The impact of sea water intrusion on the spatial variability of the physical and chemical properties of ground water in Limbe-Cameroon

A significant proportion of the population of Limbe, a coastal town in Cameroon, relies on ground water (gw) to satisfy their drinking water needs. The coastal aquifer is threatened by pollution from sea water intrusion and hence the aim of this study was to determine the suitability of the water. Physicochemical analysis were carried out on samples collected from the study area and the Revelle Index was calculated and used to determine the extent of pollution from seawater intrusion. The water quality index (WQI) was calculated using values of the pH, bicarbonate, chloride, total dissolved solids and the electrical conductivity. The results showed that 23% of the groundwater in the study area is slightly polluted by seawater intrusion. Using the WQI, it was found that about 34.4% of the ground water resources in the study area is currently good for drinking while 65.6% is either poor, very poor or unsuitable for drinking. In order to reduce the intrusion of sea water into the aquifer, a halophyte plant like mangrove should be planted at the mouth of the main river in Limbe.


INTRODUCTION
In the last few decades, there has been a tremendous increase in the demand for fresh water in the world due to the rapid growth of the population and the accelerated pace of industrialization (Rejith et al., 2009). Limbe, a coastal town in Cameroon on the Atlantic Ocean has a demand for domestic water supply that the water utility company cannot satisfy and hence a significant proportion of the population relies on ground water (gw) to meet their domestic water demands.
Under natural conditions, coastal aquifers are recharged by rainfall, and the gwflows towards the ocean preventing saltwater from encroaching into the freshwater. The global mean sea level (GMSL) increased by an average rate of 1.8 mm/year during the 20 th century *Corresponding author. E-mail: motchemien@yahoo.com. Tel: +237 677579410.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License (Douglas, 1997) and the IPCC reported with a high confidence that this rate has been increasing (Bates et al., 2008). The IPCC (2007) estimated that the GMSL increased by 3.1 mm/year from 1993 to 2003, but this change is not spatially uniform worldwide. Nicholls and Cazenave (2010) estimated a GMSL rise of approximately 3.3 mm/year for the period 1992 to 2010. Motchemien (2015) estimated on the basis of data obtained from the tide gauge installed in Limbe, a rise in the mean sea level of the Atlantic Ocean in the lower part of the Gulf of Guinea of about 10 mm / year. One effect of such an increase is sea water intrusion into coastal aquifers (Werner and Simmons, 2009). Salt water intrusion is a serious problem because about 80% of the world's population lives along the coast and utilize coastal aquifers for their water supply. In addition, over exploitation of coastal aquifers has resulted in falling groundwater levels (gwl). Sea level rise and falling water gwl have resulted in increased pollution of gw due to sea water intrusion resulting in wells previously used to for domestic water supply being abandoned. For example, in New Jersey, more than 120 wells were abandoned because of saltwater contamination (Lacombe and Carleton, 1992).In the study area, about 250 wells has been abandoned because the water has become salty (Motchemien, 2015).
Variations in the sea level and the associated wedge movement can influence the near-shore and/or largescale submarine discharge patterns and impact nutrient loading levels across the aquifer-ocean interface (Li and Jiao, 2003). While anthropogenic activities, such as over pumping and felling of trees in urbanized coastal areas, are the major causes of salt water intrusion, it is projected that increases in the sea level due to climate change would aggravate the problem (Li and Jiao, 2003). Feseker (2007) modelled the impacts of climate change and changes in land use patterns on the salt distribution in a coastal aquifer and concluded that rising sea level could induce rapid progression of salt water intrusion.
Excessive groundwater withdrawals have been reported to result in hydro-chemical changes in the physical, chemical and microbiological water quality; drop in the water table level; reverse hydraulic gradient and consequently water quality deterioration in coastal areas (Esteller et al., 2012). Poor water quality results in incidences of waterborne diseases and consequently reduces the life expectancy of the population (WHO, 2006). Thus, concern for clean and safe drinking water and protection from contamination is justified because a large proportion of the population in the study area depends on ground water for domestic purposes.
Water quality evaluation is based on the physical, chemical and biological parameters ascertaining the suitability for various uses such as domestic consumption, agricultural, recreational and industrial use (Sargaonkar and Deshpande, 2003). The traditional assessment of water quality consists of comparing the point values of water quality parameters levels with their guideline or standard values based on allocated water use or uses. This type of assessment does not provide an overall assessment of water quality of a water body which is important for managers and decision-makers. To resolve this decision-making problem, several water quality indices have been developed to transform point value water quality parameters into integrated indicator values. Many studies have demonstrated the usefulness of assessing the water quality of gw using a water quality index (WQI). Examples are presented by: Shah et al (2008) Rao and Nageswararao (2013), and Sahu and Sikdar (2008).
The aim of this study was to assess the suitability of the gw in Limbe for domestic use. The specific objectives were to: Determine the spatial variation of the physical and chemical properties of the gw; evaluate the extent of sea water intrusion into the aquifer; determine the proportion of the gw suitable for drinking and finally a propose solution to reduce the impact of sea water intrusion on the gw quality.

