Hydrogeochemistry, contaminant transport and ... - Hydrologie.org

Hydrological Sciences -Journal - des Sciences Hydrologiques, 31,2, 6/1986

Hydrogeochemistry, contaminant transport and tectonic effects in the Okposi-Uburu salt lake area of Imo State, Nigeria BONIFACE C E , EGBOKA Water Resources and Environmental Pollution Unit (WREPU), Department of Geological Sciences, Anambra State University of Technology, PMB 01660, Enugu, Nigeria

«ALU 0. UMA Department of Geology, Nsukka, Nigeria

University

of

Nigeria,

ABSTRACT Hydrogeochemical data of groundwater samples from 35 boreholes drilled in the Okposi-Uburu salt lake area are analysed. The data reveal that concentrations of dissolved geochemical constituents such as calcium (Ca + ) , manganese (Mn + ) , magnesium (Mg + ) , chloride (CI ) , and sulphate (S0,t ) ions show significant areal variations. Dissolved solids, chloride and manganese ions have concentrations up to and above the objectionable limits for drinking water in the salt lake area. Concentrations of dissolved solids in this zone are about 1200 mg 1~ . Concentrations of chloride and manganese ions are 350 mg 1" and 1.0 mg 1" respectively. These geochemical constituents and groundwater flow patterns show that transport of contaminants away from the source zone has been greatly influenced by advection, while in areas of high velocity dispersion is the controlling factor. Temperatures for the Okposi and Uburu salt springs are 34.4 and 37.5°C respectively. Bomb tritium indicated water of pre-1953 age. Deuterium and oxygen-18 showed high isotopic enrichment. The high concentrations of dissolved salts resulted from the combined effects of migration of dissolved salts through fractures at the lake floor and evaporation from the lake surface. These findings are related to the tectonic history of the Okposi-Uburu area, Hydrogeochimie, transport de matières polluantes et effets tectoniques dans la région du lac salé de OkposiUburu, Etat de Imo, Nigeria RESUME Les données hydrogéochlmiques relatives a des échantillons provenant de 35 sondages forés dans la région du lac salé de Okposi-Uburu sont étudiées. Les données révèlent que les concentrations des constituants geochimiques dissous tels que le calcium (Ca ) , le manganese ( M n 2 + ) , le magnésium ( M g 2 + ) , les ions chlorures (Cl ) et sulfates (SO4 ) présentent des variations spatiales significatives. Les solides dissous, ions chlorures et manganèse ont des concentrations qui arrivent jusqu'aux limites acceptables pour l'eau potable et même qui la 205

206

Boniface C. E. Egboka & Kalu O. Uma

dépassent dans la region salée du lac. Les concentrations de matières solides dissoutes dans cette zone sont d'environ 1200 mg 1 . Les concentations des ions chlore et manganèse sont respectivement de 350 mg 1 _ 1 et 1.0 mg 1" . La répartition de ces constituants et le schéma de l'écoulement de l'eau souterraine montrent que le transport de ces polluants depuis la zone d'origine a été largement influencé par l'advection, tandis que dans les régions à vitesses élevées la dispersion est le facteur principal qui contrôle les phénomènes. Les températures pour les sources salées d'Okposi et d'Uburu sont respectivement de 34.4 et 37.5°C. La bombe au tritium montre que l'âge de ces eaux remonte au delà de 1953. La concentration en deuterium et en oxygène-18 mettent en évidence un enrichissement isotopique élevé. Les hautes concentrations en sels dissous résultent des effets combinés de la migrations de ces sels par les fractures du fond du lac et de 1'evaporation à sa surface. Ces constatations sont en rapport avec l'histoire tectonique de la région de Okposi-Uburu.

INTRODUCTION Okposi and Uburu towns are rural communities in the Ohaozara local government area of Imo State, Nigeria (Fig.l). Fresh water resources are generally scarce in these parts of the country. The areas lie within the Imo-Cross River basin province (Egboka, 1984). The water supply problem is particularly serious because of the common occurrence of saline groundwater in the bedrock. This paper describes the hydrogeological setting and the hydrogeochemical processes that have affected the distribution of the contaminants. 9-E

STUDY

Fig. 1

Location of the study area.

