Spatio-Temporal Assessment and Water Quality Characteristics of Lake Tiga , Kano , Nigeria

The physico-chemical water quality of Lake Tiga was monitored over a two-year period (March 2009- March 2011) in order to bridge the information gap on its limnology and assess its physico-chemical condition. Turbidity, Dissolved Oxygen (DO) saturation and organic matter were significantly higher (p<0.05) in the rainy season than in the dry season, while pH and Biological Oxygen Demand were significantly higher (p<0.05) in the dry season than in the rainy season. Apparent colour, Total Solids (TS), Total Suspended Solids (TSS), K + , Cl - , total acidity, total hardness, NO3 - and PO4 3- decreased (p<0.05) from the riverine section towards the dam site, while water transparency, Dissolved Oxygen (DO), SO4 2- and Mg 2+ showed an increase (p<0.05) from the riverine section towards the dam site. Apparent colour, TS, TSS, total acidity, total hardness, Ca 2+ , NO3 - and PO4 3- increased (p<0.05) from the surface down to the bottom, while pH, Mg 2+ and DO decreased (p<0.05) from the surface down to the bottom at the lacustrine section of the lake. Cluster analysis of the parameters showed major clusters between the major ions (Ca 2+ , Na + , K + , Cl - , HCO3 - ) and the general chemical characteristics (TDS, alkalinity, conductivity, acidity and hardness) and also between the nutrient compounds (Organic matter, NO3 - and PO4 3- ) and the hydro- physical parameters (TS, TSS, apparent colour, true colour and turbidity). The water quality indices and sodium absorption ratio values in the sampled stations indicated that the water is most suitable for probable applications at the lacustrine section, towards the dam site.


INTRODUCTION
Dams, built to change natural flow regimes, are one of the most significant human interventions in the hydrological cycle (McCartney et al., 2001) and they can result in post-impoundment phenomena that are specific to reservoirs and not natural lakes (Dinar et al., 1995).The size of the dam, its location in the river system, its geographical location with respect to altitude and latitude, the detention time of the water and the source(s) of the water all influence the water quality (McCartney et al., 2001).Although oxygen demand and nutrient levels generally decrease over time as the organic matter decreases, some reservoirs require a period of more than twenty years for the development of stable water quality regimes (Petts, 1984).Eutrophication of reservoirs may occur as a consequence of large influxes of organic loading and/or nutrients (McCartney et al., 2001).This can result in water blooms of blue-green algae which can cause oxygen depletion and increased concentrations of iron and manganese in the bottom layer, as well as increased pH and oxygen in the upper layers of stratified reservoirs (Zakova et al., 1993).Downstream, the water discharged from reservoirs can be of different composition and show a different seasonal pattern to that of the natural river (McCartney et al., 2001).The salinization of water below dams in arid climates (arising from increased evaporation) is problematic and has proved to be a problem on floodplain wetlands in the absence of periodic flushing and dilution by flood water.If sufficiently high and prolonged, elevated salinity will affect aquatic organisms (Hart et al., 1991).
Even without stratification of the storage, water released through dams may be thermally out of phase with the natural regime of the river (Walker, 1979).The quality of water released from a stratified reservoir is determined by the elevation of the outflow structure relative to the different layers within the reservoir.Water released from near the surface of a stratified reservoir will be well-oxygenated, warm, nutrientdepleted water.In contrast, water released from near the bottom of a stratified reservoir will be cold, oxygen-depleted, n hydrogen 1994).
Lake T (based on Water Res (Hadejia-Ja 1989) enough to stratify both thermally and chemically using a 2-litre Van Dorn water sampler.This was based on the need to give information on both horizontal and vertical variations in the physico-chemical water condition of the lake.Water temperature and pH were determined in-situ using a mercury-in-glass bulb thermometer and a field pH meter/lovibond comparator respectively, while transparency was determined with a Secchi disc.Electrical conductivity was determined using a JENWAY conductivity meter.
Laboratory analyses: Apparent colour (of unfiltered sample) and true colour (of filtered sample) were both determined by colorimetric method while turbidity was determined by nephelometric method (APHA et al., 1995).Total Solids (TS), Total Suspended Solids (TSS) and Total Dissolved Solids (TDS) were all determined by gravimetric method (APHA et al., 1995), while titrimetric (iodometric) method was used for Dissolved Oxygen (DO) and Biological Oxygen demand (Goltermann et al., 1978).Acid-base titrimetric method was employed for both alkalinity and acidity, Mohr titrimetric method for chloride ion and complexiometric titration for both calcium and magnesium ions (Goltermann et al., 1978).Sodium and potassium were analyzed using the flame emission spectrophotometer (Goltermann et al., 1978).Sulphate was determined by using a turbidity meter which was a function of the turbidity difference produced in the water sample after adding 0.15g of Barium chloride.Bicarbonate ion was estimated from the concentration of total alkalinity (Cole, 1975) as stated in the equation (alkalinity X 1.20) mg/l).Nitrate and phosphate were both determined by spectrophotometer (Goltermann et al., 1978).Water quality index was determined based on eight parameters (Dissolved Oxygen, pH, Biochemical Oxygen Demand, temperature change, total phosphate, nitrates, turbidity and total solids) (Oram, 2012).Sodium Absorption Ratio (SAR) in the lake was calculated using the formula: SAR = Na + /√ (½Ca 2+ + ½Mg 2+ ) (All concentrations are in meq/l).

