Written by: John Jansen, Ph.D., P.G., P.Gp. Sr. Geophysicist & Hydrogeologist
Managed Aquifer Recharge (MAR) and Aquifer Storage and Recovery (ASR) are two related technologies that are both being used in the state of Colorado. Both technologies are designed to recharge aquifers and to use the aquifers as storage vessels to balance seasonal imbalances of water resources. The methods offer large advantages in cost, ecological foot print, and potential storage volume over traditional surface reservoirs making them attractive tools for future water projects.
MAR is usually accomplished by storing water in permeable zones of a surficial aquifer and recovering the water using a field of wells within, or adjacent to, the storage area. A peculiarity of Colorado water law requires that an impermeable slurry wall be placed around the storage vessel to prevent migration of stored water. To efficiently store and recover the water it is important to have a good understanding of the depth of bedrock and the composition of the unconsolidated materials. Some projects have experienced low recovery efficiency or leakage out of the storage zone because the aquifer and bedrock were not understood in sufficient detail. Generally it is impractical to drill enough boreholes to fully characterize the storage area or screen larger areas for suitable sites. Surface geophysical methods have been used very effectively to quickly screen large areas to select suitable locations for recharge basins and recovery Wells. This presentation will show several examples of how electrical resistivity, seismic refraction tomography, and electromagnetic induction methods have been used to site recharge basins and design recovery well fields. Geophysics can also be used to locate leakage zones in the slurry wall containment system and improve storage efficiency.
ASR generally involves injecting water into a deep confined aquifer through a well and recovering the water at a later date, generally from the same well. This requires an understanding of the distribution of permeable aquifer units as well as the integrity of confining units above and below the potential storage zone. While these attributes can be obtained from well logs in existing wells, predicting aquifer properties prior to drilling is difficult.
Colorado has in abundance of confined aquifers, but the permeability and thickness of those units can vary considerably from location to location. The cost of exploring for suitable storage zones by test drilling is prohibitively expensive. Surface geophysical methods can be used to identify the location of thicker and more permeable aquifer units at depths of several hundred feet to several thousand feet. This presentation will show how seismic reflection and attribute processing can be used to map permeable zones as potential ASR storage zones. Existing oil and gas data can often be reprocessed and reinterpreted to screen the major aquifers for suitable storage zones. Acquiring shallow-focused high-resolution seismic data specifically to site ASR Wells provides data that is better suited for this purpose and is often and worth the additional expense.
MAR and ASR will become a more important part of Colorado’s water future. The prudent use of surface geophysical methods will improve the performance of these systems and reduce the overall cost of siting and development.