Implications of a Multi-well Tracer Test in the Transport of Pathogens at a Riverbank Filtration Experiment Site.

This study tracks the transport of bromide and microspheres mimicking pathogens in an arid environment. The study site uses the Rio Grande that experiences significant annual fluctuations in both water quantity and quality. The pumping well is 17 m from the stream bank and the water table was 2 m below the stream surface. The aquifer is medium and fine-grained sand comprising two flow units. Observation wells are screened over 1 or 1.5 m intervals. The average hydraulic conductivity was about 2 x 10-3 m/s based on a test analysis, however, the responses indicated that sediment heterogeneities affected the hydraulic behavior. A 427 hour tracer test using bromide and fluorescent microspheres provides initial results that are relevant to the transport of pathogens through the subsurface under riverbank filtration conditions. Bromide was injected into an observation well at the channel margin. Differently colored fluorescent microspheres (0.25nm, 1?m, 6?m and 10?m) were injected into the stream bottom and into two observation wells. Conclusions from the tracer test are: 1) Both bromide and microspheres continued to be observed throughout the 18 days of the experiment. 2) The bromide recovery in the pumping well and in the deeper observation wells showed early and late peaks with a long tails indicating that the geological medium at the field site behaves like a double-porosity medium allowing the tracer to move relatively quickly through the higher conductivity units while being significantly retarded in the low hydraulic conductivity units. 3) Some wells showed consistently higher concentrations of bromide. 4) The 1? micospheres were abundant in the observation wells and allowed tracing of flowpaths. These showed multiple peaks similar to the bromide results. This indicates highly preferential transport paths in the sediment. 5) Microspheres from the three injection sites had distinctly different transport paths and rates. 6) Both bromide and microspheres appeared in the stream soon after injection, moving apparently against an 2-m head difference. 7) The 6 ? and 10 ? microspheres were observed in low concentrations and were episodically detected in the stream and in two widely spaced observation wells. The significance of these results is that: 1) Inorganic microspheres may mimic the episodic occurrence of microorganisms in wells. 2) Even in this relatively homogeneous aquifer, preferential transport within the aquifer results in highly divergent transport paths and rates. Microspheres from one of the injection sites traveled essentially perpendicular to the expected transport direction. 3) Even small variations in the sand grain size can effectively compartmentalize the aquifer. The next steps of this project will include field studies to observe the migration and persistence of selected organisms (E.coli, enterococci, coliphages, cysts, oocysts and enteroviruses) in the pumping well and observation wells under different pumping rates. Continued combined chemical sampling along with the microbial sampling will document the whether changes in water chemistry alter the behavior of the organisms.