Factors Influencing Water Of Pangani River Basin,Tanzania:Implication On Water Quality And Quantity For Human And Aquatic Ecosystem Health

Water is essential natural resource for environment,human and aquatic ecosystem.Quantity and quality of freshwater over the global is threatened by pollution and over abstraction owing to rapid population growth,land use change,economic development and climate change.Problem of water quality and quantity is serious to most of developing countries where water related sector such as agriculture is the backbone for economic development.In addition to that,most of the communities in developing countries especially those located in rural areas use untreated water from the river for drinking and other domestic activities.Tanzania is one of developing countries facing shortage of water quantity and quality.Downscaling to Pangani River Basin?PRB?as one of important basin in Tanzania;the basin has about 6 million people who use water from the river for irrigation,domestic,industrial and production of hydroelectric power.Quantity of nutrients in the river increased with time.Increase in nutrients led to the decrease in water quality.On the other hand the decrease in water quantity was associated with the decreasing rainfall.Therefore with the current global challenges such as climate change,population growth,agricultural and industrial development there was a high chance of increasing water demand with the decreasing in water quality and quantity.Decrease in water quantity and quality will have impacts to human and aquatic ecosystem health.To evaluate health risks associated with water quality and quantity,it was important to understand concentration of nutrients and factors influencing quantity and quality of water.Understanding concentration of nutrients and factors influencing quality and quantity of water can help to manage water resources for sustainable development.Therefore study was conducted to investigate concentration of nutrients,organic carbon,metals and factors influencing quality and quantity of water in PRB.Surface water,groundwater and suspended particulate matter,were sampled in October 2014?dry season?,June 2015?soon after rainy season?and May 2016?during rainy season?.Sampling was conducted three times in order to have information on how the rivers behave in dry season,soon after rainy season and during raining.Plant leaves C₃?rice leaves and grasses?and C₄?maize and sugarcane?were also sampled and analyzed for?¹³C.Analysis of particulate organic carbon?POC?,dissolved silicates,urea,major ions,metals,stable water isotopes of?¹⁸O,?²H,⁸⁷Sr/⁸⁶Sr,organic and inorganic nutrients?N,P,C?was conducted in State Key Laboratory of Estuarine and Coastal Research in East China Normal University.Various instruments were used to analyze different parameters including Multi-parameter probe?Multi 350i Set 5 made from Germany?which was used for on-site measurement of dissolved oxygen?DO?,salinity,pH,temperature and electrical conductivity?EC?.Skalar SAN^plus Continuous Flow Auto-analyzer was used to analyse nitrite,nitrate,ammonium,phosphate,dissolved silicates?DSi?,total nitrogen and phosphorus.Urea was analyzed by using UV-VIS spectrophotometer at 520 nm wave length.Thermo Finnigan isotope ratio mass spectrometer Delta^Plus XP was used to analyze?¹⁸O and?²H.Dionex Ion Chromatography System?ICS-2000?was used to analyze major ions.Neptune multicollector inductively coupled plasma mass spectrometry?MC-ICP-MS?was used to analyze strontium isotopes.Finnigan EA 1112 elemental analyzer was used to analyze POC and?¹³C,Shimadzu Total Organic Carbon?TOC?analyzer was used to analyze DOC.Metals were analyzed by high resolution inductively coupled plasma mass spectrometry?HR-ICP-MS?.The results showed that PRB had freshwater with electrical conductivity?1500?S/cm which decreased with altitude.Mean water temperature was 22.9 ^oC with high and low temperatures being recorded in 2016?23.9±4.2?and 2015?21.1±3.43?,respectively.Temperature decreased from lowlands to highlands.In general water had alkaline pH which increased in 2016 and decreased in 2014.Spatial variation showed that some samples from Mt.Kilimanjaro and groundwater had slightly acidic character but the rest of the stations had neutral to basic pH.Most of the stations had suitable dissolved oxygen of?5 mg/L except Lake Jipe,Ruvu River and bottom water sampled from Nyumba ya Mungu reservoir.Stable water isotopes of?¹⁸O and?²H together with discharge and meteorological data showed that rivers in Pangani basin depend on water from rainfall with insignificant contribution from glacier on Mt.Kilimanjaro.Since the rivers depend on rainfall,river flow increased in rainy season and decreased in dry season.River flow to Indian Ocean ranged from 8.8 to 33.2 m³/s with an average of 15.1 m³/s,the discharge was lower than most of tropical rivers discharging into Indian Ocean.Decreased rainfall in the last three decades was one of the causes to decrease quantity of water in the basin.The estimate showed that Nyumba Ya Mungu reservoir has high residence time of about 18 months.Climatic data showed that in average rate of evaporation in PRB was 5.3 mm/day.Therefore,high residence time in Nyumba ya Mungu reservoir,high evaporation rate and decrease in rainfall were major factors reducing quantity of water in PRB.Hydrograph separation in upper PRB showed that Kikuletwa River was the major source of water to the reservoir with high quantity of water being coming from Mt.Kilimanjaro.Most of irrigation activities were conducted around Kikuletwa River and it was one of human factor reducing quantity of water in PRB.In future,global warming is expected to raise sea surface temperature which will possibly increase rainfall and river flow in PRB.Even though,quantity of water is expected to increase with climate change but the increase in population,agricultural and industrial activities will also increase water demand.Therefore,management measures are needed to ensure availability of water to the users such as storage of water in rainy season in order to use in dry season,smart irrigation practice such as drip irrigation system and switching to drought resistant crops including cereals which can help