- Email: [email protected]
Geoelectric investigation of saline contamination of a chalk aquifer by mine drainage water at Tilmanstone, England
Geoexploration, 19 (1981/82) 179-192 Elsevier Scientific Publishing Company, Amsterdam -Printed
179 in The Netherlands
GEOELECTRIC INVESTIGATION OF SALINE CONTAMINATION OF A CHALK AQUIFER BY MINE DRAINAGE WATER AT TILMANSTONE, ENGLAND
A.U. GTERI’ Department of Geology and mineral Sciences, (Received March 26,1981;
U~iversi~ of Ilorin, Ilorin ~~igeria~
accepted June 22,1981)
ABSTRACT Oteri, A.U., 1981. Geoelectric investigation of saline contamination of a Chalk aquifer by mine drainage water at Tilmanstone, England. Geoexploration, 19: 179-192. Arising from the soakage of mine water pumped from a coal mine in the Kent coalfield which underlies the Chalk of East Kent in England, an area of 27 km2 of a Chalk aquifer has become contaminated with saline water having concentrations of chloride as high as 5000 mg/l. Twenty-six Schlumberger vertical electrical soundings were made as part of a hydrogeological study of the area. Interpretation of the VES data showed a spatial variation in contamination of the aquifer which correlated closely with data obtained from boreholes. Laboratory measurements of effective porosity and resistivity of Chalk core samples were made and the form of Archie’s formula was determined. A synthesis of the surface geophysieai, hydroehemical and laboratory data led to quantitative mapping of formation water salinity in the fully saturated aquifer.
INTRODUCTION
Much water was encountered in the Kent coalfield and large quantities of saline waters had to be pumped out of the ground during shaft sinking as well as for coal mining to take place. This water was discharged into unlined pits in the chalk in the vicinity of the colliery and allowed to soak into the aquifer. Disposal of mine drainage waters by this method which started in 1907 only ceased in 1974. It is estimated that 318,000 tonne of chloride were discharged into the aquifer. A large area of the aquifer has been contaminated as a result. Since 1974, many hydrogeological investigations involving borehole drilling and coring, aquifer testing as well as borehole geophysics logging, have been carried out by the Southern Water Authority and the Water Research Centre. From the results of these investigations, the contaminated aquifer was divided into shallow, intermediate and deep zones, the shallow zone (top 45 m) being the most contaminated and the deep zone (100 m-l 50 m) the least contami‘Formerly
of Department of Geology, University College, London.
0016-71~2/81~0000~000/$02.50
o 1981 Eisevier Scientific Publishing Company
180
nated (Puri and Headworth, 1976; Headworth et al., 1980). Apart from this decrease in contamination with depth at any point, it was also found that contamination decreases with distance from the coal board’s discharge pit. A surface resistivity survey was carried out in order to see whether a correlation could be made with the newly acquired hydrogeological data. Twenty-six vertical electrical soundings (VES) using the Schlumberger array were made between the 12th and 25th of October 1978. Laboratory determination of the effective porosity and electrical resistivity of Chalk core samples from the area was also carried out. The location of the vertical electrical soundings and some of the boreholes are shown in Fig. 1.
Fig. 1. Index
map showing
the survey area, location
of electrical
soundings,
and boreholes.
GEOLOGYANDHYDROGEOLOGY
The Chalk is the main productive aquifer in the study area, as well as being the major aquifer in England. Chalk is a fine-grained white limestone composed predominantly of fossil debris and microfossils. The Upper and Middle Chalk are extremely pure limestones, analysis of samples from the Upper Chalk of south England showing total acid-insoluble residues of often less than 1% by weight of sediment (Gray, 1965; Scholle, 1977). In the survey area, gamma-ray logs of boreholes to depths of 150 m showed the absence of marly or clay bands. The Chalk of Kent has a high effective porosity, 34.6-46.5% with a mean of 40.8%. The hydraulic permeability of chalk is low; Price et al. (1976) having obtained values ranging from 1.2 to 12 - 16’ cm/s. The importance of the Chalk as an aquifer is due to the development of secondary porosity in
181
the form of intersecting horizontal and high angle fissure patterns. Caliper logs show the top 58 m to be most fissured in a borehole in the T~m~stone area. The Chalk is separated from lower and older rocks by a thick gault clay. The coal occurs in the Coal Measures which consist of sands and shales intercalated with coal seams. Much of the water drained from the mine occurs in the Folkestone Beds, Hastings Beds and the Lower Oolites (Plumptre, 1959) as well as in sandstones in the Coal Measures. The area has an undulating topography with dry river valleys separated by higher rounded slopes. The ground elevation decreases from south to north and ground water flows in a northeasterly direction. Table I lists the chemical analysis of water samples from the survey area. Samples 1 and 2 represent native uncontaminated Chalk water pumped into public supply. Samples 3 and 4 represent underground water from the Tilmanstone colliery (pollutants) while samples 5 and 6 are water samples from the areas contaminated by the mine drainage waters. An analysis of sea water from Dungeness (Oteri, 1980) is included for comparison_ Native Chalk water is characterized by a low conductivity and low chloride ion concentration, the ions being dominated by calcium and bicarbonate. The pollutants have high conductivity with sodium and chloride ions being predominant. Also, they have negligible sulphate ion concentration as a result of sulphate reduction (Revelle, 1941). The contaminated waters have intermediate values of conductivity and chloride ion. The chemistry is also dominated by sodium and chloride ions. G~OELE~TRICAL
TECHNIQUE
The resistivity survey was done using an ABEM Terrameter. East Kent is an area of intensive agriculture and the survey was carried out at a time when winter crops were being sown. This restricted the amount of available surveying space. The form of the geoelectrical sounding curves obtained varies throughout the area but most of them are characterized by a final segment of positive gradient which in many cases approximates 45”. This indicates a semi-infinite basal unit of higher resistivity. This was to be expected as the effective porosity (both fissure and intergranular) in the Chalk decreases with depth (Gray, 1965; Edmunds et al., 1973; Scholle, 1977) and also the contamination of the Chalk aquifer at any point decreases with depth (Puri and Headworth, 1976). Fig. 2 shows typical curves obtained. QUALITATI~
INTERPRETATION
Qualitative interpretation of the resistivity data was done by drawing apparent resistivity maps for current electrode spacing (AB) of 60 m and 160 m; and by the comparison of vertical electrical sounding (VES) curves along two profiles, profiles I and II shown in Fig. 1. Assuming that the chalk behaves as a uniform aquifer with constant porosity,
Thornton Farm Bh. 5 Oct., 1975
Eastry borehole 9 August, 1977
Kent No.1 Coal seam
3
4
5
Sea water (off Dungeness) July 1978
1976
Mine waste Discharge water (Tilmanstone colliery)
May, 1955
47619
12220
ND*3
4150
7950
690
480
ND*3
-
25
-
chalk water;
19545
4700
18508
1230
3000
45
33
5, 6 = pollutant;
38965
ND*3
32520
ND*3
--
-
ND*”
325
(mg/l)
Cl (n&l)
;;is;ved
-
Total
area
20
20
Samples: 1, 2 = fresh water; 3, 4 = contaminated * ‘Hardness as CaCO,. *2S.W.A. = Southern Water Authority. *‘ND = Not determined.
