This decision tree delineates a step-by-step approach to characterize contamination and then employ phytoremediation and other barrier methods as a tools to block soil lead exposure.
 

The first step is to determine if the lead concentration in the soil presents a hazard or warrants concern.  If lead in the soil does need addressing, then one must determine whether to employ phytoremediation or install a barrier or both.  Then the following steps are followed:
 
 
 

2:  Land Use	3:  Site Characteristics	4:  Area Specifics
front/back lawns play areas pet areas auto park/work garbage/recycle/compost garden	climate compaction drainage sun exposure/shade soil composition debris exposed/buried	barrier aesthetics health effects grubs, insects, animals, etc. durability

These steps are described in further detail below.

Land Use classification (An area may be classified under more than one land use).  After the land uses have been determined, the "branch" of the decision tree for each land use should be followed.

The six major land use descriptions for urban residential areas are as follows:

1. front/back lawns
2. play areas
3. pet areas
4. auto park/work
5. garbage/recycle/compost
6. garden
7. playground

These areas need to have durable grasses below them because of the traffic that both humans and the vehicles will create in these areas. The durability issues will be similar to issues in other area specific sections. Because of the trampling of grass and the increased shade underneath a vehicle, a concrete pad may need to be constructed if grass is not able to grow in these areas.

Durability issues that are specific to auto park/work include chemical durability. Oils and other hydrocarbons may be deposited onto the ground from the cars if there any leaks. Grasses must be tolerant to these types of chemical loadings, otherwise, constructing a concrete pad (as stated above) may be a better alternative.
Durability issues for grasses in garbage/recycling/composting areas are similar to those presented in the auto park/work section, though durability issues may be more significant. Grass would be covered for a significant duration of time, therefore if these types of conditions will not allow for proper grass growth (which is extremely likely), a concrete pad or some other covering must be constructed to prevent exposure of barren soil.
 

Garden

Gardens will consist of various flowers, shrubs, and vegetables. Since these plants do not provide full coverage, some other method must be used to prevent this lead ingestion pathway. Wood chips, mulch, and other soil coverings may be useful to prevent this exposure. Wood chips and mulch are often used in gardens to beautify; they are also useful in preventing dust from being spread during dry periods and to trap moisture in the ground. If placed properly within the garden bed, the wood chips will allow plants to continue to grow, while limiting the amount of exposed soil.

Crushed coconut shells and other materials have been also used as soil bed coverings in gardens. Also, vinca plants have been used in many gardens to fully cover the barren soil. What is most important is that the soil is covered in some matter.

Gardens that lie along houses are subjected to problems associated with water running off the roofs during rain events and trampling the plants that are found there. Within the first foot of a house, any plants in those areas must be fairly durable; the plants can not have weak stems that may be broken from a heavy water stream. The plants also must be able to withstand the increased moisture associated with being in these areas. Furthermore, often the highest lead levels are found along the drip line. This is the result of rain run-off from the homes. If lead levels prevent plant growth, then a covering such as stone or wood chips may be needed.

Research must be done to study the accumulation of lead in the fruits or vegetables of plants grown in lead-laden soil. If lead does accumulate in the fruits of vegetables, then obviously such plant products can not be used for consumption. This is an issue in plants such as broccoli, sunflowers, alfalfa, and other plants that have been shown to be useful in phytoremediation. Though such plants can still be used, those residents will have to be careful not to eat those foods until further research is done. Residents will need to be educated on the dangers.

Playgrounds

Playgrounds submit many of the same issues that play areas in residential applications present. One additional option in playground applications may be to close down the playgorund for a year, phytoremediate, and then re-landscape to convert the land back to a viable playground. There may be negative public opinion and other issues to such a plan. Because of these issues, options such as utilizing grasses that only provide a full barrier or ideally phytoremediate and provide a full barrier are being considered.

