Tidal Trends and Magnitude of Chwaka and Uzi Bays as a Proxy of Seawater Intrusion in Jozani Groundwater Forest, Zanzibar, Tanzania

Tidal characteristics, land altitude, level of groundwater, and rainfall patterns are among the main factors determining seawater intrusions in coastal areas. As Jozani Groundwater Forest (JGWF) is a coastal forest of low altitude found between Chwaka and Uzi bays, it was assumed the forest is increasingly being intruded by seawater. This study was therefore intended to discern conditions of tidal trends and magnitude in relation to seawater intrusion into JGWF. Tidal data and rainfall patterns were collected from Tanzania Meteorological Agency, Kisauni Zanzibar. Two sets of three observation wells (OWs) were opened at two forest ends towards Chwaka and Uzi bays. The OWs were used as data collection points while Height of Instrument method with the help of SOKKIA C.3.2 level and benchmarks number 205 and 210 were used to determine elevation of ground surfaces of the OWs. GPS receiver model GARMIN etrex 10 was used for geo-referencing the OWs. Water level recorders were installed above the wells to record changes in water level (WL) around the wells. Total dissolved solids (TDS) in water samples from the OWs were measured in situ using Hanna Combo tester HI 98129. Results showed that WL in South-end OWs rises during rains and falls during dry seasons, and occasionally also rises during high water of spring tide (HWST) associated with South Easterly monsoon winds. Similar results were obtained at North-end OWs, but more frequently even during HWST that were not associated with Monsoon winds. Values of water TDS fell to a minimum of 0.7 x 10 3 and 4.9 x 10 3 ppm during rainfall and rose to a maximum of 25.5 x 10 3 and 34.1 x 10 3 ppm during dry season at South-end and North-end, respectively. It was concluded that intrusion takes place during dry seasons when seawater of HWST from Chwaka and Uzi bays through creeks reached the soil surfaces at North-end and South-end of JGWF. The water accumulated salts which were then diluted and drained off the areas by rain water.