1. Introduction

The Niger Delta (Figure 1) is a large and ecologically sensitive region in which various water species (including surface and groundwater, saline and freshwaters) are in dynamic equilibrium ^[1]. It is the second largest delta in the world with a coastline spanning about 450km terminating at the Imo River entrance. The region spans over 20,000km² and it has been described as the largest wetland in Africa and consists mainly of freshwater swamps, mangrove swamps, beaches, bars and estuaries ^[4].

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Figure 1. Location Map of the Study Area

Because of the very nature of the region, groundwater constitutes the predominant, if not the only source of water supply in the area and unless a determined effort is made to understand the nature of the groundwater in the region, serious problems would be encountered in the area of water needs of the region, in future. Although, detailed stratigraphic analysis of the various geologic/geomorphic and aquifer systems ^{[6, 7, 18]} have yielded a better understanding of the groundwater potentials of the Niger Delta region, they have not addressed the important and crucial question of over-abstraction and its associated consequences, particularly the possibility of large scale saltwater intrusion and general issue of sustainable supply of potable water in the region.

Chlorine is the most abundant of the halogens. Its compounds with common metallic elements, alkali metals and alkali earth metals are readily soluble in water. Therefore, chloride is present in all natural waters but its concentration is lower than that of sulphates and bicarbonates. The most common type of water in which chlorine is the dominant anion is one in which sodium is the predominant cation. Chloride produces salty taste in water which varies with quantity of sodium ions (Na⁺). Predominance of Na⁺ imparts highly noticeable salty taste even in small quantities of chlorine unlike when Ca²⁺ and Mg²⁺ predominate. Determination of chloride content is very important especially in coastal aquifers because the first sign of saline water intrusion is a progressive upward trend in chloride concentration in the water obtained from the affected boreholes.

2. Geomorphologic/Geologic Setting

The geomorphology of the Niger Delta has been described by many researchers ^{[19, 23]}. The topography of the area is essentially flat, sloping very gently seawards. The area is low lying (usually does not exceed 20m above sea-level) and is drained and criss-crossed by network of distributaries. The Niger Delta constitutes an extensive plain exposed to periodical inundation by flooding when the rivers and creeks overflow their banks ^[18]. A prominent feature of the rivers and creeks is the occurrence of natural levees on both banks, behind which occur vast areas of backswamps and lagoons/lakes where surface flow is negligible ^{[12, 13, 15]}.

Although various types of morphological units and depositional environments have been recognized in the area (coastal flats, ancient/modern sea, river and lagoonal beaches, sand bars, flood plains, seasonally flooded depressions, swamps, ancient creeks and river channels), the area can be sub-divided into five major geomorphological units namely:

a. Active/abandoned coastal beaches

b. Saltwater, mangrove swamps

c. Freshwater swamps, back-swamps, deltaic plain alluvium and meander belt

d. Dry deltaic plain with abundant freshwater swamps (Sombreiro-Warri deltaic plain) and

e. Dry flat land and plain.

Several authors have discussed the geology of the Niger Delta [2,3,5,7,10,11,19,21,23,25]. The influence of geology on the groundwater resources of the Niger Delta constitutes the most important factor besides that of climate in the region ^[9]. Geology has been observed to be responsible for the complex groundwater distribution, extractability and quality in the Niger Delta ^{[20, 22]}. Unfortunately, the present knowledge of the true geological condition prevailing within the groundwater domain, especially with quality status in the various aquifer zones of the Niger Delta is limited.

The geologic sequence of the Niger Delta consists of three main Tertiary subsurface lithostratigraphic units ^[21] which are overlain by various types of Quaternary deposits. From bottom to top, the Tertiary units are the Akata, the Agbada and the Benin Formations (Table 1).

Table 1. Geological units of the Niger Delta (after Short and Stauble, 1967)

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3. Methods of Study

In this study, available data and literature were gathered and categorized on the basis of the different geomorphologic zones in the Niger Delta area of Nigeria. In categorizing these, only Chloride data with depth range were utilized. The data were also compared with the World Health Organization ^[26] standards to assess their concentrations in the various geomorphic zones in the study area. Data were sourced from the Niger Delta Basin Development Authority (NDBDA), Rivers State Ministry of Water Resources (RMWR), Rivers State Water Board (RSWB) and Rural Water Supply and Sanitation Agency (RUWSSA) and from some borehole companies operating in the area. Table 2 shows the summary of chloride concentrations and locations in the different geomorphic zones of the Niger Delta.

4. Results and Discussion

Chloride values were reported in all the samples analysed. Their values range from 3mg/l – 810mg/l. WHO ^[27] specified 250mg/l as the upper limit of harmless tolerance. 68% of the samples have chloride concentration less than 48mg/l, 24% produced water with chloride concentration between 48mg/l and 250 mg/l while 8% produced water with chloride concentration greater than 250mg/l. The highest concentration of 801mg/l was obtained from a borehole at Oloma. Table 2 shows the Chloride data while Figure 2 the relationship of Chloride with depth while Figure 3 shows a chloride distribution map of the study area. From the map, the chloride content in groundwater appears generally to decrease from north to south i.e. from the hinterland to the coast. The localized chloride enrichment around the Old GRA, Port Harcourt, Borokiri, Moscow road and Central Police Station in Port Harcourt could be as a result of brackish water invasion from nearby creeks and tidal channels. Chloride again is a major quality issue especially in groundwater in these areas as well as in the mangrove swamps and the coastal beaches and ridges ^{[16, 17]}. Figure 3 shows the relationship of Chloride with depth in the area.

Table 2. Chloride data from the study area

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Figure 2. Relationships of Chloride and Depth from the Study Area

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Figure 3. Chloride distribution in groundwater samples in the study area

5. Conclusion

From the above discussions, three parameters form the main water quality issues in the study area. They are: pH, iron and chloride contents. The problem of low pH and acidic water are more pronounced in the eastern parts of the area covering Ahoada, Isiokpo and Port Harcourt, which corresponds to the results of Ngah ^[14]. Iron is a more common problem in groundwater samples from the freshwater swamps/backswamp/meander belt region as well as the mangrove swamps and coastal ridges. Chloride concentration is high particularly in the coastal areas and parts of Port Harcourt city, bordering saltwater creeks and tidal channels which have influent relationship with the local groundwater table. The chloride originates from the nearby salt/brackish water bodies and from rain water whose composition in the coast resembles diluted sea water.

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