Application of MODFLOW for simulating groundwater flow and solute ...
Undergraduate/Graduate Category: Graduate Degree: Civil Engineering Abstract ID#: 511
Application of MODFLOW for simulating groundwater flow and solute transport in the Vega Alta karst aquifer, Puerto Rico R. Ghasemizadeh, X. Yu, H. Weerasinghe, C. Butscher, I. Padilla, F. Hellweger, A. Alshawabkeh Abstract.
Karst aquifers have a high degree of heterogeneity and anisotropy in their geologic and hydrogeologic parameters which generate high levels of uncertainty and complication in modeling through such systems. Therefore, the application of an equivalent porous media (EPM) approach for modeling investigations in karst aquifers requires extra attention. Using finite difference MODFLOW and MT3D computer codes, a numerical model was developed groundwater flow and contaminant transport on the basis of a conceptualized, equivalent, continuous medium for a karst basin located in Vega Alta, Puerto Rico. The model illustrates the application of EPM approach, which does not directly consider the possible conduit or fracture flow, in karst hydrogeology. Existing hydrogeological data and previous USGS studies conducted in the area were used to define model input such as the distribution of hydraulic conductivity. Recharge conditions were adopted by monthly precipitation data provided by NOAA. Hydraulic conductivity and specific yield were further calibrated against water level data of three historical observation stations in the area. The reasonable agreement between measured and simulated heads validates the reliability of assigned hydraulic parameters such as the computed horizontal hydraulic conductivity ranged from 0.2 to 960 ft/day. Groundwater flow is mostly northward and the calibrated TCE concentration of the recharge in source area is 2.8 mg/l and the long-term average TCE influx into the aquifer is estimated to be 22 lb/yr. The established model can be used to simulate temporal and spatial variations of groundwater levels and transport of Trichloroethylene (TCE) contamination plume.
Model Outputs:
Areal recharge
Boundary Conditions
Model Inputs:
Flow directions, Equipotential contours
Specific yield
• • • •
GW withdrawals (pumping rates & well locations) Chemical properties of TCE Hydraulic conductivity Riverbed conductivities Initial conditions (head & concentration)
No Flow BC Constant Head BC No Flow BC River BC
Pumping Wells
No Flow BC Recharge Concentration BC (2.8 mg/l)
Model Calibration: Minimizing MAE and RMS, and maximizing R2 to improve the match between simulated and measured groundwater head in USGS well 182647066201700 SABANA HOYOS 2 WELL
W.T. fluctuations: Monthly water table level hydrograph in USGS well 182647066201700, for period of 1983 – 2011 W.T increases over 2.5 ft over 30 years Monthly rainfall (data from NOAA)
Daily W.T. Variation: Using refiner data (daily rainfall) produces higher resolution behavior of W.T. in response to rainfall events and give a closer match with the measured data
Estimating W.T. Recovery: Scattered plot of simulated water-level recovery vs. rainfall intensities assists to roughly predict W.T. variation knowing the rainfall intensity
Results and Discussion: • Sensitivity analyses indicated that the temporal variation in water-level elevations is most sensitive to variation in recharge, pumpage, and specific yield • Although EPR approach does not directly account for possible conduits, but satisfactory results maybe achieved in moderately karstified aquifers • The reasonable agreement between measured TCE data and simulated plume verifies the reliability of the assigned model parameters • Only the general trend (large scale) of solute transport may be predicted by the EPM approach. The local scale behavior of solute movement is highly impacted by high permeable paths
Site location map, municipalities, major streams, superfund sites, and Digital Elevation Model (DEM) map of the study area Geologic formations of Vega Alta aquifer Karst Modeling Challenges: - High heterogeneity and random anisotropy - Unknown location and pattern of conduits & fractures - High uncertainty in geologic and hydraulic parameters
This program is supported by Award Number P42ES017198 from the National Institute of Environmental Health Sciences.
Cross-section A-B Karst Modeling Approaches: - Equivalent Porous Media - Dual Porosity Model - Discrete Fracture Network - Discrete Conduit Network - Hybrid Models
TCE Plume Fate & Transport: TCE plume at Focus area, from 1990 to 2005. The initial condition for TCE concentrations in Jan 1990 changes over time and the plume become smaller
Aquifer Recharge • Precipitation (99.90 %) • Atlantic Ocean (0.06 %) • River leakage (0.04 %)
Aquifer Discharge • Withdrawals (65.08 %) • Atlantic Ocean (19.14 %) • River drainage (16.47 %)
Water level variations • Raised about 2.5 ft over the last three decades • Maximum overall water table level in 2011 • Minimum overall water table level in 1995
www.neu.edu/protect
Application of MODFLOW for simulating groundwater flow and solute transport in the Vega Alta karst aquifer, Puerto Rico R. Ghasemizadeh, X. Yu, H. Weerasinghe, C. Butscher, I. Padilla, F. Hellweger, A. Alshawabkeh Abstract.