Statistical analysis of data:
All the data obtained were subjected to appropriate various methods including descriptive statistics, regression and correlation coefficient, analysis of variance (ANOVA) and cluster analysis.
Temporal variation: Table 3 provides the descriptive statistics as well as the seasonal mean values of the investigated parameters.Most of the investigated parameters were higher in the dry season than in the rainy season, viz; water temperature, Secchi disc transparency, pH, conductivity, total alkalinity, total acidity, total hardness, Ca 2+ , Na + , K + , Cl -, SO 4 2-, HCO 3 -, BOD 5 , NO 3 -, Fe and Cu.Although water temperature was generally higher in the dry season than in the rainy season (p>0.05), the lowest value was recorded in the middle of the dry season in January (data not shown).A different seasonal trend was observed for apparent colour, turbidity, TS, TSS, Mg 2+ , organic matter, PO 4 3-, DO and Zn which were all higher in the rainy season than in the dry season.In these seasonal variation patterns, only pH, BOD 5 , turbidity and organic matter showed significant difference (p<0.05).

DISCUSSION
The lowest ambient air temperature and water temperature were recorded in the middle of the dry season due to the characteristic cool dry North-East trade wind known as Harmattan between November and February.This pattern of seasonal variation has similarly been reported in Northern Nigeria by Balarabe (1989) in Makwaye Lake, Adakole et al. (1998) in Bindare stream, Ezra and Nwankwo (2001) in Gubi Reservoir, Ajibade et al. (2008) in the major rivers of Kainji Lake National Park and Adakole et al. (2008) in Kubanni Lake.Horizontal increase in surface water temperature was a function of the time of sample collection.Collection of water samples started early in the morning from the riverine section (Stations A and E), followed by the lacustrine section (Stations B, C and D) in the afternoon.This diel surface water temperature pattern of the lake is in agreement with Chapman and Kimstach (1996) who stated that water temperature is not only influenced by seasonality but also by time of the day.Water temperature determines the concentration of many variables.As water temperature increases, the rate of chemical reactions generally increases together with the evaporation and volatilization of substances.Increased temperature also decreases the solubility of gases, such as O 2 , CO 2 , N 2 and CH 4 .The metabolic rate of aquatic organisms is also related to temperature and in warm waters, respiration rates increase leading to increased oxygen consumption and increased decomposition of organic matter (Chapman and Kimstach, 1996).
The on-set of the rain signals a radical change in the physico-chemical characteristics of tropical rivers (Chapman and Kramer, 1991), as evidenced in this study.Allochthonous sediments from run-off which have the capacity to further attenuate incident solar radiation could be the main reason for the lower values of water transparency recorded in the rainy season.Ayoade et al. (2006) in their study of Oyan and Asejire Lakes also observed lower water transparency in the rainy season and attributed this to decrease in sunlight intensity caused by the presence of heavy cloud in the atmosphere which in turn reduced the quantity of light reaching the water.Secchi disc transparency values (0.05 m-0.95 m) in this study put the lake in the eutrophic class, following Wetzel (1983) classification of lakes i.e., eutrophic lakes have 0.8 m-7.0 m Secchi disc readings, while oligotrophic and mesotrophic lakes have 5.4 m-28.3 m and 1.5 m-8.1 m, respectively.The highly significant positive correlation (p<0.01) between transparency and Dissolved Oxygen (DO) is further strengthened by the increase of both parameters from the upper basin down to the lower basin.This may be attributed to the sedimentation of suspended solids in the lower basin and the deeper layers.This must have consequently increased the penetration depth of incident solar radiation as well as the euphotic zone of the water body, hence the increased concentration of DO towards the dam site.
Biological respiration, including that related to decomposition processes, reduces DO concentrations.Waste discharges high in organic matter and nutrients can also lead to decreases in DO concentration as a result of increased microbial activity occurring during the degradation of the organic matter (Chapman and Kimsatch, 1996).Biological oxygen demand (BOD 5 ), a measure of biological activities taking place in the water was higher in the dry season than in the rainy season in spite of a higher organic matter in the rainy season.This was probably as a result of a high presence of inundated woody trees in the lake in addition to the accumulated detritus of the rainy season, whose rate of decomposition would be greatly favored by the relatively high temperature of the dry season.This also suggests that the BOD 5 is not only dependent on the concentration of organic matter in a water body but also on water temperature.
