Extent of seawater intrusion from Chwaka and Uzi bays into Jozani Groundwater Forest, Zanzibar, Tanzania.

International Journal of Scientific & Engineering Research, 2017; 8 (5): 1143-1149.

The rise in the total dissolved solids (TDS) in a coastal land is among the signs of seawater intrusion into the land. In many cases, the magnitude of the effects of seawater intrusion depends upon the proximity of the land to the coast, amount, and patterns of freshwater recharging the aquifer underneath. The intention of this study was to determine the extent to which Jozani groundwater forest (JGWF) has been affected by seawater intrusion from Chwaka and Uzi bays. Rainfall data were collected from Tanzania Meteorological Agency (TMA). Spatial and temporal TDS values were collected from temporary wells (TWs) and local wells. Desk work divided JGWF length into 11 northern and 4 eastern gridlines. Along northern gridlines, three grid points were selected on which TWs were drilled for data collection. GARMIN etrex 10 GPS was used for geo-referencing the wells. Water samples were collected at about 0.1 m depth from water surface and were tested in situ using Hanna Combo HI 98129 tester. Water tests were done in the middle of Kiangazi, Masika, Kipupwe and Vuli seasons. The area of JGWF was estimated based on JGWF elevation map. The results showed that the average TDS values in JGWF ranged from 0.4 - 25 x 103 mg L-1. The least TDS values were recorded from the inner parts of JGWF while highest TDS values were recorded from the outmost parts. It was found that rainfall patterns: rainy and dry seasons affected TDS values and their spatial trends. The TDS severity in JGWF area was assigned five categories namely none: least-, slightly-, moderately- and severely-affected areas. The range of TDS values for these categories were 0 – 0.5, 0.5 – 2, 2 - 5, 5 - 10 and >10 x 103 mg L-1 and the areas of occupation were about 342.3 ha (58.2%), 61.8 ha (10.5%), 46.8 ha (8.0%), 47.4 ha (8.1%) and 89.4 ha (15.2%), respectively. In conclusion, it was found that about 77% of JGWF is free from intrusion effects, but there were some variations of TDS values between dry and rainy seasons that also in the long run cause TDS fluctuation between years