A Guide to the Revegetation and Environmental Restoration of Closed Landfills

Chapter 11: Maintenance of Vegetation

Irrigation

In addition to the types of vegetation that will be planted, considerable planning is needed for the type of irrigation system that will be installed and the degree of irrigation that will be implemented on the site. Some projects will institute a non-irrigated grassland concept. This will usually involve a hydroseeded annual grass and an accompanying perennial grass mix for sustained coverage. Irrigation may be supplied by the natural rainfall cycle for the remainder of the project's life or occasional supplemental watering during dry periods.

For more elaborate projects involving more sensitive and water demanding plants, different types of irrigation systems are available for use. Complex irrigation systems may involve use of Polyvinyl Chloride (PVC) conveyance piping, overhead sprinkler systems, drip systems directly to the roots, timers, computers, soil moisture sensors, or other water tracking systems to moderate moisture and an available water supply. These systems can be very complex to design and install. Use of irrigation systems requires a more active monitoring of soil moisture to reach a compromise in sufficient water for the vegetation and soil moisture levels that will not compromise the cover layer with saturated soils.

Control Systems. The heart of complex irrigation control systems can be basic valves and manual operation following a prescribed schedule. Timers and remote relay switching systems can provide a second type of irrigation control. This type of system still requires an operator to monitor moisture levels by visual inspection of the landfill site. Larger facilities may employ computer operated control systems that can include moisture sensors that remotely read soil moisture levels and use this information to automatically control the water supply to the landfill site. In a naturally designed vegetative plan, these systems should become less necessary and may eventually be eliminated and removed as the vegetation community becomes established and matures.

Water dispersal systems are available as two types, listed below.

• Overhead
  These are various water projection sprinkler heads that come off the surface irrigation system supply. These can be different types of "Rainbird" heads that force water through a nozzle and rotate by various means to project water over a set area, usually in a circular coverage area. This system requires accurate coverage plans to avoid dry areas where vegetation could be missed and die. This type of system is good for nonspecific coverage, as in large grass areas. The advantage of this system is that it is economical to install and can cover large areas with relatively few sprinkler heads. Disadvantages of overhead systems are their general broadcast format is nonspecific to terrain, or other taller plants in the line of spray (trees, shrubs) that can create "shadows" where water may not reach, creating dry spots. Moisture left on leaves can cause mold or fungus growth or frost damage in cold temperatures.

Evaporative loss of water through this type of irrigation is high as it sprays through the air and to the ground surface and loss of moisture can occur on the ground from solar evaporation. In windy conditions, the water spray can be deflected or blown away from desired areas, which prohibits thorough water coverage.

• Drip Systems
  Instead of directing water as an overhead stream, drip systems use feeder lines and flexible pipe with special low-flow drip heads. The tubing can be laid out directly to specific plantings. Water is efficiently conveyed directly to the roots of plants instead of being projected through the air over the whole spray area. The drip system reduces water loss by evaporation as the water crosses from the head to the soil in an overhead sprinkler. The drip system allows for more accurate control of water flow and soil moisture monitoring and control. Drip systems dramatically reduce evaporation loss at the soil surface. They can also reduce moisture-caused disease on leaves and frost damage from water on leaves left by overhead systems.

Advantages of drip watering are that it provides direct water delivery to plants, moisture control can be high, and evaporation loss is low, since water can be deposited at the roots. Drip watering also eliminates plant damage to leaves from molds or frost. Disadvantages are that only small areas can be covered adequately; the systems do not work for large open tracts of land such as golf course fairways and greens; and they are more equipment intensive, requiring lots of conductor pipe, special drip system heads, and special planning to locate these fittings at ideal plant locations.

A good plan is to incorporate both types in a project to accommodate both open areas and specific plantings.

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Water Supplies

For a sustained landfill vegetative cover to survive, it will need a readily available source of clean irrigation water. In addressing the water supply issue, it is suggested that the potential water supply be checked for its chemical content such as dissolved salts or other potentially harmful chemicals. Many plants are sensitive to the chemical content of irrigation water. Salt in high concentration or other contaminants in untested irrigation water can weaken the plants, making them vulnerable to disease or it can outright kill them. In addition, high concentrations of these dissolved substances in irrigation water can leach into the soil and accumulate into concentrations that can render the soil useless for plant growth. Once salt accumulation has occurred, the corrective procedures can be very expensive, involving flushing out the soil (leaching) to remove the concentrated salts and possibly having to replant the vegetation with more salt tolerant plants.

• Municipal, County, Private Utilities
  This is straightforward contracting for direct water delivery to the site.
   
