2003 Progress Report: Riverbank Filtration Effectiveness in an Arid EnvironmentEPA Grant Number: R829009
Title: Riverbank Filtration Effectiveness in an Arid Environment
Investigators: Langford, Richard P. , Pillai, Surresh , Schulze-Makuch, Dirk
Institution: The University of Texas at Austin , Texas A & M University
Current Institution: The University of Texas at El Paso , Texas A & M University
EPA Project Officer: Page, Angela
Project Period: September 1, 2001 through August 31, 2004 (Extended to August 28, 2005)
Project Period Covered by this Report: September 1, 2002 through August 31, 2003
Project Amount: $437,418
RFA: Drinking Water (2000) RFA Text | Recipients Lists
Research Category: Drinking Water , Water
The objective of this research project is to determine whether bank filtration in an arid environment is effective at removing particulates and microbial pathogens. The field site for the proposed studies is unique in that it is located in an arid region and utilizes the Rio Grande River system that exhibits significant fluctuations in both water quantity and quality. The results from these studies can be significant for this region because the region relies almost exclusively on aquifers for its municipal drinking water. The research site is in a research park operated by the University of Texas at El Paso, and consists of a pumping well 17 m from an artificial channel of the Rio Grande.
We conducted a 430-hour (18-day) tracer test that combined bromide conservative tracer (bromide) and microspheres selected to mimic the transport behavior of pathogenic protozoa. Differently sized microspheres (1 mm, 6 mm, and 10 mm) in different colors (yellow-green, red, and blue) were employed. Approximately 106 of the 6 µm and the 10 µm microspheres, and 1010 of the 1 mm microspheres were injected at three locations at the site. One of the locations was in the channel and the other two were on the bank. Microsphere breakthroughs mimic the encountered pattern of Giardia lamblia and Cryptosporidium parvum occurrences in many documented environments. Our goal is to model this observed behavior to identify the parameters that control protozoa breakthrough and occurrence in field applications.
Key results were that both bromide and microspheres were found in low concentrations in observations and pumping wells over long periods of time. The microspheres showed multiple peaks similar to the bromide results. This indicates highly preferential transport paths in the sediment. The detection of larger sized microspheres indicates the potential migration of cysts and oocysts under riverbank filtration conditions. It is important to note that in some samples, especially the C (deep) and G (deep) wells, 5-10 microspheres/mL were obtained. This corresponds to approximately 1,000 1 µm micropheres/100 mL.
Figure 1. Breakthrough of Bromide in the Deep Wells (left) and Green Microspheres (right) at Selected Wells of the Field Site
Background samples were collected from the canal to identify whether indicator bacteria and viruses could be used without the need for injecting organisms. A background level of 173 pfu/100 mL (n = 7) of somatic coliphages were detected in the canal. The canal samples were positive for Escherichia coli (mean = 319 MPN/100 mL; n = 7), enterococci (108 MPN/100 mL; n = 4), and fecal coliforms (mean = 750 MPN/100 mL; n = 7). None of the samples were positive for the bacterial pathogen Salmonella spp. None of the samples were positive for C. oocysts or Giardia cysts, suggesting that either injection of these cysts and oocysts has to be performed or microspheres (as surrogates) need to be employed.
The levels of organisms during pumping conditions also were evaluated. None of the samples were positive for bacteriophages during pumping conditions. The samples collected from the wells, however, were positive for E. coli and enterococci during the pumping conditions. During a 5-day pumping period, E. coli was detectable in the C (deep) sample on all 5 days.
The results from the initial pumping study suggested that bacterial indicators can be used to study the transport of organisms during riverbank filtration conditions. The differences in the behavior of E. coli, enterococci, and bacteriophages, however, suggest that a suite of indicator organisms needs to be used to accurately assess the migration of the organisms in the subsurface during riverbank filtration conditions. There are distinct differences in the migration behavior between E. coli and enterococci, suggesting that differences in cell wall composition, dimension, and adsorptive characteristics could all be involved in dictating the transport patterns.
We will continue to study riverbank filtration in an arid environment to determine its effectiveness at removing particulates and microbial pathogens.