AREA

Hydrogeochemistry in the Okposi-Uburu area, Nigeria

207

The origin of the dissolved salts is not yet known but possible sources of the salinity include one or a combination of the following : (a) soluble salts in the matrix of sedimentary strata that now exist in the zone of active groundwater flow in the bedrock; (b) ocean water intruded into the bedrock aquifer under present hydrological conditions; and (c) brines that flow upward from deep (>1000 m) sedimentary zones. Samples used for the investigation were collected from 35 boreholes drilled in shallow aquifers by UNICEF, and those from the Okposi and Uburu Salt Lake (Fig.2). Earlier, an extensive field

Fig. 2

Location of sample points, rivers and streams.

survey and geological mapping were carried out both during the rainy season and the dry season. The shaley and clayey nature of the terrain makes the area swampy and muddy and difficult to investigate during the wet season,and caked and cracked during the dry season. Some of the surface waters and ponds are highly contaminated by chemical and biological pollutants. Hence water is scarce during the dry season but of poor quality during the rainy season. As a result, UNICEF has drilled many shallow boreholes for water supply to the rural communities. The water is of low quality. The objective of the study reported here is to use the distribution of dissolved geochemical constituents and available isotopic data to describe the source and transport of contaminants in the hydrogeological system. The transport and dispersion of the contaminants are used to assess the effects of tectonism on the origin of the salt lakes in the Uburu-Okposi area.

208 Boniface C. E. Egboka & Kalu 0. Uma

PHYSIOGRAPHY AND GEOLOGY The topography of the area can be described as comprising irregular ridges and gentle sloping hills. The highlands range from 45 to 65 m above mean sea level, and the lowlands have an average elevation of 30 m. These topographical features are controlled by the bedrock geology. The ridges and hills are capped by well-indurated argillaceous sandstones and the lowlands are underlain by shales. Surface drainage in the area is irregular and consists principally of a number of small ephemeral streams. The streams (Fig.2) generally follow a north-south course into the Asu River which is about 15 km south of the study area. The flow of the Asu River during the dry season is near zero implying a negligible baseflow contribution. Saline springs and ponds occur within a relatively narrow belt which extends in a northnortheast-southsouthwest direction. Gravity surveys (Cratchley & Jones, 1965) show features aligned in a northeast-southwest direction in the Benue trough corresponding to a series of folds and fractures that occurred during Tertiary times (Beltaro & Bojarski, 1971). The saline springs and brines occur along this tectonically-affected belt near the axes of anticlines in fractured and mineralized areas. Intrusives in the nearby Abakaliki area have yielded galena, sphalerite, barytes, fluorite and subordinate pyrite and silver, and the galena has been extensively mined for economic use. The bedrock of the Okposi-Uburu area is made up entirely of sedimentary rocks. These sedimentary rocks belong to the Asu River group of Cenomanian age. The lithology consists of an alternating succession of well-indurated argillaceous sandstones, siltstones and shales. The well-indurated sandstones are exposed at the hills. Borehole locations are confined to sandstone exposures where there is a good chance of getting high yielding shallow aquifers. Borehole logs reveal that the thickness of the sandstones ranges between 3.2 m and over 30.5 m. The sandstones are generally fine-grained but medium-grained units occur at some localities. The structural geology in the area has been described by Offodile (1976) and Orajaka (1972). Orajaka (1972) showed the area as lying within the southeastern limb of an asymmetrical anticline whose axis trends northeast-southwest. The beds of this limb strike N45°-60°E and dip 10°-50° to the southeast. A fault striking N55°W and dipping moderately to the southwest has been located at the base of the Okposi salt lake. The outlets of the water of Okposi salt lake are located linearly along the fissured zone of the fault. According to Orajaka (1972), the lake appears to be an artesian spring since the upwelling of the water is spontaneous. The age of the rock units is lower Cretaceous (Turonian) within the Asu River Group (Reyment, 1965).

