THE USE OF ANION GEOCHEMISTRY IN MAPPING GROUNDWATER FACIES IN THE PORT HARCOURT AREA OF THE NIGER DELTA , NIGERIA

The groundwater system of the Port Harcourt area is home to three anion facies, the Cl – SO4, the Cl – SO4 – HCO3 and the HCO3 – Cl – SO4. The first two types exist in both shallow – and deep – groundwater environments while the third is restricted to the deep environment. Although there are natural intermittent and, in some cases, continuous interactions between shallow and deep groundwaters, this paper establishes that the quality and facies types of the groundwaters are not exactly alike. 28 groundwater samples (12 from hand-dug wells; 16 from boreholes) were analysed for Cl, SO4, HCO3 and CO3. The concentration levels of SO4 2were analysed using the HACH Spectrophotometer equipment, model No DR/2000 while those of Cl , CO3 2and HCO3 were by titrimetric method, using the Titro Process Dosimat 665 equipment. The results indicate that, for the hand-dug wells, SO4 2levels ranged from 0.00mg/l to 67.0mg/l with an average of 16.5mg/l; HCO3 from 12.2 to 36.6 with an average of 15.25, while Cl ranged from 11.332 to 121.905 with an average of 41.22mg/l. No CO3 2was detected in all the samples. For the borehole samples, the respective figures for SO4 2, HCO3 and Cl are 0.00 to 33.0 with an average of 2.13; 12.2 to 24.4 with an average of 12.96, and 5.490 to 72.306 with an average of 17.037 mg/l. Again, no CO3 2was detected. The total absence of CO3 2conforms with the relatively high acidity of the groundwater environment of the Port Harcourt Area; and the diminutive level of SO4 2, is associated with the effect of the bacteria catalysed reduction of the ion. This absence of some ions and the low levels in others do affect the number and type of mappable facies in groundwater systems. With the results obtained, the paper notes that: (1) The shallow groundwater environment embedded less number of mappable facies than the deep groundwater environment; (2) Facies sequences mapped are tending towards the composition of seawater; and concludes that the existence of different groundwater facies is an evidence that groundwater encounters strata of different mineralogical compositions along its flow path.


INTRODUCTION
Hydrogeological practice in the Port Harcourt area of the Niger Delta has centred mainly on drilling for water supply.Details of water quality are rare except for some scanty qualitative details that appear in Amajor (1991), Etu-Efeotor (1981), Abibo (1988) and Akujieze et al. (2003).There is hitherto no literature on the aspect of groundwater facies for the Port Harcourt region.

Hydrochemical Facies
Facies are identifiable parts of different nature belonging to any genetically related body or system.Hydrochemical facies are distinct zones that have cation and anion concentrations describable within defined composition categories.Domenico (1972) notes that although specific details concerning methodology vary somewhat, the facies are studied in much the same manner as lithofacies in geology, with Piper's (1944) trilinear diagram, or some slight modification, used to identify the various chemical types.The areal distribution of constituents is shown by fence diagrams, maps showing lines of equal concentrations or concentration ratios of two constituents (Back and Hanshaw, 1965).Domenico (1972) specifies that hydrochemical facies can be studied in terms of anions, or cations, or both.These various options of study may be in consideration for the scope of the research involved or of the possible unavailability of relevant equipment or both.For example, Chebotarev (1955) used anion species only and developed his well-known sequence which states that all groundwaters tend to evolve chemically toward the composition of seawater; similarly Toth (1966b) used anion facies development in mapping groundwater discharge and recharge areas in Canada; and Amadi et al.;(1989) used both anion and cation species in mapping the groundwater facies-types housed in a north-south direction of some part of the Niger Delta region.Each option defines its scope but is directed towards achieving the same objective, that is, the delineation of facies -types embedded in groundwater systems.There is no significant advantage of one option over the others.The thrust of this paper is to present a preliminary work carried out specifically to define the facies embedded in the groundwater system of the Port Harcourt area, using only anion species of groundwater samples from both the shallow groundwater -and deep groundwater environments of the region.

Rivers/Creek
LGA Boundary

Rivers/Creek
LGA Boundary

Undeveloped Area
The study area comprises two Local Government Areas, namely, the highly industrialized Port Harcourt Local Government Area (PHALGA) and the rapidly expanding Obio/Akpor Local Government Area (OBALGA), both of which are together designated as the Port Harcourt Metropolis, or Greater Port Harcourt.The Metropolis lies between latitudes 4 0 42΄ and 4 0 57΄N and longitudes 6 0 53΄ and 7 0 08΄ E (Fig. 1) and covers an area of about 313km 2 within the Niger Delta sedimentary basin.Its characteristic area, shown in Fig. 2, lies between the freshwater swamp area that is seasonally flooded and the saltwater swamp area that is tidally flooded.