TSS contributed more to the TS load than the TDS (67.5 and 32.5% respectively).Olofin (1991) had also reported high sediment yield in the main rivers of Kano and a rapid siltation in the reservoirs.This could be attributed to the characteristic nature of the Sahel vegetation zone with widely-spaced trees and the consequent reduced capacity to check allochthonous run-off.McCartney et al. (2001) also opined that high suspended sediment load in the arid tropics could be as a result of sparse vegetation in the area, which fails to prevent erosion by intense seasonal rainfall.The role of rainfall and allochthonous input in the concentration of hydro-physical and nutrient parameters is underscored by the significant positive correlations (p<0.05)among apparent color, turbidity, NO 3 -and PO 4 3-and their higher concentrations (save NO 3 -) in the rainy season.Nitrate level in the lake was typical of freshwaters with the overall mean concentration <1.0 mg/L and the maximum concentration <5.0 mg/L and Kimstach, 1996).Nitrate was the only nutrient compound with higher concentration in the dry season while its maximum concentration was also recorded just at the beginning of the rainy season (June 2010).This agrees with Wolfhard and Reinhard (1998) that nitrates are usually built up during dry seasons and that high levels of nitrates are only observed during early rainy season.Adeyemo et al. (2008) also stated that initial rains flush out deposited nitrates from nearsurface soils and nitrate level reduces drastically as rainy season progresses.
Based on phosphate concentration, Tiga Lake can be categorized as a meso-eutrophic lake.Overall mean and range values of PO 4 3-concentration in the lake (0.21 mgL -1 and 0.08-0.39mg/L respectively) exceeded the mean and range values of mesotrophic lakes (0.08 mg/L and 0.03-0.29 mg/L, respectively) but were less than the corresponding values in eutrophic lakes (0.25 mg/L and 0.05-1.18mg/L) (Wetzel, 1983).
pH limits in the lake (6.40-7.83)exceeded the limits (6.9-7.6) reported by Adeniji and Ita (1977) in the preliminiary limnological study of the lake.They were however still within the EU recommended range of 6 to 9 for fisheries and aquatic life (Chapman and Kimstach, 1996) and the WHO pH guideline (<8.0) for drinking water for effective disinfections with chlorine (WHO, 1993).These pH values suggest that the water is suitable for drinking (after disinfections) and aquatic life.The disparity between pH readings in the upper basin stations and the lower basin stations suggests that the former are slightly acidic.This may not be unconnected with the higher concentration of organic matter in the upper basin stations and the consequent release of CO 2 gas during decomposition, as shown in the significant inverse correlation (p<0.05) between pH and organic matter.Low pH increases the solubility and toxicity of many chemical nutrients and heavy metals (DAWF, 1966;Chapman and Kimstach, 1996), which was the reason for the inverse relationships of pH with NO 3 -, PO 4 3-Cu, Zn and Mn.There appeared to be a strong relationship between metals, suspended solids, pH and organic matter as evidenced in the significant correlations (p<0.05)among these parameters.This shows that metals tend to be strongly associated with sediments in rivers, lakes and reservoirs and their release to the surrounding water is largely a function of pH, oxidation-reduction state and organic matter content of the water (Carr and Neary, 2006).
Conductivity values (51.9-695.0µS/cm) in the lake were typical of a freshwater since the electrical conductivity of most freshwater ranges from 10-1,000 µScm -1 (Chapman and Kimstach, 1996).Virtually all the major ions were lower in the rainy season than in the dry season since conductivity declines in the wet periods as the concentration of salts becomes more dilute (Carr and Neary, 2006).There were inverse correlations between the major ions (save SO 4 2-) and water depth.This corroborates the claims of Schmidt (1973), Hamilton and Lewis (1987) and Vasques (1992) that water level fluctuation increases or decreases water transparency, pH, electrical conductivity, suspended matters, concentrations of nutrients and other variables.Concentration effects of salts at this period of the year when the water level was low (Holden and Green, 1960;Egborge, 1981;Moore, 1989;Michaud, 1991;Ovie and Adeniji, 1994) and water abstraction for dry season irrigation (Smakhtin et al., 2003) could have also contributed to the higher values of these ions in the season.The concentrations of Ca 2+ , Mg 2+ , Na + and K + , Cl-, SO 4 2-and HCO 3 -were all typical of freshwaters (Chapman and Kimsatch, 1996), although relatively high concentrations of these parameters were recorded in an upper basin station (E) between December 2009 and March 2010.The lake can be categorized as a soft water since its total hardness was less than 120 mg CaCO 3 /L (Renn, 1968).The mean total hardness (23.76 mgCaCO 3 /L) however fell short of the optimal range (75-250 mg CaCO 3 /L) for aquatic life, though above the minimum concentration of 20 mg CaCO 3 L -1 (Wurts, 1992).The mean total alkalinity concentration (32 mgCaCO 3 L -1 ) of the lake was also above the minimum of 20 mgCaCO 3 L -1 required for aquatic life and this will also prevent large swings in its daily pH values (Wurts and Masser, 2004).Conductivity, total alkalinity, hardness, total dissolved solids and most of the major ions (Ca 2+ , Mg 2+ , Na + , K + , SO 4 2-, HCO 3 -, Fe, Mn and Zn) showed an increasing trend up to about 10 m depth and then showed a decline.This is probably because water is being let out close to this depth since the main outlet of the dam is submerged 16 meters below the full supply level (Hadejia-Jama'are River Basin Development Authority, 1989).
Sodium Absorption Ratio values in all the stations were generally less than 1 and decreased from the upper basin (inflows) down to the lower basin (dam site), from where the water is being let out for the Kano River irrigation project.Water quality index (Q) increased from the inflow stations towards the dam site.The overall water quality index recorded in this study (76%) implies that Lake Tiga is still good for intended uses (Oram, 2012).

