Transport and retention of fecal indicator bacteria in unsaturated porous media: effect of transient water flow

Transport and retention of fecal indicator bacteria in unsaturated porous media: effect of transient water flow

 

Rozita Soltani Tehrani^a*, Luc Hornstra^b, Jos van Dam^a, Gijsbert Cirkel^b

 

^a Department of Soil Physics and Land Management, Wageningen University and Research, P.O. Box 47, 6700 AA Wageningen, the Netherlands
^b KWR Water research Institute, Nieuwegein, the Netherlands

 

Email addresses: rozita.soltanitehrani@wur.nl (R. Soltani Tehrani), luc.hornstra@tno.nl (L. Hornstra), jos.vandam@wur.nl (J. van Dam) Gijsbert.Cirkel@kwrwater.nl (G. Cirkel)

 

Abstract

To produce clean drinking water, the processes governing bacterial remobilization in the unsaturated zone at transient water flow are critical. While managed aquifer recharge is an effective way to dispose of pathogens, there are concerns about recontamination after precipitation infiltration. To better understand how bacteria that were initially retained in porous media can be released to groundwater due to transient water content, transport experiments and modeling for Escherichia coli and Enterococcus moraviensis were conducted at the soil column scale. After inoculating dune sand columns with bacteria suspension for 4 h, three rainfall events were performed at 24 h intervals. The effluent from sand columns was collected to analyze bacteria breakthrough curves (BTCs). After the rainfall experiments, the bacteria distribution in the sand column was determined. The collected BTCs and profile retentions were modeled with HYDRUS-1D, using different model concepts: one-site kinetic attachment/detachment (M1), Langmuirian (M2), Langmuirian, and blocking (M3), and two-site attachment/detachment (M4). After inoculation, almost 99 percent of the bacteria remained in the soil, according to the findings of the experiments. The M1 and M2 bacteria models had a high agreement between observed and modeled concentrations, and attachment and detachment were two significant mechanisms for regulating bacteria movement in a porous medium with fluctuations in water flow. At the end of the experiment, the majority of bacteria were still found at the column entrance. Our experiments show that E. coli is more mobile in sandy soils than E. moraviensis. The results of this study suggest that the unsaturated zone is an important barrier between microbial contamination at the soil surface and groundwater. Bacteria that accumulate in the unsaturated zone, on the other hand, can multiply to such an extent that they can be released into the saturated zone when saturation changes due to major rain events or variations in groundwater level. A Follow-up study is needed to completely understand the variables that regulate bacteria remobilization in the unsaturated zone of dune sands.