TECHNICAL HIGHLIGHTS

Ground Water Impacts 

by 

Land Application Practices at AFOs

Region 6 “Lessons Learned”

EPA REGION 6 PROGRAM CONTACTS: EPA REGION 6 TECHNICAL CONTACTS:

Source Water Protection Branch Chief: Larry Wright (214) 665-7150 Claybourne Chesney (214) 665-7128

Ground Water Center/UIC Section Chief: Philip Dellinger (214) 665-7165 Nancy Dorsey (214) 665-2294

Chris Lister (214) 665-6672

Finalized: September 16, 2002

Contamination of ground water by nitrate (greater than 10 mg/L as nitrate-nitrogen) has been documented at a number 

of animal feeding operations (AFOs) in Region 6 and across the United States. Investigation of ground water quality

at AFOs has primarily focused on leakage from waste holding lagoons, but a review of available data shows that many 

land application areas are also contaminated. Ground water samples at New Mexico dairies and several swine

operations in Oklahoma commonly have elevated levels of nitrate in land application areas. 

The principle governing land application of organic wastes is that the wastewater will be taken up by the roots of

growing plants and incorporated into plant tissues. The amount of nutrients which are applied is calculated based on

growth rates of the specific crop planted and typically is reported as an application rate in pounds per year (the

agronomic rate). In practice, specific application procedures and schedules may have to be followed in order to

achieve full utilization of the nutrients applied and to prevent the movement of the applied fluids into ground water. In 

light of the evidence of nutrient contamination at some land application sites, staff of the Ground Water Center 

reviewed common land application practices at AFOs and identified several that may result in contamination of 

ground water as discussed below. 

 The Use of Flood Irrigation

Land application of fluids at most sites is accomplished by use of spray irrigation. However, at some dairy operations 

wastewater from lagoons is commonly applied to the land by flood irrigation. On highly permeable soils wastewater

applied in this manner infiltrates rapidly with the consequence that there is a high potential for movement of fluids

below the root zone and into the underlying aquifer. Information on ground water quality and land application

practices at a number of dairies in New Mexico shows that there is a strong correlation between flood irrigation and

high nitrate levels in ground water. The EPA health-based standard for nitrate-N in drinking water is 10 mg/L. Of 30 

dairies where flood irrigation was practiced, 45% (fourteen dairies) reported nitrate-N concentrations exceeding 10

mg/L, while none of the 8 dairies using spray or center pivot irrigation reported nitrate-N levels this high. As 

indicated by the data, flood irrigation should not be considered an acceptable practice for land application of

liquid wastes. 
 Considering Grazing as Crop Removal

As noted above, the amount of nutrients which can be land applied depends on the amount of nutrients which are

utilized by plant growth. In order for this method to work continuously the plant crop must be removed as it reaches

maturity and growth rates decline. Some AFO operators consider grazing by cattle to constitute removal of the crop

and therefore removal of nitrate. However, a bulletin issued by the Oklahoma State Extension asserts that, “Nutrients

are not effectively removed from the forage system by grazing livestock . . . nutrient removal is accomplished only by

removing forage as a hay crop and transporting the nutrients away from the application site. Grazing, for the most

part, recycles most of the nutrients back into the forage system.” Agricultural specialists who were contacted by

Region 6 staff generally agreed that grazing would recycle part of the nutrients. The use of grazing, instead of crop

removal, can lead to over-application of nutrients and the potential for contamination of ground water.

Failure to Consider Soil Characteristics

A number of Natural Resource Conservation Service (NRCS) guidance documents include consideration of soil

characteristics among the factors which determine land application practices. These considerations include the ability

of the soil to hold the applied nitrogen long enough to be taken up by the plants before it can percolate below the root 

zone. The NRCS recommends “severe” limitation on land application where soils do not hold applied liquids for a

sufficient length of time. 

Agricultural specialists contacted by the Region indicated that best management practices developed by the states and

widely used in the field, commonly do not consider all of the detail contained in the NRCS guidance. The worksheets 

used in calculation of nutrient application volumes cover available soil nutrients, crop needs and effluent content, and

for nutrient limited watersheds land slope, soil depth and phosphorus content are also considered. However, certain

factors such as intake rate, cation-exchange-capacity ratio, and permeability are seldom explicitly addressed. There

was no general agreement among the specialists as to how the NRCS category of “severe” limitation on land

application should be interpreted, presumably reflecting confusion about how land application should be modified for

soils which may be highly permeable. The vulnerability of soils to rapid infiltration should be an important

consideration in the design of any land application program.