---
7
6
Kingsdown (near Deal) 1976
2
Tilmanstone colliery
Sutton Pumping Station Dec., 1978
Temperature (“C)
in the Tilmanstone
Electrical conductivity (pmhos/cm)
of groundwater
1
character
Sample location and date
I
Sample no.
Chemical
TABLE
--
2750
300
0
203
350
25
8.4
(w/l)
so:-
10000
2140
10143
718
830
29
15.5
(mg/l)
Na’
7 = sea water comparison.
149.6
990*’
51
248*’
139
147
210**
(mg/l)
HCO
_.
450
390
ND*3
8.1
26
6.2
ND*$
(mg/l)
K’
1290
ND*$
232
26.2
268
3.2
280
ND*3
1300
155
624
84.8
ND*3
(mg/l)
(mg/l)
ND* 3
Ca2+
Mgz+
author
S.W.A.*2
Plumptre
S.W.A.*=
S.W.A.*’
S.W.A.*=
S.W.A.+=
---
Source
ti
183
Fig. 2. Typical
VES curves
obtained
in the Tilmanstone
area.
a pollution plume caused by the saline water from the mine would appear as a low resistivity zone on an apparent resistivity map. The apparent resistivity maps are shown in Fig. 3. In the discussion of these maps, two further assumptions are made: (1) the apparent resistivity measured is of a uniform thickness of the Chalk at each electrode spacing below the surface of the ground; and (2) the pollution plume is indicated by the 25 Qm contour line. The pollution plume shown in Fig. 3a (corresponding to AB = 60 m) is a narrow linear feature restricted to the main valley running from the mine to the sea. In Fig. 3b however, which is the apparent resistivity map for Al3 = 160 m, the plume has broadened around the source of contamination, spreading east and west. The plume has a larger area than that of Fig. 3a. It can therefore be concluded from the apparent resistivity maps that near the source of contamination, a thicker layer of the Chalk is contaminated than further away. Figs. 4 and 5 show the geoelectrical sounding graphs along profiles joining VES stations. Profile I joins VES 6,20, and 1; all of which are along the trough of the valley from the coal mine, VES 6 being nearest to the mine and VES 1 furthest. Profile II is an east-west profile through VES 24,26,25,1 and 3. VES 1 and 26 are in valley troughs while VES 24,25 and 3 are on high ground. In profile I, Fig. 4, the surface layer resistivities for all three VES graphs are seen to be similar. There is a deep minimum in all the curves indicating the presence of a conductive layer which is taken to be the polluted chalk layer. The abscissa of the minimum increases from VES 1 to VES 6, indicating a deeper depth to the base of polluted chalk near the source of contamination. The ordinate of the minimum point decreases from VES 1 to VES 6, indicating lower resistivity values for the contaminated chalk layer (i.e., greater pollution) near the coal mine than further away. This agrees with one of the conclusions reached by Headworth et al. (1980) from borehole,hydrogeological studies.
i
48
_j5 Fig. 3a. Map of apparent resistivity for AB/2 = 30 m. Contours resistivity for AB/2 = 80 m. Contours in R m.
in 12m. b. Map of apparent
Fig. 5 shows the sounding graphs for profile II. All the VES locations are far away from the Tilmanstone colliery. The three VES graphs on high ground are all similar, with the conductive middle layer being only slightly developed. The depth to the resistant basement is seen to increase from VES 3 through
185
.spacing
Electrode
Fig. 4. Geoeleetrieal
,o
----
sounding
10 Elaetrode
Fig. 5. Geoelectrical
/21
m
graphs along profile I.
I
6 1
iae
8paCi”g
sounding
(A+$n
103
graphs along profile II.
VES 24 to VES 25. All have high resistivity values for the middle layer. VES 26 and 1 which were on troughs of valleys have a more conductive middle layer than the previous three VES. The resistivity of the conductive layer is less at VES 1 than at VES 26 as indicated by the ordinate of the minimum point. VES 1 was located on the main valley from the colliery, while VES 26 was located on an adjacent valley, showing that there is more contamination on the main valley than at corresponding points on adjacent valleys. Transmissivity was found by Ineson (1962) to be higher for chalk in the valleys than on the higher ground. It is however thought that inter~~~~ porosity will not differ markedly. The higher resistivity values indicated by the VES graphs on the high grounds compared to the valleys is interpreted as being due to the valleys being more polluted than adjacent points on the high grounds. It is also thought that the chalk at VES 3,25, and 24 is not contaminated while contamination occurs at VES 1 and 26.