• Wells
  The operator may be able to drill a source water well nearby or purchase water from an adjacent landowner. The large volume of water that is needed for daily irrigation will have to be considered to avoid overdrawing the well source.
   
• Leachate Water
  If the landfill is producing leachates that are chemically clean enough to be used on vegetation, this is a possible candidate for irrigation. The leachate must be tested for salts and any other constituents that can be hazardous to public health and safety. If these criteria are met and there are no elements harmful to vegetation, leachate water is a possible irrigation source. Treatment of the leachate water with constructed wetlands serving as biofilters can also produce a product water for use.
   
• Other Source Water
  Water from operating facilities such as power plants, refinery process water, or cooling facility water can be used for irrigation. Many large facilities use water for processing products, cooling equipment, or in heat exchange cooling towers. This is a potential source of irrigation water for a landfill vegetation project. Prior to using water from these sources, the chemical content of the water should be tested for its safety and potential hazard to plants; salts, heavy metals, etc. Effluent water from water treatment facilities is becoming a popular source for golf courses. Incidence of diseases from this type of water resource is being reported, and the project proponent, for health and safety reasons, should address this issue prior to using this water.
   
• Adjacent natural water ways
  If there is a lake, pond or large river near the landfill, a water delivery system pumping water directly from the source may be possible. Daily water volume is a consideration for the source to avoid overdrawing the water. Obtaining water use permits may be necessary.

Figure 11 Map of California Showing Areas of Ground Water Occurrence (17 KB)

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Fertilizing and Plant Nutrition

Once the landfill vegetative cover has been established, the next phase is maintenance of the vegetation. This part of a closure maintenance program will involve some type of 30-year monitoring and supplemental program to help maintain a viable plant community. The operator will need to monitor irrigation, plant growth patterns, mortality, nutrients, and even cleanup.

In most landfill closure projects, soils for the final cap are usually of poor quality, providing little nutrient value. Fertilizing and nutrient supply will be an important consideration at initial planting and as an ongoing maintenance program to insure continued growth and survival of the vegetation that has been planted on the closed landfill. When plants or seed are first placed in the soil at the beginning of the planting phase, fertilizer is usually included with the plantings. Grasses and other hydro-seeded plants usually have their fertilizer incorporated in the seed mix or immediately following the seeding step with fertilizers and mulch.

Larger plants such as shrubs and trees will require fertilizer to be included at the time of manual planting. Fertilizer pellets or spikes will be placed in the root hole or driven as spikes at the base of the tree. These nutrient supplements can provide fertilizing ability for around five years, slowly releasing nutrient directly to the roots. As the fertilizer is exhausted, continued monitoring of the plants can provide information on needs for further supplemental fertilizing programs. As the plants mature and develop hardiness, the need for supplemental fertilizing may be less.

Plants will grow more symmetrically if staked. Tie material should be soft and flexible to avoid damaging the bark or girding the tree as it grows. Trees should not be allowed to lean as this puts stress on the roots and makes the tree vulnerable to being blown over. Trees and shrubs located at more remote areas of a site may require a water truck to deliver water directly to their roots.

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Maintaining Plant Health

A 30-year maintenance program of the vegetation on a landfill will require sustained maintenance for continued plant health. A program will be needed to safeguard against disease, insect pests, and injury due to wind damage and fire control. A pesticide spray program should help the survival of young plants until they have established themselves. This program would lessen over time, and cease as the plants matured.

Soil conditioning and supplemental fertilization will be needed to maintain moisture retention and plant health. When the plants have gained satisfactory rooting establishment, the frequency of the maintenance regimen can be reduced. Things to be considered in preparing a vegetation maintenance program are listed below.

Establishment

The operator will need to inspect the plants to insure they have been securely planted in the soil. The root ball should be entirely below the plane of the soil surface. There should not be any exposed roots above the soil line, at the trunk.

A 3-inch high soil berm around the plants should be built up, forming a circle approximately 24 inches around the plant trunk or base. This will help retain water during the plants’ establishment period. Frequent watering will be required until it is determined the plants are well established. For thin-trunked trees and taller shrubs, double staking should be installed to support the plant from being toppled or broken in strong winds.

Smaller trees with younger and less developed root systems were more able to adapt and develop shallow root systems in response to the high landfill gas environment than the more mature plants. Trees less than 1 meter (~36") tall fared better than those 2 meters or taller (~72" or more).¹⁰

Certain tree varieties tolerant to acid soils and other specific soil conditions or with moisture hardiness also exhibited greater tolerance to landfill conditions than other more sensitive varieties.

