Managing Livestock Manure to Protect Groundwater

Two-thirds of Washington state's population, and virtually all of its rural residents depend on groundwater for their drinking water. Popular interest in public health and water's beneficial uses has led to state standards that forbid degrading groundwater quality and limit contaminant concentrations in drinking water. For example, public health standards limit nitrate in drinking water to 10 milligrams per liter, expressed as nitrogen, or 45 milligrams per liter as nitrate.

For a discussion of groundwater, see Washington Groundwater: A Vital Resource, EB1622, and other bulletins in the Clean Water for Washington series available at your county extension office.

Livestock wastes are a potential source of groundwater contamination. Modern livestock production, confining larger herds in smaller spaces, has increased manure disposal problems.

Properly handled and used, manure is an asset; otherwise, it is a liability. Manure spread on land recovers nutrients and replaces much commercial fertilizer; increases soil fertility and water holding capacity; and improves soil tilth, bulk density, infiltration rate, and permeability. Collecting, storing, hauling, and applying manure to fields generally represents net production costs, but is usually the least-cost system for livestock waste disposal (Fig.1).

Unpaved feedlots develop a compacted manure and soil seal 2 inches to 4 inches thick. The seal, which reduces salt and nitrate leaching as much as 97%, includes bacterial cells, organic matter, degrading manure, and soil. Compacted manure above the manure-soil seal has almost no oxygen for aerobic decomposition. In this anaerobic condition, microorganisms reduce the amount of nitrate available to leach downward by converting the nitrate to nitrogen gas, which escapes to the atmosphere.

Inside surfaces of lagoons and earthen storages are sealed by bacterial cells and fine organic matter in the soil pores. After 1 to 3 months, manure seepage is only 0.1% to 10% of clear water seepage. Fine textured soils (silt, clay loam, clay) usually have permeabilities reduced to 0.00014 to 0.0014 inch per hour when sealed; sand permeability can exceed 0.0014 inch per hour even after sealing. Cattle manure generally seals better than hog manure.

Although manure tends to seal soil linings of earthen basins, salt concentrations sometimes increase for a few months in shallow groundwater nearby. Nitrate cannot form in lagoons and earthen storages because no dissolved oxygen is present to make ammonium into nitrate. Disease-causing microorganisms, such as bacteria or viruses, can also move with seepage before a lagoon or storage basin seals.

Therefore, seal porous soils before filling a lagoon or pond: Compact 6 to 12 inches of moist soil that is sandy loam or finer in texture, use soil additives such as bentonite, or install an impermeable membrane or other sealant. A seal is especially needed where fractured rock underlies shallow porous soil. For more information on storage lagoon design and management, ask for EB1642, Livestock Manure Lagoons Protect Water Quality, available from Washington State University Cooperative Extension offices.

Manure storages can contaminate soil and groundwater if they overflow, if spillage occurs during emptying and handling, and if runoff from aboveground solid manure storages is handled improperly. Earthen and leaking concrete storages affect soil and water about as lagoons do.

Manure applied to fields, including runoff infiltration channels and installed wetlands, can contaminate water if handled improperly. To keep runoff from surface waters, avoid spreading manure on frozen soil or on steep, poorly vegetated areas draining toward surface water. To reduce contaminated infiltration, limit applications to what the plants can use, and do not apply during heavy rainfall or while irrigating.

Water moves from the land surface through the aerated zone above the water table to the saturated zone, which is the groundwater reservoir. Then it moves to a stream and on to the sea. In the root zone, plants take soil nutrients from the water. Excess nutrients, especially nitrate-N, become contaminants that are diluted in the saturated zone but persist all the way to the sea.

Contaminants in waste solutions are already mobile, and those in solid manure stockpiles, etc., are leached when added to the soil. Contaminants dissipate by the following actions:

Decay, as used here, destroys, inactivates, or dissipates the toxicity of a soil's foreign materials. Some contaminants decay with time; others by sunlight. Toxic substances also decay by contact with oxygen in surface water or the aerated zone. Animal wastes degrade in an oxygen-rich environment that favors biochemical decomposition.

Sorption is how soil particles hold or chemically react with contaminants, so a fraction of the chemicals bond to the soil and do not move with the soil water. Clays and soils with much organic matter tend to retain many contaminants, while sands do not. Dense rocks have little surface area for sorption, allowing contaminants to move freely with percolating water.

Dilution in water lowers contaminant strength, potency, and effect. Because concentrated toxic spills may need more dilution than soil water can offer, contain and remove the contaminants before they enter the ground.

Animal manure has organic matter, plant nutrients, salts, and pathogenic microorganisms that can reduce water quality. But manure's plant nutrients (See table below) and organic matter can add to crop production and profitability.

