Modified: 12/08/98
Conventional municipal sewage 1 treatment systems is one of the options under review by the Wisconsin Department of Agriculture, Trade & Consumer Protection (DATCP). Such systems are very costly and very unpopular with the owners and operators of large livestock operations. Most municipal sewage treatment systems do not remove phosphate from effluent, the treated water released to streams, and any facility treating animal waste would require processing to remove phosphate 1 to curve eutrophication caused by nutrient-rich effluent 1 flowing into streams and lakes.
Animal wastes contain nitrogen 1 and phosphorus 2, essential plant nutrients, and are mostly organic substances resembling humus. Land application of animal wastes increase aggregate stability and soil water-holding capacity because it increases the humus content of the soil. Land application of animal wastes increase the nitrogen and phosphorus fertility of the soil.
The problem occurs when excessive amounts of animal wastes are left on the surface where it is easily eroded into streams and lakes or when the build up of phosphorus is so great that erosion of the phosphorus-rich surface soil enters streams and lakes where it causes eutrophication 1. If manure is stored, then farmers can apply it to land during late fall or early spring when the ground is thawed and no crops are growing. Land application in Fall or Spring allows the farmer to plow the manure under, reducing the risk of erosion that accompanies surface application on frozen ground in Winter, but does nothing to prevent buildup from continual disposal of nutrient-rich manure on land.Heavy uses of nitrogen fertilizers or excessive land disposal of animal manure may supply far more nitrogen than the crop can use. If the nitrogen is not originally nitrate 2, soil microbial processes will eventually convert it into nitrate through the process called nitrification 1. The lack of significant anion exchange 1 capacity in soils means that nitrate readily leaches from the soil. Land disposal of animal manure is a waste disposal problem for which there are few, if any, practical solutions. Curbing nitrogen fertilizer use places farmers at risk of reduced crop yields, accelerating the economic failure of farms. The nitrate leaching problem arises from the demand for abundant and inexpensive food.
The nitrogen 3/phosphorus 3 (N/P) ratio identifies which of the two nutrients is present in excess of plant requirements. The N/P ratio 1 in animal manure is usually lower than the N/P ratio in plant tissue, meaning manure contains more phosphorus than required to build plant biomass. Mineralization 1 of organic phosphorus in manure releases phosphate that precipitates as insoluble minerals in soil, resulting in a gradual build up of soil phosphorus levels. Erosion of phosphorus-rich soil into streams and lakes causes eutrophication.
The more shallow the lake, the less the nutrient load into the lake will be diluted.
Nitrate 4 in drinking water is not the problem. Associated with the nitrate--NO3- is nitrite--NO2-. Infants lack the enzymes that will oxidize nitrite to nitrate, it is absorbed into their blood where it binds with hemoglobin and prevents the normal binding of oxygen. Called the "blue baby syndrome", the infant suffers from anoxia 1 because its blood can no longer carry sufficient oxygen.
Biological oxygen demand (BOD 1) is a measure of "dissolved" carbon. Animal manure, lawn clippings, leaf litter, even the biomass of an algae bloom can all contribute to "dissolved" carbon when they enter streams or lakes. This carbon is the energy source of aquatic microbes. A stream or lake containing both "dissolved" carbon and nutrients, such as nitrate and phosphates, stimulates the rapid growth of aquatic microbes. These rapidly growing microbes that consume the "dissolved" carbon for energy and absorb the nutrients to build biomass are aerobic.
A stream or lake rich in "dissolved" carbon and nutrients will support such a large and active microbial population that oxygen becomes depleted and other aquatic organisms die from a lack of oxygen (anoxia 2). There is a direct correlation between the amount of carbon suspended in the water and the microbial population it can support and the likelihood that oxygen will be depleted. The carbon creates a demand for oxygen by the microbial population, hence "biological oxygen demand."The term "pesticide 1" is inclusive of chemicals used to control insects, weeds, fungi, and nematodes. These organisms are collectively called pests. Chemicals that specifically control insect infestation are insecticides, those controlling weeds are herbicides, fungicides and nematicides control fungi and nematodes, respectively.
