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An Abem SAS 1000 terrameter and ES 464 switching unit were used for the field surveys at Tutuka ash dump site. Four multicore cables and stainless steel pegs were used with the “roll- along” surveying method. Measurement of the resistivity of the ground is carried out by transmitting a controlled current (I) between two electrodes pushed into the ground, while measuring the potential (V) between two other electrodes. Direct current (DC) or a very low

frequency alternating current is used, and the method is often called DC-resistivity. The resistance (R) is calculated using Ohm’s law (After Petrik et al. 2008).

The RES2Dinv version 3.52-inversion program was used to invert the measured data after being manually and mathematically filtered. The 2-D model used by this program divides the subsurface into a number of rectangular blocks that will produce an apparent resistivity pseudo section that agrees with the actual measurements. A forward modelling subroutine is used to calculate the apparent resistivity values, and a non-linear least-squares optimisation technique is used for the inversion routine (After Petrik et al. 2008).

One profile was surveyed at Tutuka on the 21st and 22nd August 2006. The profile was surveyed using a 10 meter electrode separation with the Schlumberger-long measuring protocol. Investigation depth is approximately 80 meters. A second profile (1B) was surveyed on the 24th October 2006 with the Wenner-long measuring protocol, yielding an investigation depth of approximately 60 meters. The traverse positions are indicated in Figure 3.6. Due to steep topographical gradient at the western side of the site, topographical corrections were applied to the data on profile 1. Elevations above mean sea level were measured with a handheld GPS unit (After Petrik et al. 2008).

The Tutuka ash dump site is underlain by the natural bedrock consists of dolorite, shale and sandstone. A clayey topsoil of between 0.2 and 1 meter thickness is found over most of the Tutuka ash dump site. Some of this topsoil was removed for soil cover of the final ash placement elevation. The geophysics positions and resistivity traverse is shown in Figure 3.6 (After Petrik et al. 2008). In general it is noteworthy that the resistivity mapping shows that the dry ash system at Tutuka (Figure 3.7) is of generally high resistance (65-159 ohm.m) in ash layers, indicating a low degree of water saturation at Tutuka. This correlated with the moisture content of Tutuka ash cores (see Fig. 5.1). Profile Ash-1 (Fig. 3.7): This profile transected the ash dump from old ash placed >20 years ago on the western side of the dump with an ash depth of approximately 10 m, to fresh ash dumped within the last year (2010) on the eastern side of the dump. The shallow (near surface), approximately 18 m thick, highly resistant-layer (red-purple contours in Figure 3.7) is associated with dry disposed ashes.

Borehole Log-AMB 23

Lithology Geology Ø [mm] Yield (l/s) pH Sp.Cord [mS/m] ORP [mv] Temp [c] Do [mg/l]

0.00 – 1.00 SOIL: Dark Brown Medium, Vary Clayey

1.00-10.00 DOLERITE: Yellowish ---and fine, Vary

10.00-19.00 DOLERITE: Light ---and fine, Slightly

19.00-30.00 DOLERITE: Dark Grey, ---and fine, Fresh

0 165 0 0.40 7.02 7.38 300 600 -240 0 16.8 18.8 2 10

Figure 3.5. Geological profile of borehole at Tutuka Ash dump site (After Hodgson (1999)).

This highly resistant layer which disappears at a distance of 1280 m from the origin of the profile reappears at approximately 1550 m and attains its maximum thickness in the far eastern side of the resistivity section. This layer behaviour can be ascribed to the ash having different composition in that part of the profile. The more conductive nature (lower resistivity) of the ash observed in the area between 1280-1550 horizontal may result from the continuous surface irrigation with brine, causing moisture and salt saturation in this region of the dump and thus lowering the resistivity (this is possible as this is the fresh ash being deposited and conditioned with brine and has not yet reached equilibrium with the ingressed CO2 from atmosphere for

Figure 3.6. Ash-1 and Ash-1B traverses used in the electrical resistivity surveys of dry disposed fly ash dump at Tutuka Power Station (After Petrik et al., 2008).

Figure 3.7. Electric resistivity profile at line ASH-1 on Tutuka: Profile length: 1700 m (After Petrik et al., 2008).

The resistive layer appears again at approximately 1550 meters and obtains maximum thickness in the far western side of the ash profile, which has been most recently placed (within the last year) and is the area of current dumping and had not yet been rehabilitated at the time of the survey. The more conductive (less resistive) subsurface layer (blue-yellow contours) below the ash layer is associated with a weathered dolerite sill body of approximate 30 m thickness with apparent resistivity between 7 and 60 Ohm.m. This layer reaches minimum resistivity values on

Ash-1

the western side of the profile where it is not covered by the ash layer and consequently exposed to a higher rate of chemical weathering. Resistivity values below 10 Ohm.m are normally associated with clayey formations.

The resistive layer below this conductor can either be fresh or less weathered dolerite or a Karoo sandstone layer. The “valley” shaped anomalies in this layer at stations 320 and 1100 metres of resistivity profile is of great interest (Figure 3.7). Positions are marked with purple squares on the model and locality map (see Figure 3.6). These anomalies are probably associated with paleo-valleys in the bedrock and are thus preferential groundwater flow paths. The valley feature at 1100 metres corresponds approximately with the old stream or drainage feature on the 1:50 000 map (Figure 3.6).

Profile Ash-1b (Figure 3.8): This model is contoured with the same resistivity contour scale as Ash-1 and the same interpretation criteria apply (see Figure 3.7). This profile (Figure 3.8) appears to be considerably less resistive indicating more brine saturated conditions at the side of the dump than the centre of the ash dump (see Figure 3.7). This low resistivity zone along the side of the dump between stations 1120 and 1600 metres correlates well with areas under constant irrigation with brine water and may also receive infiltration from the brine irrigated ash adjacent to it.

Figure 3.8. Electrical resistivity profile at Line ASH-1b on Tutuka: Profile length: 1700 m (After Petrik et al., 2008).

Of interest is the lateral resistivity change between stations 240 and 320 meters at a depth profile of 1630 m (Figure 3.8). This area is natural background and had no ash over layer. The lateral change visible in the substrata of this region is most probably caused by lithology of different

electrical resistivity (e.g. Dolerite sill and Karoo layer) or vertical displacement due to a geological fault. This anomaly (lateral change) might be correlated with the downward curving contours on the far eastern side of resistive profile Ash-1. The annotated section indicates the associated geology with the different modelled subsurface resistivity. It is assumed that the site is underlain by a Karoo dolerite sill and that no ash was dumped north of the existing Tutuka dump in the past. The geophysical data (Petrik et al. 2008) was used to optimize the site selection of the borehole positions (Figure 4.3). Sites for core drilling were selected in areas with different ash age and different salt content as interpreted from the geophysics data.