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Seepage Losses from Animal Waste Lagoons: Potential Impacts On Groundwater Quality

Research Update: February 11, 1999

 Jay M. Ham, Ph.D.

Associate Professor, Department of Agronomy, Kansas State University, Manhattan, KS 66506

Introduction

Anaerobic lagoons are an integral part of the waste management system at many concentrated animal operations (CAOs). Lagoon waste contains high concentrations of nitrogen, phosphorus, salts, and other nutrients that are eventually applied to farmland as liquid fertilizer. However, while the waste is being stored and treated in the lagoon, subsurface seepage losses may affect soil and water quality near the facility. Of particular concern is the movement of nitrates into local drinking water supplies. Kansas State University is conducting research to determine how construction methods, soil type, local geology, and other factors affect the relationship between the lagoon use and groundwater quality. Components of the research project include: (1) measurement of seepage and subsurface nitrogen losses from commercial lagoons, (2) a laboratory evaluation of soil used to construct compacted liners, (3) a survey of water chemistry in wells located adjacent to CAOs, (4) computer modeling of nitrogen movement in the soil under lagoons, and (5) developing best management practices for land application of waste. This report provides an abbreviated update on measurement of seepage and nitrogen movement from lagoons in Kansas. Results represent progress to date and are not final conclusions. More detailed results are provided elsewhere (see references). Additional reports are planned for April and December, 1999.

Whole-lagoon Seepage Rates

Regulations in Kansas stipulate that soil-lined lagoons used for animal waste should be constructed so that seepage is less than 1/4 or 1/8 inch per day, depending on where and when the facility was built. Kansas State University developed instrumentation to measure whole-lagoon seepage rates using water balance methods. New research by Ham (1999) shows that this technique can measure seepage to within ± 0.02 inch per day. To date, these methods have been used to collect data from seven swine waste lagoons and two cattle feedlot lagoons. Seepage rates from the Kansas lagoons ranged from 0.01 to 0.10 inch per day; thus, seepage was below the 1/4 and 1/8 inch standards in all cases (Table 1). Data suggest that, when proper soils and clays are used to construct the liner, it is feasible to build soil-lined lagoons that will keep seepage rates below 1/16 inch per day, even when waste depths are near 20 feet and sandy soils exist beneath the compacted liner. Ham and DeSutter (1999) found that organic sludge on the bottom of the lagoon apparently reduced the permeability of soil liners, especially in medium- and coarse- textured soils. However, there was also evidence that seepage may have been more pronounced along the side embankments (shoreline) where erosion and other processes compromised the integrity of the liner. Because of side seepage, it may be impractical to build soil-lined lagoons that have seepage rates less than 1/32 inch per day. In summary, seepage losses from many earthen lagoons in Kansas are probably less than 1/10 inch per day. However, seepage rates from soil-lined lagoons are not zero, and questions remain regarding the movement of nitrogen that does penetrate the liner and move into the subsoil surrounding the facility. The implications of this process will be discussed in later sections.

Waste Chemistry

The potential impact of a lagoon on groundwater quality is not directly governed by the seepage rate, but is dependent on nitrogen input loading and the vulnerability of the local aquifer. For waste lagoons, input loading represents the rate at which ammonium, nitrate, and other soluble compounds flow from the reservoir of liquid waste into the underlying soil. Thus, the nitrogen export rate from a lagoon is essentially the seepage rate times the concentration of nitrogen in the waste. Table 2 shows the average chemical properties of lagoon waste collected from swine and cattle-feedlot operations in Kansas (DeSutter and Ham, 1999). Almost all the nitrogen in the lagoon is in the form of ammonium (NH₄⁺). There are only traces of nitrate (NO₃^-) in the liquid, which would be expected under anaerobic conditions. The most notable finding was that nitrogen concentrations in swine-waste lagoons were, on average, seven times higher than in cattle-feedlot lagoons. Waste in cattle feedlot-lagoons is diluted because it is primarily runoff from precipitation that falls on the open-air pens. Conversely, liquid in swine lagoons is waste that was collected in pits beneath the animal barns and then flushed into the lagoon (no dilution). In summary, there are differences in the nitrogen content of lagoon effluent from swine and cattle-feedlot operations. If a swine-waste lagoon and a cattle-feedlot lagoon were seeping at the same rate, the amount of nitrogen deposited into the underlying subsoil would be significantly higher at the swine site. This does not mean that swine lagoons are hazardous and cattle lagoons are not. Our data simply show that these differences exist.