Location
Limbe is located along the coastal area of Fako Division, South-West Region of Cameroon ( Figure 1). The study area is located approximately between latitudes 3 o 90´ and 4 o 05 N and longitudes 9 o 29´ and 9 o 06´ E. It is bounded in the East by Bimbia, in the North by Bonadikombo, in the South by the Atlantic Ocean and in the West by Mukundange. The population of Limbe is about 130,000 inhabitants spread over a surface area of 596 km 2 (INS, 2013).
The town is characterized by a low-lying coastal plain, rising up to a chain of horseshoe shaped hills towards the northeast and east, with the highest point at 362 m above sea level (Njabe and Fobang, 2006). Within the town, small streams flow into larger drainage channels that converge into the main river (Njenguele) that empties into the Atlantic Ocean ( Figure 2). These rivers frequently overflow their banks in the rainy season causing floods in the low-lying areas that are only 1 to 2 m above sea level (Ndille and Belle, 2014). The hills that surround the town are made up of loose ferralitic and volcanic soils that easily disintegrate when the water content is high (Nwankiti, 1983).

Climate
The climate of Limbe is the sub-equatorial type with two distinct seasons: a dry season of two months from December to January and a rainy season of 10 months that runs from February to November with a mean annual rainfall of 3,100 mm, ±1,100 standard deviation (Suh et al., 2003). The monthly rainfall frequently exceeds 500 mm and sometimes is over 1,000 mm in June, July and August. The mean annual temperature is about 26°C while the relative humidity is generally above 85% (Fombe and Molombe, 2015).

Geomorphology and hydrogeology
Geomorphologically, the study area is made up of ridges and deeply incised ravines with a W-E orientation at a high angle to the general NE-SW orientation of Mount Cameroon (Suh et al., 2003). The elevations in the study area range from 0 to about 90 m above sea level with slopes ranging from 0 to 43° ( Figure 3). The slopes on the foot of Mount Cameroon are composed of multiple porphyritic basaltic lava flows, punctuated by several strombolian pyroclastic cones to the W and NW and lahar deposit to the east of the study area (Diko et al., 2012). These ridges form part of the Limbe-Mabeta Volcanic Massif, made up of degraded and deeply weathered tertiary basaltic lava flows (Suh et al., 2003). The main rock types within this area include basalts (Figure 4), basanites, lahar deposits, and pyroclastic materials (Buh, 2009).
The hydrogeology is characterized by unfossiliferous sandstone and gravel, weathered from underlying Precambrian basement rock (Longe, 2011). It consists of coastal plain sands (CPS) and recent sediments. The CPS aquifer is the most productive and exploited aquifer in Limbe.

