Whole Document Navigator (Click Here)    ---------------------------------- Top of Document How Contaminants Move Through Soil Managing Specific Contaminants - I. Managing Pesticides -- Pathways of Pesticide Loss -- Persistence -- Sorption -- Estimating Pesticide Losses -- Selecting and Using Pesticides -II. Managing Nitrogen -- How Nitrogen Moves Through Soil -- How Climate and Irrigation Affect Nitrogen Leaching -- How Nitrate Contaminates Aquifers - III. Managing Pathogens -- How Pathogens Move Through Soil Summary Footnotes Disclaimer Copyright Infomation

How Contaminants Reach Groundwater ¹

Arthur G. Hornsby²

The movement of contaminants through soil to groundwater is affected by many variables, including properties of the contaminant itself, soil conditions and climatic factors. These combinations of factors make the likelihood of groundwater contamination a very site-specific science. Nevertheless, as thorough an understanding as possible of these processes and variables is critical to effective management of potential groundwater contaminants.

Groundwater is the source of drinking water for 50 percent of the population in the United States. In rural areas, 85-90 percent of residents obtain their drinking water from groundwater. Because the quality of drinking water supplies is important, groundwater merits protection from contamination.

How Contaminants Move Through Soil

Folklore holds that the presence of soil protects groundwater quality by "filtering" contaminants out of recharge water. Present knowledge, however, indicates that the capacity of soils and the intermediate vadose zone (the area below the crop root zone and above the permanent water table) to degrade potential contaminants as they move toward groundwater is limited. The major processes that determine the fate of pollutants in soil and their potential to leach to groundwater are now well understood, and this knowledge can be used to reduce or eliminate contamination of drinking water supplies.

There are two basic processes by which contaminants move from the earth's surface through soils and groundwater. These processes are diffusion and mass flow. Substances diffuse through soils and aquifer materials in response to differences in energy from one point to another. These energy gradients may be caused by differences in concentration or temperature within the system. The principal process of movement of contaminants in soils and groundwater is mass flow. Dissolved constituents in water move through the soil, with the water acting as carrier of the contaminants.

Diffusion and mass flow are affected by properties of the contaminants, the soil, the intermediate vadose zone and the aquifer, climatological factors; and vegetation patterns:

• Properties of contaminants that determine their movements and potential threat to water quality include water solubility, tendency to adhere to soil materials, persistency and toxicity.
• Properties of soil, the intermediate vadose zone and the aquifer that affect rate of contaminant movement include infiltration characteristics, pore size distribution, microbial population density and diversity, organic matter content, total porosity, ion exchange capacity, hydraulic properties, pH and oxygen status.
• Climatic factors include temperature; wind speed; solar radiation; and frequency, intensity and duration of rainfall.
• Vegetation may act as a sink for contaminants by uptake or assimilation, thus reducing the amount of contaminant available for transport to groundwater.

All of these properties interact to determine the rate and amount of movement of contaminants in soils and groundwater.

Managing Specific Contaminants

Since different classes of contaminants interact differently, management of these materials requires different approaches to reducing movement through soil to groundwater. Three such groups are pesticides; nitrogen forms, including fertilizers, manures and septage (organic semi-solids pumped out of septic tanks); and pathogens.

I. Managing Pesticides

The use of pesticides is important to modern agriculture and residential landscapes. During the past several years, however, concern has grown about the presence of pesticides in the environment and the threat they pose to wildlife and humans. By design, pesticides are poisons and can be particularly dangerous when misused. Fish kills and acute illnesses in humans have been attributed to pesticide exposure or ingestion, usually as a result of misapplication or careless disposal of unused pesticides and containers. Pesticide losses from areas of application, as well as contamination of such non-target sites as surface water and groundwater, represent a monetary loss to the farmer as well as a threat to the environment. Thus, careful management of pesticides to avoid environmental contamination is desired by both farmers and the general public.

Pathways of Pesticide Loss

There are four pathways by which properly applied pesticides may be removed from the targeted application site:

• vaporization (volatilization) into the atmosphere
• removal in the harvested plant
• runoff -- the physical transport of pollutants overland by water or eroded soil, caused by precipitation or irrigation that does not penetrate the soil
• leaching -- the process whereby pollutants are flushed downward through the soil by rain or irrigation water that does infiltrate the soil

In areas of the country where soils are sandy and permeable, leaching is likely to be a more serious problem than runoff.

Once applied to cropland (Figure 1) , a number of things may happen to a pesticide:

Figure 1.