The VES curves were interpreted initially with an automatic interpretation
programme after Zohdy (1974a), which uses an iterative method to invert Schlumberger VES curves into layer thicknesses and resistivities. The number of layers were further reduced by the aid of a DZchart (Zohdy, 1974b). In certain cases the calculated sounding curve from the automatic interpretation was found to differ greatly from the field curves. These were therefore reinte~reted by cube-matching of two- and three-layer master curves in conjunction with auxiliary charts (Orellana and Mooney, 1966; Rijkswaterstaat, 1969), to obtain horizontally stratified geoelectric models. The models were checked by computing a theoretical sounding curve, the model being subsequently modified to improve the fit with the observed curve (Zohdy, 1974c). A comparison of interpreted geoelectric model of VES 1 was made with the resistivity log of borehole A, 140 m away from the central location of VES 1, A close agreement in both layer resistivities and thicknesses was found to exist. However, differences existed between the interpreted model of VES 20 and the resistivity log of borehole 6, 380 m south of the central location of VES 20. The resistivities of the most contaminated and last layers were comparable for both models, but there were differences in the resistivities and thicknesses of the intervening layers. This was attributed to two causes. Firstly, the resistivity log showed the existence of ten geoelectric layers from the surface to a depth of 150 m, while VES 20 was interpreted as a 6-layer curve. Computation of the electrical sounding curve for the lo-layer model (Zohdy, 1974c) was carried out and it was found to fit the field data of VES 20. Secondly, the borehole resistivity log revealed the presence of a 12 m thick layer of resistivity greater than those of neighbouring beds (one of which was the most contaminated layer). Calculation of the effective relative thickness of this layer gave the low value of 0.4, hence it did not affect the computed sounding curve (i.e., there was no maximum on the curve corresponding to the high resistivity layer). Calculation of the longitudin~ conductance (S) for the layers between the surface layers and the last layer for VES 20 interpreted model and borehole 6 resistivity log gave values of 9.2 and 9.4 mhos, respectively. The conclusion was reached therefore that the longitudinal conductance would be the most reliable parameter characterizing the contaminated layer. Also, the resistivity of the most contaminated layer would be accurately determined from interpretation of the VES curve. Contour maps of the longitudin~ conductance and formation resistivity for the most conductive layer were drawn and are shown in Figs. 6 and 7. The maximum values of S occur in the main valley from the colliery, higher values were found nearer the source of contamination than further away. It was also found that troughs of valleys have higher values of S than the high grounds. Similarly, lower resistivity values occur at troughs of valleys, and locations nearer the source of contamination also have lower values than those further away. The low resistivity value of 42 Qm in the southe~most VES station is insignificant as lack of available space prevented the sounding being completed.
187
js
l
I
1 i4 *26”_._L-.-i_.-_._.
L
II-~
--_-L-L
Colliery Y.-q 1
1
01: 1
/ 35
Fig, 6. Map of the longitudinal in mhos.
4 8
I..-__.
2s
ll-~__-l.”
..-
Fig. 7. Map of the resistivity
conductance
_-_-L_--
(S), of the most conductive
layer. Contours
-v
of the most conductive
layer. Contours
in 12m.
188 LABORATORY SAMPLES
STUDIES
OF POROSITY
AND RESISTIVITY
OF CHALK
CORE
The effective porosity of sixteen Chalk core samples from three boreholes in the Tilmanstone area were determined in the laboratory by the liquid resaturation method using deaerated, de-ionised water (API, 1960; Price et al., 1976). The effective porosity was found to vary from 34.6% to 46.5% with a mean of 40.8%. All the core samples except two came from depths below 60 m. The Institute of Geological Sciences, London, working on Chalk core samples from one of the boreholes, found the porosity in the depth range 22-64 m to be 43-48.8% with a mean of 45.8%. The resistivity of the samples saturated with 9,573 mg/l sodium chloride solution was determined by the electrolyte-buffered, four-electrode method (Keevil and Ward, 1962; Sauck and Sumner, 1967; Emerson, 1969; Worthington and Barker, 1972). Three sets of resistivity values per sample were determined (except for sample 14 for which only two values were obtained). Each measurement was followed by a period of resaturation in the saturating electrolyte. The multiple measurements were made with the aim of attaining equilibrium between the cores and the saturating electrolyte, namely sodium chloride solution (Hoyer and Spar-m, 1975). As only one electrolyte concentration was used, the values of formation factor obtained were termed apparent, since the presence of matrix conduction was not investigated. Fig. 8 shows the plot on bilogarithmic scale, of the apparent formation factor against effective porosity for all 47 apparent formation factor values. The linear regression line
10
Fig. 8. Variation
20
(; g PoRo:“,rY
of apparent
formation
60
factor
70
80
90
100
with porosity
of Chalk core samples.
189
by the method found to be: F, =
of least squares was computed
and the equation
of the line was
(1)
1.12cp-‘-6s
where F, = apparent formation factor; 4 = effective porosity. Also shown in the figure is the apparent formation factor-porosity relationship for sixteen values of F, judged to be the equilibrium values. The equation of the line of best fit by least squares method is given by: F, = 1.224”‘” Comparing the two regression lines, it is seen that they are coincident at porosity values between 38% and 44%, while at porosity of 30% and 50%, the apparent formation factors differ by only 2.6 and 2.4% respectively. The two linear regression lines therefore give identical values of apparent formation factor for the porosity range of the Upper Chalk of the Tilmanstone area. QUANTITATIVE OF THE CHALK
MAPPING OF WATER QUALITY
IN THE MOST CONDUCTIVE
ZONE
The conductivity and chloride concentration of water samples from the Tilmanstone area were correlated. Chloride concentration in milliequivalents per litre for each water sample was plotted against its resistivity in Rm at 25°C on a bilogarithmic scale. Fig. 9 shows the plot which also includes the corresponding data for sea water off Dungeness (Oteri, 1980). The apparent formation factor corresponding to an effective porosity of 45.8% was determined from the formation-factor/porosity relation of eq. 2 above. The value of porosity used was the average porosity determined by IGS on chalk samples from the shallow zone in a borehole in the Tilmanstone area. Neutron log as well as the IGS data showed higher porosity values in the shallow zone than at the deeper horizons. Caliper logs also showed the shallow zone to be the most fissured zone in the boreholes and this zone corresponds to the most contaminated zone. At each VES station, the resistivity of the formation water of the most conductive layer was determined by dividing the formation resistivity by the apparent formation factor of 4.09. The chloride concentration corresponding to this value of water resistivity is then read from the plot of Fig. 9. To convert chloride concentration in mequiv./l to mg/l, a multiplying factor of 35.5 is used. Fig. 10 is a map showing the distribution of chloride concentration of the formation water for the most conductive layer. Also shown in this map are values of pore water and fissure water of boreholes in the area, for comparison. Pore water in the Chalk is the water in the intergranular pores in the matrix, while fissure water is the water within the joints and this is the water in a well drilled in the Chalk. Puri and Headworth (1976) noted chemical differences between pore and fissure water, in the Chalk of the Tilmanstone area. Similar differences were noted by Edmunds et al. (1973) on Upper and Middle Chalk samples from Berkshire in Southern England.
