Climate change has been attributed to an increase in frequency and intensity of extreme hydro-meteorological conditions, including droughts and overtopping experienced in the Pacific. Low-lying atolls, with elevations less than 5m and communities with a historical dependence on shallow fresh groundwater lenses, are highly susceptible to the impacts of drought and seawater inundation. Onotoa atoll, located in the southern Gilbert Islands of Kiribati, with population of 1,519), receives relatively less rainfall than other parts of Kiribati, and 85% of all households is reliant on groundwater from shallow wells(Census 2015).
In collaboration with the Kiribati Government, The Pacific Community undertook groundwater investigations using EM34 for 7 villages of Onotoa. Water quality sampling for E.Coli from household wells, installation of an automatic raingauge, and well survey data used to supplement the geophysics. The assessments helped to identify thickest parts of freshwater lens and advise the community on water resource development opportunities aimed at improving the supply of potable water during extended dry periods.
EM34 geophysics is rapid and cost-effective assessment technique, indicating lateral and vertical variation in bulk conductivity. The surveys are relatively easy to undertake, with results that are indicative of the presence and relative thickness of fresh groundwater. The confidence in interpretation of EM34 measurements is greatly improved as results can be calibrated against known freshwater lens thicknesses from monitoring bores in similar hydrogeological settings, such as found in Bonriki, South Tarawa, Kiribati.
Support was provided to the Government of Kiribati’s water resources staff to develop their capacity to plan and undertake water resource assessments. The Onotoa assessment included 48 EM-34 survey lines across width of the islets, at a separation of 200m, with measurements taken every 20m along each traverse using the horizontal dipole.
Resulting information was processed and GIS-based maps generated indicating the variability in thickness and extent of fresh groundwater lens. These were shared with the community to assist them with their planning for future water supply needs, especially during extended dry periods. Two of the villages demonstrated poor groundwater potential and are vulnerable to groundwater salinisation during droughts, which may require investigation of alternative options, including desalinisation. Survey information for the remaining villages identified areas of thicker fresh groundwater potential that could guide future water supply investments, such as infiltration galleries and communal well systems.
This case study highlights application of EM34 geophysics as a tool to identify fresh groundwater potential in atolls across the Pacific.