Modern insecticides 1 may mimic hormones that control the development of larvae or reproduction. Such compounds will have no effect on plant physiology. Conversely, herbicides 1 that disrupt plant physiology, e.g., atrazine disrupts photosynthesis in broad leaf plant species, will have no effect on insect physiology.First-generation pesticides 2 were toxic inorganic compounds 1 (calcium arsenate--CaAsO4, copper nitrate--CuNO3, copper sulfate--CuSO4, lead arsenate--PbAsO4) that will not degrade, required heavy application rates, and accumulate in soils. Second-generation pesticides were organic compounds that are highly-resistant to microbial degradation 1 and, therefore, persistent in the environment. Modern pesticides are organic compounds that are biodegradable by soil microbes, less persistent in the environment and less likely to accumulate in species high in the food chain.
Most rural households rely upon well water that taps groundwater. The vast majority of this well water is not treated to remove pathogens (bacteria or viruses) or chemicals (nitrates 5, fuels, or pesticides 3) that contaminate the groundwater. Pesticides in groundwater increase the risk of human exposure through the consumption of the untreated contaminated groundwater.
Whether a pesticide 4 is charged or neutral has more to do with its mode of action in the target organism than its implications for retention by soil colloids. Keep in mind that a pesticide that adheres strongly to soil colloids is no longer biologically available (bioavailable 1) to control pests.
Given that insect physiology is more similar to mammals than plants, modern insecticides 2 are more likely to be toxic to humans than herbicides 2.
Fungicides and insecticides 3 almost certainly kill beneficial fungi and insects along with the primary target pests. Because it is virtually impossible to accurately monitor bacterial ecology in soils, soil scientists cannot determine the general effects of pesticides 5 on bacterial communities.
One alternative to pesticides is biological control 1. Biological control employs the natural predators, parasites, or diseases to reduce or eliminate pests. Sometimes this works and sometimes it does not. A pesticide 6 may kill beneficial plants along with weeds. A plant pathogen may just as readily kill beneficial plants along with the weed species it is supposed to control.
DDT (dichlorodiphenyltrichloroethane) is an insecticide 4 while atrazine is an herbicide 3. Both are chlorinated organic compounds that makes them resistant to microbial degradation 2 and, as a result, both persist in the environment.
The fundamental reason pesticides 7 rarely kill only the target species arises from the physiological similarites between species. The hormones 1 that control insect development, metabolism and reproduction are very similar across different insect species. An insecticide 5 that disrupts the physiology 1 of the target insect specie will be toxic to beneficial insects because of these similarities. There are sufficient differences between grass and broad leaf plant species that herbicides 4 that target one will not kill the other, but an herbicide that kills grass weeds will also kill corn, wheat, rice and other crops belonging the the grass family.
Crop rotations 1 reduce but do not eliminate the need for pesticide 8. Continually growing the same crop year after year on the same plot of ground will increase the population of insect species that feed upon that crop because each new generation finds its desired host plant. Interrupting that cycle means fewer adults survive to lay eggs simply because they do not have a host. Crop rotations alternate grass and broad leaf species, corn and soybeans for instance. Herbicides used in corn kill broad-leaf weeds but are less effective on grass weeds. Conversely, herbicides used in soybeans kill grass weeds but are less effective on broad-leaf weeds. Crop rotation allows the farmer control grass and broad-leaf weeds on alternate years, resulting in a general decline in weed species of both kinds and therefore the amount of herbicide need to control weeds.
The life-span of pesticides 9 is measured in half-lives. The half-life 1 of a substance is the time span required for half of the original amount to degrade 3. For instance, the half-life of the insecticide DDT is about 100 days. If you added 1 gram of DDT to a soil today, then 100 days from now only 0.5 gram will remain and 200 days from now (2 half-lives) only 0.25 gram will remain. It would take over 7 half-lives, 700 days or about 2 years for 99% of the DDT to degrade. The herbicide 2,4-D has a half-life of less then one day while the herbicide atrazine has a half-life in soils of 700 days. Modern, third-generation, insecticides and herbicides have soil half-lives of 4-10 days. Which means that 99% of these pesticides will be gone in 70 days or about 2 months.
The simplest definition, a definition that is correct with few exceptions, relies upon presence of carbon in the compound. This definition calls "organic 1" those compounds that contain carbon and "inorganic" those compounds that do not contain carbon. There are two exceptions: carbon dioxide--CO2 and carbonate--CO32-. Carbon dioxide and carbonate are "inorganic" carbon compounds.
