Lower Aquifer System in the Coastal Area, Tanzania Said Suleiman - PowerPoint PPT Presentation

Geochemical Characterization of the Upper and Lower Aquifer System in the Coastal Area, Tanzania Said Suleiman Bakari Per Aagaard Rolf D. Vogt Dept. of Geosciences, University of Oslo, Norway Ruden Fridtjov Ruden Aquifer Development Ltd, Norway Vuai Said Ali Department of Physical Sciences, The University of Dodoma, Tanzania October 2009

This presentation focuses on Geochemical Characterization of the Upper and Lower Aquifer System in the Coastal Area, Tanzania The rapid pace of urban development, and prolonged droughts and deforestation has led to increased the demand for freshwater resources in coastal Tanzania Current Projected (2009) (2020) Population 3,000,000 > 5,000,000 Water demand 280,500 550,000 (m 3 /day) Actual water 180,000 supply (m 3 /day) Water deficit 100,500 (m 3 /day) 2

Searching for a groundwater source to augment surface water as a source for public water supply The regional neogene aquifer was discovered in the alluvial plain of coastal area in Dar es Salaam 3

To use groundwater in a sustainable manner in this region, an accurate estimation of the discovered aquifer and the existing groundwater was conducted; Our specif ific ic objective ctives -to o examin amine e closely sely - and to asses s the water er the e hydrogeoch ogeochemi mical cal quali ality ty and cross-fl flow ow char harac acteristics teristics of con ontam amin inat atio ion betw twee een n grou oundwa dwater ter in the the e aquifer ifer units ts and coas oasta tal l aquif ifer er syst stems ems surfa face ce pollu luti tion 4

Study Area and Hydrogeological Setting Stud udy area ≈10.000 km 2 Loca cati tion on 39 o E 39 E - 39 39 o 45’E 6 o 30’S -7 o 30’S Tempera ratu ture re ranges ges 18 to 32 o C C mean n annua ual 1100 0 mm preci ecipita tati tion on mean n annua ual valu lue of of 1900 0 mm. actu tual evapora rati tion on 5

Physio- chemic ical al: 3-River River pH, EC, water ter DO, Temp, Mat aterial rial 6-deep ep Major Water er an and wells Analysed d anions s (200- sampli mpling paramete ters & Methods hods 610m) cations 13- 13 shall allow Trace ce Characte well lls s Geoche metal: l: (<100 100m) Mn Mn acterisati chemical risation cal Piper diagram were re used to c classify fy and characte acterize rize the ground ndwa wate ter quality. y. 6

The main Kimbiji production well (PW6) Artesian flow > 3 l/s  Average saturated thickness of the aquifer ≈ 1000 m  Flow direction: W-E driven by artesian pressure-western hinterland 7

Sam ampl ple Village age Dept pth Temp mp. DO ORP EC Code de (Area) ea) (m) (m) ( ( o C ) (mg/L) /L) (mV) (µS/c /cm) m) pH pH Water er Type Groundwater samples from Lower Aquifer System PW1 Mpiji 200 7.71 30.2 2.13 nm 1057 Na-HCO3-Cl PW5 Mpera 200 6.16 28.5 1.89 nm 362.0 Na-Cl-HCO3 PW7 Kimbiji 200 7.85 30.1 1.39 -51.1 1154 Na-HCO3-Cl PW3 Mpiji 610 7.76 29.7 2.46 nm 2570 Na-Cl PW4 Mpera 600 6.35 30.8 2.48 nm 359.0 Na-Cl-HCO3 PW6 Kimbiji 612 8.33 34.6 0.49 -81.2 866.0 Na-HCO3-Cl Groundwater samples from Upper Aquifer System PW2 Mpiji 80 7.79 29.6 6.35 nm 848.0 Na-Ca-Cl-HCO3 PW8 Kijichi 50 6.88 29.3 5.44 nm 358.0 Na-Ca-HCO3-Cl PW9 Mbagala 70 6.32 28.1 4.84 nm 1050 Na-Cl-NO3 PW10 Tandika 53 5.80 28.5 3.50 88.7 1197 Na-Ca-Mg-NO3-Cl PW11 Tazara 65 6.76 28.0 6.64 28.2 860.0 Na-Mg-Ca-Cl-NO3 PW12 Tabata 60 6.87 28.9 6.56 25.2 1211 Na-Cl PW13 Airport 100 7.53 28.7 4.01 -16.6 1168 Na-Cl-HCO3 PW14 Kigogo 60 7.17 29.5 4.46 42.0 3870 Na-Ca-Cl PW15 Ocean-Rd 11 7.43 28.4 4.40 -11.0 1097 Ca-Na-HCO3-Cl PW16 M’nyamala 50 7.08 28.0 3.69 9.50 1696 Na-Ca-Cl-HCO3 PW17 Tegeta 65 6.92 28.6 4.46 19.1 6130 Na-Ca-Cl-SO4 PW18 Kibaha 95 7.31 27.3 2.01 -4.30 5100 Na-Cl PW19 Ubungo 70 6.23 26.8 6.13 59.9 1790 Na-Mg-Cl Riverwater Samples R1 Kizinga 7.38 27.6 5.65 nm 651.0 Na-Ca-Cl-HCO3 R2 Ruvu 7.80 26.5 7.09 -33.3 97.40 Ca-Mg-Na-HCO3 R3 Mpiji 7.82 28.4 5.80 nm 688.0 Ca-HCO3 8

