USGS Groundwater Information
Fate And Transport of Bacterial, Viral, and Protozoan Pathogens During ASR Operations- What Microorganisms Do We Need To Worry About And Why?
By David Metge
Deteriorating ground-water quality has stimulated new research initiatives; that, in part focus on the subsurface fate and transport behavior of pathogens. Increasing application of aquifer storage and recovery (ASR) operations has brought the issue of fate and transport of microbial pathogens into sharper focus. Because ASR practices are becoming widespread, they are found within diverse geohydrologic regimes. This is coupled with modest, recent regulatory guidance and limited understanding of how pathogens within the subsurface are affected by this type of water handling. These factors make the study of pathogen fate and transport within ASR systems extremely difficult. A major challenge in assessing the utility of ASR is to identify the pathogens that may pose a threat in each ASR setting. Another concern is how best to study the role of pathogens upon water quality during ASR operations.
There are several factors that influence survival of enteric microorganisms within ground water matrices. These include temperature, salinity, dissolved oxygen (DO), pH, microbial size, predation, metal/nutrient availability, and microbial growth. Dissolved oxygen and temperature are among the most significant factors in loss of enteric virus infectivity and bacterial activity. Below 10°C, many viruses can survive for months or even years. In contrast, at higher temperatures greater than 20°C, inactivation of most viruses occurs rapidly. In Florida, ASR temperatures range from 22-28°C; these conditions promote both enteric bacterial and viral die off. Currently, there is little data on enteric protozoa (i.e., Cryptosporidium, and Giardia lamblia) survival in ground water. Thus, different conditions may promote or inhibit microbial survival and may be site-specific. Nevertheless, regular screening for coliform bacteria, indicator virus and Cryptosporidium should be undertaken.
In order to adequately describe subsurface transport behavior of microbial pathogens and address public safety and ground-water quality issues, surrogates (well-characterized microbial-sized microspheres; non-pathogenic, fluorescently labeled bacteria and protozoa; and bacteriophage) can be used in lieu of microbial pathogens in flow-through column and field injection and recovery experiments. Controlled field and lab experiments are generally conducted to study effects of trace organic contaminants (e.g., surfactants) and microbial properties (surface chemistry, size, and buoyant density) on movement of microbial surrogates through aquifer sediments. Results are then applied to the predicted subsurface transport behavior of pathogens, assuming that the pathogens would be affected by these factors in a similar manner. These results also provide attenuation rates for selected pathogens within groundwater at artificial recharge sites. A recently developed stochastic model indicated that the degree of aquifer heterogeneity helped govern the degree of viral transport within granular media; these types of data would help water utilities and regulators determine pretreatment requirements and predict water quality changes within aquifer storage and recovery systems.
Current studies show that slight differences in ground-water matrices impact the degree of subsurface microbial transport. It should be recognized that surface waters injected underground during ASR operations can have different physicochemical characteristics than the ground-water they displace. However, it is unknown if introduction of waters with different geochemical characteristics accelerate, inhibit or modify naturally occurring rock-water interactions and microbial-sediment interactions. These modifications may be operational on regional scales. A further complication is that different states have different regulations about ASR practices. Adequate water quality is most often obtained by chlorination before water is placed into aquifers. Though this prevents clogging and allows introduction of potable water to the aquifer, chlorination is well known to be less effective against viruses and protozoa (specifically Cryptosporidium). Therefore, the survival and fate these microorganisms and enteric bacteria in the vicinity of ASR schemes requires closer scrutiny.
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