CONDUCTIVITY
(micro-mhos
/cm)
1
Fig. 9. Plot of chloride concentration Tilmanstone area.
against resistivity vf ground water samples in the
Fig. IO. Map of the chloride ion concentration
of formation water, of most conductive layer.
191 CONCLUSIONS
Both qualitative and quantitative interpretation of vertical electrical resistivity data delineated the contaminated Chalk as a low resistivity layer. Good correlation was found to exist between the hydrogeological and surface geophysical data. The Chalk is more polluted near the colliery discharge pit, pollution decreasing with distance away from the pit. Near the colliery, both valleys and high ground (spurs) are polluted while further away, the pollution is confined to the troughs of the valleys. The longitudinal conductance of the contaminated zone is the parameter most precisely determined from the interpretation of the VES curves. As a result of suppression, values of the resistivity and thickness of some layers could not be accurately determined for certain VES graphs. A quanti~tive mapping of water quality in the most contaminated zone of the Chalk was carried out using an apparent formation factor of 4.09 for a constant effective porosity of 45.8%. Quite good agreement was shown to exist between chloride values obtained from surface VES interpretation and those obtained from borehole water sampling, bearing in mind the limitations in precision of the electrical method, the assumption of a constant porosity coupled with absence of matrix conduction, and the nature of the Chalk aquifer, especially the differences observed in chemistry between pore and fissure waters. ACKNOWLEDGEMENTS
The author is grateful to the Association of Commonwealth Universities London and the Nigerian National Petroleum Corporation for financial support of his research at University College London. The cooperation and financial support of the Southern Water Authority for the field work as well as permission to publish some of their data is gratefully acknowledged. The author is also grateful to Mr. G.P. Jones who supervised the project and Dr. A. Thomas-Betts of the Geophysics Department, Imperial College, London who kindly edited the manuscript making constructive comments and suggestions. REFERENCES American Petroleum Institute, 1960. Recommended practice for core-analysis procedure. Am, Petrol. Inst. Rept., RP40. Edmunds, W.M., Lovelock, P.E. and Gray, D.A., 1973. Interstitial water chemistry and aquifer properties in the Upper and Middle Chalk of Berkshire England. J. Hydrol., 19: 21-31. Emerson, D.W., 1969. Laboratory efectrical resistivity measurements of rocks. Proc. Aust. Inst. Mining Met., 230: 51-62. Gray, D.A., 1965. Tbe stratigraphical marker bands in the Cretaceous strata of the Leatherhead (Fetcham Mill) borehole Surrey. Butl. Geof. Surv., Gt. Britain, 23: 65-116.
192 Headworth, H.G., Puri, S. and Rampling, B.H., 1980. Contamination of a Chalk aquifer by mine drainage at Tilmanstone, East Kent, U.K. Quart. J. Eng. Geol., 13: 105-117. Hoyer, W.A. and Spann, M.M., 1975. Comments on obtaining accurate electrical properties of cores. Trans. Sot. Profess. Well Log Analysts, Pap. B, 11 pp. Keevil, N.B. Jr. and Ward, S.H., 1962. Electrolyte activity: its effect on induced polarization. Geophysics, 27: 677+90. Orellana, E. and Mooney, H.M., 1966. Master Tables and Curves for Vertical Electrical Sounding over Layered Structures. Intersciencia, Madrid, 150 pp. Oteri, A.U.J., 1980. Geophysical and Hydrogeological Investigations of Contaminated Aquifers in Southeast England. Thesis, University of London, 424 pp. (unpublished). Plumptre, J.H., 1959. Underground waters of the Kent coalfield. Inst. Mining Eng., 119: 155-169. Price, M., Bird, M.J. and Foster, S.S.D., 1976. Chalk pore-size measurements and their significance. Water Services, 80: 596400. Puri, S. and Headworth, H.G., 1976. Tilmanstone Investigation -Report on Test Pumping of Observation Boreholes, October 1975 to January 1976. Kent Area Resource Planning Office, Southern Water Authority, 26 pp. (unpublished). Revelle, R., 1941. Criteria for recognition of sea water in ground waters. Trans. Am. Geophys. Union, 22: 593-597. Rijkswaterstaat, 1969. Standard graphs for resistivity prospecting. European Assoc. Expl. Geophys., The Hague. Sauck, W.A. and Sumner, J.S., 1967. Laboratory experiments in Induced Polarization. Intern. SEG Meeting, 37th, Oklahoma City, 31 pp. Scholle, P.A., 1977. Chalk diagenesis and its relation to petroleum exploration: oil from chalks, a modern miracle. Am. Assoc. Petrol. Geol., Bull., 61: 982-1009. Worthington, P.F. and Barker, R.D., 1972. Methods for the calculation of true formation factors in the Bunter sandstone of northwest England. Eng. Geol., 6: 213-228. Zohdy, A.A.R., 1974a. A computer program for the automatic interpretation of Schlumberger sounding curves over horizontally stratified media. U.S. Geol. Surv. Rep., USGS-GD-74-017; PB-232 703. Zohdy, A.A.R., 1974b. Use of Dar Zarrouk curves in the interpretation of vertical electrical sounding data. U.S. Geol. Surv. Bull., 1313-D. Zohdy, A.A.R., 1974c. A computer program for the calculation of vertical electrical sounding curves by convolution. U.S. Geol. Surv. Rep., USGS-GD-74-010; PB-232 056.
