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• • • • • SECTION 6.0 • Evaluation of Remedy Performance • • • 6.1 Tree Health • • The first portion of this section presents a review of tree health after 10 years of remedial • action, discusses the success of phytoremediation in terms of establishing groves of trees, and provides recommendations regarding continued remedial action.The next subsections • contain the annual evaluation of the health of individual planting areas since last reported • in the 2010 Annual Groundwater Monitoring Report(CH2M HILL,2010). • 6.1.1 Overall Tree Health —Ten-year Review The RAP(CH2M HILL,2000b) defines one of the criteria for success of phytoremediation as • "established groves of mature poplar trees."Since remedial action was initiated,approxi- mately 370 hybrid poplar trees remain onsite,compared to the 1,600 poplar trees that • were planted originally.However,more diverse groves consisting of eight different tree • species have been established at the site. • As initially discussed in the 2002 Annual Groundwater Monitoring Report(CH2M HILL,2003), • the loss of poplar trees is largely attributed to infestation by the western poplar clearwing moth.The western poplar clearwing moth is a wood-boring insect and a pest of birch, • poplar, and willow trees in southern California and the Central Valley.The selected • contingency action to address tree loss is to replant with tree species resistant to infestation • by the western poplar clearwing moth and,in doing so,establish mature but diverse groves of trees to prevent future wide-scale losses from a pest or disease. • Replanting with resistant tree species has been ongoing since 2003.Tree species resistant to • the western poplar clearwing moth that have been planted onsite include Paulownia • (Paulownia fortunei),sycamore maple(Acer pseudoplatanus),sycamore(Platanus acerifolia), • fruitless mulberry(Morus alba),black locust(Robina pseudoacacia),sour gum(Nyssa sylvatica),and water gum(Tristania lavrina).Approximately 310 replacement trees were • planted between 2006 and 2010.The new plantings are well established,and the total • number of living trees observed in August 2011 has increased to approximately 80 to 85 percent of the number of trees initially planted. No replacement trees were planted since • November 2010,but additional replacement trees will be planted during the 2011 to 2012 • dormant season.Approximately 12 percent of the existing poplars(or 51 trees) died between • August 2010 and August 2011.Although continued losses of poplars are expected, the poplars that survive continue to take up nitrogen from the groundwater and soil.In com- parison,only 1 to 2 percent of resistant tree species were lost between August 2010 and • August 2011.In addition,new volunteer poplars occasionally sprout from existing poplar trunks.Some of these young poplars also exhibit signs of infestation,but several do not. • These will be monitored to see if they show signs of infestation in the future. • Nitrogen concentrations rebounded in groundwater underlying the original test grove • between 2002 and 2004,which corresponds to removal and replanting of the original test • • RD01112370001(CAH5003.DOCX) 6-1 ES�t511W90BPLd • • •