Effects of Clay-lined Manure Storage Systems on
Ground Water Quality in Minnesota: A Summary

David B. Wall, Paul Trapp and Randy Ellingboe
Minnesota Pollution Control Agency

The use of manure storage systems can have many benefits for producers and for protection of surface water quality. However, concerns have long been raised about the effect of manure storage systems on ground water quality, and many people have asked, "Do earthen manure storage basins and lagoons result in ground water contamination?" To answer this question, it is important to understand that there are many different kinds of manure storage system designs and construction techniques. These construction requirements vary greatly across the United States and have changed considerably in many states during the past decade. Numerous manure storage facilities have been constructed with minimal compacted clay liners or no liner at all.

Soil and ground water monitoring studies conducted throughout the country have determined effects on ground water from earthen manure storage basins which were constructed without a two-foot thick minimum clay-liner or synthetic liner material. Results from 42 such monitored basins reported in the literature¹ show that most of these sites have some evidence of elevated chloride and nitrogen in ground water or soil water resulting from the manure storage systems. The degree of reported ground water contamination varies widely, ranging from very slight elevations in contaminant concentrations to some sites with total nitrogen concentrations over 100 mg/l above background levels. No ground water contamination or only slight evidence of degradation was reported at about half of the monitored facilities, with the other half showing total nitrogen concentrations at least 10 mg/l above background.

Several different factors are expected to affect seepage rates and measured ground water degradation from manure storage systems, such as soil type, basin age, manure properties, climate, basin maintenance and management. The wide ranging reported effects on ground water from unlined basins could be due to these factors, along with the type of monitoring system used and the location of monitoring wells. Since the seepage from basins is not expected to be uniform, a limited number of monitoring wells may miss contaminant plumes. While all of the variables affecting seepage are not understood from the literature, the studies clearly demonstrate the potential for ground water degradation from unlined or poorly-lined manure storage systems.

With the concerns about seepage from unlined basins, the Minnesota Pollution Control Agency (MPCA) has required, with some exceptions, that earthen basins and lagoons be lined with a compacted cohesive soil liner. The requirements² have become more specific and stringent during recent years, and since the early 1990's the goals of design and construction of manure storage systems have been to:

• Ensure that if the basin were filled with water, it would seep no more than 1/56 inch per day
• Prevent problems in construction and operation caused by high seasonal ground water levels
• Prevent damage to cohesive soil liners from occurring after construction

All earthen basins constructed in Minnesota need to be permitted by the MPCA prior to construction. Permit applications must include a soils investigation report which indicates soil type, texture and depth of seasonal saturation as indicated by soil coloration. Drainage systems are required for liner protection where a high seasonal water table is evident. All design plans and specifications for the drainage system and storage structure must be prepared by or under direct supervision of a registered professional engineer. The plans must include a construction QA/QC plan and an operation and maintenance plan. A construction report is required upon completion.

While the Minnesota requirements for construction of earthen basins are among the most protective in the country, there has remained some concern regarding the impacts of clay-lined manure storage systems on ground water quality, especially with the marked increase in the size of these systems constructed recently. Some concerns stem from a shortage of monitoring data for clay-lined manure storage systems and several uncertainties about seepage rates, pollutant treatment, liner damage and construction oversight.

It is widely recognized that there is a sealing effect of manure at the manure/soil interface, which further reduces seepage below the designed seepage rates. However, this sealing effect varies with manure type and has not been quantified for entire manure storage basins. Also, there are uncertainties regarding the amount of pollutant reduction occurring as manure seeps through the soil liner and underlying soils. The liner sealing by manure and treatment of pollutants passing through the liner both work to reduce ground water impacts from the manure storage system. However, these benefits can be counteracted by damage to the cohesive soil liner caused by wetting/drying cycles, worms, roots, freeze/thaw cycles, agitation during pumping, etc.

To begin to resolve some of the questions regarding the effects of earthen manure storage basins and lagoons being constructed in Minnesota and ensure that ground water is being protected, the MPCA began to require ground water monitoring around certain manure storage facilities and initiated studies to determine liquid quantity and quality passing through cohesive soil liners.

Macro-lysimeter Research

A cooperative research effort by the Natural Resources Conservation Service, University of Minnesota, Morrison County and the Minnesota Pollution Control Agency was initiated in 1993. The objectives were to collect seepage waters, which move through a large portion of a cohesive soil liner and measure the volume and chemistry of these seepage waters. The farm chosen for the study was a 100 cow dairy operation in central Minnesota, where a 600,000 gallon earthen basin (130'x115' top dimensions) was to be constructed during the fall of 1993. The glacial till at the site was classified as sandy clay and silt loam soils.

