Evaluation of the effect of geochemical and physical heterogeneity on Cryptosporidium parvum oocyst transport in saturated porous media

To mitigate the public health concern of pathogenic microbes in many rivers used as potable water supplies, water utilities are using riverbank filtration to remove microbes. Predicting microbe removal during riverbank filtration is difficult because alluvial valley aquifers are typically physically and geochemically heterogeneous.; To test the effect of geochemical heterogeneity on microorganism transport in saturated porous media, we measured the removal of two microorganisms, the bacteriophage PRD1 and oocysts of the protozoan parasite Cryptosporidium parvum, in flow-through columns of quartz sand coated with different amounts of a ferric oxyhydroxide and in solutions of different pH and different amounts of dissolved organic matter. The experiments were conducted over ranges of ferric oxyhydroxide fractional surface coverage (lambda) from 0 to 0.12 for PRD1 and from 0 to 0.32 for the oocysts at pH 5.6-5.8 and 10-4 M NaCl. For oocysts, column experiments were conducted over a range of pH of 5.7 to 10.0 and 0 to 20 mg L-1 fulvic acid at a ferric oxyhydroxide coating fraction of 0.04. Collision (attachment) efficiencies (alpha) increased as the fraction of ferric oxyhydroxide-coated quartz sand increased from alpha = 0.0071 to 0.13 over lambda = 0 to 0.12 for PRD1, and from alpha = 0.059 to 0.75 over lambda = 0 to 0.32 for the oocysts. Increasing the pH from 5.7 to 10.0 resulted in a decrease in the oocyst collision efficiency as the pH exceeded the expected point of zero charge of the ferric oxyhydroxide coatings. Increasing fulvic acid concentration decreased oocysts deposition. The decrease in oocysts deposition is attributed to fulvic acid adsorption to the ferric oxyhydroxide coatings, and not to an alteration of the oocyst surface.; To test the effect of physical heterogeneity on the amount of Cryptosporidium parvum oocysts retained in a column due to physical-chemical filtration and straining, removal of oocysts was measured under unfavorable (repulsive) electrostatic conditions of low ionic strength. Experiments were performed in columns packed with clean-quartz sand using porous media of grain size ranging from 0.10 to 2.2 mm. Oocyst retention on the sand was inversely proportional to grain size with an unexpectedly high number of retained oocysts in quartz sand of d50 < 0.92 mm under the experimental conditions. Decreasing solution ionic strength and increasing flow velocity decreased the amount of retained oocysts in all grain sizes significantly. Minimum oocysts retention was observed when column packed with grains of d50 < 0.45 mm and conducted at high flow velocity accompanied with column vibration. Oocysts retention in small grains d50 < 0.45 mm might still be attributed to straining and to deposition in the secondary minimum of the potential energy profile between oocysts and grains. The overall conclusion from these experiments is that most of the retained oocysts were attached to the secondary minimum, in spite of the unfavorable conditions for attachment.