A more accurate definition, one with no exceptions, relates to the type of chemical bonds. Organic compounds are those where carbon atoms are bonded to carbon atoms, which excludes carbon dioxide and carbonate.All life processes in organisms involve chemical reactions that are promoted and regulated by a class of biological molecules called enzymes 1. Some of these enzymes, called metalloenzymes, contain specific metals that give the enzyme its special function. These metals are essential, but only in trace quantities. Both plants and animals require these so-called trace nutrients 1. Two familiar examples are the iron-containing enzyme hemoglobin that carries oxygen in blood and the magnesium-containing enzyme chlorophyll required for photosynthesis in plants.
The metalloenzymes regulate important physiological 2 functions and they perform these functions only if they contain a specific metal. You can replace the iron in hemoglobin with other metals, for instance, but replacing the iron changes its function because it binds oxygen too weakly or too strongly. Metal toxicity in plants and animals occurs when sufficient amounts of the metal are absorbed into its tissue to replace the trace metals in metalloenzymes and, thereby, disrupt critical physiological processes. Some plants are very sensitive to toxic elements, while others have mechanisms that allow them to accumulate toxic elements in their tissue without apparent harmful effect. Scientists are studying the potential utility of growing these tolerant plants on soil contaminated by toxic elements and harvesting them to remove the contaminants accumulated in their tissue. This is called phytoremediation 1, "phyto-" meaning "of or by plants". Human exposure can occur when they consume plant tissue that has accumulated toxic elements 2. The likelihood of exposure increases as the soil content of these toxic elements increase. For that reason, environmental regulatory agencies limit land disposal of materials containing toxic elements.Cations 1 are positively-charged atoms. Anions 1 are negatively-charged atoms.
The surfaces of all soil particles are charged, positive under acidic conditions and negative under alkaline conditions. Because positively-charged particles surfaces attract and bind negatively-charged atoms (anions) they are called anion exchangers 2. Negatively-charged particles surfaces are likewise called cation exchangers 1. Soil clays and humus account for most of the ion exchange 1 capacity because they have a much larger surface area per weight than coarser particles (sand or silt).
Ion exchange removes ions from solution, immobilizing them on particle surfaces. This retains nutrient and toxicant ions in the soil, keeping they from being washed away by the water that percolates through the soil. Ion exchange also reduces biological availability 2, storing nutrients or reducing the toxicity of contaminants. Water lacks this capacity, which is why water contamination is more serious then soil contamination. Non-toxic soil components always are more abundant than toxic contaminants 3. This means that when one adds up the contaminant ions held on soil colloids by ion exchange, they account for only a small percentage of the total. Nutrient ions far exceed the number of toxic ions in all but the most highly contaminated sites.There are chemical measurements that can determine this number. The exact number is not important for our purposes. Simply put, the number of cations 2 and anions 2 adhering to the surfaces of soil particles far exceeds the number actually dissolved in water. Ion exchange 2 acts like a reserve.
Perhaps the best analogy is to picture a cafeteria with some people seated (bound) at tables (particles) and some people waiting to be seated (dissolved). The demand for seating is so great that all vacant seats are immediately occupied (law of charge neutrality 1). The ion exchange capacity is the number of seats. A soil is like a cafeteria where the ratio between the number of persons seated to the number waiting to be seated is 100-to-1.Clay-sized particles have a much higher ratio of surface area 1 to volume than sand-sized particles. This means that weight for weight, clays add far more surface area to a soil than sand. Ion exchange 3 is proportional to surface area, so increasing the clay content will dramatically increase the capacity to retain ions.
Changes in soil acidity can alter the solubility of toxic compounds 4 or alter the retention of ions by ion exchange 4. Acidifying a soil tends to increase the solubility of most compounds, including toxic compounds. Acidifying 1 a soil will increase anion exchange 3 capacity, reducing the leachability of toxic anions, while decreasing the cation exchange 2 capacity, increasing the leachability of toxic cations. Adding calcium carbonate (lime) to an acid soil will make it more alkaline 1, reversing the effects just described.
Biological activity 1 is the primary source of soil acidity 2 while mineral weathering is the source of alkalinity 2. Respiration by organisms produces carbon dioxide, a weak acid. Plant roots release acidity into soils at the same time they absorb nutrient ions in order to maintain the charge balance within their tissue. Plant roots also release acidity so dissolve minerals the store essential nutrients. All of these cause soils to become more acid. This is important because pH affects biological activity, it affects the solubility of nutrient elements and toxic elements, and it affects the ion exchange 5 that binds nutrients and toxicants to soil particles.