179 in The Netherlands
GEOELECTRIC INVESTIGATION OF SALINE CONTAMINATION OF A CHALK AQUIFER BY MINE DRAINAGE WATER AT TILMANSTONE, ENGLAND
A.U. GTERI’ Department of Geology and mineral Sciences, (Received March 26,1981;
U~iversi~ of Ilorin, Ilorin ~~igeria~
accepted June 22,1981)
ABSTRACT Oteri, A.U., 1981. Geoelectric investigation of saline contamination of a Chalk aquifer by mine drainage water at Tilmanstone, England. Geoexploration, 19: 179-192. Arising from the soakage of mine water pumped from a coal mine in the Kent coalfield which underlies the Chalk of East Kent in England, an area of 27 km2 of a Chalk aquifer has become contaminated with saline water having concentrations of chloride as high as 5000 mg/l. Twenty-six Schlumberger vertical electrical soundings were made as part of a hydrogeological study of the area. Interpretation of the VES data showed a spatial variation in contamination of the aquifer which correlated closely with data obtained from boreholes. Laboratory measurements of effective porosity and resistivity of Chalk core samples were made and the form of Archie’s formula was determined. A synthesis of the surface geophysieai, hydroehemical and laboratory data led to quantitative mapping of formation water salinity in the fully saturated aquifer.
INTRODUCTION
Much water was encountered in the Kent coalfield and large quantities of saline waters had to be pumped out of the ground during shaft sinking as well as for coal mining to take place. This water was discharged into unlined pits in the chalk in the vicinity of the colliery and allowed to soak into the aquifer. Disposal of mine drainage waters by this method which started in 1907 only ceased in 1974. It is estimated that 318,000 tonne of chloride were discharged into the aquifer. A large area of the aquifer has been contaminated as a result. Since 1974, many hydrogeological investigations involving borehole drilling and coring, aquifer testing as well as borehole geophysics logging, have been carried out by the Southern Water Authority and the Water Research Centre. From the results of these investigations, the contaminated aquifer was divided into shallow, intermediate and deep zones, the shallow zone (top 45 m) being the most contaminated and the deep zone (100 m-l 50 m) the least contami‘Formerly
of Department of Geology, University College, London.
0016-71~2/81~0000~000/$02.50
o 1981 Eisevier Scientific Publishing Company
180
nated (Puri and Headworth, 1976; Headworth et al., 1980). Apart from this decrease in contamination with depth at any point, it was also found that contamination decreases with distance from the coal board’s discharge pit. A surface resistivity survey was carried out in order to see whether a correlation could be made with the newly acquired hydrogeological data. Twenty-six vertical electrical soundings (VES) using the Schlumberger array were made between the 12th and 25th of October 1978. Laboratory determination of the effective porosity and electrical resistivity of Chalk core samples from the area was also carried out. The location of the vertical electrical soundings and some of the boreholes are shown in Fig. 1.
Fig. 1. Index
map showing
the survey area, location
of electrical
soundings,
and boreholes.
GEOLOGYANDHYDROGEOLOGY
The Chalk is the main productive aquifer in the study area, as well as being the major aquifer in England. Chalk is a fine-grained white limestone composed predominantly of fossil debris and microfossils. The Upper and Middle Chalk are extremely pure limestones, analysis of samples from the Upper Chalk of south England showing total acid-insoluble residues of often less than 1% by weight of sediment (Gray, 1965; Scholle, 1977). In the survey area, gamma-ray logs of boreholes to depths of 150 m showed the absence of marly or clay bands. The Chalk of Kent has a high effective porosity, 34.6-46.5% with a mean of 40.8%. The hydraulic permeability of chalk is low; Price et al. (1976) having obtained values ranging from 1.2 to 12 - 16’ cm/s. The importance of the Chalk as an aquifer is due to the development of secondary porosity in
181
the form of intersecting horizontal and high angle fissure patterns. Caliper logs show the top 58 m to be most fissured in a borehole in the T~m~stone area. The Chalk is separated from lower and older rocks by a thick gault clay. The coal occurs in the Coal Measures which consist of sands and shales intercalated with coal seams. Much of the water drained from the mine occurs in the Folkestone Beds, Hastings Beds and the Lower Oolites (Plumptre, 1959) as well as in sandstones in the Coal Measures. The area has an undulating topography with dry river valleys separated by higher rounded slopes. The ground elevation decreases from south to north and ground water flows in a northeasterly direction. Table I lists the chemical analysis of water samples from the survey area. Samples 1 and 2 represent native uncontaminated Chalk water pumped into public supply. Samples 3 and 4 represent underground water from the Tilmanstone colliery (pollutants) while samples 5 and 6 are water samples from the areas contaminated by the mine drainage waters. An analysis of sea water from Dungeness (Oteri, 1980) is included for comparison_ Native Chalk water is characterized by a low conductivity and low chloride ion concentration, the ions being dominated by calcium and bicarbonate. The pollutants have high conductivity with sodium and chloride ions being predominant. Also, they have negligible sulphate ion concentration as a result of sulphate reduction (Revelle, 1941). The contaminated waters have intermediate values of conductivity and chloride ion. The chemistry is also dominated by sodium and chloride ions. G~OELE~TRICAL
TECHNIQUE
The resistivity survey was done using an ABEM Terrameter. East Kent is an area of intensive agriculture and the survey was carried out at a time when winter crops were being sown. This restricted the amount of available surveying space. The form of the geoelectrical sounding curves obtained varies throughout the area but most of them are characterized by a final segment of positive gradient which in many cases approximates 45”. This indicates a semi-infinite basal unit of higher resistivity. This was to be expected as the effective porosity (both fissure and intergranular) in the Chalk decreases with depth (Gray, 1965; Edmunds et al., 1973; Scholle, 1977) and also the contamination of the Chalk aquifer at any point decreases with depth (Puri and Headworth, 1976). Fig. 2 shows typical curves obtained. QUALITATI~
INTERPRETATION
Qualitative interpretation of the resistivity data was done by drawing apparent resistivity maps for current electrode spacing (AB) of 60 m and 160 m; and by the comparison of vertical electrical sounding (VES) curves along two profiles, profiles I and II shown in Fig. 1. Assuming that the chalk behaves as a uniform aquifer with constant porosity,
Thornton Farm Bh. 5 Oct., 1975
Eastry borehole 9 August, 1977
Kent No.1 Coal seam
3
4
5
Sea water (off Dungeness) July 1978
1976
Mine waste Discharge water (Tilmanstone colliery)
May, 1955
47619
12220
ND*3
4150
7950
690
480
ND*3
-
25
-
chalk water;
19545
4700
18508
1230
3000
45
33
5, 6 = pollutant;
38965
ND*3
32520
ND*3
--
-
ND*”
325
(mg/l)
Cl (n&l)
;;is;ved
-
Total
area
20
20
Samples: 1, 2 = fresh water; 3, 4 = contaminated * ‘Hardness as CaCO,. *2S.W.A. = Southern Water Authority. *‘ND = Not determined.
