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1 .0 Introduction and Background This work plan describes the soil sampling activities planned as a part of a ten-year evaluation of the remedial action that was implemented at the Koppel Stockton Terminal in Stockton,California in 2000.The remedial action(phytoremediation)was implemented in accordance with the Remedial Action Plan(RAP) (CH2M HILL,2000)and subsequent conditional approval from the Regional Water Quality Control Board(Water Board)of the RAP(Water Board,2000).In addition,this work plan describes the additional investigation proposed to define the potential nitrate source area at the former bagging plant near Monitoring Well KP-3 at the Koppel Stockton Terminal. The Koppel Stockton Terminal is a former dry fertilizer warehouse.The site is located in Stockton west of Ventura Avenue,between the extensions of Hazelton Avenue and Sonora Street,approximately 1 mile west-southwest of the I-5/1-lighway 4 interchange,next to the Port of Stockton,California.The site layout of the Koppel Stockton Terminal is presented on Figure 1. The site has been the subject of numerous investigations since the early 1990s to identify the extent of nitrogen contamination in the soil and groundwater.Soil types encountered onsite include discontinuous beds of sand,gravel,clay,and silt deposited in a continental alluvial fan environment. It is possible that some or all of the site has been covered with fill.The water table is approximately 3 to 10 feet below the ground surface(bgs)(0 to-7 feet mean sea level).Groundwater flow direction is generally southerly/southeasterly but can vary across the site in possible response to rainfall,nearby pumping,tidal fluctuations,and recharge of ponded runoff(CH2M HILL,2000). A test grove of 267 poplar trees was planted on approximately 1 acre on the southern end of the site in May 1998 to evaluate the effectiveness of phytoremediation in removing nitrogen from soil and groundwater.An additional 1,250 hybrid poplar trees and 230 willow trees were planted near the turnaround area,stormwater pond,former bagging plant,and in the area north of the warehouse from March through June 2000(CH2M HILL,2006).The original test grove,turnaround area,stormwater pond,former bagging plant,and north of the warehouse planting locations are presented on Figure 1. As described in the RAP,soil sampling was proposed to be performed at five year intervals to assess the success of phytoremediation in removing nitrogen from soil.The first five-year sampling event was conducted in July 2006 and the results are documented in Five-year Evaluation and 2006 Annual Groundwater Monitoring Reporting(CH2M HILL 2006).The ten- year sampling event,the scope of which is described in this work plan,is scheduled to be performed in July 2011.The results from the ten-year sampling event will be compared to data collected during the 2006 five-year sampling event and to baseline concentration data collected in 2000. At the former bagging plant,nitrate concentrations in Monitoring Well KP-3 have been steadily increasing since 2002(CH2M HILL,2010).To determine if a nitrate source area exists around Monitoring Well KP-3 at the former bagging plant and to define its extent,an is