The Bétaré-Oya gold district is located in the Sanaga basin between latitudes 5˚30 '07'' and 5˚46'01'' North, and longitudes 14˚04'04'' and 14˚28'06'' East (Figure 1). The study area varies from 665 m to 1050 m altitude. The climate is a typical equatorial transition type, characterized by two seasons consisting of a long rainy season from March to November, a short dry season from December to February. The hydrologic regime of the river Lom in Bétaré-Oya is controlled by rainfall, the lowest monthly flow rate is observed in February (56 m³/s), while the maximum flow rate is observed in October (328 m³/s). In the 2015-2016 hydrologic year, the mean annual flow rate was 150 m³/s, corresponding to a specific discharge of 13.50 l/s/km⁻². This corresponds to runoff which flows at 397.55 mm for a flow coefficient of 28%; with an average annual rainfall of 1440 mm. The average inter-annual temperature is 24.58˚C. The hottest month is March with an average temperature of 26.00˚C and the coldest month is in July (23.50˚C).

Bétaré-Oya belongs to the Panafrican belt of Cameroon, limited to the North by the Adamawa shear zone and to the South, by the Sanaga shear zone [15] [16] .

The geological formations are dominated by meta-sedimentary rocks, metavolcanics and intrusive granites known as the “Lom series” [17] . They consist of schists, micaschists, orthogneiss of Wakaso and Ndokayo, quartzites, cross-cut by Pan-African granitoids and conglomerates of Mari, post-tectonic granite of Ngibi and Kongolo (Figure 2) [18] [19] . These lithological units have a NE-SW steeply dipping foliation reflecting a dextral and senestral shear direction that is related to the central shear zone of Cameroon [19] . The Lom series is also well known for its gold mineralization as well as other minerals associated with granitic intrusions such as Pb, Bi and Mo [20] [21] . Two types of gold mineralization are known in Bétaré-Oya [16] : (1) mineralization in quartz veins associated with pyrite and (2) alluvial and eluvial deposits, which have been intensively exploited in the area. The latter are located in the flats of the River Lom and its tributaries such as Mari, Nakoyo, Mbal. The geological formations of Bétaré-Oya are overlain mainly by thick ferrallitic soils with brown or red color. Hydromorphic soils occur in marshy areas as well as alluvial flats and the dark color on the upper part of the profile is indicative of the abundance of organic matter [22] .

The river Lom and some of its tributaries (Mari, Mbal, Nakoyo) drain the vast majority of the Bétaré-Oya mining sites. In the present study, sampling was carried out in the Lom, Mari and some of its tributaries which drains a representative area of all the mining sites in the locality. Sixteen sampling points were identified in Lom River and the Mari River and its tributaries. The points include:

three on the Lom River, five points on the main Mari River, three and five points respectively on the left and right bank of Mari (Table 1). These points were identified using Global Positioning System (GPS) materialized on a SRTM satellite image with 30 m of resolution (Figure 3). All these points were selected based on criteria such as accessibility and their proximity or not in the vicinity of the mining sites. All of the sixteen sample points were used for physicochemical analysis (EC, turbidity, TSS, Na⁺, , K⁺, Mg²⁺, Ca²⁺, , Cl⁻, F⁻, , ,) and four of them were selected for the control of heavy metals and cyanide. This includes two points of the Mari River and it right bank (MMR₃, MMR₅) and two points from the Lom River (LOM₂, LOM₃). These points are either located close to the mining sites or around the area where washing and panning is carried out. A total of 71 samples were collected from April 2015 to February 2016, taking into account the different seasons of the hydrological year of the study area.

Samples were collected manually once time at <1 m depth at the centre of the river preferentially where the flow velocity was high enough to allow a good homogenization of the solid particles and dissolved materials [5] . The samples were collected in 1.5 L polyethylene bottles washed with ultrapure acid, previously rinsed with distilled and MilliQ deionized waters, and finally rinsed three

times with sampling water according to standard procedures [23] [24] .