HYDR0GE0L0GICAL SURVEY The Okposi-Uburu areas lie within the Rain Forest Belt of eastern Nigeria. Annual precipitation averages about 1800 mm. Surface manifestations of groundwater discharge are limited to a few ephemeral springs and road-cut seepages with very low flow


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Table 2 Hydrogeochemical and isotope data of Okposi-Uburu salt lakes (Loehnert, E.P., 1982, unpublished data Sample no. Location S 544 545

Date of collection Lat. N

Temp. Long. E (°C) pH

_ _ _ _ _ _ _ Okposi-Okwu salt spring (pond) 22/3/82 Uburu salt spring 22/3/82

Isotopes 544 2.3±1.7TU 545 1.8 + 1.7TU

6°02

7°47.5 34.4

6° 01.5 7°44.5 37.5

6D:-6.3%„ 6D:-19.1%o

6.8

Na+ K+ |mg I"1 ) (mg I"' I _

_

14918 260.5 6677

122.4

Ca" (mg T1 ) ,

:

Mg 2+ CI" HC0 3 ~ (mg I"1 ) (mg T1 ) (mg T1 ) ___

447.5

45.5

53 750 3355

174.9

19.3

22520

2745

61BO:-0.77% 61!0:-4.15%

Chloride ions range from 362 to 1.0 mg 1 . Other anions are less significant and generally have concentrations that are less than 20 mg 1 . Manganese ions vary between 1.0 and 0.1 mg 1~ . Major cations such as Ca + , Na + , and K + were not tested for but an indication of high Ca + is given by the high calcium hardness (325 mg 1~ ) . High conductivity values indicate high concentration of N a + and chloride ions. Field conductivity measurements gave conductivity values of over 1000 yS cm - for the Okposi salt lake. Boreholes that penetrated deeply into the shale unit were found to contain more saline groundwater than adjacent ones that ended more or less in the sandstone. For instance, boreholes 22/83 with 5 m penetration into the black shale has a TDS of 127 mg 1~ while borehole 24/83 which is less than 1 km away from borehole 22/83 but with a penetration of 26.67 m into the shale has a TDS concentration of 328 mg 1 _ 1 . Table 2 shows the hydrogeochemical and isotope data of OkposiUkwu and Uburu salt springs of the Ohozara area (Loehnert, E.P., unpublished data). The remarkable points of significance are: (a) high temperatures; (b) high Na + , Cl~ and HCO3 concentrations; (c) very low bomb tritium values; (d) enrichment of deuterium and oxygen-18 values; and (e) higher enrichment in deuterium and oxygen-18 values for the Okposi-Ukwu salt spring (pond, S44) when compared to those of Uburu even though the temperature for Uburu is higher than that of Okposi. The high temperature classifies the Okposi and Uburu salt lakes to be warm springs such as the Ikogosi of western Nigeria, and other warm springs in the Benue valley (Loehnert, 1984). The high Na , C I - and HCO3 concentrations are indicators of salinity. The very low to near-absence of bomb tritium in the water samples indicate old waters of pre-bomb or pre-1953 age. This means that the lake water has not been recharged to any significant degree by young post-bomb water since 1953 and that an older subterranean spring water recharges the lake. The enrichment in deuterium and oxygen-18 may be caused by the effects of evaporation and the high-temperature effects of the warm spring. These would deplete the lighter isotopes, hydrogen ( H) and oxygen-16 ( 1 6 0 ) , that evaporate; H and 8 0 , the heavier isotopes, would be enriched in the lake water while the lighter isotopes would be evaporated from the lake water.

214 Boniface C, E. Egboka & Kalu 0. Uma

DISCUSSION Figures 6 and 7 display the spatial distribution of the dissolved geochemical constituents. They show the presence of a northeastsouthwest trending source zone that terminates southwestward at the Okposi salt lake. The major geochemical constituents such as TDS, Cl, Ca-hardness and Mg-hardness have maximum concentrations below the source zone and decrease in the direction of groundwater flow.

