6.5 range helps reduce metal solubility, but is not a guarantee metals will not move in all soils (remember that anionic contaminants such as As are more soluble at higher pH).
Land managers need to keep track of how much waste and metals have been applied to their lads, and stay well below EPA limits to safeguard the productivity of their lands. • For most people, the term water quality brings to mind chemical concentrations and healthy bounds for those concentrations. • For example, they might know that the U.S. Environmental Protection Agency (EPA) recommends that drinking water contain no more than 10.0 mg/L of nitrate (as N). Or they may know that most fish begin to suffocate when dissolved oxygen levels drop below 4 mg/L. • Agencies like the EPA and the World Health Organization have developed guidelines and regulations regarding safe concentrations of around 100 inorganic and organic chemicals within drinking water. • These guidelines have been developed through laboratory toxicity testing, epidemiological studies, and even taste and odor tests. In fact, regulations of certain chemicals are not targeted at human health but rather at preventing pipe corrosion.
A Few Examples • A wastewater contains 200 ppm plant-available N. • Calculate the inches of water that can be spray-applied in order to supply 300 lbs-N/acre to a growing crop over a one-year period. • Try this again with water containing 50 ppm available-N, and supply 500 lbs N/ac/yr.
A sewage sludge contains 25% biosolids (that is, 25 lbs solid material per 100 lbs wet sludge). • The total N content of the solids is 4%; about 25\% of this N will mineralize in the first year after application. • The Cd content is 22 ppm on a dry weight (solids) basis. • Calculate the tons of wet sludge that should be applied per acre in order to supply 250 lbs of available N to a crop during the first year after application.
Calculate the Cd (in lbs/acre) that would be applied. • Indicate whether this is an acceptable load, given that the maximum EPA load rate for Cd is 2 lbs/ac/yr. • Calculate how many year this sludge could be applied if the EPA maximum lifetime loading for Cd is 40 lbs/acre. • Try this problem again with different sludges (10% solids, 3% N, Cd = 12 ppm), using the same N mineralization rate. • It can be easy, and fun. Just keep track of your units, and identify what units you want to achieve in the end.
Chapter 14 Quiz 1. Match: _____ Class C landfill a. Municipal Waste _____ Class D landfill b. Hazardous Waste 2. What two pathways pose the greatest risks for organic contaminants? 3. What two pathways pose the greatest risks for metals? 4. Besides metals and organics, what other class of contaminants should we be worried about? 5. What are the two primary risk pathways for this class? 6. [True / False] DNAPLs are found above LNAPLs.
