A key to protecting groundwater is understanding the role of soil in removing contaminants. Contaminants reach groundwater by moving with percolating water through the soil pores. If soil did not remove these materials, daily human activities would rapidly pollute groundwater. Fortunately, the soil cleanses water of most contaminants as the water moves through the soil. Some contaminants become attached (adsorbed) to the surfaces of soil particles. Microorganisms that live on soil particle surfaces break down other contaminants into harmless materials.

Different types of soils vary greatly in their ability to bind and break down contaminants. Likewise, contaminants differ in their ability to persist and leach in the soil environment. Other factors also affect the soil's ability to remove contaminants. These include climate, irrigation, how much potential contaminant is applied, and how soil and crops are managed.

The most common groundwater contaminants in agricultural areas are nitrate (from fertilizer, manure, and septic systems) and pesticides. Disease-causing microorganisms (pathogens) from septic systems can also contaminate groundwater. To protect groundwater, users of fertilizers and pesticides, and designers of septic systems must take into account the soil's ability to remove the contaminants.

Soil is a loose mass of chemically weathered rock fragments mixed with organic matter. It is biologically active­a home to countless microorganisms and plant roots. It varies in depth from a few inches to 6 or more feet. Soil is roughly 50% pore space. This space forms a complex network of pores of varying sizes, much like those in a sponge. The pores provide water and air to the plant roots and microorganisms that live in the soil.

Much can be learned about a soil by examining a soil profile in an excavation or road cut. We can observe the following properties and use them to estimate how well the soil will protect groundwater.

Soil texture. Texture describes how coarse or fine a soil is. The coarsest soil particles are sand. Clay particles are the finest, and silt is intermediate in size (Fig. 1). Soils that contain a large amount of sand feel gritty, while silty soils feel smooth. Clay soils feel hard when dry, and sticky and plastic (moldable) when moist. Sand particles resemble small rocks, and silt particles are like even smaller rocks. Silt and sand particles are not very active chemically; they contribute little to the ability of the soil to adsorb (bind) contaminants. Most clay particles are structurally and chemically quite different from sand and silt, and are smaller. Clay is responsible for much of the chemical activity and water holding capacity in soils.

Fig. 1. Sand, silt, and clay particles. Magnification not proportional.

Texture influences the porosity as well as the chemical activity of a soil. Sandy soils contain mostly large pores. They hold little water, and excess water drains through them easily. The combination of low chemical activity and rapid water movement through sandy soils makes them more vulnerable to leaching of contaminants than finer-textured soils. Soils which are mostly silt or clay have mostly small pores that do not drain water readily. The risk of groundwater contamination is much less in these soils. They must be managed carefully, however, to prevent runoff and surface water contamination.

A loam is a soil that contains a roughly balanced mixture of sand, silt, and clay (Fig. 2). Loamy soils have more chemical activity than sandy soils, and hold more water. They offer more protection to groundwater. Also, water tends to infiltrate through them more readily than through fine-textured soils, so the risk of runoff is less.

Structure. Individual particles of sand, silt, and clay tend to become clustered together in soil. This clustering of particles into aggregates gives structure to the soil. The granules of soil that we see hanging to grass roots when we dig into sod is a type of soil structure. Examples of structure are shown in Fig. 3.

Fig. 2. Soil textural classes.

Structure is important because it increases the number of large pores in a soil. In fine-textured soils, structure is essential to movement of water (infiltration) and air into the soil. Good structure may lead to the deeper leaching of some contaminants, resulting in an increased risk to groundwater. In general, however, good soil structure is desirable because it increases soil aeration, improves productivity, and reduces runoff.

Organic matter. Organic matter is formed from the decomposed remains of plants, animals, and microorganisms. Well-decomposed organic matter is called humus. Humus gives topsoil its dark color. Organic matter plays an important role in forming structure in soils by helping to bind soil particles into aggregates. Organic matter also resembles clay in that it is chemically active. Organic matter is especially effective at binding many pesticides, and plays a key role in keeping pesticides out of groundwater. Increasing the amount of organic matter in a soil can reduce the risk of pesticide leaching.

Subsurface color and high water tables. Soils with high water tables often have gray or mottled colors in the subsurface, while well-drained soils are a more uniform brown color. In well-drained soils, enough air generally occurs in the soil pores to keep soil microbes supplied with oxygen. Under these conditions, pathogens from septic systems are killed rapidly and do not leach to groundwater. When a high water table exists, soil microbes cannot get much air. They are then much less effective at removing pathogens from sewage.

Soil depth. To protect groundwater, we need to know the depth of soil that provides contaminant removal. Once a contaminant leaches below the soil, microbial activity is much lower. The contaminant tends to persist much longer, increasing the threat to groundwater. Sometimes a high water table limits soil depth. In other soils, we encounter a clayey or compacted layer at a shallow depth. Often, little water can move through this layer, which helps protect the underlying groundwater. But, surface water may be at risk if contaminated water moves across this layer to a spring, stream, or lake. When shallow soils are underlain by loose sand or gravel, very little binding or microbial breakdown of contaminants occurs in the sand or gravel. In addition, leaching is rapid, making groundwater vulnerable to pollution.

Soil variability. Sometimes soil properties vary considerably over short distances. Soils at the base of a hill are usually much wetter than soils near the crest, because water runs down slope and collects at the bottom. Soils near the crest of the hill also tend to be shallower than those at the base due to erosion. Some variability is due to changes in subsurface features that are not visible from the soil surface. Soil texture and depth can vary considerably over a field that looks uniform at the surface. A key to groundwater protection is identifying areas that have more vulnerable soils, and managing them properly.