Subsurface Nitrogen Losses Into Soil Under Lagoons

The movement of effluent-nitrogen into the soil surrounding the lagoon is not only dependent on the seepage rate and the nitrogen concentration, but also is affected by the chemical and physical properties of the soil. Ammonium has a positive charge, while clay particles in soil are negatively charged. Objects with opposite charge attract; thus, NH₄⁺ ions that leak from a lagoon are often strongly adsorbed onto the surface of clay particles in the soil profile. Conversely, negatively charges ions, such as chloride, are not attracted to soil particles and tend to move through the soil profile unimpeded. The ability of a soil to adsorb positively charged ions is described by the Cation Exchange Capacity (CEC). Soils with high clay contents have CECs near 30 meq/100 g and very sandy soils have CECs near 5 meq/100 g. If two lagoons were seeping at the same rate, but one was built above a sandy soil and the other above a clayey soil, one might expect the NH₄⁺ to travel 6 times farther from the lagoon built at the sandy site. This is not exactly what happens in the field because other factors affect solute transport, but it does demonstrate the importance of soil CEC. To gain a better understanding of subsurface nitrogen dynamics, Kansas State University plans to sample and analyze the soil beneath lagoons. Figure 1 shows the average NH₄⁺ concentration from four soil cores collected at an 11-year old cattle-feedlot lagoon in southwestern Kansas. The lagoon had been dried and the organic sludge removed prior to sampling. Ammonium concentrations were near 400 ppm near the original bottom of the lagoon and then decreased rapidly to about 30 ppm at 16 feet. The shape of the concentration curve demonstrates how NH₄⁺ was adsorbed in the soil profile. There were essentially no nitrates in any of the soil samples. Thus, almost all the nitrogen that had been lost from the lagoon was still in the NH₄⁺ form and about 90% of that nitrogen was still within 10 feet of the soil liner. However, in one area of the lagoon the subsoil was very sandy, and NH₄⁺ concentrations were 66 ppm at 16 feet. This shows how a lower CEC allowed nitrogen to move to lower depths. Ammonium could potentially move directly into the groundwater at sites built above shallow aquifers in sandy soils. In summary, preliminary data suggest that nitrogen losses through a lagoon liner will, in many cases, be deposited as NH₄⁺ in a rather shallow soil zone near the periphery of the lagoon liner. The amount of nitrogen and size of the deposit will be dependent on the seepage rate, concentrations of nitrogen in the waste, CEC of the underlying soil, local geology, and lagoon age.

Lagoon Closure

Field measurements have shown that seepage losses from many lagoons occur very slowly. However, over 20 to 40 years of operation, even a low seepage rate can deposit a large mass of nitrogen beneath a lagoon. For example, Ham and Desutter (1999) showed that the total nitrogen deposited in soil beneath a 5-acre swine lagoon could potentially exceed 250,000 lbs. over a 20 year period. When a lagoon is eventually emptied and closed, the nutrient-laden zone of soil under the lagoon will tend to become dry and aerobic , especially in western Kansas where potential evaporation is much greater than precipitation. Under dry soil conditions the NH₄⁺ may convert to NO₃^-, which is very mobile in the soil (Figure 2). Over time, seasonal precipitation and intermittent water movement (drainage) through the soil profile could transport this newly formed NO₃^- toward the groundwater. However, a fraction of the nitrogen may be converted to harmless N₂ gas and released into the atmosphere (denitrification). It is difficult to predict the ultimate fate of nitrogen in the NH₄-laden soil surrounding lagoons. It may be feasible to phytoremediate the soil profile with plants. Salt tolerant crops like barley or perhaps constructed wetlands might be capable of absorbing large portions of the nitrogen and also stimulate denitrification. Furthermore, it is not clear if the nutrient-laden soil under a lagoon poses a significant risk to the groundwater, especially when the depth to groundwater is large (e.g., 100 ft). Much of the nitrogen may be lost to the atmosphere even without a phytoremediation plan. In summary, older lagoons that are closed and abandoned will initially have a deposit of NH₄⁺nitrogen in the soil under the facility. Additional research is needed to determine if this nitrogen will affect groundwater quality, and how the risk of contamination is affected by soil and geologic conditions. Best management practices for lagoon closure should be explored.

Future Research

The Kansas State University research team will continue the study of animal waste lagoons in 1999. Research priorities include: (1) measuring whole-lagoon seepage in different regions of Kansas to evaluate the effect of soil type and geology on lagoon performance; (2) periodically measuring whole-lagoon seepage at new facilities to document the change in seepage over time; (3) collecting soil cores beneath older lagoons to document the extent of side seepage and size of the NH₄-laden soil zone; (4) modeling nitrogen movement in the soil and groundwater surrounding lagoons using computer simulation; (5) formulating strategies for lagoon closure and remediation; and (6) measuring the movement of ammonia and other odorous gases emitted from the surface of lagoons.

References

DeSutter T.M., and J.M. Ham. Survey of waste chemistry in anaerobic lagoons at swine units and cattle feedlots. Technical Report. Department of Agronomy, Kansas State University, Manhattan, KS 66506.

Ham, J.M. 1999. Measuring evaporation and seepage losses from lagoons used to contain animal waste. KAES no. 99-326-J. Trans. ASAE (submitted)

Ham, J.M. and T.M. DeSutter. 1999. Seepage losses and nitrogen export from swine waste lagoons: A water balance study. KAES no. 99-138-J. J. Environ. Qual. (in press)

Ham, J.M., L. Reddi, C.W. Rice and J.P. Murphy. 1998. Evaluation of lagoons for the containment of animal waste. A report submitted to the Kansas Department of Health and the Environment. Kansas Center for Agric. Resources and the Environment. Kansas State University, Manhattan, KS 66506.

Table 1. Whole-lagoon seepage rates from lagoons in Kansas.

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* waste depth was near maximum capacity

Table 2. Average chemical characteristics of lagoon waste from swine units and cattle feedlots.

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Measured

Parameters                 Swine          Cattle

(mg L^-1)

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NO₃^--N                             1.0             0.5

NH₄⁺+NH₃-N         672.8           98.3

Total N                         792.4          184.2

Organic N                 118.8            85.6

Calcium                           79.8          144.9

Magnesium                   19.3            87.8

Potassium                         647.0          551.9

Sodium                         270.3          147.7

Total P                           42.5            47.5

Chloride                          275.6          568.8

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Figure 1. Ammonium-nitrogen profile in soil beneath an 11-year-old cattle feedlot lagoon.

Figure 2. Potential conditions after lagoon closure.