Some in-situ parameters such as pH, electrical conductivity (EC) were measured on unfiltered water using calibrated WTW 315i instruments. The samples for trace elements analysis were filtered on-site through filters on acetate cellulose 0.45 μm into 250 ml polyethylene bottle and immediately acidified to pH < 2 with ultrapure nitric acid. All samples were stored in an ice-chest at temperature less than 4˚C until they were transported to the laboratories in Yaoundé (Cameroon) and analyzed within ten days. Chemical analyses of water samples were performed in the Laboratory of Geochemical and Water Analysis (LAGE) at Nkolbisson (Cameroon). The major cations (Na⁺, , K⁺, Mg²⁺, Ca²⁺) was performedby Dionex ICS-90 ion chromatography and major anions (Cl⁻, F⁻, , ,) by Dionex ICS-1100 ion chromatography. The turbidity was measured using the turbid meter 966 Orbeco Hellige model. Alkalinity was measured by automatic titration 862 Compact Metrohm model. Total suspended solids (TSS) were obtained by filtration and weighing. The ultrapure water (MQ) with the resistivity of 18.2 MΩ・cm was used for all analyses also used as blank. Reagent and procedural blanks were determined in parallel to the sample treatments using identical procedures. Each calibration curve was evaluated by analyses of the quality control standards before, during and after the analyses of a set of samples. The chemical results were only accepted when the charge balance error was within ±5%. The essential trace metals (Cr, Cd, Zn, Fe, Cu, Pb, As, Mn) and CN⁻, were determined using MACHEREY-NAGEL photometric method at the ‘‘Cabinet d’étude SOS environement’’ in Soa, Cameroon. Arsenic was determined by Quantofix method, Pb and Cd by Nanocolor and Cu, Zn, Cr, Mn, Fe and CN⁻ by Visocolor ECO.

Data obtained in this study was processed using standard method of hydrochemistry (Diagram software) and the calculation of Water Quality Index (WQI) and Heavy Metal Pollution Index (HPI). The parameters such as SAR and percent sodium (Na%) were also calculated to evaluate the suitability of the water quality for agricultural purposes.

The summary statistics of the physicochemical parameters in the study area are given in Table 4. The trends of seasonal variation are represented by Box plots in Figure 4. The pH values of Bétaré-Oya varied from 5.77 to 8.84 in the dry season and 5.42 to 7.58 during the wet seasons. These results show that water samples from rivers have acidic to basic properties. In general, high pH values were observed at the sampling sites LOM₁, LOM₃ and MLB₅. These basic pH values could be linked to the high gold mining activities which is carried out in the upstream portion, which experiences oil spills or leakages from excavation machinery and transportation vehicles [36] [37] . Additionally, samples taken in the dry period have high pH values compared to the wet period (Figure 4(a)). This difference is due to a high concentration of chemical products used during exploitation which are often diffused during the rainy season. The basic to acidic pH in Bétaré-Oya are generally higher than those observed in the other forest areas of South Cameroon [38] [39] . They are of the same order like those in gold

Min.: minimum, Max: maximum, Std: standard deviation, Med: median.

mining basins of Lower Pra in Ghana and in the South eastern part of Senegal [36] [37] .

The EC values varied within a range of 12.50 - 62.90 μS/cm in dry period and 11.60 - 122.10 μS/cm in wet period. This type of water is very weakly to weakly mineralized [40] like most of the water in the forest part of South Cameroon flowing on the plutono-metamorphic basement [41] [42] . The maximum EC values were determined at MLB₄, MMR₄, MLB₅ sites, during the wet period (Figure 4(b)), and appear to depend on increasing ion contents related to pollutants from extensive mining activity in the study area. It is also important to note that EC varies from one point to another. This spatial variation is due to the importance and the intensity of mining activities around each sampling point.

The turbidity and the TSS of the studied water varied respectively from 1.22 to 276.00 NTU and from 6.60 to 4886.00 mg/L, in the dry period. During the wet season, these elements varied between 2.30 to 4758.00 NTU and 2.00 to 8996.00 mg/L. The variation of TSS is sensibly proportional to that of turbidity [43] . The high turbidity and TSS mainly observed around mining sites, such as MMR₄, MMR₅ and MLB₄ especially in wet period (Figure 4(c) and Figure 4(d)). Around these sample points, the river bed dug by excavators are washed and panned to extract gold. However, TSS and turbidity decreases in the Lom, particularly in LOM₂ and LOM₃ points, found in the mining zones. This decrease in turbidity and TSS in the Lom could be due to a high dilution. The TSS of thepresent study are higher than those obtained by [44] [45] in the Nyong, Ntem, Dja and Boumba respectively (6 ≤ TSS ≤ 30 mg/L), similar to values in forest zone of South Cameroon. The TSS and turbidity are equally higher than those of streams in the forest/savana contact zone such as the Sanaga and the Mbam (5 ≤ TSS ≤ 82 mg/L and 6 ≤TSS ≤ 350 mg/L respectively) [5] . On the contrary, the high TSS and turbidity values are similar to those found in the mining sites of Amazonia, 318 < TSS < 2468 mg/L and 424 < Turb < 2874 NTU [46] . It can therefore be said that these high TSS and turbidity values were presumably a result of the activities of gold miners in this zone alongside deforestation, digging of river beds, dumping of solid and liquid waste resulting from gold washing (panning) and the high soil leaching as well as the barren materials during the rainy season [47] .