• It may be taken up by plants or ingested and metabolized by animals, insects, worms or microorganisms in the soil.
• It may move downward in the soil and either adhere to soil particles or dissolve.
• It may vaporize and enter the atmosphere.
• It may break down via microbial and chemical pathways into other less-toxic compounds.
• It may be leached out of the root zone or washed off the surface of land by rain or irrigation water.
• It may flow upward with water as a result of evaporation of water at the ground surface. In the United States, this process is not likely to be as important as downward leaching.

In addition to the mass flow components controlled by climate and the water-holding properties of the soil, the fate of a pesticide applied to soil depends largely on two of its properties: persistence and sorption . Table 1 presents persistence and partition coefficients of selected pesticides in soil.

Persistence

Persistence defines the life and activity of a pesticide. Most pesticides degrade or become inactive over time as a result of chemical and microbiological reactions in soil:

• Sunlight breaks down some pesticides.
• Soil microorganisms can completely break down many pesticides to carbon dioxide, water and other inorganic constituents.
• Some pesticides produce intermediate substances, called metabolites, as they degrade. The biological activity of these substances may have environmental significance.

Persistence is expressed as a half-life. Half-life is the amount of time (in days) it takes for one-half the original amount of a pesticide applied to be deactivated in the soil. Half-life is sometimes defined as the time required for half the amount of applied pesticide to be completely degraded and released as carbon dioxide and water. Usually, the degradation halflife of a pesticide measured by this "complete degradation" factor is longer than that based on deactivation only.

Sorption

Probably the single most important property influencing a pesticide's movement with water is its sorptivity , or tendency to stick to soil particles. Soil is a complex mixture of solids, liquids and gases that provides the life-support system for roots of growing plants and such microorganisms as bacteria. When a pesticide enters soil, some of it will stick to soil particles, particularly organic matter, through a process called sorption .

Some of the pesticide will dissolve and mix with the water between soil particles, called soil-water. As more water enters the soil through rainfall or irrigation, the pesticide molecules will move down and may enter soil-water through a process called desorption. The relationship between water flow, sorption and desorption is a dynamic process. The solubility of a pesticide and its sorption on soil are generally inversely related, with increased solubility resulting in less sorption.

One of the most useful indices for quantifying pesticide sorption on soils is the partition coefficient (PC). This PC value is defined as the ratio of pesticide concentration bound to soil organic matter particles to that dissolved in the soil-water. Thus, for a given pesticide application, the smaller the PC value, the greater the concentration of pesticide in soil-water. Pesticides with small PC values are more likely to be leached than those with large PC values.

Table 2 considers pesticide persistence and sorption in determining its potential to contaminate groundwater and surface water.

Estimating Pesticide Losses

In estimating pesticide losses from soils and their potential to contaminate groundwater or surface water, it is essential to consider simultaneously both persistence and sorption. Quantitative estimation of pesticide losses via runoff or leaching requires complex mathematical models. These models are used to analyze site-specific soil, crop, management and climatological information. In the absence of such information, however, a qualitative assessment of a pesticide's potential to contaminate surface water or groundwater is possible.

Pesticides with high persistence and a strong sorption rate are likely to remain near the soil surface, increasing the chances of being carried to a stream or lake via surface runoff. In contrast, pesticides with high persistence and a weak sorption rate may be readily leached through the soil and are more likely to contaminate groundwater.

For nonpersistent pesticides, the possibility of surface water or groundwater contamination depends primarily on whether heavy rains or irrigation occur soon after pesticide application. Without water to move them downward, pesticides with short half-lives are more likely to remain within the biologically active crop root zone and may be degraded readily. In terms of water quality, then, pesticides with intermediate sorption values and low persistence values may be considered generally nonthreatening to health, because they are not readily leached and are degraded fairly rapidly.

Selecting and Using Pesticides

Agricultural use of pesticides should be part of an overall pest management strategy that includes biological controls, cultural methods, pest monitoring and other applicable practices, referred to altogether as integrated pest management or IPM. When a pesticide is needed, its selection should be based on effectiveness, toxicity to nontarget species, cost and site characteristics, as well as solubility and persistence.

In addition to pesticide sorptivity and soil permeability, it is important that the pesticide's toxicity to nontarget species be considered. The use of some pesticides is severely restricted due to acute toxicity or long half-lives. These pesticides present serious concerns if they are leached through the soil to groundwater. The U.S. Environmental Protection Agency has issued health advisory levels for pesticides that pose a potential threat to groundwater. Selection of pesticides based on their mobility and persistency should be done only according to instructions on the pesticide label.

II. Managing Nitrogen

Nitrogen is an essential element in life. It is a normal part of the human environment. The atmosphere consists of approximately 78 percent nitrogen gas. Amino acids, which are nitrogen-containing organic acids, form the building blocks of protein in our bodies. Nitrogen is also an integral part of chlorophyll production. Chlorophyll converts radiant energy from the sun to carbohydrates, which are essential dietary components of human health.

