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• -3- Mitigation Plan The objective of the mitigation plan is to eliminate the nuisance of flowing oil to the residents of the Alden Park area. We are also developing a plan to monitor water quality. Liquid oil has been found in a crescent shape around the north, east , and south borders of the park. We will install liquids pumping and bioventing wells to eliminate this nuisance. This process removes the volatile components of the oil and leaves behind an immobilized asphalt. Mitigation will begin in mid-1989 with the existing well. We will collect additional design data from this well and then install the full-scale system in mid- 1990. Results of Work Included in the Preliminary Mitigation Plan Trees When we started the site assessment work, there were trees dying in Alden Park . Tracy residents were concerned that oily dirt might have been responsible. We designed a special tree planter (Figure 1) to protect the tree roots from oil and to provide drainage. We installed six of these planters in Alden Park and filled each one with clean pot- ting soil . However , the trees did not grow as well as we expected. After six months , some of the trees died, and all of them had leaf burn. Oil is not killing the trees, because the planter prevents oil from contacting the roots. The problem appears to be standing water which is drowning the tree roots. The tree planter pene- trates the concrete bottom of the old oil reservoir to provide drainage. However, the clay below the planter does not drain very easily, and the grass in the park is watered frequently. We will continue to work with the City of Tracy to resolve this problem. Oil Seeps In several locations small amounts of thick oil were seeping to the surface. These oil seeps were a nuisance to the residents in the park area.