1. Workshop on “Coastal Aquifer Management in the Caribbean” 14 th - 16 th December 2011 Trinidad and Tobago Workshop on “Coastal Aquifer Management in the Caribbean” 14 th - 16 th December 2011 Trinidad and Tobago Yallahs Alluvium Aquifer in Jamaica Presented by Angella Graham –Water Resources Authority, Jamaica

2. Outline of Presentation   Background- Project Area   Model – Conceptual, Input Parameters   Model Result   Next Steps

4.   Comprises an area of 200 km2. km, and is drained by the Yallahs River.   The Yallahs River flows south- southeast until it reaches the southern coastal plain, and forms an alluvial delta.   Elevation vary from 2000 m in the north to 0 m in the south.

5. Hydrostratigraphy 82% Basal aquiclude 11% Alluvial aquifer 7.2 % Limestone aquiclude and aquifer

6. Climate Rainfall -approx. 2085 mm/year; ET- 1,045 mm/year

7. Diversion of the Yallahs River   Since the mid-1980s, 17.52 Mm 3 /yr of water from the Yallahs River has been diverted to augment freshwater supply in the Kingston Metropolitan Area.   Diversion has led to the conversion of Yallahs River from a perennial to seasonal river and has seriously affected water availability for downstream users in the area.   Irrigation has been especially affected since it relied entirely on surface water.   Diversion has increased reliance on groundwater in the alluvial aquifer of the lower part of the Yallahs River WMU.

8. Water Resources Groundwater Safe Yield - 23.1 Mm 3 /yr Reliable Surface - 13.2 Mm 3 /yr Available Water Resources – 36.3 Mm 3 /yr Total Licenced Abstraction- 49.42 Mm 3 /yr

9. Total Demand – 11.9 Mm 3 /year

10. Modeling Methodology   Mathematical modeling in conjunction with geological and hydrogeological data.   The software used is VisualMODFLOW Pro 2009(version 4.3)

11. Study Area - Yallahs Alluvial Aquifer- Conceptual Model- Geology and structure   Area -16.6 km 2.   The aquifer is bound by the Caribbean Sea in the south, by salt marshes in the west, by the Morant River WMU in the east and by aquicludes in the north-east and north- west   Coastal in nature

12. Cross-section of alluvium  Aquifer is unconfined  Alluvium is comprised of coarse gravel, cobbles and boulders containing patches of fine-grained sand and silt at places.  Thickness vary from 13 m - 50m

13. Static Water Level

14. Boundary Conditions   The southern boundary was designated as a constant ‐ head boundary with the head at sea level.   Salt marshes in the west- constant head.   Eastern boundary was also considered impervious.   Boundaries to the northeast and northwest were made impervious.

15. Active/Inactive Zones

16. Input Parameters Rainfall – 2,085 mm/year Recharge- Rainfall and subsurface flow of the Yallahs River- 9.3 *10 ^6 m 3 /year- 564 mm/year Storage value - 0.2 Evapotranspiration-1,045 mm/year 30 ‐

17. Recharge Areas

18. Hydraulic Conductivity Value range; red -20 m/day, blue- 35 m/day; green- 40 m/day

19. Location of Pumping and Observation Wells 5 – Pumping well ; 4,000 m 3 /day- 10,000 m 3 /day; PWL 69 mamsl - 2 mamsl 6 Observation well- water level vary from 1- 40 mamsl.

20. Water Balance

21. Calibration Result

22. Next Step 1. Sensitivity analysis-Y to account for the uncertainties in the estimates of aquifer parameters and recharge values 2. Model Climate Change Scenarios- PRECIS Model

23. Climate Change Scenario – Rainfall

25. Impact on the Aquifer   Less water for groundwater recharge   Increase ET   Reduction in streamflow   Increase in Salinity- Use SEAWAT to determine the threat of saline intrusion

26. Aquifer Management Strategies 1. Revision of Abstraction Rates – Wet and Dry season abstraction rates 2. Artificial Recharge 3. Monitoring of activities in the upper watershed

27. Thank You

28. References   Baptiste, J., Hydrogeological study of the lower Yallah’s Basin, St. Thomas, Jamaica, W.I., Ph.D thesis (monograph), University of the West Indies, Department of Geology, faculty of Natural Resources, Campus: Mona, 1996   Modflow Model of the Yallahs Alluvium Aquifer- Dr. Faisal Butt.   Water Resources Master Plan 2005 (Draft)- Water Resources Authority