1. Application of Geophysical Surveying Techniques to Characterization of Seawater Intrusion Presented by Karl Payne

2. CIMH Mandate “… to assist in improving and developing the Meteorological and Hydrological Services as well as providing the awareness of the benefits of Meteorology and Hydrology for the economic well-being of the CIMH member states. This is achieved through training, research, investigations and the provision of related specialized services and advice”.

3. Caribbean Meteorological Institute (CMI) established August 23, 1967 under special funding from UNDP and WMO –16 Commonwealth Caribbean Governments that comprise the Caribbean Meteorological Organization operate CMI CMI becomes affiliated with UWI in 1973 –Undergraduate major in meteorology established in Faculty of Natural Sciences CMI designated by WMO as a Regional Meteorological Training Centre in 1978 Caribbean Operational Hydrology Institute (COHI) established in 1982 and located at CMI CIMH established in 1987 by combining CMI & COHI CIMH History

4. Anguilla, Antigua and Barbuda Barbados Belize British Virgin Islands Cayman Islands Dominica Grenada Guyana Jamaica Montserrat St. Kitts/Nevis St. Lucia St. Vincent and the Grenadines Trinidad and Tobago Turks and Caicos Islands Member Countries

5. Train meteorological and hydrological personnel Operate as a centre of research in meteorology, hydrology and associated sciences Data collection, storage, & dissemination Maintain, repair, and calibrate meteorological & hydrological instruments Advise regional governments and partner agencies on hydrological & meteorological matters Provide consulting services to industry Key Functions

6. Introduction Seawater intrusion into coastal freshwater aquifers reduces aquifer quality and sustainability. In many Caribbean islands, seawater intrusion into coastal karst freshwater aquifers is exacerbated by groundwater abstraction for public supply and irrigation. Apart from over-pumping, water quality in coastal aquifers is under threat from the anticipated consequences of climate change.

7. In 2002, the Inter-American Agency for Cooperation and Development (IACD) initiated funding for a three-year project to address groundwater management in coastal karst environments on a number of Caribbean islands, including Barbados. As part of the project, a pilot site was established along a portion of the west coast of Barbados to test and evaluate strategies and methodologies to improve groundwater management on the island. Introduction

8. An integral component in the development of a groundwater management strategy for an aquifer is a conceptual model describing the hydrologic processes at the site. Unfortunately, few data existed at the start of the project to adequately describe these processes. To address this shortcoming, a geophysical characterization program was initiated. Challenges

9. Sustainable Aquifer Management Framework Site Characterization Conceptual Groundwater Model Development Numerical Groundwater Model Implementation Aquifer Management/ Decision Support System

10. Non-invasive geophysical surveys were performed at the pilot site on Barbados as part of the geologic and hydrologic characterization program designed to support the development of a long-term sustainable management program for coastal karst aquifers. Methodology

11. Time-domain electromagnetic soundings Schlumberger D.C. resistivity depth soundings Multi-electrode dipole-dipole D.C. resistivity profiling These methods were chosen based on the significant differences in electrical resistivity between freshwater and seawater and the documented success of these techniques for integrated geophysical interpretation (Sandberg, 1993). Geophysical Surveys

12. Electrical Properties of Rocks Electrical resistivity(ρ, rho) is a measure of resistance to current flow. Conductivity is the ability of a material to conduct current under an applied electric force i.e. voltage. Fig 1: Range of resistivity values for different rocks

13. Site Description Fig 2. Location of the Porters and Trents groundwater management zones at the Barbados pilot site

14. Site Description (Land Use, Geology & Hydrology) Land use varies across the pilot site: Hotel complexes, private residences, golf courses, agricultural, and pasture/scrub vegetation. Geology and Hydrology: Pleistocene limestone represents the primary rock type at the pilot site. Adjacent to the coast, the LCR unit may be covered by a mixture of marine beach and modern dune deposits. Fig 3. Geology of the pilot site (Poole and Barker, 1982). The purple contours on the map represent the elevation of the base of limestone.

15. Site Geology & Hydrology Geology: The Pleistocene limestone can be divided into three terraces separated by the First High Cliff and the Second High Cliff. The Lower Coral Reef (LCR), the Middle Coral Reef (MCR) and the Upper Coral Reef (UCR). The Pleistocene limestone units sit on top of sediments of the Oceanic Group. This group consists of deep-sea clay and marl deposits. Available geologic data indicate that there are no significant faults at the pilot site.

16. Site Hydrology Potable groundwater at the Barbados pilot site occurs primarily in karst limestone formations. This water occurs as a thin freshwater lens, which may be on the order of several meters to a few tens of meters. Because of the potential for karst solution features to be present in the limestone, groundwater flow in the sheetwater zone may occur as diffuse (i.e., matrix) flow and/or focused (i.e., conduit) flow

17. Results(High-Resolution Electrical Resistivity Tomographic Imaging ) Fig 4 Geophysical surveys near the Trents playing field and St. James Secondary School. Dipole resistivity surveys were performed along 9 transects, using an electrode spacing of 5m. The length of the survey lines ranged from 95-215, with the maximum sounding depth being 30-35m

18. Results Fig 5. Resistivity image along dipole-dipole Profile #1,#9 and #8 respectively. The transition from electrically conductive wet beach sand and dune deposits in the west to electrically resistive dry limestone in the east, which occurs at the 25 m mark, is evident. Line 9 demonstrates the karst nature of the limestone bedrock. The low resistivity near the surface is consistent with the assumption that the soil is less resistive than the limestone. In addition, the high electrical conductivity of the soil may result from the high moisture conditions indicative of recharge zones

19. Sinkhole Fig 6. Image of sinkhole (Nationnews)

20. Time-Domain Electromagnetic & Resistivity Depth Soundings TDEM and Schlumberger resistivity depth soundings were used to characterize the pilot site geology down to depths greater than 50m. According to Fitterman et al. (1999) for studies conducted in karst aquifers in Florida: Seawater (<10 ohm-m) Freshwater(>30 ohm-m)

21. Survey Lines (Transects)

22. Results Fig 6. Resistivity model developed from TDEM data. Superimposed on the figure are cut-off electrical resistivites based on Fitterman et al (1999).

23. Summary of Results (TDEM Survey) Transition from freshwater to seawater occurs over a narrow region near the coast. This transition zone appears to thicken to the east. It could be possible that the mixing is induced by the high velocity streamwater flows entering the sheetwater zone Measurements suggest that the transition zone can attain a thickness of 5 m to 15 m

24. Conclusions The application of electrical and electromagnetic resistivity sounding techniques at the Barbados pilot site successfully delineated the freshwater- seawater interface. The transition zone is relatively narrow (5m- 15m). Site characterization identified possible karst conduits in the subsurface.