Organic matter in rainfall or snowmelt runoff breaks down in streams and ponds by biochemical reactions. Those reactions can use enough dissolved oxygen to cause a fish kill. Organic matter is filtered from water at the surface or in the soil. So, organic matter often reaches surface water, but groundwater is largely protected.

Microorganisms abound in animal intestinal tracts. Although documented cases of human disease from livestock waste are rare, transmissible livestock diseases from pathogens do threaten humans.

The soil-climate environment is hostile to such microorganisms. As is the case with organic matter, they are filtered by soil above groundwater. Microorganisms seldom seep to groundwater from land-applied manure, properly functioning lagoons, or earthen manure storages. However, they remain a threat where they can pass through a shallow layer of coarse sand to fractured rock.

Soil microorganisms decompose manure, forming humus and releasing plant growth elements. Soil treats biodegradable wastes better than any method yet devised.

Plant nutrients in livestock waste, especially nitrogen and phosphorus, readily flow with runoff to surface water. The risk to groundwater varies with soil type and the specific nutrient.

Soil fixes and immobilizes phosphorus in a few hours. Phosphorus also is not very soluble in water. It rarely moves with percolating water to groundwater. But phosphorus can pass through clean, clay-free sand. Phosphorus is a threat to surface water quality because clay-bound phosphorus will move with eroding soil.

Nitrogen is also processed in the soil:

Nitrification converts ammonium nitrogen (NH₄) to nitrite (NO₂), and then nitrate (NO₃). Nitrification requires microbes, ammonium nitrogen, free oxygen (O₂), and the carbon in organic matter.

• Ammonium nitrogen is positively charged and binds to clay particles by sorption. It is nitrified by soil microbes using free oxygen, which is usually available in soils that can support crops.
• Nitrite is water soluble, seldom occurs in significant amounts, and is usually rapidly converted to nitrate.
• Nitrate is water soluble and therefore leachable, moves with soil water, and can contaminate groundwater unless it is used up by plants. Denitrification reduces nitrate and nitrite nitrogen to volatile gases, mainly nitrous oxide and molecular nitrogen.

Denitrification requires microbes, nitrate, carbon, and no free oxygen.

• Microbes take oxygen from nitrate, making nitrite and nitrogen gas. The escaping nitrogen reduces both fertilizer value and potential groundwater contamination.
• Because saturated soils have little or no free oxygen, denitrification occurs during wet seasons and in areas such as wet feedlots.
• If the land area is insufficient for manure disposal, nitrogen loss by denitrification helps protect groundwater from nitrate.
  
  Because nitrification and especially denitrification are difficult to control, limit potential nitrate in groundwater by managing nitrogen applied in wastes and chemical fertilizers.

The nitrogen demand by plants sets how much manure to spread without groundwater contamination.

Groundwater quality benefits from proper rates, placement, and timing of fertilizer application. Such management can reduce nitrogen and phosphorus losses on cropland by 50% to 90%. Managing nutrients is the most practical way to reduce both nonpoint source pollution and nitrate groundwater contamination from soluble nutrients on cropland.

Test well water near livestock operations for microorganisms, nitrate, total dissolved salts, and pH to assure safety for drinking. Nitrate concentration is a base figure for checking how changing farm operations affect water quality. Contact your local health department and your county extension office for help.

Soil tests and manure tests are the two most important livestock waste management tools. They help determine nutrient levels and manure application rates. Test soil for nitrate in the fall west of the Cascades. Conduct nitrate tests in fall and early spring east of the Cascades according to crops. Spring testing determines the carryover from the previous crop.

For manure tests:

• Nutrient analysis by a competent laboratory is best.
• A calibrated hydrometer is low cost, gives reasonable nutrient estimates, but requires lagoon solids contents of 2% or more. A Swedish tester costs more, is more accurate, but analyzes only nitrogen. Testers are available from Agri-Waste Technology, Inc., 3504 Sloan Court, Raleigh NC 27606; phone 919/851-8528.
• Also test manure for salt content and for the types and amounts of weed seeds.

Store solid or semi-solid manure on a concrete floor. Install a perimeter wall or dike to contain drainage and rainfall. As a less costly alternative, the floor and dike can be compacted impermeable soil.

Timing manure applications can minimize how long nutrients are available for loss to surface or groundwater. Fall applications tend to increase nitrate leaching, but allow organic nitrogen to convert into mineral nitrogen that will be available to plants in the spring. Reduce nitrogen losses and leaching to groundwater by waiting until fall soil temperature is less than 50°F.