All of the factors discussed above are important in determining a soil's ability to prevent groundwater contamination. Following are examples of how these soil properties affect the potential for removal of pesticides, pathogens, and nitrate from percolating water.

Pesticides. Pesticides are most likely to leach through sandy soils containing little organic matter. Pesticides are not bound tightly to these soils. Breakdown is slower because there are fewer microbes, and leaching can be rapid through the large soil pores. Pesticides must be managed carefully in sandy soils to prevent contamination. The coarse texture of these soils cannot be changed, but the ability to bind pesticides may be improved by building up the organic matter.

Pathogens. Pathogens from septic systems do not survive long in microbially active, aerated soil. But, in two situations they can threaten groundwater. First, when soils are extremely coarse-textured (gravel and sand), wastewater from septic systems may percolate so rapidly through them that the pathogens are not all removed by the time they reach groundwater. Second, soils with high water tables lack enough air in the soil pores to keep soil microbes actively removing pathogens from the wastewater. In either case, contaminating a well or spring with disease-causing bacteria or viruses is a risk.

Nitrate. Soil microbes produce nitrate from nitrogen in fertilizer, manure, and septic system wastewater. Nitrate does not bind to soil particles, and most soil microbes are not capable of breaking it down. In many sandy soils nitrate leaches rapidly to groundwater.

Nitrate is removed from soil in two ways. First, plants use it as a nutrient. The key to preventing nitrate contamination is to rely on plants as the removal mechanism. Apply only as much available nitrogen in fertilizers or manure as crops can use during the growing season. Second, some microbes can remove nitrate from wet soils, reducing the risk of nitrate contamination of groundwater in these soils. In well-drained soils, little nitrate will be removed once it leaches below the root zone, and careful fertilizer and irrigation management are essential to prevent contamination.

Fig. 3. Soil structure and water movement.

Management. Other fact sheets in this series outline ways to manage areas having vulnerable soils. In general, avoid using highly leachable pesticides in areas made up of vulnerable soils. Choose and use other pesticides carefully, and plan pest management to reduce overall pesticide use. Calculate nitrogen fertilizer and manure applications to meet crop needs. For septic systems, it is best to avoid wet soils and very coarse-textured soils altogether. You can use alternative types of septic systems in some vulnerable soils to improve their wastewater treatment capabilities.

The best way to gather site-specific soils information is by directly observing soils in the field. Because detailed site-specific information is needed for septic system design, soils are evaluated in pits or borings before any septic system is designed.

Soil survey reports are used to supplement site-specific information, and when broader information is needed over a larger land area. The Natural Resources Conservation Service (NRCS) produces and publishes these reports for every county in the state. Each soil survey contains maps, tables, and text describing soils in the county. Soil types are called series. Soils with similar properties are grouped and mapped as one series. Soil survey maps show the locations of each series in the county. Tables and texts describe properties, use, and suggested management for each series. The tables and text also include information on slope, depth, drainage, texture, landscapes, and parent materials for each soil series. In addition, the soil survey report gives information on the variability we can expect to find in the field. Soil surveys are available from NRCS offices located in each county. The county office can assist you in using the soil survey to identify ways to improve groundwater protection.

Personnel from four federal and six Washington State agencies prepared this guide. It helps evaluate the impact of recommended farming practices on the quality of ground and surface water.

Two sections of this guide relate to soils. Pesticide runoff and leaching potential ratings are given for all soils in the state. The ratings show each soil's relative potential for protecting water from pesticide leaching and runoff. A pesticide database also is included in the water quality guide. It gives ratings for pesticide leaching potentials. A worksheet helps todetermine the overall rating for particular pesticides on a specific soil. A producer can evaluate several different pesticide programs for the soil, and choose a program based on water quality protection as well as pest management.

Another section evaluates movement of soluble nutrients, such as nitrate. This movement also depends largely on soil type.

The water quality guide can be seen in NRCS and WSU Cooperative Extension offices. The guide will be updated as new material becomes available.

In order to protect groundwater in agricultural areas, we must take into account many environmental, site, and management factors. Soil is a key factor because different soil types have quite different abilities to remove contaminants. Also, soil is often the last line of defense against groundwater contamination. By fitting agricultural and waste management practices to the properties and limitations of soils, we can protect the groundwater resource.

For further reading, ask about other fact sheets in this series.

Acknowledgments. Partial funding for publications in this series on Groundwater Protection was obtained through U.S. Environmental Protection Agency nonpoint source pollution grants administered by the Washington State Department of Ecology.

By Carl F. Engle, Ph.D., Washington State University Extension Soil Scientist, Pullman; Craig G. Cogger, Ph.D., WSU Extension Soil Scientist, WSU Pullman; Robert G. Stevens, Ph.D., WSU Extension Soil Scientist, WSU Prosser.

The authors acknowledge the contributions of Christopher F. Feise, Ph.D., Washington State University Extension Western Washington Water Quality Coordinator and Groundwater Fact Sheet Project Coordinator, WSU­Puyallup Research and Extension Center; John H. Pedersen, Ph.D., P.E., Consulting Technical Editor and retired manager of the Midwest Plan Service, Iowa State University, Ames; and Ronald E. Hermanson, Ph.D., P.E., WSU Extension Agricultural Engineer and Water Quality Project Leader, WSU­Pullman.

WSU Cooperative Extension bulletins contain material written and produced for public distribution. You may reprint written material, provided you do not use it to endorse a commercial product. Alternate formats of our educational materials are available upon request for persons with disabilities. Please contact the Information Department, College of Agriculture and Home Economics, Washington State University.

Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended. Published August 1991. Reprinted October 1993. Subject code 376 A EB1633