The seasonal distribution of physico-chemical parameters are presented in Table 4. Generally, the decreasing order of magnitude of cations in Bétaré-Oya were in the following order Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ >. Sodium was the dominant cation with concentrations up to 310.27 µeq/L, several times higher than other major cations. The total cationic charge TZ⁺ varied from 72.52 to 845.16 μeq/L for an arithmetic mean of 243.10 ± 165.65 μeq/L. Bicarbonates and nitrates were major anions with concentrations up to 927.87 μeq /L and 276.59 μeq/L respectively. The concentration of anions decreases in the order: > > > Cl⁻ > F⁻ >. The value of major equivalent anions (TZ⁻) varied between 37.65 μeq/L and 977.40 μeq/L with an average value of 339.86 ± 260.47 μeq/L.

When comparing the wet period analysis results with dry period, higher concentrations were measured in the wet period for all major ions except for, K⁺ and that showed high concentrations during the dry season than rainy season (Figure 4(k) and Figure 4(l)). This phenomenon is related to materials issued from the atmosphere, vegetation or from an anthropogenic source as the case of nitrate ions during the raining season [48] .

Charge balance error was calculated for all the samples. In general the charge balance errors less than ±5% were accepted. According to the present study, 48% of samples have charge balances between 5 and 10%. Meanwhile, 52% of samples have ion balance slightly higher than 10% with a cationic deficit. In these cases, the deviations are attributed to an anion excess caused by a suspended contaminated load from mining and neighbouring agricultural zones around the study area [49] . These results are different from those obtained in the southern Cameroon forest area by [41] [42] where anionic deficiencies are attributed to organic anions.

Two main water types have been revealed in the study area (Figure 5) using the Piper Trilinear diagram [50] . The CaMg-HCO₃ water type represents 66% and NaK-HCO₃ type represents 34%. The predominance of these water types is indicative of the process of mineral dissolution particularly the silicate minerals. The predominance of CaMg-HCO₃ and NaK-HCO₃ is in accordance with those

of several authors who have worked in the same geological environments in Africa and in some parts of the world.

The concentration of the eight heavy metals (Pb, Cd, As, Zn, Cu, Cr, Mn, Fe) and CN⁻ in Betare-Oya gold mining area are shown in Table 5. The order of abundance was: Fe > Mn > Pb > Cr > Cu > Zn > Cd > As > CN. Among them, only concentrations of As, Cu, Zn and CN⁻ do not exceed WHO 2011 guidelines for water intended for human consumption. The metal concentrations were significantly different between sampling locations. The high concentration of heavy metals was along the streams where the separation mined minerals was carried out (gold washing pool).

Generally, the tailings from gold extraction are a major source of heavy metals in the water as well as chemical products used during the separation of gold and excavating machines (excavators and heavy duty trucks) [51] [52] . However, sometimes these elements can be derived from geological units, e.g. Cd exists in spharelite (ZnS) accompanied by Zn. Zinc also occurs in gold ore bodies in the form of sphalerite (ZnS) which is often associated with galena. Arsenic from oxidation of sulfide minerals pyrite (FeS₂), arsenopyrite (FeAsS) in gold bearing rock [53] .