Fig. 6

Total dissolved solids (TDS) concentration--variation of contours.

Fig. 7

Chloride concentration — variation of contours.


215

These decreases in concentration along the flow path can be attributed to one or all of the following processes: (a) increasing concentration of the contaminants at the source (input) zone; (b) advection; and (c) hydrodynamic dispersion. The increase in concentration-strength of the contaminants at the source may have contributed to the observed general trends of decreasing contaminant concentration in the aquifer. The extent to which increasing strength of contaminants at the input zone during recharge from below has affected major ions in the plume cannot be quantitatively ascertained and it is therefore a cause of uncertainty in the discussion of contaminants transport. Advection and hydrodynamic dispersion are the major physical processes that control solute migration in groundwater environments (Egboka, 1981; Egboka et al., 1983; Sudicky et al., 1983). Advection is the transport of dissolved constituents by the bulk motion of the flowing groundwater. The constituents are thus transported at a rate equal to the average linear groundwater velocity. Hydrodynamic dispersion involves the spreading of the solutes away from the expected flow paths. It comprises two components: mechanical dispersion due to pore water velocity variations, and molecular diffusion due to thermal/kinetic energy of the solute particles (equations (1) and (2)). Mechanical dispersion is predominant at high groundwater velocities while molecular diffusion is significant at low velocities. The onedimensional equation (equation (1)) shown below has been used widely to estimate the dispersivity of aquifers in contaminant transport (Egboka, 1981; Bear, 1972; Egboka et al., 1983; Sudicky et al., 1983). The estimation of the hydrodynamic dispersion coefficient and dispersivity enables a description of the transport of contaminants along a groundwater flow system to be made. The one dimensional-differential equation for transport of a single dissolved geochemical constituent through a homogeneous, isotropic, granular aquifer is given as (Freeze & Cherry, 1979): ^C 3t

= D x

9fc _ 3x2

v x

9C 3x

(1)

subject to: C = CQ 3C: 0 3x c = 0

at

x = 0

at

x = 0

at

t = 0

where : C = solute concentration in solution at time t; C 0 = initial solute concentration; D x = coefficient of hydrodynamic dispersion in the longitudinal section along the flow path; V x = average linear groundwater velocity. To establish accurately the influence of any of these parameters in the transport of the dissolved geochemical constituents in the

216

Boniface C. E. Egboka & Kalu O. Uma

study area, it is necessary to establish input functions at the source zone, initial concentrations of these geochemical constituents in the groundwater regime prior to pollution, and the time lapse since the arrival of the contaminated plume at the source zone. The data relating to these are lacking and thus no quantitative assessment could be made. However, the concentration trends shown on Figs 6, 7, 8 and 9 reveal that advection and longitudinal hydrodynamic dispersion processes have been significant in the spreading of the saline water from the input zone. Along the principal direction of flow (Figs 7 and 8), the 50 mg l -1 contour for the CI"

(a)

Along of

the

principal

flow

direction

(A1 A 2 i

2 Km

(b)

Section

transverse

direction

of

flow

to

principal

( C1 C2 )

172W

(c)

Section

transverse

direction

of

to

flow ( A l

principal B)

TifKm Fig. 8

Principal and transverse directional migrations of chloride.

ion has travelled about 2 km away from the source zone whereas the corresponding distance in the transverse directions are 0.80 and 0.72 km. The spreading along the transverse directions is believed to be due to the effects of transverse dispersion. Similar observations are obtained for TDS (Figs 6 and 9). To obtain a more quantitative impression of the influence of advection and hydrodynamic dispersion, the following assumptions were made : (a) the input of chloride at the source zone is 500 mg 1~ ; (b) the threshold concentration of chloride prior to contamination was less than 5.0 mg 1~ and is negligible relative to the concentration at the source; and (c) the geological materials are homogeneous along the principal flow direction with a hydraulic conductivity of 1.0 x 10~3 to 1.0 x 10~5 cm s - 1 and a porosity of 0.25. These assumptions approximate field situations (Table 1 and Fig.7). Under these conditions, the average linear groundwater velocity is from 2 x 10 to 2 x 10 m year , and the 50 mg 1 chloride contour will take

Hydrogeochemïstry in the Okposi-Uburu area, Nigeria

(a)

Along

the

direction

217

principal of

flow

I AT A 2 )

1.8 Km

(b)

Section direction

ï

transverse of

flow

to

principal

(AI B )

1

0-85 Km

(c)

Section direction

transverse to of

flow

principal IA1 C !