Data
Water samples were collected monthly between July 2017 and June 2018 from thirty hand dug wells located along three transects as shown on Figure 1. The sample size was determined using the formulae presented in Equations 1 and 2 (Howell, 1999): (1) Where: m = the sample size. n = the correction sample size for a limited population. N = the population. z= the value related with confidence level (1.96 for 95% confidence level). p= the degree of variance between the elements of population (0.5). ε= the maximum error (0.07).
The electrical conductivity (EC), the pH and the total dissolved solids (TDS) were measured in situusing a portable hand held, pH-028 six in one monitor. Water samplesfor laboratory analysis were collected in clean 150 ml polyethylene bottles and preserved in ice chests for analyses of chloride and bicarbonate using standard methods (APHA, 1998).
The co-ordinates of the sampled wells were recorded using a Global Positioning System (GPS) and thereafter were plotted using ArcGIS software on the geomorphological map of Limbe. To get a comprehensive picture of the overall quality of groundwater, the WQI was used. WQI is defined as a rating reflecting the composite influence of different water quality parameters on the overall quality of water. Revelle (1941) proposed the following equation for calculating the Revelle Index (RI) which is used to assess groundwater pollution from seawater:  According to Revelle (1941) when RI < 0.5, this indicates there is no sea water intrusion; when RI is between 0.5 and 6.6, this indicates the gw is slightly affected and when its greater than 6.6 it indicates its strongly affected.
The drinking water quality was assessed using a water quality index (WQI) and the World Health Organization (2006) standard. The stages of calculating the WQI are as follows: Where:qn= quality rating for the n th water quality parameter; n= the water quality parameter and quality rating or sub index (qn) corresponding to n th parameter, that is, a number reflecting the relative value of this parameter with respect to its standard (maximum permissible value); Vn= estimated value of the n th parameter at a given the sampling point; Sn= standard permissible value of the n th parameter; Vio = ideal value of n th parameter in pure water, that is, 0 for all other parameters except pH and dissolvedoxygen (7.0 and 14.6 mg/l respectively).
The unit weight of the n th parameter (Wn) was calculated by a value inversely proportional to the recommended standard value (Sn) of the corresponding parameter.
The WQI wasthencalculated using Equation 6: WQI = (6) Table 1 shows the classification of water based on the WQI from the point of potability.The coordinates of each sample were determined using a Garmin GPSmap 78S. The values of the RevelleIndex and the Water Quality Index were then exported to the ArcGIS 6.0 software and used to generate maps of their spatial distribution over the study area. The areas referring to each water quality class and the Revelle Index were circumscribed on the map using ArcGIS6.0. They were automatically generated with this software surface function. Finally, slicing options were applied using these ranges of values with five groups of water quality classes to generate a spatial distribution of water quality map (Chatterji and Raziuddin, 2002). The statistical analysis of the examined groundwater parameters was computed using STATA software version 6.0.  The pH of the gw varied from 6.2 to 7.49 with a mean value of 6.94. The EC values varied from 120.00 to 808.33 Table 3. Spatial variation of the mean values of some physico-chemical parameters of gw along three transects. µS/cm with a mean value of 405.10 μS/cm which is higher than the norm indicating a high amount of dissolved salts in the water. The values of EC in all wells in transect 1 exceeded the minimum recommended value. The total dissolved solids ranged from 82.50 to 566.67 ppm with a mean value of 280.11 mg/l which is about 50% lower than the norm. Most of the values of the TDS were within the recommended range for drinking water except for values obtained from wells P 2 , P 4 and P 5 which exceeded the recommended value. The temperature varied between 26.57and 28.09 0 C with a mean value of 27.51. The bicarbonate level varied between 75.00 and 196.83 mg/l with mean value of 121.71mg/l. Chloride values were found to vary from 22.43 to 140.4 mg/l with mean value of 57.94. These values of chloride are within the recommended standard level. A high level of chloride in freshwater is an indicator of pollution (Chandra et al., 2012). The Secondary Maximum Contaminant Limit (SMCL) for chloride is 250 mg/l. This is the level above which the taste of the water may become objectionable to the consumer. In addition, high chloride concentration levels in the water contribute to the deterioration of domestic plumbing materials, water heaters and equipment in municipal water works.The levels of TDS, EC, and Cl decreased significantly from transect 1 to 3 as shown in Table 3 while the level of bicarbonate ions increased significantly. This indicated that saline intrusion was more pronounced in sites near the ocean.