METHODOLOGY
Considering the objective of this work, groundwater samples were collected with scrupulously clean polyethylene plastic bottles.The samples were unacidified but were filtered through a 0.45µm pore size disposable filter paper, and collected in pre-cleaned plastic bottles.To accommodate both shallow -and deep-groundwater environments, 12 samples were collected from hand-dug wells and 16 from boreholes.The target species were SO 42-, Cl -, CO 3 2-and HCO 3 -.The concentration levels of SO 2 4 were determined using the HACH Spectrophotometer equipment model No. DR/2000 while those of Cl, CO

STAGES IN MAPPING GROUNDWATER FACIES
Four stages were followed in mapping the groundwater facies.

Stage (i):
Results of the chemical analyses in milligrams per litre were converted to values in milliequivalents per litre.

Stage (ii):
The resulting values of (HCO 3 + CO 3 ) and those of (Cl + SO 4 ) were then respectively expressed as percentages of all anions.

Stage (iii):
To define the facies, the resulting percentages were matched with the guidelines proposed by Back (1996), using the anion column in Table 7.

Stage (iv):
The direction of facies change was thereafter determined by fitting the facies types into the anion diamond field of Domenico (1972), (Fig. 3).

RESULTS AND DISCUSSION
The anion values of the sampled wells and boreholes are presented in Tables 1 and 2 respectively; and the results converted to values in milliequivalents per litre are presented in Tables 3 and 4. Values of (HCO 3 + CO 3 ) and those of (Cl + SO 4 as percentages of all anions are presented in Tables 5 and 6.Domenico (1972) notes that hydrochemical facies can be studied in terms of anions, or cations, or both and Back (1966) gives a classification guide shown in Table 7.

Environmental Controls on the Anion Concentration Levels
Tables 1 and 2 show that among the anions, SO 4 2-was detected in only 50% of the 12 well samples and in only 12.5% of the 16 borehole samples.While no CO 3 2-was detected in all 28 samples, HCO 3 -and Cl - were detected in all.The SO 4 2-values in the 50% of well samples ranged from 1mgl/l to 67 mgl/l, with an average of 33 mgl/l; while the values in the other fraction of borehole samples ranged from 1mgl/l to 33 mgl/l with an average of 17mgl/l.The absence of SO 4 2-in some of the samples could be attributed to sulphate -reduction, a process engineered by certain bacteria.Davis and DeWiest (1966) note that some types of sulphatereducing bacteria are found in soil horizons and that groundwater may contain less than 1 ppm of sulphate if sulphate-reducing bacteria are active in the soil through which recharge water percolates.Domenico (1972) notes that sulphate reduction accounts for diminishing quantities of sulphate in groundwater.This process therefore controls the level of occurrence of SO 4 2-in groundwater.For CO 3 2-, its absence in all 28 samples conforms with the relatively high acidity of the groundwater system of the Port Harcourt region.In Table 1, the pH range is from 4.46 to 6.30; in     dissociation of HCO 3 -.According to Davis and DeWriest (1966) the process is only effective largely above a pH of 8.2 below this pH, most of the CO 3 2-add H + to become HCO 3 : H + + CO 3 2-= HCO 3 -.In fact, the dependence of individual CO 2 forms on pH is shown in Table 8.It is therefore not surprising that CO 3 2-is completely undetected in all the samples.The occurrence of HCO 3 -in all samples (Tables 1 and 2) is favoured by the pH conditions of the groundwater system.
In the shallow groundwater environment (the wells), HCO 3 -values range from 12.2mg/l to 36.6 mg/l with an average of 15.25mg/l while the range in the deep water environment (the boreholes) is from 12.2 to 24.4 with an average of 12.96mg/l.
It is known that below pH of 8.2, HCO 3 -forms CO 3 2-by addition of H + .The maximum pH value recorded in this research for the shallow and deep water environments are 6.30 and 6.14 respectively (Tables 1 &  2) and this favours the occurrence of the bicarbonate ion.The relatively high values of Cl -in all samples is tied to the fact that irrespective of its source, Cl -is

THE USE OF ANION GEOCHEMISTRY IN MAPPING GROUNDWATE
conservative.It does not react easily with aquifer materials and tends to be closely associated with water molecules (Mercado, 1985).These qualities preclude Cl -from being quickly removed from solution and enhances its relatively easy occurrence in groundwater.In the shallow water environment (Table 1) the ion ranges from 11.332mg/l to 121.905mg/l with an average of 41.22mg/l.The respective values in the deep water environment (Table 2) are 5.490 to 72.306 with an average of 17.037 mg/l.It is obvious that the absence of some ions and the varied concentration levels of others should affect the types and number of mappable facies in groundwater systems.