---
7
6
Kingsdown (near Deal) 1976
2
Tilmanstone colliery
Sutton Pumping Station Dec., 1978
Temperature (“C)
in the Tilmanstone
Electrical conductivity (pmhos/cm)
of groundwater
1
character
Sample location and date
I
Sample no.
Chemical
TABLE
--
2750
300
0
203
350
25
8.4
(w/l)
so:-
10000
2140
10143
718
830
29
15.5
(mg/l)
Na’
7 = sea water comparison.
149.6
990*’
51
248*’
139
147
210**
(mg/l)
HCO
_.
450
390
ND*3
8.1
26
6.2
ND*$
(mg/l)
K’
1290
ND*$
232
26.2
268
3.2
280
ND*3
1300
155
624
84.8
ND*3
(mg/l)
(mg/l)
ND* 3
Ca2+
Mgz+
author
S.W.A.*2
Plumptre
S.W.A.*=
S.W.A.*’
S.W.A.*=
S.W.A.+=
---
Source
ti
183
Fig. 2. Typical
VES curves
obtained
in the Tilmanstone
area.
a pollution plume caused by the saline water from the mine would appear as a low resistivity zone on an apparent resistivity map. The apparent resistivity maps are shown in Fig. 3. In the discussion of these maps, two further assumptions are made: (1) the apparent resistivity measured is of a uniform thickness of the Chalk at each electrode spacing below the surface of the ground; and (2) the pollution plume is indicated by the 25 Qm contour line. The pollution plume shown in Fig. 3a (corresponding to AB = 60 m) is a narrow linear feature restricted to the main valley running from the mine to the sea. In Fig. 3b however, which is the apparent resistivity map for Al3 = 160 m, the plume has broadened around the source of contamination, spreading east and west. The plume has a larger area than that of Fig. 3a. It can therefore be concluded from the apparent resistivity maps that near the source of contamination, a thicker layer of the Chalk is contaminated than further away. Figs. 4 and 5 show the geoelectrical sounding graphs along profiles joining VES stations. Profile I joins VES 6,20, and 1; all of which are along the trough of the valley from the coal mine, VES 6 being nearest to the mine and VES 1 furthest. Profile II is an east-west profile through VES 24,26,25,1 and 3. VES 1 and 26 are in valley troughs while VES 24,25 and 3 are on high ground. In profile I, Fig. 4, the surface layer resistivities for all three VES graphs are seen to be similar. There is a deep minimum in all the curves indicating the presence of a conductive layer which is taken to be the polluted chalk layer. The abscissa of the minimum increases from VES 1 to VES 6, indicating a deeper depth to the base of polluted chalk near the source of contamination. The ordinate of the minimum point decreases from VES 1 to VES 6, indicating lower resistivity values for the contaminated chalk layer (i.e., greater pollution) near the coal mine than further away. This agrees with one of the conclusions reached by Headworth et al. (1980) from borehole,hydrogeological studies.
i
48
_j5 Fig. 3a. Map of apparent resistivity for AB/2 = 30 m. Contours resistivity for AB/2 = 80 m. Contours in R m.
in 12m. b. Map of apparent
Fig. 5 shows the sounding graphs for profile II. All the VES locations are far away from the Tilmanstone colliery. The three VES graphs on high ground are all similar, with the conductive middle layer being only slightly developed. The depth to the resistant basement is seen to increase from VES 3 through
185
.spacing
Electrode
Fig. 4. Geoeleetrieal
,o
----
sounding
10 Elaetrode
Fig. 5. Geoelectrical
/21
m
graphs along profile I.
I
6 1
iae
8paCi”g
sounding
(A+$n
103
graphs along profile II.
VES 24 to VES 25. All have high resistivity values for the middle layer. VES 26 and 1 which were on troughs of valleys have a more conductive middle layer than the previous three VES. The resistivity of the conductive layer is less at VES 1 than at VES 26 as indicated by the ordinate of the minimum point. VES 1 was located on the main valley from the colliery, while VES 26 was located on an adjacent valley, showing that there is more contamination on the main valley than at corresponding points on adjacent valleys. Transmissivity was found by Ineson (1962) to be higher for chalk in the valleys than on the higher ground. It is however thought that inter~~~~ porosity will not differ markedly. The higher resistivity values indicated by the VES graphs on the high grounds compared to the valleys is interpreted as being due to the valleys being more polluted than adjacent points on the high grounds. It is also thought that the chalk at VES 3,25, and 24 is not contaminated while contamination occurs at VES 1 and 26.