Extent of sea water intrusion in the coastal aquifer
In the study area, RI varied from 0.126 to1.551 as shown in Table 4. According to Revelle (1941), this suggests that some areas in the study area have not been affected (RI less than 0.5) while others have been slightly affected (RI is between 0.5 and 6.6). The relationship between the ratios of indicates a strong positive linear relation with Cl concentrations (r = 0.94, p < 0.01).This linear relationship indicates the mixing of saline water and fresh groundwater (Zaharin et al., 2009). Figure 5 shows the extent of the groundwater salinization in the study area. From Table 5, about 77% of the groundwater in the study area was unaffected by sea water intrusion, while 23% of the aquifer was slightly affected by pollution from sea water. The hotspots include locations of wells P 1 , P 2 , P 3 , P 4 and P 5 . Thus, efforts should be made to reduce the pollution of ground water due to sea level rise in the area.

Extent of the gw suitable for drinking
The suitability of groundwater for drinking purposes in the study area was determined using WHO (2006) guidelines. The computed Water Quality Index (WQI) ranged from 28 to 115 as indicated in Table 6. The spatial distribution of water quality in the aquifer is presented in Figure 6. The EC, pH, TDS, Cl and HCO -3 all contributed to the WQI values. However, values of chloride and electrical conductivity were the main parameters responsible for the high values of WQI. In some locations, the TDS also significantly increased the WQI.
The area covered by different water quality classes were calculated from the WQI maps and are presented in Table 7. About 34.4% of the gw is currently considered to be good for drinking while 65.6% is either poor, very poor or unsuitable for drinking.
Hot spots that require attention are wells P 2 , P 3 , P 4 and P 5, all along transect 1, which is closest to the sea.

Solution to reduce sea water intrusion
About 23% of the aquifer of the study area is affected by sea water intrusion rendering the water non-potable. In order to reduce the intrusion of ocean water, there are two possibilities; construction of dykes or barriers and planting a halophyte plant. Anti-salt barriers or dykes can be used to combat saline intrusion (Barry, 1988). However, they are limited in that they are effective only in controlling surface sea water flow but, they do not prevent the intrusion of underground sea water (Diawara, 1988).
A more effective solution is the planting of halophyte plants like mangrove trees of the Rhizophoraceae family. They should be planted on both sides of the mouth of the River Njenguele over a distance of 500 m. Mangrove trees stabilize the coastline and serve as a barrier against erosion due to swell by reducing the energy of the waves  (Furukawa et al., 1997). Mangrove trees have a root system that filters water, and can thus exclude up to 90% of salts from interstitial waters thanks to glands located in their leaves (Tymbery et al., 2019). Mangrove trees planted a short distance offshore, control high energy waves which erode the coastline and prevent the meeting of coastal marine waters and fresh waters. In addition, the complex geochemical reactions taking place in the sediment around the trees can immobilize toxic metals and consequently purify coastal waters. Furthermore, mangrove trees serve as a natural barrier against the gradual rise in mean sea level and saline intrusion into aquifers and arable land. They also have a significant refuge value and hence they enhance aquatic biodiversity.

Conclusion
Based on the methodology used in this study and the results obtained, it can be concluded that: (1) About 23% of the ground water resources in the study area is presently slightly polluted by seawater intrusion.
(3) About 34.4% of the ground water resources in the study area is currently considered to be good for drinking while 65.6 % is either poor, very poor or unsuitable for drinking.
(4) To minimize the impact of sea water intrusion on the quality of gw in the study area a halophyte plant like mangrove should be planted at the mouth of River Njenguele, which is the main natural drain in the study area.

DISCLOSURE STATEMENT
No potential conflict of interest was reported by the authors.