Karst aquifers have a high degree of heterogeneity and anisotropy in their geologic and hydrogeologic parameters which generate high levels of uncertainty and complication in modeling through such systems. Therefore, the application of an equivalent porous media (EPM) approach for modeling investigations in karst aquifers requires extra attention. Using finite difference MODFLOW and MT3D computer codes, a numerical model was developed groundwater flow and contaminant transport on the basis of a conceptualized, equivalent, continuous medium for a karst basin located in Vega Alta, Puerto Rico. The model illustrates the application of EPM approach, which does not directly consider the possible conduit or fracture flow, in karst hydrogeology. Existing hydrogeological data and previous USGS studies conducted in the area were used to define model input such as the distribution of hydraulic conductivity. Recharge conditions were adopted by monthly precipitation data provided by NOAA. Hydraulic conductivity and specific yield were further calibrated against water level data of three historical observation stations in the area. The reasonable agreement between measured and simulated heads validates the reliability of assigned hydraulic parameters such as the computed horizontal hydraulic conductivity ranged from 0.2 to 960 ft/day. Groundwater flow is mostly northward and the calibrated TCE concentration of the recharge in source area is 2.8 mg/l and the long-term average TCE influx into the aquifer is estimated to be 22 lb/yr. The established model can be used to simulate temporal and spatial variations of groundwater levels and transport of Trichloroethylene (TCE) contamination plume.
Model Outputs:
Areal recharge
Boundary Conditions
Model Inputs:
Flow directions, Equipotential contours
Specific yield
• • • •
GW withdrawals (pumping rates & well locations) Chemical properties of TCE Hydraulic conductivity Riverbed conductivities Initial conditions (head & concentration)
No Flow BC Constant Head BC No Flow BC River BC
Pumping Wells
No Flow BC Recharge Concentration BC (2.8 mg/l)
Model Calibration: Minimizing MAE and RMS, and maximizing R2 to improve the match between simulated and measured groundwater head in USGS well 182647066201700 SABANA HOYOS 2 WELL
W.T. fluctuations: Monthly water table level hydrograph in USGS well 182647066201700, for period of 1983 – 2011 W.T increases over 2.5 ft over 30 years Monthly rainfall (data from NOAA)
Daily W.T. Variation: Using refiner data (daily rainfall) produces higher resolution behavior of W.T. in response to rainfall events and give a closer match with the measured data
Estimating W.T. Recovery: Scattered plot of simulated water-level recovery vs. rainfall intensities assists to roughly predict W.T. variation knowing the rainfall intensity
Results and Discussion: • Sensitivity analyses indicated that the temporal variation in water-level elevations is most sensitive to variation in recharge, pumpage, and specific yield • Although EPR approach does not directly account for possible conduits, but satisfactory results maybe achieved in moderately karstified aquifers • The reasonable agreement between measured TCE data and simulated plume verifies the reliability of the assigned model parameters • Only the general trend (large scale) of solute transport may be predicted by the EPM approach. The local scale behavior of solute movement is highly impacted by high permeable paths
Site location map, municipalities, major streams, superfund sites, and Digital Elevation Model (DEM) map of the study area Geologic formations of Vega Alta aquifer Karst Modeling Challenges: - High heterogeneity and random anisotropy - Unknown location and pattern of conduits & fractures - High uncertainty in geologic and hydraulic parameters
This program is supported by Award Number P42ES017198 from the National Institute of Environmental Health Sciences.
Cross-section A-B Karst Modeling Approaches: - Equivalent Porous Media - Dual Porosity Model - Discrete Fracture Network - Discrete Conduit Network - Hybrid Models
TCE Plume Fate & Transport: TCE plume at Focus area, from 1990 to 2005. The initial condition for TCE concentrations in Jan 1990 changes over time and the plume become smaller
Aquifer Recharge • Precipitation (99.90 %) • Atlantic Ocean (0.06 %) • River leakage (0.04 %)
Aquifer Discharge • Withdrawals (65.08 %) • Atlantic Ocean (19.14 %) • River drainage (16.47 %)
Water level variations • Raised about 2.5 ft over the last three decades • Maximum overall water table level in 2011 • Minimum overall water table level in 1995
www.neu.edu/protect