The VES curves were interpreted initially with an automatic interpretation
programme after Zohdy (1974a), which uses an iterative method to invert Schlumberger VES curves into layer thicknesses and resistivities. The number of layers were further reduced by the aid of a DZchart (Zohdy, 1974b). In certain cases the calculated sounding curve from the automatic interpretation was found to differ greatly from the field curves. These were therefore reinte~reted by cube-matching of two- and three-layer master curves in conjunction with auxiliary charts (Orellana and Mooney, 1966; Rijkswaterstaat, 1969), to obtain horizontally stratified geoelectric models. The models were checked by computing a theoretical sounding curve, the model being subsequently modified to improve the fit with the observed curve (Zohdy, 1974c). A comparison of interpreted geoelectric model of VES 1 was made with the resistivity log of borehole A, 140 m away from the central location of VES 1, A close agreement in both layer resistivities and thicknesses was found to exist. However, differences existed between the interpreted model of VES 20 and the resistivity log of borehole 6, 380 m south of the central location of VES 20. The resistivities of the most contaminated and last layers were comparable for both models, but there were differences in the resistivities and thicknesses of the intervening layers. This was attributed to two causes. Firstly, the resistivity log showed the existence of ten geoelectric layers from the surface to a depth of 150 m, while VES 20 was interpreted as a 6-layer curve. Computation of the electrical sounding curve for the lo-layer model (Zohdy, 1974c) was carried out and it was found to fit the field data of VES 20. Secondly, the borehole resistivity log revealed the presence of a 12 m thick layer of resistivity greater than those of neighbouring beds (one of which was the most contaminated layer). Calculation of the effective relative thickness of this layer gave the low value of 0.4, hence it did not affect the computed sounding curve (i.e., there was no maximum on the curve corresponding to the high resistivity layer). Calculation of the longitudin~ conductance (S) for the layers between the surface layers and the last layer for VES 20 interpreted model and borehole 6 resistivity log gave values of 9.2 and 9.4 mhos, respectively. The conclusion was reached therefore that the longitudinal conductance would be the most reliable parameter characterizing the contaminated layer. Also, the resistivity of the most contaminated layer would be accurately determined from interpretation of the VES curve. Contour maps of the longitudin~ conductance and formation resistivity for the most conductive layer were drawn and are shown in Figs. 6 and 7. The maximum values of S occur in the main valley from the colliery, higher values were found nearer the source of contamination than further away. It was also found that troughs of valleys have higher values of S than the high grounds. Similarly, lower resistivity values occur at troughs of valleys, and locations nearer the source of contamination also have lower values than those further away. The low resistivity value of 42 Qm in the southe~most VES station is insignificant as lack of available space prevented the sounding being completed.
187
js
l
I
1 i4 *26”_._L-.-i_.-_._.
L
II-~
--_-L-L
Colliery Y.-q 1
1
01: 1
/ 35
Fig, 6. Map of the longitudinal in mhos.
4 8
I..-__.
2s
ll-~__-l.”
..-
Fig. 7. Map of the resistivity
conductance
_-_-L_--
(S), of the most conductive
layer. Contours
-v
of the most conductive
layer. Contours
in 12m.
188 LABORATORY SAMPLES
STUDIES
OF POROSITY
AND RESISTIVITY
OF CHALK
CORE
The effective porosity of sixteen Chalk core samples from three boreholes in the Tilmanstone area were determined in the laboratory by the liquid resaturation method using deaerated, de-ionised water (API, 1960; Price et al., 1976). The effective porosity was found to vary from 34.6% to 46.5% with a mean of 40.8%. All the core samples except two came from depths below 60 m. The Institute of Geological Sciences, London, working on Chalk core samples from one of the boreholes, found the porosity in the depth range 22-64 m to be 43-48.8% with a mean of 45.8%. The resistivity of the samples saturated with 9,573 mg/l sodium chloride solution was determined by the electrolyte-buffered, four-electrode method (Keevil and Ward, 1962; Sauck and Sumner, 1967; Emerson, 1969; Worthington and Barker, 1972). Three sets of resistivity values per sample were determined (except for sample 14 for which only two values were obtained). Each measurement was followed by a period of resaturation in the saturating electrolyte. The multiple measurements were made with the aim of attaining equilibrium between the cores and the saturating electrolyte, namely sodium chloride solution (Hoyer and Spar-m, 1975). As only one electrolyte concentration was used, the values of formation factor obtained were termed apparent, since the presence of matrix conduction was not investigated. Fig. 8 shows the plot on bilogarithmic scale, of the apparent formation factor against effective porosity for all 47 apparent formation factor values. The linear regression line
10
Fig. 8. Variation
20
(; g PoRo:“,rY
of apparent
formation
60
factor
70
80
90
100
with porosity
of Chalk core samples.
189
by the method found to be: F, =
of least squares was computed
and the equation
of the line was
(1)
1.12cp-‘-6s
where F, = apparent formation factor; 4 = effective porosity. Also shown in the figure is the apparent formation factor-porosity relationship for sixteen values of F, judged to be the equilibrium values. The equation of the line of best fit by least squares method is given by: F, = 1.224”‘” Comparing the two regression lines, it is seen that they are coincident at porosity values between 38% and 44%, while at porosity of 30% and 50%, the apparent formation factors differ by only 2.6 and 2.4% respectively. The two linear regression lines therefore give identical values of apparent formation factor for the porosity range of the Upper Chalk of the Tilmanstone area. QUANTITATIVE OF THE CHALK
MAPPING OF WATER QUALITY
IN THE MOST CONDUCTIVE
ZONE
The conductivity and chloride concentration of water samples from the Tilmanstone area were correlated. Chloride concentration in milliequivalents per litre for each water sample was plotted against its resistivity in Rm at 25°C on a bilogarithmic scale. Fig. 9 shows the plot which also includes the corresponding data for sea water off Dungeness (Oteri, 1980). The apparent formation factor corresponding to an effective porosity of 45.8% was determined from the formation-factor/porosity relation of eq. 2 above. The value of porosity used was the average porosity determined by IGS on chalk samples from the shallow zone in a borehole in the Tilmanstone area. Neutron log as well as the IGS data showed higher porosity values in the shallow zone than at the deeper horizons. Caliper logs also showed the shallow zone to be the most fissured zone in the boreholes and this zone corresponds to the most contaminated zone. At each VES station, the resistivity of the formation water of the most conductive layer was determined by dividing the formation resistivity by the apparent formation factor of 4.09. The chloride concentration corresponding to this value of water resistivity is then read from the plot of Fig. 9. To convert chloride concentration in mequiv./l to mg/l, a multiplying factor of 35.5 is used. Fig. 10 is a map showing the distribution of chloride concentration of the formation water for the most conductive layer. Also shown in this map are values of pore water and fissure water of boreholes in the area, for comparison. Pore water in the Chalk is the water in the intergranular pores in the matrix, while fissure water is the water within the joints and this is the water in a well drilled in the Chalk. Puri and Headworth (1976) noted chemical differences between pore and fissure water, in the Chalk of the Tilmanstone area. Similar differences were noted by Edmunds et al. (1973) on Upper and Middle Chalk samples from Berkshire in Southern England.
