Pathogen prevalence in fractured versus granular aquifers and the role of forward flow stagnation zones on pore-scale delivery to surfaces

Authors: RASMUSON, ANNA - University Of Utah; ERICKSON, BROCK - University Of Utah; Borchardt, Mark; MULDOON, MAUREEN - University Of Wisconsin; JOHNSON, WILLIAM - University Of Utah

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/26/2019
Publication Date: 11/26/2019
Citation: Rasmuson, A., Erickson, B., Borchardt, M.A., Muldoon, M., Johnson, W.P. 2019. Pathogen prevalence in fractured versus granular aquifers and the role of forward flow stagnation zones on pore-scale delivery to surfaces. Environmental Science and Technology. 54:137-145. https://doi.org/10.1021/acs.est.9b03274.
DOI: https://doi.org/10.1021/acs.est.9b03274

Interpretive Summary: For groundwater to become contaminated with microorganisms from the land surface, for example from land –applied livestock manure, a microorganism must first move downward through the soil before it reaches the water table. Soil could potentially remove (“filter out”) downward moving microorganisms, but different soil types likely differ in their removal potential. We investigated soils overlying two important aquifers in Wisconsin, the Central Sands region and the Silurian dolomite in the northeast. Using microscopic plastic microspheres to mimic microorganisms we found the Central Sands soil removed nearly all microspheres over a three foot distance. Removal was highly effective regardless of microsphere size. In contrast, the soil overlying the Silurian dolomite aquifer was not nearly as effective in microsphere removal. Twenty-three feet of soil was needed to remove virus-sized microspheres to the same level as the Central Sands. With 33 feet of soil, the longest distance investigated, microspheres the same sizes as bacteria and protozoa (e.g. Cryptosporidium) were removed only 99% and 93%, respectively. These findings are consistent with computer models that predict microorganism attachment to soil grains and consistent with the microorganism contamination rates measured in the Central Sands and Silurian dolomite aquifers. These findings are useful for establishing minimum distances between microorganism contamination sources on or near the land surface (e.g. septic system drain fields) and the water table to prevent groundwater contamination.

Technical Abstract: Groundwater susceptibility to pathogens generally corresponds to transport distances (and residence times) between sources and receptors. However, two aquifers in Wisconsin, USA with similar pathogen source proximity to water supply wells show vastly different pathogen prevalence. Whereas pathogen prevalence is often characterized in terms of residence time prior to reaching receptors, this study shows that differences in rates of filtration (delivery to surfaces) among granular sand versus macropore-dominated till/dolomite is responsible for the contrasting prevalences. Column transport experiments in representative media from the two regions were conducted with nano- to micro-sized (0.1 to 4.2 µm) polystyrene latex microspheres reflecting the range of pathogen sizes from viruses to protozoa. Column transport experiments demonstrated that several orders of magnitude greater removal of all colloid sizes occurred in sand relative to till/dolomite. Distinct trends in observed retention with colloid size occurred in each media, with minimum retention for the ca. 1 µm colloids (bacterial size range) in sand versus minimum retention for ca. 5 µm colloids (protistan size range) in glacial till. To demonstrate the importance of delivery to surfaces, particle trajectory simulations were conducted in Happel sphere-in-cell and parallel-plate geometries to represent granular media and macropore-dominated till/dolomite, respectively. Simulations captured experimental trends in retention as a function of colloid size, as well as greater capture in the granular versus macropore-dominated media, demonstrating that higher water quality in the granular relative to macropore-dominated media results from more effective delivery to surfaces rather than decreased residence time in the macropore-dominated media.