Nitrogen accumulates in soils during the process of soil formation, through deposition from rainfall, and through plant and microbial fixation of nitrogen as from the atmosphere. Nitrogen accumulated in soil organic matter is produced from decaying plant and animal matter.

Worldwide, nitrogen is the plant nutrient most critical to the production of food and fiber. Throughout recorded history, humans have added nitrogen to crops through animal manures, legume crops or fertilizers.

There are four major factors that affect the behavior of nitrogen in the environment and subsequently its potential to contaminate drinking water supplies. These factors include:

• the amount and forms of nitrogen entering the soil
• the soils that overlay the aquifers
• assimilation of nitrogen by plants, microbes and other soil organisms
• local climate conditions and irrigation practices

These factors interact to determine the fate of applied nitrogen fertilizers, animal wastes, sewage sludge and septage.

The most common forms of nitrogen in fertilizers are ammonium, nitrate, urea and natural organics. Forms of nitrogen in septic tank effluent include ammonia, ammonium, organic nitrogen, nitrate and nitrite.

Nitrogen can be lost from the soil by various pathways, some of which reduce the potential for nitrate to contaminate groundwater. Pathways of loss include volatilization as gases to the atmosphere. plant uptake, microbial metabolism and leaching.

How Nitrogen Moves Through Soil

Soils are the medium in which we grow most of our crops. Soils provide a reservoir for the nutrients, water and microbes necessary for economic production of crops. Soils differ in their capacities to retain water and nitrogen and thus must be managed differently to maximize production and minimize water and nutrient leaching.

Deep sandy soils require more frequent water and nutrient applications due to their very limited capacity to retain these inputs. Excessive leaching is the rule rather than the exception, unless very careful management practices are followed. No soil will retain heavy, continuous applications of nitrogen exceeding crop requirements. Thus, high leaching potentials occur under these conditions.

Poorly drained soils, on the other hand, may require artificial drainage to be productive. Leaching to groundwater may occur if confining layers are discontinuous or if drainage or irrigation ditches cut through the confining layer. Medium and heavy textured soils and organic soils have good water and nitrogen retention capacities. Nevertheless, careful management of water and nitrogen application practices is necessary.

How Climate and Irrigation Affect Nitrogen Leaching

Rainfall is dissipated by streamflow and groundwater recharge. Rainfall patterns vary with season in different regions of the country. Even in areas with ample annual rainfall, drought can occur, causing production losses or failure unless irrigation is used. Soils with poor water retention capacity also may require frequent irrigation. Managing the soil-water deficit to prevent plant-water stress and excess leaching of nutrients from the root zone is an onerous task in these soils.

Whether in arid or humid regions, management practices to reduce groundwater contamination must be based on the following:

• a good understanding of the water-holding capacity of the soil
• a good estimation of soil-water deficits
• irrigation systems that deliver precisely the water required to replenish the soil-water deficit

How Nitrate Contaminates Aquifers

Nitrate is very soluble and mobile in water. Forms of nitrogen fertilizer other than nitrate are transformed readily into nitrate and thus become subject to leaching to groundwater. Consequently, forms of nitrogen fertilizer other than nitrate are seldom found in aquifers.

Studies indicate that such conversion can take place within 30 days in warm, moist soils. Soluble nutrients are carried with the water through soils. Excessive rainfall or irrigation will tend to leach nitrogen below the root zone and ultimately to groundwater. This results in both an economic loss of nitrogen and deterioration of water quality in drinking water supplies. For these reasons nitrogen and water management practices must be considered jointly in farm management decision-making. Soil testing for residual soil nitrogen, crop nutrient requirements, realistic yield goals and irrigation efficiency are concepts that must be integrated to develop a crop production system that avoids excessive nitrogen leaching.

III. Managing Pathogens

Pathogens (bacteria and viruses) may occur in sewage sludge, septage, animal wastes, some food-processing wastes and septic tank effluent. These waste streams enter the soil environment in several ways. Some are applied to the land as fertilizers, some are disposed into landfills, and others seep into the soil either by design or happenstance.

How Pathogens Move Through Soil

Pathogens are carried in suspension with water through soil. These suspended organisms are removed from the soil water by filtration and adsorption. Two factors that significantly affect the mobility of bacteria and viruses in the underground environment are the size of the water-filled pores, including cracks, fissures and solution channels; and the actual velocity of the water in those pores. Other factors affecting the survival of pathogens in the vadose zone (the unsaturated zone extending from the earth's surface to the permanent water table) are pH, temperature and oxygen concentration.