Oxidation 1 and reduction 1 are chemical processes that change the number of electrons bound to an element. Logan describes it very well in the essay "Kaolin" through the analogy between the red color of oxygen-rich blood and the blue color of oxygen-depleted blood or the warm colors of pottery fired in oxygen-rich kilns and the cool colors of pottery fired in oxygen-deprived kilns.
These are visual changes that correspond to chemical changes that can drastically alter the solubility of certain elements. Water solubility determines biological availability 3. Some elements, such as iron and manganese, become more soluble when they are reduced, i.e., electrons are removed. Other elements, such as chromium, become less soluble when reduced.Aerobic 1 microbes require molecular oxygen--O2 to respire and become dormant when O2 is absent. Anaerobic 1 microbes can survive in the absence of O2 because they do not require O2 to respire. Instead they can use oxidized forms of other elements: iron--Fe3+, manganese--Mn4+, nitrate--NO3-, sulfate--SO42-, carbonate--CO32-.
When water fills soil pores, driving out oxygen, aerobic 2 microbial communities become dormant 1 for lack of oxygen (anoxia 3) and formerly dormant anaerobic 2 microbial communities become active. Anaerobic microbes cannot tolerate oxygen and will remain active only if oxygen is lacking. As soon as water drains from the soil pores and oxygen reenters the soil, the anaerobic communities become dormant and aerobic communities become active. In reality, some regions of the soil are poorly aerated even in well-drained soils and anaerobic communities will be active in those regions.
This is a surprisingly difficult question to answer. In fact, it is one of the most important questions environmental scientists are studying today. It is relatively easy to measure the amount of a toxic element 5 in soil, but toxicity is determined by biological availability 4. It is far more difficult to measure the biological availability of a toxic element than the total quantity. Because soil contains large quantities of natural organic substances, humus, it is extremely difficult to measure the amount of organic contaminants in soils.
One reason chlorinated solvents pollute the environment is because they are so widely used by industry, creating the opportunity for environmental contamination. Pollution, however, is a vague term and carries connotations of both contamination and toxicity. Xenobiotic (nonbiological, synthetic) organic contaminants 2 containing chlorine are highly resistant to microbial degradation 4. Once they contaminate soil, they persist for a very long time. These compounds may or may not be toxic to plants and animals. If they are toxic, that toxicity may be acute or chronic.
Soil scientists can easily catalog the effects of pollution on plants, animals and multicellar invertebrates. It is far more difficult to determine and accurately measure the effect on microbial ecology. Soil microbiologists cannot catalog the microbes active nor determine the active microbial biomass in healthy soils, let alone contaminated soils. In short, the level of biological activity 2 within the microbial community may be unaffected by pollution but the microbial population may loose diversity. Biological diversity 1 may be as important as the overall level of biological activity.
Soil is porous material and the rate water flows through porous material depends on the size and connectivity of the pores. Soil pores are like tiny pipes. The rate water flows through these pipes depends on their diameter. If an equal pressure is applied to two pipes, the one with the larger diameter will carry more water. Similarly, if the force driving water through a soil is the same in two different soils, the rate of water flow (hydraulic conductivity 1) will be higher in soils with larger pores (permeability 1).
Here are some examples from the text (Miller & Gardiner, p. 131, Detail 4-1). The hydraulic conductivity of sand is about 0.01 to 10 millimeters per second. The hydraulic conductivity of silt is a thousand times slower than sand while the conductivity of clay is a million times slower than sand. Topography has not effect on the intrinsic conductivity of a soil, but it does affect the gravitational driving force that pulls water through soil.The permeability 2 of sand is very high because pore diameters are large and, as a result, the rate of groundwater flow through sand is exceeded only by flow through gravel. Typically sand contains little clay or humus, the soil colloids that bind contaminants and retard their transport by percolating water. Retardation factors 1 in sand are exceedingly low. This combination, high permeability and low retardation factors, mean contaminant transport is very rapid through sand.
A retardation factor is the ratio of water flow rate to contaminant migration rate. The simplest way to think of a retardation factor is the time a contaminant is dissolved in moving water relative to the time it is immobilized on soil particles. If the retardation factor is 0.5, meaning the contaminant moves at half the rate of groundwater flow, the contaminant spends roughly half its time dissolved in moving water and half its time bound to soil particles. Ion exchange is one way contaminants adhere to soil particles, hence any increase in ion exchange will likely slow contaminant migration.Molecules are much smaller than the smallest soil pores and their size has no effect on their rate of movement. Contaminant flow rates depend on how strongly the contaminant binds to soil clays and humus as measured by the retardation factor 2. A "conservative constituent 1" is a substance that either does not bind to soil particles or adheres so weakly that it spends most of its time dissolved in moving water and little time bound to soil particles. As a result, a "conservative constituent" moves at about the same rate as the groundwater and can be used to measure the rate of flow.