CONCLUSION
Lake Tiga is low in electrical conductivity and may also be categorized as a soft water.Comparatively, the physico-chemical nature of the lacustrine section of Lake Tiga was distinctively different from that of its riverine section.Water quality increased while sodium absorption ratio decreased from the riverine section towards the dam site.This also suggests that the lake undergoes self-purification process as water flows downstream towards the dam site and as suspended solids are deposited in the deep layers.This study has revealed that Lake Tiga is still suitable for intended applications, namely irrigation, fisheries and aquatic life.
The various physico-chemical indices of water quality in the lake suggest that it is less impacted by human activities, obviously because it is some distance (about 70 km) upstream of the commercial and industrial city of Kano.However, the sediment load of the reservoir was relatively high and successive loading can accelerate its aging process.Adequate monitoring of the water quality and regulation of anthropogenic activities in and around the basin are recommended in order to slow down the aging process of the lake and conserve it for a longer period.

Table 1 :
Horizontal variation of physico-chemical water parameters in Lake Tiga

Table 2 :
Vertical variation of physico-chemical water parameters in the lacustrine section of Lake Tiga s.e.m : Standard error of mean; *: significant; **: highly significant; ***: very highly significant

Table 3 :
Descriptive statistics and seasonal mean values of physico-chemical water parameters in the sampled stations

Table 4 :
Sodium absorption ratio values and water quality indices in the sampled stations

Table 5 :
Comparison of the physico-chemical water quality of Lake Tiga with conventional standards Use