Following excavation of the basin, and prior to construction of the cohesive soil liner, a 35'X70' geomembrane was installed in a position to separately collect seepage waters from a portion of the basin bottom and a portion of the sidewall. The purpose of the geomembrane was to intercept liquids, which pass through the cohesive soil liner, and route these seepage waters to a collection sump located at the side of the basin. A blanket of sand was placed on top of the geomembrane to act as a drainage material and the cohesive soil liner was then constructed on top of the sand blanket. The liner is two feet thick on the bottom and was constructed in ten-foot wide horizontal lifts on the sidewall. A similar monitoring system design was used at two additional systems constructed in southern Minnesota during 1996³.

The macro-lysimeter was visited every 2 to 3 weeks during the first five months of operation and has since been visited approximately every 4 to 6 weeks. Seepage water samples are taken during site visits and analyzed at a laboratory for nutrients and other major cations and anions. Results from the first three years of monitoring (August 1994 to September 1997) indicate that the liquids seeping through the cohesive soil liner and the underlying sand blanket contain only a small fraction of nitrogen concentrations found in the manure. Total nitrogen, phosphorus, potassium and sulfate concentrations in seepage waters have averaged 0.2%, 0.03%, 1.0%, and 5%, of respective concentrations in manure. Sodium and chloride has shown up at relatively higher concentrations in seepage waters, averaging 25% and 44% of respective manure concentrations. Seepage rates have remained much greater through the basin sidewall compared to the basin bottom. The overall seepage rate has averaged 5 gallons per day from the basin bottom and 102 gallons per day from the sidewall during the first three years of operation.

While this project has provided some preliminary indications about seepage quantity and chemistry, few definitive conclusions have been drawn. We intend to continue this research project for an additional 5 to 12 years and to replicate this type of monitoring at the other two monitoring sites constructed during 1996. The two additional sites and continued monitoring should provide more certain answers regarding the long-term seepage through different types of cohesive soil liners.

Since 1994, ground water monitoring has been required at 13 newly constructed earthen basins and lagoons in Minnesota. Eleven of the sites contain swine manure and two of the sites are dairy operations. All facilities have a design capacity of between 3 and 10 million gallons. Soils at the sites are mostly fine textured, but typically contain some veins and lenses of coarse-textured soils.

The ground water monitoring design for each facility was developed by consultants for the producer and later approved by the MPCA. Five sites use only monitoring wells (3-6 wells per site), 3 sites use only the drainage tile lines installed around the perimeter of the basin/lagoon, and 5 sites use a combination of perimeter tile lines and monitoring wells for the ground water quality effects evaluation.

With the exception of one facility, the site owners were required to have 4 rounds of baseline ground water samples taken prior to adding any manure into the storage system. Quarterly sampling began after manure was added and has continued for 2 to 3.5 years at the 13 facilities. The following parameters have been measured: nitrate plus nitrite, ammonium, total Kjeldahl nitrogen, chloride, sulfate, fecal coliform bacteria, phosphorus, and field parameters.

The estimated costs for the first five years of monitoring a system with wells and a perimeter tile, including monitoring system design, construction, sampling labor, laboratory analytic, reporting, and well permits, is between $16,000 and $35,000.

Results from the baseline sampling have shown that nitrate levels were above drinking water standards at most sites prior to adding any manure to the manure storage system. Eleven of the 13 sites had nitrate-N concentrations in excess of 10 mg/l in at least one well or perimeter tile due to sources not related to the manure storage system. The overall average baseline nitrate-N concentration for all sites was 16 mg/l.

Quarterly sampling following manure addition has shown little evidence that seepage is affecting ground water at 12 of the 13 sites. Contaminant concentration trends have not increased from pre-basin operation to post-basin operation or from upgradient to downgradient wells. The one site, which appears that it may be impacted by a manure storage system, has had perimeter tile line water with elevated ammonia and chloride concentrations. The source of contaminants needs further investigation at this site. Fecal coliform bacteria have been detected in tile line water or monitoring wells at some facilities. Because the detections have been infrequent, at low-levels, and have not corresponded with increases of other contaminants, it is likely that the bacteria could be from sources not related to the manure storage system.

Past monitoring in other states has demonstrated that unlined manure storage basins will often result in ground water degradation. Initial indications from limited research and monitoring in Minnesota are that earthen manure storage basins and lagoons meeting current MPCA regulatory requirements are generally protective of ground water quality during the first few years of operation. Many more years of monitoring and carefully selected additional monitoring sites are needed to determine whether the cohesive soil liners provide long-term ground water protection.

Ground water monitoring requirements are costly for producers and regulatory agencies. Since it is better to prevent pollution than to monitor it, the resources required for ground water monitoring may be better spent on site selection, system design, and improved construction and maintenance, and project construction oversight. Design and construction requirements can be adjusted, if needed, following the results of sound research conducted by extensive monitoring at a limited number of sites.

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