Overall, any grasses that are used must be highly durable and should be able to withstand constant traffic. There are four major land descriptions that will be considered for urban playgrounds: baseball fields, soccer/other fields, swing sets/monkey bars, and jungle gyms. All these areas will need to be subjected to the background information queries (climate, drainage, etc.) that residential land descriptions were classified under. Once the background information has been classified, area specific factors for the various land uses must then be addressed. Such information is required to ensure that plants that are placed in an area will be able to endure other outside stresses that may hinder plant advancement.

Grass in the outfields and on the infield must be very durable. Areas of concern are where the right, center, and left fielders typically stand during a game. Such areas may be susceptible to degradation and possible soil exposure if an extremely durable grass is not chosen and maintained. If such an area can not be maintained, new topsoil or sand may need to be placed in these areas. Other than the areas of concern in the outfield, all other grassy areas on a baseball field can be considered similar to residential play areas.  Dirt on the pitcher’s mound and along the baseball diamond is of major concern because there is exposed soil/dirt that may be contaminated. When the area is tested, it will be determined if the infield dirt poses a threat. If there is lead that exceeds the playground limits, then there is one of two options: remove all the dirt/sand and put new material in the infield or put down topsoil and plant grass throughout the infield.

This section also includes general grassy areas throughout the playgrounds. The grass for soccer fields will be quite similar to the grass for the outfields and infields of a baseball field. The grass must be durable and able withstand the prolonged traffic that a soccer, football, or other game may produce. Some areas of concern are where the goalies stand during a soccer match. Like the various outfield positions, such areas may be susceptible to degradation and possible soil exposure if an extremely durable grass is not chosen and maintained.

Swing sets, monkey bars, slides, see-saws, and other playground equipment that are stand-alone will have severe durability issues if grass is used around such apparatus. For example, the areas right below the seats of a see-saw will endure a constant battering from the seat moving up and down, pounding on the ground often several times. Because of these severe durability issues, materials such as sand or rubberized playground coverings should be used around portions of or around the entire apparatus. This will prevent bare soil exposure.

Similar to swing set/monkey bar areas, jungle gyms will need coverings to prevent bare soil exposure. These coverings should enclose the whole area, since jungle gyms may have multiple points of entry and are often more extensive than stand-alone apparatus such as monkey bars. Furthermore, the coverings should be child-safe materials that will cushion a fall if a child should happen to stumble or trip. Rubberized playground materials are often used in such applications.

There are several major site characteristics that should be labeled for each and every land description in residential areas. These background characteristics are:

1. climate
2. compaction
3. drainage
4. sun exposure/shade
5. soil composition
6. debris buried/exposed

There are seven major climates that have been classified for North America. Climate is a general term for the average weather conditions in an area over a long period of time. These weather conditions may include the temperature, the amount of precipitation, and the average wind speeds. Climate data is important because plants typically have a range of climatic variables that it must fall within to survive.

Climate data will allow for certain plants to be eliminated from the list of possible plants for an application, but for a plant to be put into use, the microclimate must also be examined. Within a climate, there may be extraordinary conditions that will not allow a plant that typically grows in a climate to thrive in a particular portion of an area. For example, a community that is near a large body of water typically has temperatures that are several degrees lower or higher than the norm for that climatic area, depending on the season. These temperature differences may not allow a plant that typically grows in a climatic area to thrive in that specific community.

Compaction:

There are three degrees of soil compaction that will be considered when evaluating the various sites: heavy, average/medium, and loose. Compaction is an issue that needs to be addressed because the degree to which the soil is compacted effects the amount of water and air movement in the soil and influences the rate at which soils drain. This is also very important in phytoremediation because of the role of microorganisms in soils. The heavier the compaction, the less ease that water or air moves through the soil; the looser the soil, the greater ease that water and air can move. Heavy compaction may also lead to "ponding" effects where water accumulates within depressions in the surface soil.  A loose soil does not automatically result in proper growing conditions. There is an optimal compactness that allows for proper water and air movement within the medium, important factors that effect the growth of plants. Also, when a soil is too loose, there is not proper soil/seed contact to facilitate optimal growth. Decisions need to be made by a landscaping expert regarding the relative degree of compaction.

