Hydrogeology of the Middle Sigatoka Valley, Southwest Viti Levu, Fiji.
Thesis DisciplineEngineering Geology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Bedrock geology of the middle Sigatoka valley, located ~ 20 km inland of the township of Sigatoka, southwest of Viti Levu, Fiji, includes the steeply inclined Tari Fomation and uplifted Qalimare Limestone of the Wainimala Group of Oligocene to Middle Miocene age. These units are juxtaposed against the steeply inclined and moderately-highly fractured Cici Sandstone and Takaro Conglomerate/Rudite of the Tuva Sedimentary Group of Late Miocene age, northwest of the study area. Surface geology is largely expressed by the sinistral Nasovatava Fault, and its associated faults splays, and unconsolidated Quaternary to Recent Alluvium that fills the incised valley. The valley includes a ~40 km² rural area, from Bilalevu to Dubalevu, characterized by semi-commercial agricultural land-use practices. This area is subject to high water resource demand and utilisation, particularly from groundwater sources. Geophysical surveys, through electromagnetic and electrical resistivity methods, at Dubalevu, Tubakeli and Bila Rd, permitted the estimation of depth to bedrock, thickness and extent of alluvial materials and the presence of fractured systems within the mapped units. Groundwater drilling, through mud-rotary circulation, permitted the characterization of the major hydrogeological units, namely surficial confining unit, alluvial aquifer system, and intermediate confining unit and fractured aquifer systems. The hydrogeological framework is capped by low permeability surficial confining unit comprising sandy silt and silty loam with 7.8x10⁻⁸ m/s at Dubalevu, 2.2x10⁻⁷ m/s at Tubakeli and 4.3x10⁻⁸ m/s at Bila Rd. The alluvial aquifer system, comprising unconsolidated coarse-medium gravels with some fine-medium sand in Dubalevu and Bilalevu have an estimated transmissivity and conductivity of 600 m²/d⁻¹ and 30 m/d⁻¹ and 1644 m²/d⁻¹ and 274.8 m/d⁻¹, respectively. The intermediate confining unit is composed of the fresh and unweathered sedimentary rock mass. Groundwater bore pump tests showed the fractured sedimentary system has an average transmissivity, storativity and conductivity of 10.9 m²/d⁻¹, 3.7x10⁻⁴, 1.16 m/d⁻¹ for the fractured Cici Sandstone at Dubalevu, 26.2 m²/d⁻¹, 6.1x10⁻⁴, 6.1 m/d⁻¹ for the fractured Takaro Conglomerate/Rudite at Tubakeli and 48.7 m²/d⁻¹, 1.7x10⁻³, 9.1 m/d⁻¹ for the fractured Tari Formation at Bila Rd. Hydrogeochemical classification of groundwaters, springs and surface sources, showed the dominance of Ca(HCO₃)₂ type water suggesting the dissolution of calcite in the sedimentary units with variable sources of Na⁺ and Mg⁺². Stable hydrogen and oxygen isotope analyses of groundwater, surface water and precipitation show a local meteoric water line slope of 4.3, suggesting that water resources in the middle Sigatoka Valley are meteoric in origin with variable modifications via evaporation and mixing with older residual evaporated and connate groundwater. Chloride concentrations confirm the dominance of meteoric rechargeinduced dilution. Groundwater recharge estimation, through physical and chemical mass balance models, yielded different results of 0.08 m/month (physical) and 0.2-0.6 m/month (chemical) in the wet season and -0.01 m/month (physical) and 0.1-0.2 m/month (chemical) during the dry season. Groundwater recharge mechanisms in the Sigatoka Valley, include moderate-rapidly dispersing meteoric-derived waters along fractured flow paths and seasonally variable leakage, from surface sources, dispersing along alluvial macro-poses. Groundwater protection and sustainable management in the study area is likely to be threatened by unregulated groundwater drilling, excess groundwater abstraction, absence of groundwater legislation and growing concerns of climatic variability. This thesis concludes by presenting several recommendations for groundwater protection and sustainable management, including sound groundwater exploration and evaluation of geological units, establishing an effective and sustainable legal frameworks, and increasing awareness of inherent groundwater issues such as climatic seasonality, climate change, vulnerability to contamination and unsustainable abstraction.