0-76 Km

Fig. 9

Principal and transverse directional migrations of total dissolved solids.

about 10 to 10 years to travel the 2.0 km. Assuming for the transverse directions that mechanical dispersion is insignificant and that diffusion is the major transport process, using diffusion coefficients of 10~ 10 to 10"** mz s - 1 (common in geological materials) and Fick's equation (equation 2)), the diffusion time taken for the 50 mg 1 chloride contour to travel 0.72 km is in the order of 3 x 10 to 3 x 10 years. In the absence of mechanical dispersion, the contaminants will travel as a plug flow and the advection time for the 50 mg 1~ contour to travel 2 km in the principal direction of flow will be equal to (within calculation and assumption error) the diffusion time to travel 0.72 km in the transverse direction. The pronounced discrepancy between the advection and diffusion times therefore indicate that mechanical dispersion is an important factor in the distribution of the contaminants. An in situ field transport experiment is being planned to obtain the coefficients of longitudinal and transverse dispersion that may be used to estimate the actual behaviour. With these values the effects of hydrodynamic dispersion will be quantified. Time and seasonal variations of the dissolved geochemical constituents in the study areas are currently under investigation by the authors. It is hoped that the data collected would enable a more quantitative assessment to be made of the several factors that have affected migration of contaminants in the area.

ORIGIN OF THE CONTAMINANTS AND TRANSPORT Many hypotheses have been presented to account for the origin of

218 Boniface C. E. Egboka & Kalu 0. Uma

saline waters in the study area (Orajaka, 1972; Offodile, 1976). The concensus is that groundwater, under considerable (negative) piezometric pressure, flows through salt-bearing sediments and, having leached the salt, issues as springs along the flanks of the asymmetric anticlines. The existence of salt-bearing sediments is supported by the presence of highly conductive shales (specific electrical conductance of about 400 ]iS cm- ) below the shallow sandstone aquifers. The major question that remains to be answered, however, is the origin of the Na + , CI - and other ions within the shales and the persistence of these ions in pore water for millions of years in a zone of active groundwater circulation. For the first part of the question, it is possible that the Na , CI - and the other ions are remnants of the marine environment in which the shale strata were deposited in the Lower Cretaceous (Albian) times. These ions probably occur in the pore spaces in the matrix of argillaceous sandstones, as dissolved constituents in shale pore fluid and possibly as NaCl precipitate. Two possible mechanisms whereby dissolved salts from the saltbearing strata enter into the groundwater flow regime are: (a) groundwater flow through the shale matrix into active groundwater flow in the fractures or fissures; and (b) molecular diffusion. Active groundwater flow in the shale occurs through fractures in the shale. Relative to flow in fractures, flow in the unfractured rock is insignificant. For instance, a shale with a hydraulic conductivity of 10~ m s - 1 and hydraulic gradient of 10~l has a specific discharge of 0.003 mm year . The matrix of the shale is, therefore, not capable of providing, by means of groundwater flow, significant amounts of water and dissolved solids to the active groundwater flow system in the fractures. As long as water circulates through fractures rapidly enough, a solute concentration gradient will occur from the shale matrix towards the fractures. According to Fick's first law, the concentration gradient will cause, as a result of molecular diffusion, a flux of solute from the matrix to the fracture. The distribution of solute in the shale matrix with time can be obtained from an analytical solution of Fick's second law:

^ = - D*^ U 3t 9x2

(2) ^ >

where C = relative concentration; D* = effective diffusion coefficient; x = axis orthogonal to the fracture; t = time from the start of the diffusion process. Dakin et al. (1983) have indicated that it is reasonable to expect that shale with considerable salt content in its matrix would produce a flux of salt into groundwater flowing through fractures, and that an appreciable period of geological time would be required before this flux would deplete the available salt in the matrix. The salt would be depleted most rapidly in the shallowest zones where fracture spacing is smallest and groundwater circulation most


219

rapid. Diffusion of geochemical constituents through the shale matrix and upward groundwater flux through fractures may thus have contributed to the concentration of salts at the near-surface geological materials. Besides the sedimentary origin of the salt water, it is possible that a much more deep-seated saline water is finding an outlet through the fractures under artesian conditions. The absence of post-1953 recharge for this water suggests little or no hydrological continuity with young meteoric waters. This observation may be confirmed by future detailed geophysical mapping, deep-borehole drilling and detailed isotopic budget analysis. Such a programme is now being planned.

TECTONIC EFFECTS Orajaka (1972), Falconer (1911), Tattam (1942), Okezie (1966) and Phoenix & Kiser (1966) gave detailed descriptions of the structural geology and tectonics of the salt lake areas. The lakes occur within the area close to the Afikpo anticlinorium. Within the large anticlinorium are found minor asymmetrical anticlines and synclines. Many faults were also described. The fault in Okposi lake strikes N55 W dipping southwest and displaces the sandstone and siltstone within the lake (Orajaka, 1972). The fault plane is fissured. The sandstone forms the aquifer which is confined by the siltstone-shale sequence. The hydrogeochemical data (Tables 1 and 2) from both the boreholes and the salt ponds show evidence of tectonic effects in the area. As shown in Figs 6 and 7, there is some linear distribution of the dissolved geochemical constituents along the northeastsouthwest directions. Two zones of high TDS and CI - concentrations are observed, one of them terminating at the salt pond where a fault was recorded. It is assumed that this fault which outcrops at the lake bottom continues below the ground surface for some distance where it is not yet exposed. This is indicated by the linear high salinity contours and the orientation of the lines Oi-Oi and O2-O2 (Figs 6 and 7). It is believed that the movement of saline waters up these unexposed fractures has imparted the salinity to the overlying groundwater. The implication of this observation is that, with a detailed hydrogeochemical map and drilling defining the salinity-concentration distributions, it is possible to establish subsurface fractures that have not been exposed as have been done for the Okposi-Uburu salt lake areas. This may be combined with geological and geophysical data in an integrated manner to define in great detail the tectonics, origin and distributions of salt beds and lakes in the Uburu-Okposi areas of Imo State and the Awgu areas of Anambra State,

WATER QUALITY PROBLEM The pollution of surface waters in the Uburu-Okposi areas (ponds, streams, rivers and lakes) by flood water during the rainy season, its inherent salinity, and biological contamination emanating from

220 Boniface C. E. Eguoka & Kalu O. Uma

anthropogenic or human activities have rendered water supplies for domestic consumption poor in quality. Water-borne diseases such as diarrhoea and guineaworm are prevalent in the study area of Imo State and in adjoining areas of the Abakaliki zone in Anambra State. In an attempt to improve the quantity and quality of water supplies to the populace, UNICEF has executed many shallow borehole schemes from which the people have continued to get their domestic water supplies. The present study shows that the shallow groundwater is not of the best or desirable quality with respect to some dissolved geochemical constituents such as chloride and TDS. The contamination is believed to have been imparted by the saline water flowing up from fractures beneath the soil zone and from weathered shale sediments of the area. The groundwater problem is also worsened during the dry season when water levels and the discharge from wells fall due to the intense aridity. Despite the salinity, the groundwater is of better quality when compared with surface water and of immense use to the rural dwellers. Women and children no longer travel for many kilometres with clay pots or plastic containers on their heads, wasting many hours, to fetch water for use. The groundwater is presently supplied without treatment. It is suggested that small-scale reservoirs of concrete tanks could be built near each borehole. The groundwater would be pumped into such systems and treated to reduce the high-concentration dissolved constituents before being supplied to the people. The installation of a reservoir and subsequent treatment would improve the water quality and prevent the threats of contamination by humans as people fetch water and do laundry. Also the people could be educated to store the water in large containers and keep these under the sun for two main reasons: to allow settling of any undissolved materials and to enable the sun's rays to kill off any constituent microbiological pollutants picked up by the shallow groundwater during recharge. Finally, the quality of the water could be enhanced by boiling it before use in situations where it is not treated as is the case now.

