Objective: conceptual model definition and steady state simulation of groundwater flow

Coastal Aquifer Management in the Caribbean-II Groundwater modelling Purpose of modelling!!!!!!!!

Coastal Aquifer Management in the Caribbean-II Conceptual model: a numerical model may constitute a good ‘real system’ representation if it is based in a solid conceptual model (built with available data and scientific knowledge) -Aquifer type : water table, confined, multilayer….. -Porous/fracutered media -flow regime: steady-state, transient. -Geometry -Boundary conditions: impervious, flow… -Change of properties: homogeneous, … -Natural: recharge, discharge -Artificial: pumping. Physical aspects Actions -Flow: recharge, discharge -transport: pumping. Relevant processes Data provided by a classical hydrogeologic study (site specific) Numerical model structure (code selection)

Coastal Aquifer Management in the Caribbean-II Spatial discretisation of domain Network definition Parameters: K, S Geology, recharge Pumping wells

Coastal Aquifer Management in the Caribbean-II Time discretisation Pumping,Climate,River stages, lateral inflows, etc Dimension: 1D, 2D, 3D

Outputs Piezometric level

For a: GW flow numerical model definition (Visual MODFLOW Pro) ERIN AQUIFER- ERIN AQUIFER- conceptual model

Aquifer geometry and hydrostratigraphic units characteristion is obtained through: - geological methods (geologic logs) - geochemical methods (hydrochemistry) - geophysical methods (resistivity, logs…) - field surveys

AQUIFER: Geometry (II)

AQUIFER : Hydrostratigraphic aquifer Units

-D: detritic (unconfined) Semiconfined layers FC-124: sandy layer (190 m thickness) FC-200: sandy layer (135 m thickness) (hydraulically connected by the existing faults, lateral facies changes and also at the NE of the study area) Parameters Quatitative hydraulic parameters have not been obtained. (No pumping tests available). Average estimates of Hydraulic conductivity(K) for the Erin Sands range from 0.5 – 12 m/day (ETI)

Piezometric level Water level observations and production wells abstraction data are integrated values for the great majority of the wells, as wells casing have been screened in all productive areas encountered along drilled depths.

AQUIFER: Conceptual model

NUMERICAL FLOW MODEL

Visual MODFLOW.PRO Study area model grid and boundary conditions. 500 x 500 m grid size L. Candela, K.Tamoh, J. Jimenez-Martínez, F.J. Elorza. Technical University of Catalonia (Spain) R. Ramdin, R. Sankar, J. Francis, M. Julien, N. Mc Intosh, C. Santana. Water Authority and Sanitation-(WASA) S. D’Angelo.UNESCO-IHP

NUMERICAL FLOW MODEL The five defined layers of the Erin aquifer numerical model

NUMERICAL FLOW MODEL cross-section of the Erin aquifer numerical model Tridimensional shape of the modeled area

NUMERICAL FLOW MODEL Location of abstraction wells

RESULTS Calibration plot

RESULTS final calibrated values of hydraulic parameters for the Erin hydrostratigraphic units

RESULTS Layer 1, Detritic. Head potential

RESULTS Layer 3, FC124. head potential

RESULTS Layer 5, FC200. head potential

RESULTS Simulated water balance

SENSITIVITY ANALYSIS : Effects of hydraulic conductivity

SENSITIVITY ANALYSIS : Effects of recharge

CONCLUDING REMARKS:  Geologic characterisation of the unit proved to be non-sufficient to properly define the aquifer system conceptual model  Recharge is the most important parameter conditioning groundwater level  20% decrease of natural recharge from precipitation could imply : 1) a decrease of groundwater level between 0.7 and 4.3 m 2) seawater input may increase into the aquifer up to 120 % m 3  An accurate DEM is needed for model definition  Results are only representative for steady state conditions and for the calibration period  Piezometric data acquisition along time (hydrographs) and specific aquifer water level is needed for future transient simulations.  Meteorological data for climate change impacts simulation under selected scenarios have not been provided and future impacts cannot be simulated.