Danelle Bertrand

Research Objective:  Assess the persistence of THMs and key factors affecting their fate and transport in basalt aquifer ASR systems

            ASR is the storage of water in natural aquifers with injection and recovery through pumped wells.  Treated water is injected into the aquifer through ASR wells during the wet season, when water supply is ample, and recovered during the dry season to meet peak water demands.  During the storage period a buffer zone of mixing is established which separates the stored water from the native groundwater as shown in Figure 1 which depicts a typical ASR cross-section in a confined aquifer.
           

Natural attenuation of contaminants can often be increased in aquifers with ASR systems.  ASR injection introduces aerated water into aquifers which are often oxygen deficient, creating redox gradients progressing from the aerobic condition at the ASR well to the ambient redox condition of the aquifer.  Figure 2 depicts the typical redox zones established after ASR injection into a methanogenic aquifer, progressing from aerobic at the injection well then successively through zones of nitrate reduction, iron reduction, sulfate reduction, and finally methanogenesis.  When injected water migrates through the redox gradient, contaminants are exposed to a succession of microbial communities, each of which may be capable of degrading different contaminants, increasing the probability of contaminant degradation.  During ASR storage, the redox gradient declines until the ambient aquifer redox zone is reestablished throughout the system.   

Disinfection by-products (DBPs) are formed when chemical disinfectants react with naturally occurring organic matter (NOM) during disinfection typically preceding aquifer storage and recovery (ASR) injection.  Oregon law requires treatment of ASR injectant water to drinking water standards including disinfection with approved methods, the most prevalent of which is chlorination.  Trihalomethanes (THMs) and haloacetic acids (HAAs), the dominant DBPs formed as a result of chlorine disinfection, are both suspected carcinogens which are regulated by the EPA with MCLs of 80 and 60µg/L, respectively for THMs and HAAs (Dillon and Toze, 2005). 

The impacts of ASR systems in basalt aquifers has become increasingly relevant in Oregon due to the ongoing investigations into using ASR to restore groundwater levels in the Umatilla Basin, which were depleted to dangerous levels by over-pumping.  Basalt aquifers are also particularly well-suited as ASR reservoirs due to their high storage and yield capacities and are likely to become more widely used in the future for ASR.  In addition, effects of climate change on the availability of water in the dry summer months, due to earlier snow melt and precipitation falling as rain instead of snow, will likely increase groundwater depletion in the Pacific Northwest and increase the need for ASR to meet water demands.

This research is focused on the persistence of THMs specifically in basalt aquifer ASR systems to identify key factors controlling their fate and mobility.  Although there has been significant research on the fate of THMs in alluvial and limestone ASR aquifers, research in basalt ASR systems is lacking.  Furthermore, due to the unique structure and geochemistry of basalts, application of research results from alluvial and limestone systems to basalt systems may not be appropriate, presenting a need for direct research in basalt aquifers.