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Evaluation of NaturalAltenuation_ 7500 West,Eleventh Street, Tracy, CA. Page 38 tri 7.2.2.7 Nitrates J' Decreased nitrate concentration in anaerobic areas within a plume may indicate use of nitrate as an electron acceptor for anaerobic biodegradation of petroleum hydrocarbons. However, at the Navarra Site, such data should be used with caution because SJC is aware, from the monitoring of water in potable water supply wells that it performs for numerous clients in the close neighborhood of the 7500 West Eleventh Street site, that the shallow aquifers beneath the area are heavily affected by nitrates and nitrites that have j been generated by agricultural fertilizers and by septic tank systems that are present in significant numbers in the neighborhood. Figure 49 shows plots of nitrate and BTEX concentrations in groundwater along section <� line C-C'. Consistent with the evidence developed above, the depleted concentrations of nitrates in the vicinity of Monitoring Wells MWFP-1 and MW-7 supports the interpretation that anaerobic biodegradation is in progress in that area of the plume. The } plots on Figure 50, which show the distributions of the same parameters along section A- A', support that same interpretation. > � However, a striking feature of the plot of nitrate distribution with distance down-gradient along section A-A' is the noticeably elevated concentration of nitrates, at 74 mg/L, in the sample of groundwater recovered from Monitoring Well MW-16, which is situated 380 ft. down-gradient from the location at which the hydrocarbons were released. As was discussed in Section 2.1 of this report, SJC believes that groundwater in the vicinity of Monitoring Well MW-16 has been affected by failure of a septic tank leach field. The high concentration of nitrates in water recovered from that well is consistent with that hypothesis and illustrates the difficulties with which the analyst may be confronted when .<; attempting to interpret nitrate concentration distributions in plumes of components of fuel hydrocarbons in areas where use of agricultural fertilizer and sewage management by use of septic tanks is prevalent. (Note: As can be seen by examining Table 5, MW-16 also exhibits other anomalous parameters. Although they do not in any way contradict other is= geochemical data evidence of vigorous natural attenuation of hydrocarbons at the Navarra Site, the condition of groundwater in Monitoring Well MW- 16 reflects a number of other geochemical anomalies. For example, groundwater from that well has unexpectedly high concentrations of ferric iron, sulfate and total organic carbon. SJC attributes those findings to local effects of discharges from the failed septic tank leach field that has generated a groundwater mound in the vicinity of that well.) 7.2.3 Other Geochemical Indicators To provide an exhaustive suite of geochemical parameters for assessing the characteristics of the natural attenuation processes active at the Navarra Site, SJC's field and laboratory testing program included, in addition to the parameters discussed above, measurements of the pH, temperature, alkalinity, total organic carbons, total dissolved sic