Anion Facies in the Groundwaters
The percentage of anions concentration in well samples (Table 5) and in borehole samples (Table 6) when matched with the guidelines in the right hand column of Table 7, resulted in the following groundwater anion facies as presented in Tables 9 and 10 respectively.Both tables show that there is a clear dominance of the Cl -SO 2 4 -HCO 3 facies over others.Over 80 per cent of the wells and 75 percent of the boreholes are dominated by this facies-type.When the facies types are fitted into the anion diamond field of Domenico (1972), as shown in Fig. 3, there is indication of a facies change, for the well water, from Cl -SO 4 -HCO 3 type toward the Cl -SO 4 type, (Fig. 4).In the borehole environment, there is also a facies change from the HCO 3 -Cl-SO 4 type, again towards the Cl -SO 4 type as shown in Fig. 5. Chebotarev (1955), using anion species only, develops his well-known sequence which states that all groundwaters tend to evolve chemically towards the composition of seawater, an evolution which according to him is normally accompanied by the regional changes in dominant anion species: Travel along flow path Cl - Increasing age     (1966b), from investigations in Canada, describes the anion facies development and reports a general tendency for a shift from a pure HCO 3 -facies in recharge areas to a SO 4 2-facies in discharge areas.Within the scope of this work, it is noted that purely HCO 3 --water does not exist in the Port Harcourt region.The facies defined in the present work do indicate that the groundwaters are actually tending towards the composition of seawater in some direction (Figs. 4 and  5).This work also reveals that in general, the shallow groundwaters house less number of mappable facies than the deep groundwaters as is obvious from their respective facies: The evolution of these groundwater facies can be reasonably explained by the order of encounter proposed by Freeze and Cherry (1979).This theory briefly states that the order in which groundwaters encounter strata of different mineralogical composition can exert an important control on the final water chemistry.As groundwater flows through strata of different mineralogical composition, the water composition undergoes adjustments caused by the imposition of new mineralogically controlled thermodynamic constraints.Domenico (1972) observes that the type of facies that develops is controlled largely by the mineralogy of the rocks and its distribution is controlled by the flow pattern.

CONCLUSIONS
In this work, the use of anion geochemistry has aided in mapping the groundwater facies embedded in the Port Harcourt region of the Niger Delta, Nigeria.Both shallow and deep groundwater environments were accommodated and that informed the use of hand-dug wells and boreholes within the same geographical environment.28 water samples (12 from wells and 16 from boreholes) were analysed for the relevant anions Cl -, SO 4 2-HCO 3 -and CO 3 2-using standard equipment.The concentration levels of each ion have been discussed and related to the environmental controls on their availability.The result of the analyses show that the groundwater system of the Port Harcourt region is home to three anion facies: Cl -SO 4 , the Cl -SO 4 -HCO 3 and the HCO 3 -Cl -SO 4 .Over 80% of the wells and over 75% of the boreholes, house the Cl -SO 4 -HCO 3 facies.The paper finally draws three conclusions that: (1) The shallow groundwater environment embedded less number of mappable facies than the deep groundwater environment.
(2) the existence of different groundwater facies is an evidence that groundwater encounters strata of different mineralogical compositions along its flow path and that (3) The facies sequences mapped are tending towards the composition of seawater.
However, the mapped facies sequences and the tendency towards the seawater composition must be viewed, like many others in the geological sciences, in terms of scale and with the normal provisions for interruption and incompletion.

ACKNOWLEDGEMENT
Our primary acknowledgements are to the imaginative and creative scientists of many nations, who, over the past several decades, have shaped and reshaped the ideas of hydrochemical facies as we now know them.We are grateful to the Shell Petroleum Development Company (Nig.)Ltd, for the sponsorship of the analysis of our water samples.

FIGURE CAPTIONS
Location map showing positions of wells and boreholes Figure 2 Characteristic areas Figure 3 Nomenclature for hydrochemical facies Figure 4 Anion facies in hand-dug wells ABI-BEZAM Hydrogeology Section, Department of Geology University of Port Harcourt, P.M.B. 5323, Port harcourt, Nigeria C.E. EGBOKA, BONIFACE Department of Geological Sciences Nnamdi Azikiwe University P.M.B. 5025 Awka,

Fig 1 :
Fig 1: Location Map Showing Positions of Wells and Boreholes

Fig
Fig. 4 Anion Facies in shallow groundwaterFig.5AnionFacies in deep groundwater environmentand Toth (1966b), from investigations in Canada, describes the anion facies development and reports a general tendency for a shift from a pure HCO 3 -facies in recharge areas to a SO 4 2-facies in discharge areas.Within the scope of this work, it is noted that purely HCO 3 --water does not exist in the Port Harcourt region.The facies defined in the present work do indicate that the groundwaters are actually tending towards the composition of seawater in some direction(Figs.4 and  5).This work also reveals that in general, the shallow groundwaters house less number of mappable facies than the deep groundwaters as is obvious from their respective facies: Figure 5Anion facies in boreholes 166

Table 2
P. AMADI ABI-BEZAM and C.E. EGBOKA BONIFACE THE USE OF ANION GEOCHEMISTRY IN MAPPING