Abstract:
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 103 and 4.9 x 103 ppm during rainfall and rose to a maximum of 25.5 x 103 and 34.1 x 103 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.