Trimming

As plants such as trees and shrubs mature, their growth patterns will change. If unattended, the branching patterns may grow haphazardly, creating a tangled crown that can interfere with growth and development of the tree or shrub. An occasional inspection and trimming of trees and shrubs will help prevent dense, scraggly foliage. Branches that cross over each other and low branches at the trunk’s base in larger trees should be trimmed.

Water suckers, shoots sprouting directly from the trunk base, and similar growths sprouting from limbs should be removed. These offshoots take away important nutrients and water from the main plant. Damaged branches with partial fractures of the limb should be properly removed. Trees should be occasionally thinned of unnecessary branches for better faster growth. As the trees mature and the maintenance program reaches its later years, these activities will be reduced so that natural processes will take over.

Disease

The larger trees and shrubs in the project should be inspected routinely for signs of disease and insect pests. Strange marks on leaves or distorted leaves are good signs of fungi, viruses or other microbial dangers. Insects can bore under tree bark or into the wood, strip and distort leaves and create destructive havoc on unattended vegetation. Certain nematode worms can attack the roots and weaken the tree or shrub.

Occasional spraying of trees and shrubs can reduce the danger of disease and insect damage. Removal of excessive plant litter can reduce the potential for disease from insects, ground borne molds, and fungi as well as ground dwelling animal pests such as gophers, moles, and other rodents. Judgement should be exercised in cleanup activities on a landfill postclosure site. Removal of too much material can jeopardize the nutritive regeneration qualities of ground litter. Excessive cleanup of grasses or plant detritus can also remove an added means of soil protection and moisture retention ability of the natural soil cover. Large fallen branches could be removed and chipped; these being returned to the site as an amendment to the site soil. A balance between fire protection, and the symbiosis of soils protection and forest floor ecology must be struck. Removal of too much floor detritus can interfere with this ecosystem. Insufficient watering, as with excessive watering, can stress a plant, making it susceptible to disease or insect pests.

Fire

When a landfill has just been vegetated, the plant profile may be primarily grass and maybe small tree and shrub plantings. As the vegetation community matures and trees grow larger, a fire prevention program will become necessary. The first element to reducing fire danger in a matured project is keeping the area relatively clean of plant litter resulting from dead or fallen branches, broken treetops, excessively accumulated leaf litter, or excessive undergrowth. Occasional removal of some of these materials will reduce the potential of kindling materials that could provide fuel for lightning fire, accidental or intentional (arson) fire incidents.

Again, a balance must be reached between the regulatory requirements of landfill maintenance and the creation of the planned environmental remediation. An accessible network of fire control hydrants or an available fire fighting resource should be made available for fire emergencies. Again, excessive removal of smaller debris should be avoided to prevent loss of surface nutrient source material when this debris decays.

Soil Amendments

Young plantings, when newly installed, can be vulnerable to the elements of the weather and nutrient needs. Young trees and shrubs can be sensitive to the desiccating impacts of the sun and wind, especially in warmer, drier climates. This can damage the roots or lead to water loss through evaporation.

To combat soil moisture loss at the root zone, it is advisable to place a soil amendment of some type in the water retention basin at the base of the trunk. The material can be any sort of composted organic plant material that has been properly matured, or properly prepared mulch.

This will prevent possible phytotoxic damage to the plants from improperly prepared mulches or composts. These improperly cured composts possess acids and other chemicals that can interfere with growth of the plant, even killing it. Poorly composted material will contain pest weed seeds that can germinate, leading to weed control problems. Wood chips can provide mulch for new plantings and for moisture retention. Other amendments such as potash, ash, or fertilizers such as manure or chemical based fertilizers can be employed.

Certain formulations can be used for specific purposes or deficiencies in the soil. The use of these materials should decrease over time as the vegetation establishes itself, making it less dependent on outside nutrient sources and more reliant on its own root development.

Soil amendments to enhance the physical characteristics of the soil can improve chances of vigorous plant growth. In preparing topsoil, gypsum added and tilled into the soil can help soften the top layer to promote better root penetration. Gypsum can be acquired from suppliers or it can be recovered from recycled sheet rock or wallboard. This helps break up excessively clayey soils.

• Composts. Composts are primarily vegetative materials that have been processed and allowed to decompose and mature, creating a soft humus-like product. Composts provide nutrients, aeration qualities, moisture retentive abilities and some pest controlling abilities to the soil. Weed seeds in the compost should be rendered infertile.
   
• Sawdust. The product of lumber mill processing, this finer grained material can provide some aeration benefits to topsoils that may possess excessive proportions of clay. The smaller particle size will facilitate decomposition. A nutrient amendment with sawdust will help prevent nitrogen deficiency of plants.
   