When plants are growing actively and their nutrient need is greatest, less nitrate leaches from the root zone before plant uptake. When possible, split manure applications to match crop needs and reduce leaching. Consider spreading more than once per year (split applications) to improve plant nutrient use and to reduce nitrogen losses to the atmosphere or groundwater. For example, side-dressing 1-month-old corn with a tank wagon and injectors reduces odor and saves about 25% of the runoff and atmospheric nitrogen losses.

Apply manure to match crop needs. A field's crop, fertilizer, and manure histories permit managing field nitrogen use. Other factors to consider include crop yield goal and soil type. Nitrogen management can reduce the amount of fertilizer required by accounting for manure nutrient carry-over to later years and for nitrogen contribution of legumes to following crops. For more information, request PNW239, How to Calculate Manure Application Rates in the Pacific Northwest, from your county extension office.

Apply manure uniformly by mixing liquid manure as much as practical and by using good equipment.

Spread manure on land used each year for crops, including pastures, which can benefit greatly from manure nitrogen. Watch for increased runoff due to decreases in the infiltration rate of untilled grassland. When manure is applied year after year without tillage, fibers in dairy cattle manure can cause matting. Establish well-designed buffer strips along waterways. Avoid steep slopes during rainy seasons, and avoid spreading manure on frozen ground. Do not spread manure on sandy or sandy loam soils over fractured bedrock, because water percolation can carry too much nitrate to groundwater.

Apply manure to sandy soil as near planting time as possible to minimize nitrate leaching. Consider split applications.

Record manure and chemical fertilizer applications, crop yields, soil and manure test results, and soil data for each field in a permanent file. The records will help with future planning. Adequate records may also assist in defending against a complaint.

Incorporate liquid, slurry, or solid manure promptly­­on the day of application when possible. Incorporation reduces nitrogen loss by volatilization, runoff to surface waters, odor, and unfortunately, solar pathogen decay. Surface application with delayed incorporation encourages sunlight decay of pathogens.

Select BMPs that achieve clean water goals for your operations. Consider:

• Overall costs and benefits.
• Short-term and long-term effects on water quality.
• Suitability of the practice to your site and your production.
• Acceptability of the practice to you and your neighbors.
• Compatibility of livestock numbers with manure loading capacity of the land, or be prepared to haul manure off-site to other land.

• Store manure at least 100 feet from domestic wells, depending on topography, geology, and well construction.
• Plug unused wells. Repair poorly sealed wells. Contact a licensed well driller for ways to prevent direct flow of contaminated water to groundwater.
• Test well water near livestock operations for drinking water safety. Compare test results with those from previous tests to check the results of adopting these BMPs.
• Seal permeable soils for manure lagoons and holding ponds.
• Store manure to conserve nutrients for crop use. Control drainage and runoff from storages.
• Do not disturb the manure-soil seal when cleaning a feedlot.
• Divert roof runoff and surface drainage away from storages, feedlots, and lagoons to reduce the amount of contaminated water to be handled.
• Minimize milking center wastes.
• Dispose of dead animals to avoid groundwater contamination by composting, incinerating, approved burying, or removal by a commercial service. Disposal sites must be at least 100 feet from domestic wells or watercourses. Select composting and burial site so seasonal high water tables are no less than 2 feet below buried animals and the composting site. Cover buried animals with at least 2 feet of soil. Composting is practical for small animals, such as poultry and young pigs.

• Test soil and manure for nutrients, salt content, and the types and amounts of weed seeds. Test soil for nitrate in fall and early spring east of the Cascades, and in the fall west of the Cascades.
• Account for nitrogen from legumes, other organic wastes, and carry-over from manure and fertilizers.
• Determine soil types from USDA Soil Conservation Service soil surveys.
• Set realistic yield goals. Apply manure to fit crop needs.
• Apply manure uniformly with calibrated equipment.
• Do not apply manure to shallow sandy soils over fractured bedrock.
• Apply manure to sandy soil near planting to minimize nitrate leaching. Split applications are better than a heavy one each year.
• Delay fall applications until soil temperature is below 50°F.
• Record manure and chemical fertilizer applications, crop data, and soil, well water and manure test results.

The Washington Department of Ecology (Ecology) and a dairy advisory committee are developing a permit system to aid in controlling dairy farm discharges that could adversely affect surface water or groundwater quality. Ecology expects to begin issuing permits in the fall of 1992.

For more information on manure management contact your local office of WSU Cooperative Extension, Conservation District, or USDA Soil Conservation Service.

By Ronald E. Hermanson, Ph.D., P.E., Washington State University Cooperative Extension Agricultural Engineer and Eddie L. Thomason, M.S., WSU Cooperative Extension Area Dairy Agent, Yakima County.

The author acknowledges 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.

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 1992. Subject Codes 120, 376. X. EB1717