Within the compaction section will be classifications for the type of soil at the site. Typically soil falls within a range from clay to sandy, and categories between. The sandier the soil, the coarser the soil is.

The compaction may be altered through use of a compacting apparatus to further condense the soil or by rototillers that can loosen up the soil. Rototillers typically can loosen up the top six inches of soil to allow for planting of the seed.

Drainage:

Drainage has similar properties with respect to the effects of compaction on plant growth. Though compaction does have an effect on the degree of drainage (for example, a soil with heavy compaction typically has poor drainage), drainage is more of a composite of various soil variables. The degree of drainage will greatly influence the soil moisture for a plot of land and hence, it will effect the plants that grow on that plot.   A landscaping specialist should determine the relative degree of drainage. Such a specialist can determine what if any drainage problems may exist and can prescribe a course of action to amend the situation (i.e. grading, conveyance).

Sunlight:

One very important factor to consider is the amount of sunlight an area has. Since light exposure is directly related to metabolic growth, it is imperative that sun exposure be studied so that the plants there can prosper. Since sun exposure is difficult to decipher from climatic data, it deserves further study in a separate area.

Different plants require different degrees of sun exposure. Some plants require constant sunlight while others actually prefer shade for the most part. Though sun exposure is somewhat dependent on the macroclimate in an area, exposure to sunlight is also a microclimate issue. Being in the sun shadow of a tree, building or other structure may have an immense effect on a plants ability to grow in that particular area. For the purposes of this project, a sunny area will be defined as being exposed to five of more hours a day of direct sunlight. An area in partial shade will be exposed to sun around one to five hours a day. A shade area will include any areas that receive less than one hour per day.

Soil Composition:

It is very important to determine is the "soil composition" of the site. What is meant by the soil composition is the various compounds and elemental components of the soil, along with other soil quality indicators. Characteristics that should be tested for included the pH, nutrient content, organic matter, metals, ions and salinity, and minerals. These characteristics are important for plant growth and/or optimal phytoremediation. For example, pH is an important factor in both metal mobility and plant survival. Both have optimal ranges at which should be accommodated.

Contamination is listed on a separate "box" on the decision tree, but mainly lies under soil characteristics because it is another characteristic that will need soil testing. It is important to classify soil contamination since such contamination may effect plant growth. Also, testing for compounds such as toluene, TCE, PAH’s, solvents, and other chemical contamination may show the need for further remediation. It is also important to classify the areas at which there are elevated lead levels so that phytoremediation is not haphazardly used and is only implemented where necessary.

Debris:

     Debris buried is fairly self-explanatory. If there is large amounts of foreign material (large or small pieces of metals, glass, plastics, and other trash/debris) in the soil, optimal growing may be hindered. Near the roadways where debris may be kicked up off of the streets, this may be a major problem. Broken glass and other debris may be deposited off the streets and then trampled into the ground.Exposed debris will need to be removed so that proper planting may occur. Often in an urban environment, trash, small appliances, and other debris may be found in backyards. This debris will need to be cleared so that such obstacles do not prevent proper seeding.
 
 

Area Specifics. Though the background information may state that an area will be favorable for a certain plant to grow, the area specific information will further determine if the plant will develop for a prolonged period of time. Background information may also need to be further specified for the specific area or land description.

Barrier:

    Information under "area specifics" also addresses areas where use of plants may not be the most effective way to prevent lead exposure or where phytoremediation does not produce a complete barrier. In such areas, stones, wood chips, or some other material may be used where exposed soil is a concern, either because of incomplete plant coverage or the inability to obtain coverage in an area.

Aesthetics:

    Finally, special consideration must be given to issues such as aesthetics and health effects. Since the plants used for phytoremediation will be placed near people’s homes, it is important that the plants look physically pleasing. Also, issues surrounding the allergenic effects of some of the plants must be considered. Though ragweed may remove lead from the soil, it will not be useful if it is causing children other health problems.