• Mulches. Mulches are usually non-composted wood chips or other non-processed wood products of chipping or hammer mill processing. These materials provide primarily moisture retentive qualities and aeration for the soil. They help keep weeds down when applied thickly around tree or shrub trunks. Mulches can provide some nutrient value as they decompose over time. "Tan bark," the bark remains from lumber bark stripping, can provide the same function.
   
• Fertilizers. Manufactured chemical-based fertilizers provide nutrients to the plants for enhanced growth. Liquid and pellet, or granular forms are available, which can be spread with rotary dispersers. Many fertilizers provide potassium, phosphorus, and nitrogen to plants. Spikes or tablets are another available choice.
   
• Manures. Animal manure is a good source of fertilizer material for plants. They should be cured and dried to avoid harmful acids that can damage plants. Manure should be cured to kill off seeds ingested and passed through the digestive tract of livestock. Cattle yards are ready sources for inexpensive fertilizer. Chicken manure is also available but is harder to locate. It is not as bulky as cattle manure and provides no "loft" to soil, which is important for air circulation. Chicken manure mixed with straw can improve its lofting abilities.
   
• Ash and Other Amendments. Ash can be used as a good amendment to soil to improve sources of potassium and other essential plant nutrients. Care must be taken to assure contaminant materials or metals are not present in the ash planned for use. Certain trace metals such as selenium, boron or other similar materials should be checked as these can harm plants if they are present in excessive concentrations. These harmful chemicals can leach into, and accumulate in the soil, rendering it inhospitable to sensitive plants.

A study conducted at Bowling Green State University demonstrated that coal fly ash transferred arsenic, boron, cobalt, chromium, copper, mercury, molybdenum, nickel, lead, and zinc into plant tissues. This could migrate up through the food chain. These metals were also found in soil leachates taken from the test site, indicating potential to contaminate associated watersheds.

An experiment with fly ash used test plots with fly ash integrated into clayey soils. Results of the experiment showed improved moisture retention of the soil from its original 12 percent retention, up to 25 percent. Boron and soluble salts leached rapidly from the soil. Trace quantities of cadmium, lead, copper, and zinc were detected in the soil leachate.

Another experiment demonstrated usefulness of bottom ash as an amendment to sodic spoil. This enhanced soil fertility when ash was tilled into the soil over 30 cm deep. Ash should be turned into the soil when possible to prevent wind borne fugitive dust and to improve plant uptake. Some experiments showed positive results, whereas most soil experiments that dealt with ash containing any of the above listed metals showed phytotoxic tendencies and adverse bio-accumulative properties.

• Biosolids. The solid or semisolid by-product of sewerage processing facilities contains high nutrient value material for potential use as plant fertilizer. Since this material also comes from municipal sources, biosolids can have high concentrations of heavy metals, petroleum, and chemical products, as well as biohazards if not treated correctly. If biosolids are to be used as a fertilizer, these contaminants need to be addressed and tested, as they can injure or kill vegetation, accumulate in soil, or create a danger to human health and safety. Use of biosolids is currently controlled under federal regulations.
   
• Accumulation of these materials in the soil can occur over time, rendering it hazardous for plants as well as posing a potential health hazard for the public and wildlife. Most biosolids application to soil involves one of several surface spreading operations followed by discing of the newly introduced material into the soil. This procedure helps to prevent fugitive dust transport of the dried sludge offsite. The procedure is commonly used on fresh undeveloped lands, before planting has occurred.

Biosolids are generally not applied where they may contact plant leaves by spraying or spreading. This procedure is intended to avoid potential health and safety problems. This is especially true for lands that may be used for livestock grazing, to prevent potential ingestion by grazing animals and on lands for food crops for human consumption. Cases of hepatitis and E-coli have resulted from imported food crops contaminated with sludge residues.

One experiment conducted by Colorado State University showed that runoff sediment concentrations from the test plot increased with application rate of sludge. Traces of heavy metals also increased. These properties could adversely affect nearby watersheds.

• Process Water. Water from facilities such as geothermal operations, and other processing facilities that use large volumes of water, may be candidates for irrigation water. The important consideration, again, is dissolved substances in the water that could be toxic to plants. A test of geothermal water from drilling operations in the Salton Sea geothermal area provided water to test its applicability in agriculture. Ammonia in the water could provide a built in nitrogen source beneficial for plants. This condensate, commingled with Class I irrigation water, yielded healthier crops than the irrigation water alone. Boron in the test water could be toxic to certain other plants.

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Last updated: April 18, 2008