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• • • SECTION 6.6 EVALUATION OF REMEDY PERFORMANCE • • 6.3 Former Bagging Plant Area • • 6.3.1 Groundwater • The grab groundwater sampling performed in the former bagging plant area has better • defined the extent of the elevated concentrations of nitrate (greater than 500 mg/L)in • groundwater.However,there is still some uncertainty remaining with respect to the extent of elevated concentrations (500 mg/L contour on Figure 5-5),particularly to the east and • west.The nitrate results suggest that the extent of nitrate concentrations greater than • 1,000 mg/L is limited to a small area immediately around monitoring well KP-3. Ammonia concentrations in grab groundwater samples are generally low in all areas,except for • relatively higher concentrations at K11-06. • 6.3.2 Soil • The 2011 former bagging plant area investigation results suggest that there are generally • higher concentrations of nitrate-N in soil in the former bagging plant area,relative to other • areas of the Koppel Stockton Terminal site.For soil samples collected in 2011,24 of the • 30 soil samples,excluding duplicates,in the former bagging plant area had nitrate-N concentrations greater than 50 mg/kg. In comparison,only 2 of the 15 samples,excluding • duplicates,outside the former bagging plant area had nitrate-N concentrations greater than • 50 mg/kg.Conclusions regarding areas that could be acting as a potential source of the • observed increases of nitrate concentrations in groundwater cannot be drawn with certainty based on the nitrate-N concentrations detected in soil in 2011.However,the highest concen- • trations of soluble ammonium and exchangeable ammonium were detected in soil samples • collected from the 3K soil sampling area,suggesting that soil sampling area 3K may be a source area. In addition,the decrease in the pH of soil at the 3K soil sampling area is an • indicator that ammonium has been oxidizing to nitrate,which causes the soil to become • more acidic.Soil sampling area 3K appears to be in the general location where skid loaders • operated and fertilizer pellets may have spilled during routine operations. In early to mid 2000,poplar trees were planted and an irrigation system was installed to • provide water to the trees as part of the phytoremediation remedy.As the poplar trees • matured,the root system developed both laterally and vertically;however,in 2001 to 2002, • the clearwing moth infestation was observed in the poplars,which ultimately resulted in the • death of most of the poplars.The roots of dead trees have likely decayed,which may have created preferential pathways and channels for water from precipitation and irrigation to • flush nitrate and ammonium in soil into the underlying aquifer.This could be one of the • reasons nitrate concentrations started to increase in groundwater beginning in 2002,as observed in downgradient monitoring well KP-3. It is also possible that soil was much drier • or a duripan/cemented layer or concrete pads and footings existed prior to the planting of • the trees and installation of an irrigation system,limiting percolation through the system, • and nitrate/ammonium remained immobile. The low pH of soil is likely associated with conversion of ammonium to nitrate,and • possibly hydrolysis of aluminum where free aluminum is present. Low pH can reduce • availability of certain required plant nutrients,such as phosphorus,and increase availability • of micronutrients and other elements,such as aluminum,to levels that may become toxic to • • RDD1112370001(CAH5003.DOCX) • Esomsnaz D • • •