The pore size distribution of soils acts in two ways to reduce pathogen movement. Soils with predominantly small pores will provide greater filtering characteristics and greater residence time to allow adsorption and deactivation than will coarse-textured soils. If the texture is too fine, however, infiltration and internal drainage will be insufficient to permit reasonable loadings of effluent. (Such soils would fail percolation tests.) Coarse sandy soils provide little opportunity for filtration or adsorption due to larger pore sizes, more rapid water movement and less exchange surface for adsorption. Thus, removal of pathogens depends strongly on soil properties, which are known to vary widely, and on controlling rate of flow through the soil.

Methods to reduce contaminant movement to groundwater depend on water management, appropriate loading rate and soil and vadose zone properties. Some level of contamination occurs in groundwater due to natural processes of precipitation, infiltration and recharge to aquifers. Understanding the processes that control movement of contaminants is necessary for developing any successful strategy to protect groundwater from contamination levels that pose significant health risks.

Summary

Methods to reduce contaminant movement to groundwater depend on water management, appropriate loading rates, and soil and vadose zone properties. Some level of contamination occurs in groundwater due to natural processes of precipitation, infiltration and recharge to aquifers. Understanding the processes that control movement of contaminants is necessary for developing any successful strategy to protect groundwater from contamination levels that pose significant health risks.

Tables

Table 1. Persistence and Partition Coefficient (PC) of selected pesticides in soil.

Common Name	Trade Name	Partition Coefficient	Half-life (Days)
NONPERSISTENT (half-life 30 days or less)
dalapon	Basfopon, Dowpon	1	30
dicamba	Banvel	2	14
chloramben	Amiben	15	15
oxamyl	Vydate	25	4
aldicarb	Temik	30	30
2,4,5-T	Dacamine 4T, Trioxone	80	30
alachlor	Alanex	170	15
cyanazine	Bladex	190	14
captan	Orthocide, Captanex	200	2.5
propham	Ban-Hoe, Chem0Hoe	200	10
diphenamid	Enide, Rideon	210	30
carbaryl	Sevin	300	10
malathion	Cythion	1,800	1
methyl parathion	Penncap-M, Metacide	5,100	5
chlorpyrifos	Lorsban, Dursban	6,070	30
fluvalinate	Mavrik, Spur	1000,000	7
MODERATELY PERSISTENT (half-life 31-99 days)
picloram	Tordon	16	90
carbofuran	Furadan, Curterr	22	50
bromacil	Hyvar, Bromax	32	60
atrazine	Attrex	100	60
chlorimuron-ethyl	Classic	110	40
simazine	Princep	130	60
ametryne	Evik	300	60
prometryn	Caparol, Primatol Q	400	60
dichlobanil	Casoron	400	60
linuron	Lorox, Aflon	400	60
diuron	Basudin, Spectracide	480	90
chlorbromuron	Maloran	500	40
fonofos	Dyfonate	870	40
phorate	Thimet	1,000	60
chloroxuron	Tenoran, Nortex	3,000	60
ethafluralin	Solanan	4,000	60
esfenvalerate	Asana	5,300	35
fenvalerate	Extrin, Sumitox	5,300	35
trifluralin	Treflan	8,000	60
cacodylic acid	Bolate, Bolls-Eye	10,000	50
glyphosate	Roundup	24,000	47
PERSISTENT (half-life100 days or more)
terbacil	Sinbar	55	120
fomesafen	Reflex	60	100
prometon	Pramitol	150	500
propazine	Milogard, Primatol-P	154	135
isofenphos	Oftanol	600	150
lindane	Isotox	1,100	400
isoxaben	Gallery, Knock out	1,400	100
chloroneb	Terraneb	1,650	130
neburon	Kloben	2,500	120
ethion	Ethion	10,000	150
mirex	Mirex, Dechlorane	100,000	3,000

Table 2. Pesticide persistence and sorption: its potential impact on groundwater.

PERSISTENCE	SORPTION	POTENTIAL IMPACT
		Groundwater	Surface Water
Nonpersistent	Low-moderate	Low	Moderate
Nonpersistent	Moderate-high	Low	Moderate
Moderately persistent	Moderate-high	Moderate	Moderate
Moderately persistent	Low-moderate	High	Moderate
Persistent	Moderate-high	Moderate	High
Moderately persistent and persistent	Low-high	Site-specific conditions determine groundwater or surface water impacts

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Footnotes

1. This document is SL143, one of a series of the Soil and Water Science, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Published: March 1999. Reviewed: September 2003. A version of this fact sheet was published in 1990 as part of a series developed in the Groundwater Policy Education Project funded by the W. K. Kellogg Foundation in cooperation with the Farm Foundation. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.

2. Arthur G. Hornsby, professor emeritus, Soil and Water Science Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

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