Aquifer characterization for ASR (aquifer storage and recovery) projects
Aquifer characterization is the most fundamental part of ASR project for the simple reason: if hydrogeological conditions are not favorable ASR system will not perform well and serve its purpose. Aquifer characterization includes all aquifer conditions that can potentially impact ASR project and according to Maliva and Missamer 2010 it includes:
• Aquifer three dimensional geometry and extent • Location and properties of vertical and horizontal confining units and strata • Overall aquifer water budget and water level trends (both annually and seasonally) • Amount, location and seasonality of existing aquifer water use (pumpage) • Aquifer hydraulic parameters • Aquifer heterogenety and anisotropy • Local storage zone water quality (chemistry) • Water quality in adjoining starta • Water quality variations within storage zone • Mineralogy of the storage zone and adjoining confining strata • Compatibility of recharged water with native groundwater and aquifer rock or sediment • Potential fate and transport of microorganisms and chemicals present in recharged water
Aquifer characterization includes five main categories:
1. Formation testing 2. Aquifer hydraulic testing 3. Geophysical methods 4. Groundwater modeling 5. Geochemical analyses and modeling
In this paper we will focus on groundwater modeling as important part of aquifer characterization for ASR projects
Groundwater modeling as part of aquifer characterization for ASR projects
Short history of software for simulating groundwater flow and transport
The first software that has been documented in detail and widely accessible was the PLASM. This software package was designed for 2D simulations using the numerical method of finite differences. Numerous authors progressively implemented various additional options and in 1981 the option “Random Walk” for contamination transport simulation was added. This software package constitutes the beginning of wider use of numerical methods in hydrogeology of that time.
Considering contamination transport the first successful simulations are related to USGS MOC software package. It was meant for 2D simulations of contamination transport by the method of characteristics and was well documented as well.
Next step represents the launch of MODFLOW software package in 1988 by USGS. This software package has been adopted rapidly throughout the world for many reasons and being well documented may be the quite important one. In the middle of nineties the first GUI (graphical user interface) packages appeared such as Groundwater Vistas, Visual MODFLOW and Model CAD working under WINDOWS operating system and making a precondition for comfort application of 3D numerical modelling.
As an open to all (free of charge) software package the MODFLOW obtained many improvements and upgrades, many new physical processes have been implemented as various contamination transport simulations (MT3DMS, RT3D, PHT3D, SEAWAT) too. The current state of the software package is available on USGS website. The most significant fact is that the basic concept represented by continuous layers, orthogonal grid and classical formulation of underground water flow[3], hasn’t been changed since 1968.
In the early 2013 the official version of MODFLOW USG software package appeared on USGS website. The abbreviation USG is for unstructured grid what represents a new approach in mathematical modelling of underground water. The reaction of GUI (graphical user interface) software developers was rapid and the new version of Groundwater Vistas (ver. 6) implemented this software package.
Numerical experiment with dual purpose well – ASR well
With purpose in mind to make some aspects of work of ASR wells more clear, simple simulation was done in software Groundwater Vistas version 6. Model is with dimensions 9.5 km x 7.5 km and the cells are 500×500 meters on the edge of the model and up to 10 x 10 meters in center of the model
To avoid long simulation only 2 D case was considered with elevation of top of aquifer at 35 mabsl and elevation of bottom at 15 mabsl. Groundwater heads are with gradient 0.003 in the direction west to east. Specific storage is S=1e-4, and starting hydraulic conductivity is 8.64 m/d while longitudinal dispersion is 2 meters. Besides program MODFLOW program MT3DMS is used and starting concentration for the whole model area is 1000 mg/l with which water from the aquifer will be labeled.
TO BE CONTINUED……