Given the fact that their contents were above the WHO standards, of some elements, its origin can be influenced by anthropogenic activities such as mining. Metals such as Mn. Cd, Cr, Pb are mostly leached from tailing waste, gold mine effluents or materials used during exploitation. Pb and Cr could be from exhaust pipes of heavy duty trucks, vehicles andmining machinery, Cd could be from the Ni-Cd batteries that are used in gold mining sites [49] [51] . Iron generally results from the leaching of ferrallitic soils with strong complexation with humic acids. This explains the high contents of Fe in surface water [38] [39] . CN⁻ mainly originates from artisanal small-scale mining

(semi-mechanized manner) from gold processing (cyanidation process) which consists of leaching gold from the ore as a gold-cyanide complex and recovering the gold by precipitation [54] .

However, the solubility of these heavy metals is strongly governed by pH through precipitation of their oxides and hydroxides [55] . In the study area most of the pH values tend to neutral-alkaline which does not allow the phenomenon of acid mine drainage but rather contaminated neutral drainage. These phenomena may contribute to significant concentrations of metals in these samples [56] .

To understand the mineralization process of the water in the Mari catchment, twomethods have been applied. These are Gibbs’ model [57] and saturation indices (SI).

To assess water quality of the river, pH, EC, Turbidity, TSS, , Cl⁻, ,

Min: minimuim; Max: maximuim; Std: standard deviation.

Na⁺, Ca²⁺, Mg²⁺, , F⁻ were taken into account for calculations of Water Quality Index (WQI) (Table 7). Furthermore, the World Health Organization (WHO, 2011) limits were used as standard. The computed values of WQI for the 71 water samples are between 12.589 and 5137.40; the water quality varied from “excellent” to “unsuitable for drinking”. 34% of samples represent excellent water; 17% represents good water; 21% is poor water; 11% and 17% respectively for the very poor and unsuitable water for drinking. Highest WQI values were recorded at site MMR₃; MMR₄; MMR₅; MLB₅; LOM₂ where an extensive mining activity is carried out. The spatial variations of WQI (Figure 7) suggested significant decrease of the water quality from upstream to downstream along the Mari and Lom rivers during the study period. The WQI value for each group of samples (Lom and Mari) shows that the sites in the Lom River (Figure 8(a)) and the main Mari River (Figure 8(b)) were the two most polluted sites in Bétaré-Oya. These results suggest that water quality in all of the above sites is controlled mainly by TSS and turbidity.

The water quality of the left tributary of Mari (Figure 8(d)) appear more deteriorated in relation to the right bank which has 66% of “excellent water” (Figure 8(c)). This zone is not affected by mining activities (deforestation, digging of river beds, dumping of solid and liquid waste resulting from gold washing). Similar results were obtained by [16] in the Nile river in Egypt. According to this author, the water quality goes from good in the upstream to unsuitable for drinking in the down-stream, reflecting the impact of industrial activity domination on the deterioration of Nile water quality.

For the HPI, the weightage (Wi) for all the four sampling points are presented in Table 8 and Table 9. The overall heavy metal pollution index of the Mari and

Min: minimuim; Max: maximuim; VC: variation coefficient.

Lom rivers was found to be 1195 on average, greater than the critical pollution index value of 100, above which the overall pollution level should be considered unacceptable. At all sampling points, HPI calculated is far above the critical in- dex limit of 100 (Table 8). This indicates that the water is critically polluted with respect to heavy metals. Similar results were obtained by [31] in India; and [61]

in Nigeria.

Fe has the highest concentration in the Mari catchment and Lom River. Nevertheless, the weightage (Wi) given to Fe is very small. Therefore, in evaluation of HPI, this parameter does not contribute much on HPI value. However, heavy metals like Pb, Cd, As, and Cr have been given no relaxation in drinking water standard and have been given high weightage (Wi) value in HPI calculation by [31] . Hence, even their smaller concentration in water samples makes the water of poor quality and gives high values of HPI [33] . The presence of these undesirable elements constitutes a potential danger to human health, to the aquatic biodiversity of this area and even to the water supply project for the town of Yaounde, from the river Sanaga.

According to the percentage of sodium and electrical conductivity (Figure 9(a)), all samples are suitable (excellent) for irrigation. The SAR data plotted on the US Salinity Diagram (Figure 9(b)) showed that 99% of the samples were categorized in low, class indicating low sodium and low to moderate salinity water, which can be used safely for irrigation purposes in almost all types of soils [62] .

However, the presence of some metallic elements in low contents in the surface water of Bétaré-Oya can be phytotoxic or could become phytotoxic when their concentrations eventually become higher [63] .