Table 9 Resistivity distribution from 2-D inversion of VES data from Sonning Way landfill site. Columns 2 to 10 give the
Ž .
resistivities in V m of the respective blocks at various depths. Only the top 29 m of the 2-D model is shown. The dashed horizontal lines indicate the approximate intra-fill boundaries, the solid lines show the approximate base of fill, and the
symbol W indicates the resistivity block at depth where groundwater seepage was encountered in boreholes close to the VES Ž
. stations. The upper zone ca. 1–2 m depth shown by italicised bold-faced numbers is the interpreted clay seal of the cover
Ž .
sequence. The interpreted saturated zones allowing for water-level fluctuations are also highlighted using bold-face Ž
. Ž
numbers 4 m depth . The topography at the respective stations was incorporated in the inversion process the quantity in .
brackets underneath each station name is the height in metres relative to station G1 Ž .
Ž .
Ž .
Ž .
Ž .
Ž .
Ž .
Ž .
Ž .
Ž .
Block bottom m G1 0.0
G2 0.3 G3 0.7
G4 1.45 G5 1.45
G6 1.5 G7 1.5
G8 1.5 G9 1.5
0.13 146.6
96.5 105
104 160
83 41
33.8 32.5
0.29 146.6
96.5 105
104 160
83 41
33.8 32.5
0.49 109.6
61.4 83.6
63.1 141.5
53.6 31.9
31.2 36.6
0.75 73.8
36 54.8
33.3 77.8
31 24.7
31.6 45.2
1.1 57.7
25.7 36.4
19.7 39.5
21.6 23.5
36.1 56.2
1.5 56.1
25.2 28.9
16.1 27.6
20.9 27.7
41.8 61.1
2.1 56.2
29.3 26.5
17.9 27.3
23.8 32.6
40.8 53.1
2.8 46.2
31.3 24.4
21.4 30.5
24.4 30
30.6 38.6
3.7 30.2
27.8 20.9
22.9 31.5
20.9 20.7
19.2 27.3
4.8 18.8
22.7 17.4
21.3 28.4
16.5 12.8
12.2 22
6.3 14.2
20.4 16
18.4 23
13.7 W 9.2
10 22.4
8.2 15.2
22.4 17.2
16.2 W 17.7
12.7 8.9
11.7 29.2
10.6 21.6
28.9 21.1
15.2 13.9
12.7 11.3
18 44
13.7 32.9
37.7 26.1
15.3 12
13.3 15.4
27.7 61.4
17.6 43.6
43.7 29.9
16.7 12.3
14.6 19.6
34.1 64.4
22.7 45.8
43.3 31.2
20.4 15.5
17.3 22.6
32.3 48.8
29.2 39.1
29 22.3
25.4 30.5
34.6 30.1
24.7 25.7
mental interpretation of these resistivity models. An attempt is made here to suggest how the
information contained in the resistivity models may be maximized or given ‘‘added value’’ by
Table 10 Ž
An illustration of a conceptual predictive scheme for approximate environmental interpretation of saturated fill or invaded .
Ž .
substrate resistivity. The reconstructed resistivity of saturated fill or invaded substrate r
is used to predict the TDS via
b
Ž . Ž .
Ž . Ž .
Eq. 4 as in a or via Eq. 3 as in b ; these may be regarded as the lower and upper bounds on the parameter estimates. Ž .
Ž . The fluid conductivity is then predicted using Eq. 1 and the chloride content may be obtained from Eq. 2 . Water content
Ž . Ž
. is approximated from Eq. 6 , and age in years is inferred from data given in Table 4 using TDS and Cl as the determinants
Ž .
Landfill site s
mSrm Predicted TDS
Predicted s Predicted Cl
Estimated W Estimated age
b w
Ž .
Ž .
Ž .
Ž .
Ž .
Ž .
r in V m mgrl
mSrm mgrl
vol. of fill years
b
a Ž .
Leicester suburb 142.86 7
3099.79 448.29
843.08 54.78
10–20 Ž
. Mallard North
139.9 7.15 3008.5
435.28 813.81
55.02 10–20
Ž .