Grubs:

    In many urban communities, grubs, moles, mice and other insects and animals can damage front and back yard lawns. Animals such as moles often burrow through the soil and leave small holes and mounds where they enter and exit the subsurface. These holes can lead to bare spots in a front or back lawn, leaving exposed soil that can create a potential pathway for lead exposure. Grubs can infest a lawn and cause patches of dead grass in one’s lawn. Grubs feast on the root systems of grasses, killing the turf and leaving exposed soil. Furthermore, if grubs are present in a lawn, more animal damage may occur since grubs are often a favorite food of insect eating animals (including moles) (Purdue, 2000). Grubs are controlled through use of chemical and non-chemical means. Pesticides are effective against grubs, but they also may eradicate other insects that have beneficial properties for the soil and grass. Some non-chemical means include using lawn spikes to aerate the soil and trample the grubs. This must be done after either a heavy rain or watering when grubs come to the surface. Also, soil nematodes (which are microorganisms that kill grubs) can be introduced to a lawn to remove grubs.

Durability:

    There are certain areas on the front and back lawns that may see considerable traffic or may endure other durability issues. Areas near pathways, sidewalks, streets, exterior stairways or other areas where people may frequently travel along may be significantly trampled. A durable grass will be needed near such areas so that the grass can endure such strains. A typical grass mixture that is considered "durable" would contain approximately 80% Kentucky bluegrass and 20% Ryegrass. Often durable seed blends are labeled right on the bag as "durable"; also, landscaping experts can determine if a blend is durable. These areas that have durability issues may need additional stone or other covering material placed near the pathways if a durable grass can not prosper under such conditions.    Other durability issues include grass areas near roadways, which will be exposed to salt deposition from the streets. Though briefly discussed in the Background Information section, salinity needs to be addressed not only through testing and soil amendment, but also by using tolerant grasses and other plants near roadways. Durability is even more of a concern for play areas since play areas often endure a constant battering from children’s activities in these areas. Grasses will need to be chosen that will be able to withstand high levels of traffic and must be able to give a full coverage barrier. Since play areas are where children may be sitting on the grass or performing other activities in very close proximity to the ground, there is a distinct possibility that lead ingestion may occur if a proper barrier is not formed.  Areas of extreme traffic or areas that will not support grass will need to have some sort of barrier that will prevent lead contaminated soil from being exposed. For example, areas under swing sets often are barren because grass is torn up from children stopping and starting the swing. In such areas, materials such as sand, rubberized playground material, or wood chips may be placed as a barrier to the barren soil. Such materials will need to be replenished as the sand or wood chips are displaced over time.
 
    Durability is an issue in pet areas because of the constant traffic and other abuse that a pet may create. Besides typical walking or running in a pet area, pets may also dig up or bury items in the ground. This will create barren soil, similar to some of the effects that animal/insect pests had on front and back lawns. If a pet tends to perform such activities, these areas will need to be repaired as soon as possible. Durable grasses will need to be used in these areas. A unique concern in pet areas is bioaccumulation. Some pets may eat the grasses in their areas, possibly ingesting lead in the process. If lead accumulates in the shoots of the grasses, this is a very real possibility. Research will need to be done to see if there are grasses that will accumulate and stabilize lead in their root system without uptake into the shoots. Though this will not allow for the lead to be "harvested", it will prevent lead ingestion by the pets. Ingestion by pets through consumption grass may not be a concern if pets do not eat the grass or if it does not stay in their systems. Often when dogs eat grass, the grass is "thrown-up" through reverse peristalsis. The same can not be said for animals such as rabbits, which may also be kept as pets. If a special grass that only accumulates in the roots can not be found, then a designated pet area where no grass is grown may be a feasible option. An enclosed section of a yard may be prepared with pea gravel to prevent exposure to lead from the barren soil that is often the case in pet areas. This should provide a safe area for pets that will prevent lead exposure.