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NMSU tests poplar trees for phytoremediation of contaminated former oil refinery site

FARMINGTON, N.M. - New Mexico State University's Agricultural Science Center at Farmington is taking its phytoremediation research into the real world of soil contamination to see if poplar trees can help stop the migration of an underground plume of petroleum contaminants.

Two men looking at a tree branch.
Mick O'Neill, New Mexico State University agronomy professor, right, and Sam Allen, NMSU agricultural research scientist, both from NMSU's Agricultural Science Center at Farmington, look at the new growth on a poplar tree planted in a phytoremediation project on a former oil refinery site in San Juan County. (NMSU photo by Jane Moorman)

A dozen old storage tanks are all that remain on a site in San Juan County that once housed an oil refinery and associated storage facilities. An array of monitoring wells tracks the status of the petroleum contaminants that permeate the soil and float on the groundwater five feet below the surface.

Most of the contaminant has been removed from the site, but a small amount remains.

"It's the final portion of contamination that is a concern for us," said Mick O'Neill, NMSU professor of agronomy at the Farmington science center. "We want to establish an underground barrier that will help prevent movement of contaminant from the site."

Hybrid poplar trees have been used for phytoremediation in other areas of the country, so O'Neill decided to see if the trees can help in this situation.

Phytoremediation is the treatment of environmental problems through the use of plants that mitigate the problem without the need to excavate, transport and dispose of contaminated soil.

"This is a great opportunity," said Sam Allen, NMSU agricultural research scientist at Farmington. "We can't recreate this type of contamination in our soil at the farm. This is a multi-level, multi-year study with three key questions to answer. First, can the trees live in this environment? Second, to what level will the plants remove the contaminant from the soil? Third, can the root system act as a barrier to further contaminant movement?"

To address the first question, nearly 700 trees have been planted over a three-year period. The first planting in 2010 consisted of 233 bare-rooted six-foot whips of local cottonwood and hybrid poplars, plus the xeric woody perennial four-wing saltbush. A drip irrigation system was installed that supplies moderately to severely saline water from a 1,500-foot well. Irrigation was used to establish the root system from the whips, which were planted at a three-foot depth.

"The first year we had no idea what would grow," O'Neill said. "From that planting, we found out we needed to use only the OP-367 hybrid poplar clone, a cross between the Populus deltoides, the regular cottonwood, and the Populus nigra, which came from Italy. This clone has been used in the United States since the mid-1920s and does very well in arid and semi-arid regions."

Plantings in 2011 and 2012 used 18-foot poles of the OP-367 clone raised at the NMSU Farmington farm.

"This allowed us to get the poles into the water table at five feet below the soil surface and have a substantial trunk to initiate the root system," O'Neill said.

"The first plot is doing well," Allen said. "The second year plot is outstanding with a lot of branch development."

The trees planted in 2012 are in the most contaminated area. They did passably well during their initial year, but suffered from the cold 2013 winter, which killed much of the top growth. But there is new growth coming from the base of the trunks.

"This tells us that the root system has survived," O'Neill said. "Which answers our question, can they survive this level of contamination?"

In preparation for the second phase of the study, soil samples have been taken for a baseline of toxicity levels. Future samples will be compared to determine if the contaminants are being removed.

"Long-term monitoring of the plant tissue, soil and groundwater is needed to determine if the poplars are impacting the site," Allen said.

"This gives us a real-world laboratory in which we can test the viability of using hybrid poplar for phytoremediation of a petroleum-contaminated site," O'Neill said. "There are many similar sites in New Mexico and in other areas of the arid western United States where trees could be used in combination with brackish water, where available, to clean contaminated soil and groundwater."