to reduce water needs.Conservation of water sources including rivers and springs can also help to ensure sustainable flow of water.To minimize conflicts among water users by laws can be developed which will clearly describe on sustainable share of water resources to various stakeholders.Investing and use of groundwater can also help to reduce relying on surface flowing water.Analysis of major ions showed that water in PRB was NaHCO₃ type with moderate level of total dissolved solids.Chemistry of water was dominated by rock weathering with small atmospheric and anthropogenic input.Rate of weathering was 57.8mmol/km²/year which increased to 66.1 mmol/km²/year in dry season and decreased to49.5 mmol/km²/year in wet season.High rate in dry season suggest that temperature had positive relation with the rate of weathering whereas rainfall had negative relation.Altitude,pH and geology were some of the other factors influencing rate of weathering,increasing pH led to the increasing rate of weathering.Basin morphology influenced rate of weathering in which higher concentration of most major ions was found downstream than upstream.High rate of weathering in downstream was associated with gentle slope and leaching from fine soils found downstream while steep slopes reduced rate of weathering in upstream.Rate of weathering in PRB was lower than most of tropical rivers.Low discharge and highly stable young volcanic rocks played role to reduce rate of weathering.PRB was dominated by carbonate weathering compared to silicate weathering.Level of Sr was higher than global average whereas ⁸⁷Sr/⁸⁶Sr was lower than global average.Low ⁸⁷Sr/⁸⁶Sr values suggest that weathering in PRB was mainly dominated by signature from volcanic rocks.Trend of increasing ⁸⁷Sr/⁸⁶Sr from upstream to river mouth reveal that age of the rocks increased from upstream to river mouth.Seasonal variation showed that the increase in the rate of weathering in dry season and evaporation caused to increase concentration of major ions in dry season.Decreased rate of weathering in rainy season and dilution led to the decreasing level of major ions.Average level of dissolved silicates?DSi?in PRB was 615.7?M;the level was higher than other inorganic nutrients in the basin and higher than most of the tropical rivers in the world.Volcanic rocks rich in silicate minerals were one of the main factors associated with the high level of DSi in PRB.Since diatoms depend on DSi,high level of DSi made PRB to be favorable area for diatoms.Spatial distribution of nutrients showed that more nutrients were found downstream than upstream.Seasonal variation showed that there was higher level of nitrogen and phosphorus in wet season compared to dry season whereas opposite was the case for DSi.The level of nutrients?nitrogen and phosphorus?in PRB was higher than pristine level suggesting that there was anthropogenic input.PRB had higher level of inorganic than organic nitrogen and phosphorus nutrients.High level of inorganic nutrients reveals that most of the nutrients were coming from fertilizers,industrial and domestic wastes.The increase of phosphorus and nitrogen nutrients in wet seasons was associated with surface flowing water collecting those nutrients from point and non-points sources to the rivers.In this regard rainfall,temperature,anthropogenic activities were main factors regulating concentration of nutrients in PRB.Metal elements?Pb,Al,V,Cr,Mn,Fe,Co,Ni,Cu,and Zn?showed almost similar trend to major ions and nutrients increasing from upstream to downstream.Finding showed that there was high concentration of metal elements in particulate form compared to the dissolved form.Lithogenic source was the dominant source of metals compared to the anthropogenic source.Biogeochemical processes controlled by redox reaction,pH,DOC and electrical charges played role in spatial distribution of metals in PRB.Distribution of sediment ranged from clay to sand with high composition of clays and silts in downstream compared to upstream.Nyumba ya Mungu reservoir was one of human activities influenced normal flow of water and nutrients from upstream to downstream.The reservoir had high trap efficiency ranging from 20%to 90%.High trap efficiency and high residence time caused the reservoir to be a sink of nutrients,organic carbon and metals.In this case Nyumba ya Mungu reservoir influenced normal flow of nutrients,organic carbon and metals from upstream to downstream.Sink of nutrients in Nyumba ya Mungu was one of the causes to reduce dissolved oxygen?DO?;measured surface and bottom DO in reservoir were 7.82 and 0.78 mg/L,respectively.Hypoxia condition in bottom water can create unfavourable environment for aquatic organisms.Finding from organic carbon and carbon isotopes showed that the basin was getting carbon mainly from allochthonous sources dominated by C₃ plants whereas for lakes there was input from autochthonous sources.Rainfall,weathering,soil and altitude were some of the main factors influencing spatial and seasonal distribution of organic carbon.Concentration of organic carbon?DOC and POC?increased from upstream to downstream.Yield of nutrients,organic carbon and metals from PRB to Indian Ocean was lower than most of the tropical rivers.Low yield was associated with low rate of weathering,low discharge,sink in reservoir and low anthropogenic input.Water quality index showed that most of station in PRB had good water quality for human and aquatic ecosystem use.Most of the stations in upstream were good to excellent whereas stations in downstream were good to medium.Most of groundwater stations were good to excellent.Maji ya Chai was the only station with poor water for human and aquatic ecosystem use.Even though,most of the stations had good water but future increasing population,agricultural and industrial activities in the basin give alert to the quality of water.This study recommends frequent monitoring in order to get reliable information for management purpose since anthropogenic activities can change quality of water at any time.Deforestation,inappropriate application of fertilizers and poor farming practice should be discouraged in order to reduce soil erosion and uptake of nutrients from agricultural areas to the rivers.