Modeling Transport of Microbes in Ten Undisturbed Soils under Effluent Irrigation

Abstract

means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. ABBREVIATIONS: BTC, breakthrough curve; cfu, colony-forming unit, a unit indicating the number of bacteria present in a water sample; DSE, dairy-shed effl uent; PV, pore volume. ORIGINAL RESEARCHThe HYDRUS-1D mobile-immobile water model (MIM) was used to evaluate the transport of fecal coliforms, Salmonella bacteriophage, and Br in 10 soils. At a fl ux of 5 mm h −1 , a pulse of dairy shed effl uent was applied to 30 large undisturbed lysimeters, followed by water irrigation. Soil types included clayey gley soil, clay loam, silt loam, silt loam over gravels, fi ne sandy loam, dune sand soil, pumice soil, and allophanic soil. Except for dune sand, modeling results showed lower mobile water contents and dispersivities for microbes than for Br, indicating the exclusion of microbes from smaller pores. The MIM-derived removal rates were in the order: volcanic soils > greywacke-derived silt loams > granular young sandy soils, and were the most variable in clayey gley loam and silt loam over gravels. Microbial reduction was 100% in allophanic soil, 16 to 18 log m −1 in pumice soil (where the unit log is the log 10 reduction in maximum concentration compared with the original concentration), and was lowest in clayey gley soil (0.1-2 log m −1 ). For most of the other soils, the reduction was 2 to 3 log m −1 , except for 9 to 10 log m −1 for fecal coliforms in a fi ne sandy loam. The detachment rate was only 1% of the attachment rate, indicating irreversible attachment of microbes. Soil structure (macroporosity) appeared to play the most important role in the transport of microbes and Br, while soil lithology had the greatest infl uence on attenuation and mass exchange. The general pattern of predicted mobile water content agrees with the measured macroporosity, which is positively related to leaching vulnerability but negatively related to dispersivity.

Bacteria transport and retention in intact calcareous soil columns under saturated flow conditions

“…Bacteria can only access a smaller range of larger pores due to size exclusion, whereas the chloride can access a wider range of pore sizes. Some other investigators have also observed an earlier breakthrough of bacteria than chloride in intact soils (Guber et al, 2005;Levy et al, 2007;Pang et al, 2008).…”

“…Some researchers have recognized straining as a potentially important mechanism for bacteria deposition (e.g. Bradford et al, 2004;Gargiulo et al, 2008;Pang et al, 2008). The calculated values of E show that two-site model provides a better performance than one-site model to predict bacteria retention profile (Table 4).…”

“…Straining occurs when the ratio of the colloid to medium grain diameter, D p /d, is >0.5% (Bradford et al, 2004), and will be significant when the ratio is >8% (McDowell-Boyer et al, 1986). The criteria for straining were developed from uniform porous media grain sizes and these criteria do not consider the effect of soil aggregates and macropores (Pang et al, 2008). The current knowledge of straining processes is still incomplete and the effect of soil aggregation and macropores on straining is unknown.…”

“…Several studies (e.g. Cey et al, 2009;Darnault et al, 2004;Guber et al, 2005;Jiang et al, 2010;Pang et al, 2008;Pivetz et al, 1996;Unc and Goss, 2003;Wang et al, 2013) have revealed that microorganisms and colloids can transport within preferential flow pathways.…”

“…Their results demonstrated that in quartz sand coated with calcite in varying fractions (0, 5%, 10%, 20%, 40%, 70%, and 100%), the sticking efficiencies of E. coli increases with increasing the fraction of quartz sand coated with calcite. Pang et al (2008) studied transport of fecal coliforms, Salmonella bacteriophage, and Br in undisturbed lysimeters soils. The results indicated that soil structure plays the most important function in the transport of microbes and Br, while soil lithology had the greatest influence on mass exchange.…”