The piper diagram revealed that the hydrogeochemical composition of the upper groundwater is affected by seawater intrusion 80 80 The upper groundwater Legend: fall under marine origin 60 60 Lower Aquifer site, dominated by Upper Aquifer 40 40 Na-Ca-Cl, as indication River water of seawater intrusion 20 20 Mg SO4 80 80 60 60 40 40 20 20 80 60 40 20 20 40 60 80 Ca Na HCO3 Cl The better water quality are in the lower groundwater dominated by Na – HCO 3 water 9

4.5 12 Lowe Aquifer (200 -610m) Lower Aquifer (200-610m) Upper Aquifer (<100m) Upper Aquifer (<100m) 4 10 a b 3.5 - (meq ratio) - (meq ratio) 8 3 2.5 6 + /Cl 3 2+ /HCO Na 2 4 Ca 1.5 2 1 0.5 0 0 10 20 30 40 50 0 10 20 30 40 50 Cl - (meq/L) Cl - (meq/L) the e increa crease sed disc scharge harge and addit ition onal l recha harg rge e has a profou found nd effec ect t on the hydroc rochemis hemistr try y of the aquifer ifer, , and induc uced groun undwa water ter flus ushi hing ng and dilu luti tion. n. 10

Cl - (mg/L) 0 500 1000 1500 0 100 Upper Aquifer Lower Aquifer 200 Well depth (m) 300 Nitrate Chloride 400 Cl - from PW3 - originated 500 from relict seawater in the lower aquifer system 600 during the Pleistocene time 0 100 200 300 400 500 - (mg/L) NO 3 11

500 Lower Aquifer (200-610m) Upper Aquifer (<100m) 400 300 -1 ) - (mgL 3 NO 200 100 0 0 500 1000 1500 Cl - (mgL -1 ) - with relation to Cl - , is evident that biogeochemical Low NO 3 processes is attenuating nitrate in the lower aquifer system 12

500 500 Lower Aquifer (200 - 610m) Upper Aquifer ( <100m) 400 400 300 300 _ (mg/L) _ (mg/L) NO 3 200 200 NO 3 100 100 0 0 0 1 2 3 4 5 6 7 0 50 100 150 200 250 300 350 400 _ (mg/L) DO (mg/L) HCO 3 _ in the lower aquifer indicating that dissolution Low DO and high HCO 3 of the carbonate minerals via biodegradation of organic matter 13

- Sample Well HCO 3 Saturation Index (SI) for selected minerals Code Depth (m) (mmoles/L) Calcite Dolomite Gypsum Groundwater samples from Lower Aquifer System PW1 200 4.294 -0.39 -1.33 -1.02 PW5 200 0.803 -0.25 -0.91 -1.14 PW7 200 5.917 -0.45 -1.33 -1.22 PW3 610 3.016 -0.15 -0.85 -0.97 _ - controlled PW4 600 1.115 0.15 0.01 -1.11 HCO 3 PW6 612 5.720 -0.36 -1.16 -1.18 Groundwater samples from Upper Aquifer System by dissolution of the PW2 80 2.614 -0.01 -0.52 -0.97 carbonate minerals PW8 50 1.770 0.82 0.85 -0.75 via biodegradation of PW9 70 1.016 0.64 0.37 -0.74 PW10 53 0.344 0.43 0.05 -0.74 organic matter PW11 65 1.492 0.87 0.57 -0.71 PW12 60 1.885 0.51 -0.19 -0.71 PW13 100 4.376 0.72 0.45 -0.72 PW14 60 4.737 0.42 -0.10 -0.70 PW15 11 4.327 -0.16 -1.47 -1.22 PW16 50 3.491 0.04 -0.41 -1.07 PW17 65 3.508 -0.48 -1.53 -1.06 PW18 95 5.556 -0.18 -0.92 -1.05 PW19 70 1.115 -0.32 -1.13 -0.89 Riverwater Samples R1 1.328 -0.20 -0.91 -1.08 R2 0.574 -0.40 -1.29 -1.04 R3 2.065 -0.04 -1.03 -0.38 14

• Most of the groundwater from the upper aquifer is not suitable for drinking water because of its high salinity which is mainly caused by saltwater intrusion • Hydrogeochemical data of shallow well samples are dominated by Na-Ca-Cl type with chloride ( Cl - ) as the dominant anion, while other type of waters are also observed • The better water quality was found at lower confined aquifer, where groundwater of Na – HCO 3 type prevails • Our findings show that the cation exchange reaction and dissolution of carbonate minerals are mainly the geochemical processes in the aquifer system - in groundwater revealed that at the moment there are • Concentration of NO 3 no cross-flow contamination between the upper and lower aquifer system 15

• We thank the staff of the;-  Dar es Salaam water supply authority (DAWASA) and the Ardhi University-Dar es Salaam for their support during the sampling campaign.  Technicians in the Department of Geosciences laboratory, University of Oslo for helpful water samples analysis. • This research was supported financially by the Quator Scheme Programme. 16

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