Hydrocarbon Impact on Trees
I am a general contractor who built a new K-8 school in Washougal, Washington, completed in August 2017. As part of the project, we had to construct a soil berm and plant more than 100 large screening trees along it. These trees are predominately Douglas Fir, Red Oak, Western Red Cedar, and Red Alder. In this berm area, approximately 15 of the trees died in the first year, and a couple of others show signs of stress. Our Landscape Subcontractor saw an oily substance in standing water at the berm and suspected that the soil was contaminated. They had two soil tests done that did note the presence of Diesel and Lube Oil. They do not want to re-plant trees along this berm if the soil is harming them. Recently, a did an additional 10 soil samples that I had tested with the NWTPH-DX method for hydrocarbons. All of the tests had some hydrocarbons, ranging from 123 parts per million to 208 ppm, with most being closer to 150 ppm. The problem now is that I don't really know what I would normally find in soils, or what limits of hydrocarbons might impact tree sustainability. Any assistance in interpreting the test results that I received would be greatly appreciated.
Clark County Washington soil contamination
Thank you for your question. I am assuming the hydrocarbons you are referencing are polycyclic aromatic hydrocarbons (https://superfund.oregonstate.edu/all-about-pahs). Here is an excerpt from an article with information on unhealthy levels of PAHs in soil (and remediation efforts), that you will need to extrapolate from in order to interpret your soil test (or ask the testing lab):
PAHs often degrade naturally in the soil. However, if the concentration is very high (like at a current or former industrial site) and solid particles are present, this process is impeded. This process is also a slow one because of the structure of the molecules. PAH molecules have low levels of water solubility (which is positive because it prevents the infiltration of ground water) and bind easily to organic materials which makes them less susceptible to biodegradation. PAH molecules with three or less rings degrade more readily than molecules with four or more rings which have been found to be present after degradation takes place. (1).
A Dutch study determined the acceptable levels of PAHs. If levels go beyond these thresholds, remediation action is vital for the safety of humans. Many means are available to clean up contaminated sites.
Recommended “limit values” for PAHs in soil
The values apply to the total content of the seven selected PAHs.
· Soil quality criterion: 1.5 mg of PAH/kg of soil. The criterion indicates a safe level for contact with soil (children playing, home owners).
· Cut-off criterion: 15 mg of PAH/kg of soil. This criterion indicates the level at which all contact with soil should be cut off if the land use of the area is sensitive. (1).
Some studies have suggested that phytoremidiation can be used to clean up contaminated sites as PAHs cling to organic matter. (2). This method of clean-up can avoid the use of further chemicals or energy/technology intensive processes. However, studies and experiments with this method are limited and high-levels of effectiveness have not been proven. However, Michigan State University has been conducting on-going tests at an auto plant to determine the best plants for use in the phytoremediation of contaminated industrial sites. (2).
Soil washing, in which soils are agitated and cleansed with a chemical solution are also an option. (4) In a study done by a Mexican University on sites contaminated by gasoline and diesel, researchers mixed a solvent and the contaminated soil and agitated the mixture. The work brought the acceptable levels of PAHs and other contaminants into line with government standards. However, the method is labor and cost-intensive.
Bioremediation, using microorganisms to clean the soil are also particularly effective as these organisms feed on PAHs and cause them to quickly degrade.
This can be done by the mixing of animal manure into the contaminated soil. Bioremediation has also proven effective in cases where 4, 5 or 6-ring PAHs are present. (5). When plants are incorporated in these techniques, results are more effective and efficient as they create an ideal environment for microbial growth. Researchers are also investigating using a hydrocarbon solvent in addition to bioremediation to improve results.
Thermal and chemical destruction of PAHs in the soil is often considered too slow and expensive to consider for large areas of contaminated land."
I am still researching whether there have been published studies indicating which plants are more or less susceptible to PAHs in the soil, and I will get back to you tomorrow.
I've done a bit more research to determine if there are tree species that may be able to tolerate contaminated soil which has not been remediated. I don't know if the area you planted the dead trees in has enough water, but the following article indicates that willow trees actually become more resistant to some problems when grown in contaminated soil: https://www.sciencedaily.com/releases/2015/10/151015083436.htm
Just a thought. Good luck!