Lucas Heights 222.2 4.5
5779.1 828.11
1760.79 50.04
5–10 b
Ž . Leicester suburb
142.86 7 7168.35
1023.92 2271.53
35.47 5–10
Ž .
Mallard North 139.9 7.15
6936.65 991.30
2184.97 35.67
5–10 Ž
. Lucas Heights
222.2 4.5 14216.0
2010.15 5103.57
31.14 0–5
Table 11 Sample illustration of the conceptual estimation of hydrochemical parameters for saturated fill using 2-D inversion result for
Ž .
Ž .
the Sonning Way site. The inferred bulk conductivity s of fill and fluid conductivity s
are in mSrm. TDS and
b w
Ž .
Ž .
Ž .
chloride Cl concentrations are in mgrl. Water content W is in volume percent. The average age in years is estimated only for G3 to G8 where saturated fill is inferred to be present. For each parameter, the top set of values was obtained using
Ž . Ž .
Eq. 4 while the bottom set was obtained using Eq. 3 Parameter
G1 G2
G3 G4
G5 G6
G7 G8
G9 s
70.4 49.0
62.5 65.8
83.3 78.7
112.4 85.5
45.5
b
TDS 1143.5
686.2 966.4
1038.9 1449.8
1338.4 2209.5
1502.5 616.9
2395.4 1366.3
1990.9 2155.6
3109.3 2847.8
4940.9 3233.8
1215.4 s
167.8 101.5
142.2 152.7
212.0 196.0
321.1 219.6
104.3
w
347.7 200.0
298.9 313.4
449.7 412.4
709.6 467.4
206.1 Cl
259.3 141.8
212.6 231.5
343.3 312.4
565.0 358.1
125 621.6
320.1 499.5
548.7 846.2
762.7 1463.0
886.3 278.7
W 58.1
63.4 59.8
59.1 61.2
56.6 51.9
55.5 64.6
36.9 41.5
38.3 37.7
41.2 35.5
31.6 37.0
42.6 Estimated age
20 20
20 20
10–20 20
10–20 10–20
10–20 10–20
10–20 10–20
predicting some hydrochemical parameters us- ing empirical relations such as those described
in a previous section. Table 10 illustrates the favoured approach to interpreting resistivity
models of saturated fill. While such results are approximate, the approach described here may
be useful for drawing qualitative geoenviron- mental inferences. For example, note the simi-
larity in the computed parameters for Mallard North landfill and the landfill site near Leices-
ter. Coincidentally, the Mallard North landfill accepted refuse from 1970 to 1974 and the
resistivity soundings were made in the summers
Ž .
of 1988 and 1989 Carpenter et al., 1991 while the landfill near Leicester was backfilled be-
tween 1970 and 1976 and the resistivity sound- ing was carried out in August 1992. It is appar-
ent from this table that the fill at Lucas Heights
Ž is younger relative to the date of the relevant
. electrical sounding than those at the other two
sites. Note that two sets of hydrochemical pa- Ž
. rameters were computed
see Table 10 ; the Ž .
values determined using Eq. 3 are about twice Ž .
those determined via Eq. 4 and both schemes may thus be considered as yielding the maxi-
mum and minimum predictions in this study. In the 2-D case, the minimum resistivities in
Ž the inferred saturated zone of the model Table
. 9 have been used for the prediction of TDS,
fluid conductivity, chloride content and water Ž
. content see Table 11 . The estimated values of
Ž . TDS from the preferred Eq. 4 range from 960
to 2200 mgrl in the 2-D model for stations G3 to G8. The predicted fluid conductivity range
Ž from 160 to 320 mrS with chloride content of
. 140–565 mgrl for these stations. An average
pre-survey age of about 20 years is estimated for the fill on the basis of the predicted TDS
and chloride content and the age-compositional
Ž .
classification of Farquhar 1989 . This is in
Ž .
accord with the known age of fill 17–22 years , relative to the date of the VES measurements. It
is stressed that while the conceptual scheme proposed here may be a feasible predictive tool,
caution should be exercised when using such qualitative results especially as the age-related
characteristics detailed in Table 4 may not be valid for all landfill sites.
4. Discussion
