Cap depth as interpreted from resistivity – discussion and problems

Due to the heterogeneous nature of this landfill and resistivity measurements, it proved to be difficult to determine the depth to the base of the landfill cap / top of the refuse in some areas.

Resistivity soundings could not delineate the landfill cap thickness in areas where either very high or low resistivity was present at the surface, or where the resistivity contrasts between the cap and underlying refuse were minor. Cap thickness estimates could be obtained only in areas where no surficial features were present (Figure 16). Lower resistivity surficial features may indicate the presence of leachate near the surface – as was evident near at least one location on Line 4. In order to determine if the conductivity data could provide clues to cap thickness and cohesiveness where the resistivity data could not, the apparent conductivity and cap thickness maps were compared to see if a correlation between the thickness of the cap and surficial high apparent conductivity exists.

56 Table 4 Conductivity values derived from ground water sampling of wells near Area C.

(measurements are in mS/m)

Table 5 Apparent resistivity with reciprocal in calculated conductivity Measured apparent

57 Figure 95 Apparent conductivity (σam) of Line 2 (top) and near surface apparent resistivity (ρam) of Line 2 (bottom)

Figure 106 Apparent conductivity (σam) of Line 4 (top) and near surface apparent resistivity (ρam) of Line 4 (bottom)

58 4.5.1 Comparison of conductivity and cap thickness maps

Using the parameters described above, the cap thickness in the baseline area (Area A1) was determined to range between 0 and ~3 m (Figure 17). The same method resulted in estimates of cap thickness in Area C from 0 to >4 meters. However, as mentioned previously, evaluating the thickness was not possible in all areas. An inverse distance weighted prediction map was constructed using ArcGIS software showing the spatial distribution of the cap thickness variations. This map of estimated cap thickness was then compared with the apparent conductivity maps (Figure 17) to see if the high conductivity areas noted on the maps would correlate with the areas where the cap was thin or missing, and thus to evaluate whether or not the conductivity measurements could be used to extend our knowledge of cap thickness and

potentially be used for remediation purposes.

A general correlation between estimated cap thickness and conductivity is apparent. The high conductivity ( >140 mS/m) areas to the northwest of the survey area correlated to cap thicknesses of <1.5 meters, while mid range conductivity values (60-100mS/m, east/central of line 2 and 3) generally correlated with cap depths of 1.5 to 3 meters. Given this pattern, it would be expected that areas of low conductivity would correlate with a thicker cap; however, this did not prove to be entirely true (e.g. most of Line 1). However, many of the areas with low conductivity had highly variable surficial resistivity features where cap thickness could not be determined. (e.g.

eastern 1/3 of lines 1 and 6). The correlation between cap thickness and conductivity may have been hindered by the shortcomings in the method of estimating cap thickness. A more accurate estimation of landfill cap thickness may be possible but in this case there were too many

unexplained surfical inconsistencies. Increased accuracy could be obtained with a better data set and a different method for picking depths.

59 Figure 17Approximate location of Dillon boreholes on 2m PRP with survey lines

Figure 18 High and low resistivity surficial features along survey lines which prevented accurate prediction of cap depth

60 Figure 119 Comparison of cap thickness (top) and conductivity (bottom).

61 5.0 CONCLUSIONS

The primary goals of this study were twofold: to assess whether or not the geophysical methods could be used to determine cap thickness, and to determine if there was any evidence for leachate leakage from the landfill into the surrounding area.

The results from the use of the resistivity models were mixed. A comparison of the subsurface models in the problematic area to the baseline North line in area A1 suggests that several areas of the clay cap are compromised. There are additional lines of evidence, such as the presence of Phragmites, and liquid, with an oily appearance and chemical smell pooling near the surface near

Line 4 which may indicate a possible leachate spring in this area. Efforts at correlation of the cap thickness to the apparent conductivity maps showed higher conductivity in areas where the clay cap was thin (<1.5m) indicating near surface leachate, and may also indicate that the cap has been compromised in these areas. Thus, although there is evidence to support the assumption that the cap is not contiguous, the use of geoelectrical methods to assess cap thickness had limited success, as it was not possible to obtain a measurement of cap thickness in all areas. Other non- geophysical methods, such as boreholes, would be helpful to confirm results.

There is some evidence for potential leachate leakage southwards into the adjacent farmer‟s field.

The information obtained from the 1986 Dillon report stated that there was likely contact between refuse and the interbedded zone. The sand content in this zone would provide an excellent migration pathway for leachate to flow outside the landfill border. There are also several high conductivity features noted in the field, including linear features of higher conductivity that apparently extend into the field from the landfill. Such linear features occur at intervals along the whole length of the map (Figure 9). However, the presence of a gas pipeline in the eastern half of the field could also have produced high conductivity values that would mask any evidence of leachate leakage. Therefore, while the data suggests that leachate leakage could be occurring, however more research and/or ground truthing is needed to confirm this.

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67 CHAPTER III