A desktop study of the groundwater resources of Cousine and Cousin Islands, Seychelles: Funded by MSP GEF and prepared for Nature Seychelles

Adrian Werner, Ben Van Den Akker, Danica Jakovovic, Anis Jacob, Erick Bestland

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    Abstract

    The groundwater resources of Cousine and Cousin Islands has previously not been investigated, although these resources are accessed via bore pumping to support tourism, residents, and visitors to the islands, in terms of domestic use, drinking water, lawn irrigation and other anthropogenic requirements. It is also likely that groundwater plays an important role in the health of the highly valued ecosystems of these two islands, given the existence of springs and other surface water bodies that depend on groundwater influxes.
    This report describes a desktop study of the groundwater resources of Cousine and Cousin Islands, and aims to address a number of issues, summarised as:
    1. Describe general considerations of importance in the management of groundwater resources of Cousine and Cousine Islands
    2. Collect and collate existing knowledge and data on the groundwater resources of Cousine and Cousin Islands
    3. Within data availability constraints, development groundwater models of the islands to explore various water supply questions relating to mainly to the sustainability of groundwater extraction
    4. Provide advice on water treatment options, hydrological monitoring, and areas of future investigation
    The report provides an overview of the water resources of small islands. The existence of fresh groundwater on small islands is mostly controlled by island size and climate, but also topography, geology, vegetation and soil type. A review of existing guidelines on island water resources indicated that Cousin and Cousine Island are classified as “very small islands” in the typology of UNESCO (1991). Islands of size similar to Cousine and Cousin Islands are considered by the authors of the UNESCO (1991) guidelines to be unlikely to contain significant fresh groundwater resources, which are expected to occur as a very thin wedge overlying saline (seawater) groundwater. It is therefore somewhat surprising that fresh groundwater supplies are abstracted from both islands in non-trivial quantities.
    The data collection phase of the study captured information relating to physiography, topography, climate, vegetation and land use, soils and geology, hydrogeology and groundwater use, and some groundwater quality observations. This information was used to develop conceptual models of the islands’ hydrogeology, and these served to develop computer models of the groundwater systems. The available dataset pertaining to the islands’ aquifers was considered to be deficient for the development of predictive management models, and further information needs to be obtained through field investigation, monitoring, sampling and analysis before reliable modelling results, sufficient to guide groundwater management decision, can be provided. Nonetheless, a groundwater modelling investigation was undertaken to assist in guiding management decisions and to demonstrate the capability of contemporary modelling methods for future studies.
    The Cousine Island groundwater model is a state-of-the-art three-dimensional groundwater flow and seawater intrusion model, which is based on cutting-edge modelling software (i.e. the MODHMS code) and GIS-based modelling techniques (e.g. Groundwater Vistas software). The model simulates the density effects of seawater-freshwater interaction, and uses interpolated surfaces to explicitly represent the hydrostratigraphic units of sand and granite, of which the island is predominantly comprised. Models were developed that simulate either long-term (548 years) or time-variant (i.e. climatic variations) conditions, and were used to evaluate both pumping and no pumping groundwater conditions. Simulations using the Cousine Island model demonstrated that pumping from several shallow bores is more likely to yield lower salinity groundwater than that obtained from a single deeper bore. This outcome is well aligned to “skimmer well” approaches adopted in similar settings of fresh groundwater overlying seawater. The Cousine Island study also demonstrated the potential for climatic variability in both water quality and watertable level, and showed that it is quite likely that a large variability in groundwater salinity could be expected from one year to the next. Simulations of well pumping, at rates similar to those historically recorded on Cousine Island, indicate that groundwater pumping has indeed induced a landward movement of saline groundwater towards the points of extraction. Further modelling is required to better optimise rates of extraction and sites of future bore construction.
    The model of Cousin Island adopted an approach that was more closely aligned to the limited available information, and more routine methods were applied (e.g. MODFLOW and PMWIN software). A “single-layer” approach (i.e. two-dimensional) was adopted and only groundwater flow was modelled in the absence of salt-related density effects. The results of the Cousin Island modelling study also provided some indication of the benefit of using multiple points of shallow groundwater extraction, although the proximity to the shoreline also proved to be a critical consideration.
    A review of water treatment options for the island water was discussed. It is clear that existing water chemistry and microbiological information are insufficient to properly plan an optimal water treatment strategy for each island. Further, more clarity of water use needs, in terms of quality and quantity, is required, especially for Cousin Island. A preliminary overview of water treatment options identified that such treatments as pH correction, filtration, reverse osmosis and/or disinfection could be considered to provide for improved water quality to users from Cousine Island, while it was not possible to predict specific water treatment works for Cousin Island with the current available information. However, it is recommended that proper analysis of water quality be undertaken before the purchase of water treatment infrastructure is further considered.
    Finally, the report outlines a preliminary proposal for future investigation of the islands groundwater resources and water quality, aimed at completing the study predominantly through field investigations, thereby allowing for more reliable estimates of sustainable groundwater use and for the provision of more specific advice relating to water treatment options.
    Original languageEnglish
    Place of PublicationBedford Park
    PublisherFlinders University
    Commissioning bodyNature Seychelles
    Number of pages76
    Publication statusPublished - Dec 2007

    Keywords

    • Groundwater model
    • Freshwater lens
    • Seychelles

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