Mapping of the Basseterre Valley Aquifer

Transcription

1 Mapping of the Basseterre Valley Aquifer Coastal Aquifer Management in Caribbean SIDS Workshop, St. Kitts-Nevis Halla Sahely, Ph.D., P.Eng St. Kitts Water Services Department October 11, 2010

2 Participating Authors Sandy Nettles N.S. Nettles and Associates, Florida, USA Dr. Ravidya Burrowes Environmental Management Consultants Caribbean Inc., Jamaica Dr. Glenn Haas and Dr. Bob Aukerman Aukerman, Haas and Associates, Colorado, USA Funded by GEF and the gov t of SKN

3 Outline Project objectives Characteristics of the Basseterre Valley Land use survey Review of legal enabling environment National park as a means of protection Hydrogeologic survey Challenges and next steps

4 Project objectives Demonstrate proper management and protection of the Basseterre Aquifer and well field by: Mitigating threats from contaminants Protecting the aquifer, well field, and associated ecosystems Improving the user resource management interface

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6 (2,220 acres) (3,010 acres)

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8 (2,220 acres) (3,010 acres)

9 Natural Resources Survey

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12 Natural Resources Survey

13 Land Use Survey Pressure to develop sugar lands 70-80% of land in the BVW are crown lands Trend towards medium to high density subdivisions Increase in informal settlements 2006 St. Kitts National Development Plan Approximately 1200 acres in the BVW slotted for development (~500 acres for National Park) Focus on housing projects in the upper watershed area Some more urban development as Basseterre grows

14 Pollution threats 1. Improper waste disposal practices (solid and wastewater) No centralized collection and treatment of wastewater Dumping of solid waste on vacant land and waterways ( ghauts )

15 Pollution threats 2. Industrial and commercial discharges and urban runoff No discharge permits required for disposal of wastewater from various sectors such as the power plant, mechanic shops etc. Increase in impermeable areas especially near to the airport may increase hydrocarbon loading

16 Pollution threats 3. Agricultural pollution Decline in plantation agriculture has reduced inputs of pesticides and fertilizers in the system Nutrient loading possible from informal livestock farming especially in sensitive areas

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18 Enabling Environment Water policy/legislation/institutional review Outdated legislation Groundwater not specifically protected under the law No statutory requirement for master planning No licensing of private water abstraction No discharge licenses or effluent standards No formal water quality standards Several agencies involved in water issues but not one to oversee or coordinate

19 LEGAL ACTION Enabling Environment New water act to be drafted soon Designation of the well-field as a NATIONAL PARK as a means of aquifer protection

20 STAKEHOLDER CONSULTATION National Park Plan Wide consultations even at early stages of project Protect the well-field and the BVA Restore and maintain a native forest Provide open green space in an increasingly urban setting High-valued tourist attraction Contribute to economic welfare and development Serve as an outdoor classroom and laboratory

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23 Hydrogeologic Survey 1. Review and evaluation of all existing hydrogeologic data 2. Mapping of the aquifer using a novel geophysical technique 3. Recording of water levels and video survey of existing wells 4. Sampling of wells for various water quality parameters 5. Construction of a computer simulation model.

24 Hydrogeologic Survey Basseterre Valley Aquifer (BVA) Unconfined aquifer recharged by rainfall Delineation of fresh / salt water interface critical 10 wells pumping between GPM Assumed safe yield is about 2.5 MGD First study in the 1977 (Dr. Christmas) Further wells drilled in 1980s (CIDA funded) Masters research by Williams in 1999 Most of the historical data is focused on only 10% of watershed

25 Safe Sustainable Yield Known Factors Delineation of the watershed basin hydrologic boundary. Topography and geomorphology. Rainfall rate and distribution. Soils type, distribution and hydrologic characteristics. Groundwater withdrawal locations, rates and periodicity. Unknown Factors Definition of the underlying geology and aquifer units. Determination of aquifer hydrologic parameters of Transmissivity, Storage, porosity, water level elevation, gradient and fluctuation. Evapotranspiration rates and distribution. Surface water runoff. Groundwater recharge rate.

26 Hydrogeologic Survey Geophysical Mapping The thickness and distribution of sediments throughout the aquifer Zones of increased porosity Zones of possible contamination The fresh/salt water interface Seawater is one-fortieth more dense than freshwater Ghyben-Herzberg principle

27 Hydrogeologic Survey Multi-Electrode Resistivity (MER) Resistivity is the property of a material to resist the flow of electrical current Differences in resistivity of earth materials Electric current is introduced into the ground Electrical fields that flow through the layers of earth in the subsurface are observed Resistivity of earth materials is very sensitive to water content

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29 Hydrogeologic Survey Three distinct units identified using MER: Unit I: A high resistivity unit of dry sands, clayey sands and volcanic rock (~5.5 m. thick) Unit II: Similar to Unit I but partially saturated with water (~14 m. thick). Unit III: Water storage unit of aquifer (~ 22 m. thick) Gravels, coarse sands and boulder rocks Lower part of Unit III is the saltwater saturated part of the aquifer Interface of fresh / salt water identified

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34 Hydrogeologic Survey Water quality sampling Ten supply wells and three surface water samples No measurable levels of pesticides, herbicides, semi-volatile or volatile organics in well water Clear increase over time of total dissolved solids (TDS) and chloride Correlate with decreasing water levels Early signs of saltwater intrusion

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38 Groundwater modeling Adjusting the efficiency and/or location of some wells within current wellfield is a viable option for improvement Areas north and west of the airport are a potential good location for additional wells with minimal impact to the aquifer and minimal pollutant intake Combination of new wells and improved efficiency of the current wells is the key to maintaining the aquifer More data needed!

39 Challenges and next steps Need to fill in crucial data gaps Do we have an accurate picture of the aquifer? Can we rely on historical data to establish trends? Is the decline in static water levels as steep as is suggested? Does it matter? Further mapping of upper watershed Extensive monitoring of aquifer is essential Estimation of safe sustainable yield is key

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41 Challenges and next steps Condition assessment of the well-field Likely operating at capacity Lack of optimal screen and gravel pack construction has led to decreased well efficiency Lack of well construction descriptions and aquifer testing methodologies from previous work provide only a limited understanding of the true hydraulic characteristics of the aquifer

42 Challenges and next steps Condition assessment of the well-field Partial penetration effects Is rehabilitation possible and cost-effective? New wells needed

43 Challenges and next steps Need to exploit additional groundwater Deep well drilling in the next year Accommodate growing agricultural and tourism sectors Adaptation / readiness for climate change More effective management of WATER Demand management Water loss prevention

44 Thank you! Special thank you to GEF-IWCAM PCU Questions???