REFERENCES Bear, J. (1972) Dynamics of Fluids in Porous Media. American Elsevier, New York. Beltaro, F. & Bojarski, R. (1971) Possiblity of salt industry in Nigeria. Geol. Surv. of Nigeria. (Unpublished (?)). Bouwer, H. (1978) Groundwater Hydrology. McGraw-Hill Inc., Tokyo. Cratchley, C.R. & Jones, G.P. (1965) An interpretation of the geology and gravity anomalies of the Benue Valley, Nigeria. Overseas Geol, Surveys, Geophys. Div., Pap. no.l. Dakin, R.A., Farvolden, R.N., Cherry, J.A. & Fritz, P. (1983) Origin of dissolved solids in groundwater of Mayne Island, British Columbia, Canada. J. Hydrol. 63, 233-270. Egboka, B.C.E. (1981) Distribution patterns of bomb tritium, chloride, sulphate, oxygen-18 and deuterium in two shallow sand aquifers. The Geochem. J. Japan 15, 305-314. Egboka, B.C.E. (1984) The hydrogeological provinces of Nigeria. Nigerian Field J. (accepted for publication).


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Egboka, B.C.E., Cherry, J.A., Farvolden, R.N. & Frind, E.O. (1983) Migration of contaminants in groundwater at a landfill: a case study. 3. Tritium as an indicator of dispersion and recharge. In: Migration of Contaminants in Groundwater at a Landfill.A Case Study (guest editor J.A.Cherry). J. Hydro!.. 63, 51-80. Falconer, J.D. (1911) The geology and geography of northern Nigeria. Geol. Surv. of Nigeria (unpublished report). Freeze, R.A. & Cherry, J.A. (1979) Groundwater. Prentice-Hall, Inc., Englewoods Cliffs, New Jersey, USA. Harleman, D.R.E., Mehlhorn, P.F. & Rumer, R.R. (1963) Dispersionpermeability correlation in porous media. J. Hydraul. Div. ASCE HY2, 89, 67-85. Hazen, A. (1983) Some physical properties of sands and gravels. Mass. State Board of Health, 24th Annual Report. Loehnert , E.P. (1984) Hydrochemical and Isotope Data on Ikogosi Warm Spring, Southwestern Nigeria. Theophrastus Publ. S.A., Athens, Greece (in press). Offodile, M.E. (1976) Hydrogeochemical interpretation of the middle Benue and Abakaliki brine fields, J. «in. Geol. 13(2), 79. Okezie, C.N. (1966) Proposed investigation of Ameri brine, Abakaliki Division, Eastern Nigeria. Conf. Memo. Geol. Surv. Nigeria. Orajaka, S. (1972) Salt water resources of east Central State of Nigeria. J. Min. Geol. 7(2). 35-41. Phoenix, D.A. & Kiser, R.T. (1966) Salt springs, Lafia Division, Benue Province, eastern Nigeria. Geol. Surv. Nigeria Report no.1436. Reyment, R.A. (1965) Aspects of the Geology of Nigeria. Ibadan University Press, Ibadan, Nigeria. Sudicky, E.A., Cherry, J.A. & Frind, E.O. (1983) Migration of contaminants in groundwater in a landfill: a case study. 4. A natural gradient test.

In:

Migration

of

Contaminants

in

Groundwater at a Landfill : A Case Study (guest editor J.A.Cherry. J. Hydrol. 63, 81-108. Tattam, C M . (1942) Preliminary report upon the salt industries: eastern Nigeria. Geol. Surv. Nigeria Rep no.778. Received

19 November 1984;

accepted

14 November

1985.