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Report of Results investigation of Agricultural Chemicals in Soil PureGroBrea Facility 1905 North Broadway Ave.,Stockton,CA Sierra-Pacific Project No.94-055.08 5.0 FINDINGS Soil types encountered during this investigation included primarily sandy silts, with fine to medium- grained silty sands and slightly clayey silts also present locally. Continuous sand layers were not correlated with depth between borings, except in some adjacent borings. Moisture was found to generally increase with depth. Hard zones exhibiting mineral cementation were noted in several borings. The origin of the cementation is thought to be precipitation of mineral salts by descending interstitial water, in large part owing to the presence of fertilizer compounds in the near-surface soils throughout the site, as described immediately below. The soils were likely emplaced as flood overbank deposits at generally low hydraulic energy levels. The sands are generally fine to medium grained, with fine sands predominating where sands are present. This high degree of sorting (poor grading) and the presence of micaceous minerals, chiefly muscovite, reflects a consistently low energy level in the depositional environment. 5.1 Nitrate Results Nitrate as nitrogen was detected in all samples collected during this investigation, at concentrations that ranged from 0.72 milligrams per kilogram (mg/kg) to 630 mg/kg. The concentrations of nitrate as nitrogen generally attenuated with depth in each boring (see Table 1). The highest concentrations were found in soil at each end of the current wash rack area(NSB-5 and NSB-6), and around the old tank farm area(NSB-2 and NSB-3). In these borings, concentrations of nitrates as nitrogen exceeded 100 mg/kg in samples NSB-2-5,NSB-3-5,NSB-3-10,NSB-5-5, NSB-5-10,NSB-5-20,NSB-6-5, and NSB-6-10. The attenuation of nitrate with depth is attributable to a decrease in available oxygen with depth. Oxygen-deficient conditions at depth are reflected in the lack of oxidation of soil minerals in freshly collected samples from 30 and 50 feet bgs, notably the gray coloration of clay blebs and sands, which turn tan to buff within a couple of hours of exposure to air at the surface. Ammoniacal nitrogen introduced as fertilizers at the surface is partially converted to nitrates by chemical oxidation and microbial action in the upper oxidized zone. Sierra-Pacific Groundwater Consultants, Inc. 1I