ARCHIVED REPORTS XR0011755 - Laserfiche WebLink

Home Browse Search

Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, C4• Page 36 Yr Those calculations have been made and the results of the concentrations of ferrous oxide in the groundwater at the Navarra Site are included in Table 5. Ferrous oxide (FeO) was detected in groundwater extracted from only two of the 27 groundwater-quality monitoring wells. The samples recovered from Monitoring Well MW-7 and MWFP-1 contained 0.12 mg/L and 1.93 mg/L of ferrous oxide, respectively. This finding shows that anaerobic bioremediation is active in the area around those two monitoring wells, consistent with the high concentrations of the components of fuel hydrocarbons that are known to be present in that area (see Table 2). i l Figure 37 plots concentrations of BTEX and ferrous oxide in groundwater in monitoring wells MW-1, MW-3, MW-13, MW-7, MWFP-1, MWFP-4, MW-14, MW-lb, MW-18, MW-11, and MW-19 with distance down-gradient across section A-A' (see Figure 3 for {__! location of section line). The plots clearly show that the ferrous oxide occurs only in the area where the concentrations of BTEX are at their peak. The same relationship is shown in Figure 38, in which the concentrations of BTEX and ferrous oxide are plotted with distance to the east and west of the axis of the primary plume at cross section C-C' (see Figure 3 for location). Those plots also indicate that anaerobic bioremediation processes are aggressively digesting the high concentrations of BTEX in the area of the plume around monitoring wells MWFP-1 and MW-7. i 7.2.2.2 Total Dissolved.It-on An increase in dissolved iron within a contaminant plume compared to the ambient conditions in the groundwater in the vicinity of the site is an indication that Fe+3 has been < 7 y used as an electron acceptor during anaerobic biodegradation of petroleum hydrocarbons. Figure 39 is a plot of the concentration of dissolved iron in groundwater with distance :;:•� across section C-C'. That plot shows that concentration of total iron in groundwater is significantly elevated in the vicinity of Monitoring Wells MWFP-1 and MW-7, in which area the concentrations of BTEX are also elevated. This relationship further supports the evidence developed in 7.2.2.1, above, that anaerobic processes are actively attenuating the concentrations of analytes of concern in that area of the plume. It is interesting to compare the plots on Figure 39 with those on Figure 40, which shows that total iron ( concentrations across that section (B-B') are very low and do not correlate with the distribution of BTEX compounds in the groundwater there. This demonstrates that natural attenuation processes active along Section B-B' are dominantly aerobic as opposed to the anaerobic conditions prevailing along section C-C'. 7.2.2.3 Manganese Increased concentrations of manganese salts within a plume compared to their ambient concentrations in surrounding groundwater can indicate that manganese dioxide is being used as a terminal electron acceptor during anaerobic biodegradation of petroleum hydrocarbons. Consistent with that type of reaction, as is shown on Figure 41, concentrations of manganese in groundwater along section C-C' are elevated in the area around Monitoring Wells MWFP-1 and MW-7, where BTEX concentrations are also sic