La flexibilidad del sistema para tener diferentes lapsos de rotaciones

Samples were collected in 40 ml Sun Sri EPA sample vials with Teflon/silicon septa. Sample vials were prepared by adding in excess of 0.4 g of TSP (for sample overflow during sampling) to each 40 ml vial.

In the field, wells were purged using a peristaltic pump (Watson Marlow 302S peristaltic pump powered with a DC to AC power inverter) until stable field parameters such as pH and Ec were reached. Field parameters were determined using a HANNA HI 991301 portable pH, electrical conductivity (Ec), temperature and total dissolved solids meter.

Sample vials were completely filled taking special care not to introduce any air bubbles into the vial. All samples were immediately packed on ice in the field and refrigerated at the laboratory (~4 ºC). Samples were preferably analyzed as soon as possible or within the holding time of 14 days.

Figure 5.1: A view of the groundwater sampling setup

5.3 Results and Discussion

Wells were purged for an average time of approximately 15 minutes after which stable field readings were obtained and samples collected. The field parameters recorded for the wells are presented in Table 5.1.

Table 5.1: Field parameters of wells

Well* pH Ec (mS/m) TDS (ppt) Temp (ºC) BH1 7.40 73 0.36 20.4 BH2 7.05 59 0.29 19.9 NMW1 7.05 71 0.35 17.8 NMW2 6.60 98 0.49 21.4 NMW3 7.44 45 0.22 21.1 NMW4 7.61 26 0.13 18.6 MW1 7.07 97 - 21.3 MW2 6.97 96 - 20.4 MW3 7.04 58 0.29 18.5 MW4 7.33 41 0.20 19.8 MW5 6.85 0.69 0.34 20.2 MW6 7.35 69 0.31 17.8

(*) Refer to Figure 3.1 for well locations (-) data not available

The first sampling round took place in August 2008 which involved the collection of samples from all the wells in the vicinity of the fuel station and the adjacent primary school. The analytical results of this round of sampling are illustrated in Table 5.2 below.

Table 5.2: Concentration results of first sampling round (August 2008)

Sample MTBE conc. (ppm) TBA conc. (ppm) BH1 BDL BDL BH2 BDL BDL NMW1 BDL BDL NMW2 2.346 0.636 NMW3 BDL BDL NMW4 BDL BDL MW1 6.387 1.774 MW2 38.907 9.074 MW3 BDL BDL MW4 BDL BDL MW5 BDL BDL MW6 BDL BDL        

Figure 5.2: Example of a typical contaminated sample chromatogram

Figure 5.2 is an example of a typical chromatogram acquired from the analysis of a groundwater sample where the good selectivity of this method is proven. Despite the sample containing a range of contaminants, no interference or coelution is observed.

10 of the 13 samples analysed displayed concentrations below the method detection limit. Wells MW1, MW2 and NMW2 contained significant concentrations of MTBE and TBA and were thus resampled four months later. The concentrations are illustrated in Table 5.3 below.

Table 5.3: 2nd round of analysis (December 2008)

MW1 MW2 NMW2 MTBE conc. (ppm) 7.026 36.338 BDL TBA conc. (ppm) 1.351 35.693 0.973        

After gathering this data, it was decided that there was no longer a need for over time monitoring of MTBE and TBA levels at this site. MW1 and MW2 are not positioned on the site being studied but are rather at the adjacent fuel station. NMW2 is the only well available on the site that contained a detectable concentration. Upon the second analysis of MW1, MW2 and NMW3, significant levels were only detected at the source and none on the site. The MTBE level of 2.346 ppm has now degraded to below detection limit.

MTBE analysis performed by ERM at this time showed that all the wells on the school premises were below detection limit except for MW3 and MW5 which contained trace levels of 1.1 and 0.37 ppb respectively. Samples collected by ERM are analyzed using a GC/MS method with a quantification limit of 0.3 ppb. It is therefore of no use to continue monitoring this site with a less sensitive method (ppm range detection), given that nothing will be detected. Table 5.4 summarizes the MTBE levels accumulated by ERM (2008).

Table 5.4: MTBE concentrations (ERM, 2008)

Well MTBE ppb MW1 8 300 MW2 35 000 MW3 1.1 MW4 _<0.30 MW5 _0.37 MW6 _<0.30 MW7 _<0.30 MW8 _<0.30 BH1 _<0.30 BH2 _<0.30        

The initial MTBE concentration of 2.4 ppm in BH1 led to the assumption that much higher concentrations would persist on the school field between the fuel station and BH1 (ERM, 2007). One year later, no MTBE is detected except for trace levels in only two of the wells. Examination and monitoring of MNA parameters on the site showed that natural attenuation processes were the driving force behind the rapid decrease in concentrations.

Prior to this study, BH1 was used extensively for watering the vegetable garden and the school field. The excessive abstraction from this well could have either drawn the plume towards BH1 or the excessive extraction could have caused the plume to dive and reach the subsurface extraction point of BH1.

Natural attenuation is a slow process and ERM claimed that the leak potentially occurred 10 years ago. If natural attenuation is the driving force behind this cleanup, then it would be safe to assume that the concentration level detected in BH1 was one that has already decreased significantly as a result of 10 years of natural attenuation. The school field is directly linked to the boundary wall of the fuel station (source). It is possible that the plume could have been making its way towards the school buildings for years without being detected since this area of about 80 metres forms part of the school field. A groundwater flow rate of 4-7 metres per year in this area would justify the odour only being detected about 10 years later due to the plume only reaching that distance at that time.

That the TBA levels are also below the detection limit is an indication that TBA is not accumulating at this site. If it was, then BH1 should have displayed a high concentration of TBA. This does not imply that MTBE degradation is not occurring but quite to the contrary.

TBA levels in MW1 and MW2 prove that MTBE transformation to TBA is undoubtedly evident. However, when comparing these levels, one would note that MTBE and TBA levels are balanced and proportional. In other words, if the degradation process resulted in an accumulation of TBA, then TBA levels should be significantly higher than MTBE levels. The natural subsurface conditions in this area, however, create favourable reaction conditions for the complete breakdown of MTBE.