Contaminant migration with groundwater increases cleanup costs because a larger volume of soil and subsurface material becomes contaminated. Any plan for cleanup must begin with an assessment of the type and extent of contamination. Given the possibility that groundwater is transporting contaminants, assessment requires information on the direction and dimensions of the contaminant plume 1 as well as the quantity of moving contaminant. Predicting the rate contaminants are moving requires some knowledge of groundwater flow rates and directions. Several factors determine the ultimate cleanup cost, including the level of contamination, the volume of contaminated soil, and the type of contaminant.
Toxic metals 6 do not degrade, but remain in a potentially toxic form as long as they reside in the soil. The higher the toxic metal content of a waste being disposed on land, the more rapid the rate of buildup and increasing risk of exposure. Plants may absorb toxic elements into their tissue, becoming a part of the "ash" or "plant minerals" stored in their tissue until they are subsequently released (mineralized 2) when the plant residue decomposes. If the plant tissue containing the toxic metals is harvested and removed from the land, the cycle is broken and buildup is reversed. This is the basis for phytoremediation 2. Otherwise, the toxic metals are continuously cycled from soil to plant and back again.
When water carrying contaminants enters a wetland, the contaminants dissolved in the water bind to the abundant humus in the wetland soil. The water continues to pass through, leaving the contaminants behind. This process cleans the groundwater but contaminates the wetland, potentially exposing the wetland biota to the toxic effects of the contamination. So long as the humus in the wetland exists, the contaminant will remain there, bound to the humus.
Draining the wetland will have two effects. Drainage may significantly alter groundwater flow through the wetland, potentially reducing the removal of contaminants. Drainage will allow aeration of the wetland soil. Organic contaminants 3 that might otherwise persist because they degrade 5 very slowly under anaerobic 3 conditions would not degrade more rapidly. Aeration will change the oxidation 2 state of toxic elements 7, in most cases reducing their solubility. The changes associated with wetland drainage have both positive and negative implications.Biological remediation and humification 1 both extract energy from organic compounds through the respiration of a living organism. Humification typically works on residues derived from biomass while bioremediation works on organic compounds derived either from fossil fuels or industrial synthesis. Microbes degrade organic contaminants 4 because they release the chemical energy stored in the compound through a process called respiration (oxidation 3). Organic compounds derived from fossil fuels are easily degraded because they are fundamentally natural biological compounds. Many synthetic organic compounds are very stable compounds that resist both abiotic chemical degradation and biological degradation. That resistance derives from subtle characteristics of their chemical structure. Some microbes somehow develop the capacity to degrade even the most resistant compounds and, if they grow in contaminated soil, they will slowly degrade resistant organic compounds. Part of a successful bioremediation depends on finding the right microbes.
Microbes use organic contaminants 5 as their source of energy. They can be coaxed to degrade resistant compounds if all other factors--moisture content, aeration, temperature, pH and nutrient availability--are optimal. Microbes seem to need nitrogen 6 more than phosphorus 4 for growth, but optimal growth requires a balanced supply of all essential nutrients.
There are many strategies for remediation. Bioremediation 2 covers all remediation strategies that involve organisms, but most practitioners associate the term with microbial degradation 6. Phytoremediation 3 relies upon plants, usually to absorb toxic elements from soil permitting the removal of the toxic elements accumulated in plant tissue by somehow harvesting plant tissue.
Soil washing is a process that passes water, perhaps containing chemical agents that extract the contaminant, through contaminated soil followed by collecting and treating the water. Soil washing may leave the soil in place, purging the soil with water injected into wells, or may require excavation and washing in specially reaction vessels. There are several treatment processes that immobilize the contaminant by mixing soil with cement or adding chemicals to a soil that react with the contaminant to produce a product that is insoluble and, therefore, less biologically available 5. Excavation and burial of contaminated soil in special landfills is always an option that may be impractical because of cost. It may clean up the site, but the contamination is simply transferred to a more secure setting.