CONDUCTIVITY
(micro-mhos
/cm)
1
Fig. 9. Plot of chloride concentration Tilmanstone area.
against resistivity vf ground water samples in the
Fig. IO. Map of the chloride ion concentration
of formation water, of most conductive layer.
191 CONCLUSIONS
Both qualitative and quantitative interpretation of vertical electrical resistivity data delineated the contaminated Chalk as a low resistivity layer. Good correlation was found to exist between the hydrogeological and surface geophysical data. The Chalk is more polluted near the colliery discharge pit, pollution decreasing with distance away from the pit. Near the colliery, both valleys and high ground (spurs) are polluted while further away, the pollution is confined to the troughs of the valleys. The longitudinal conductance of the contaminated zone is the parameter most precisely determined from the interpretation of the VES curves. As a result of suppression, values of the resistivity and thickness of some layers could not be accurately determined for certain VES graphs. A quanti~tive mapping of water quality in the most contaminated zone of the Chalk was carried out using an apparent formation factor of 4.09 for a constant effective porosity of 45.8%. Quite good agreement was shown to exist between chloride values obtained from surface VES interpretation and those obtained from borehole water sampling, bearing in mind the limitations in precision of the electrical method, the assumption of a constant porosity coupled with absence of matrix conduction, and the nature of the Chalk aquifer, especially the differences observed in chemistry between pore and fissure waters. ACKNOWLEDGEMENTS
The author is grateful to the Association of Commonwealth Universities London and the Nigerian National Petroleum Corporation for financial support of his research at University College London. The cooperation and financial support of the Southern Water Authority for the field work as well as permission to publish some of their data is gratefully acknowledged. The author is also grateful to Mr. G.P. Jones who supervised the project and Dr. A. Thomas-Betts of the Geophysics Department, Imperial College, London who kindly edited the manuscript making constructive comments and suggestions. REFERENCES American Petroleum Institute, 1960. Recommended practice for core-analysis procedure. Am, Petrol. Inst. Rept., RP40. Edmunds, W.M., Lovelock, P.E. and Gray, D.A., 1973. Interstitial water chemistry and aquifer properties in the Upper and Middle Chalk of Berkshire England. J. Hydrol., 19: 21-31. Emerson, D.W., 1969. Laboratory efectrical resistivity measurements of rocks. Proc. Aust. Inst. Mining Met., 230: 51-62. Gray, D.A., 1965. Tbe stratigraphical marker bands in the Cretaceous strata of the Leatherhead (Fetcham Mill) borehole Surrey. Butl. Geof. Surv., Gt. Britain, 23: 65-116.
192 Headworth, H.G., Puri, S. and Rampling, B.H., 1980. Contamination of a Chalk aquifer by mine drainage at Tilmanstone, East Kent, U.K. Quart. J. Eng. Geol., 13: 105-117. Hoyer, W.A. and Spann, M.M., 1975. Comments on obtaining accurate electrical properties of cores. Trans. Sot. Profess. Well Log Analysts, Pap. B, 11 pp. Keevil, N.B. Jr. and Ward, S.H., 1962. Electrolyte activity: its effect on induced polarization. Geophysics, 27: 677+90. Orellana, E. and Mooney, H.M., 1966. Master Tables and Curves for Vertical Electrical Sounding over Layered Structures. Intersciencia, Madrid, 150 pp. Oteri, A.U.J., 1980. Geophysical and Hydrogeological Investigations of Contaminated Aquifers in Southeast England. Thesis, University of London, 424 pp. (unpublished). Plumptre, J.H., 1959. Underground waters of the Kent coalfield. Inst. Mining Eng., 119: 155-169. Price, M., Bird, M.J. and Foster, S.S.D., 1976. Chalk pore-size measurements and their significance. Water Services, 80: 596400. Puri, S. and Headworth, H.G., 1976. Tilmanstone Investigation -Report on Test Pumping of Observation Boreholes, October 1975 to January 1976. Kent Area Resource Planning Office, Southern Water Authority, 26 pp. (unpublished). Revelle, R., 1941. Criteria for recognition of sea water in ground waters. Trans. Am. Geophys. Union, 22: 593-597. Rijkswaterstaat, 1969. Standard graphs for resistivity prospecting. European Assoc. Expl. Geophys., The Hague. Sauck, W.A. and Sumner, J.S., 1967. Laboratory experiments in Induced Polarization. Intern. SEG Meeting, 37th, Oklahoma City, 31 pp. Scholle, P.A., 1977. Chalk diagenesis and its relation to petroleum exploration: oil from chalks, a modern miracle. Am. Assoc. Petrol. Geol., Bull., 61: 982-1009. Worthington, P.F. and Barker, R.D., 1972. Methods for the calculation of true formation factors in the Bunter sandstone of northwest England. Eng. Geol., 6: 213-228. Zohdy, A.A.R., 1974a. A computer program for the automatic interpretation of Schlumberger sounding curves over horizontally stratified media. U.S. Geol. Surv. Rep., USGS-GD-74-017; PB-232 703. Zohdy, A.A.R., 1974b. Use of Dar Zarrouk curves in the interpretation of vertical electrical sounding data. U.S. Geol. Surv. Bull., 1313-D. Zohdy, A.A.R., 1974c. A computer program for the calculation of vertical electrical sounding curves by convolution. U.S. Geol. Surv. Rep., USGS-GD-74-010; PB-232 056.