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Abstract
The vadose zone can function as both a filter and a passage for bacteria. This review evaluates when and why either effect will apply based on available literature. It summarizes theories and experimental research that address the related, underlying bacterial attenuation processes, the applied macro-scale modeling approaches, and the influencing factors - including the cell, soil, solution and system characteristics. Results point to that the relative importance of each removal mechanism depends on the moisture content and the solution ionic strength. The limitations of available modeling approaches are discussed. It remains unclear in which contexts these are reliable for predictions. The temporal first-order kinetic Escherichia coli (E. coli) removal coefficient ranges three orders of magnitude, from 10−4 to 10−1/min. Results suggest that this rate depends on the pore-water velocity. Spatial filtration of E. coli increases with slower flow and higher collector surface heterogeneity. It could be insignificant in the case of heavy and sudden infiltration and subsequent transport in preferential flow paths, induced, for example, by plant roots or cracks in clayey soils. Future research thus needs to address transport as an effect of extreme weather events such as droughts and subsequent floods.
NOMENCLATURE
| = | Units | |
| L | = | Length |
| M | = | Mass |
| N | = | Number of particles |
| T | = | Time |
Variables and Coefficients
| ap | = | Particle diameter [L] |
| C | = | Concentration of particles in water [NL−3] |
| C0 | = | Influent concentration of particles [NL−3] |
| CAatt | = | Concentration of air-phase attached particles [NL−3] |
| CAattmax | = | Maximum air-phase concentration of attached particles [NL−3] |
| Cim | = | Concentration of particles in the immobile region [NL−3] |
| Cm | = | Concentration of particles in the mobile region [NL−3] |
| CSAdep | = | Concentration of deposited particles [NM−1] |
| = | Concentration of deposited particles for compartment i [NM−1] | |
| CSatt | = | Concentration of solid-phase attached particles [NM−1] |
| Cstr | = | Concentration of strained particles [NM−1] |
| Cstrmax | = | Maximum of the strained concentration of particles [NM−1] |
| Csub | = | Concentration of substrate in the fluid [ML−3] |
| D | = | Dispersion coefficient [L2T−1] |
| Dm | = | Dispersion coefficient in the mobile phase [L2T−1] |
| d50 | = | Median grain diameter [L] |
| E | = | Colloid pore exclusion factor [−] |
| h | = | Pressure head [L] |
| kAatt | = | Air-phase attachment coefficient [T−1] |
| kgro | = | Growth coefficient [T−1] |
| kgromax | = | Maximum specific utilization rate [T−1] |
| Khalf | = | Half-velocity coefficient [ML−3] |
| kina | = | Inactivation coefficient in solution [T−1] |
| Kr(h) | = | Relative hydraulic conductivity [−] |
| kremt | = | Total temporal kinetic first-order removal coefficient [T−1] |
| kremx | = | Total spatial kinetic first-order removal coefficient [L−1] |
| KS | = | Saturated hydraulic conductivity [LT−1] |
| kSAdep | = | Total solid- and air-phase deposition coefficient [T−1] |
| kSArel | = | Total release from solid- and air-phase coefficient [T−1] |
| = | Release coefficient for compartment i [T−1] | |
| kSatt | = | Solid-phase attachment coefficient [T−1] |
| kSdet | = | Solid-phase detachment coefficient [T−1] |
| kSdep | = | Solid-phase deposition (attachment and straining) coefficient [T−1] |
| KSeqatt | = | Solid-phase equilibrium deposition coefficient [L3M−1] |
| kstr | = | Straining coefficient [T−1] |
| l | = | Metabolic time lag [−] |
| m | = | van Genuchten parameter [−] |
| N | = | Parameter relating colloid retention to pore-velocity [(LT)−0.5] |
| N2 | = | Parameter relating colloid release to the pore-velocity [L-κTκ−1] |
| NC | = | Number of compartments [−] |
| q | = | Darcy velocity [LT−1] |
| S | = | Source/sink term [T−1] |
| SM | = | Effective microbial saturation (microbial over void volume) [−] |
| ST | = | Effective total saturation (sum of water and microbial saturations) [−] |
| SW | = | Effective water saturation [−] |
| t | = | Time [T] |
| v | = | Average interstitial (pore) velocity [LT−1] |
| x | = | Vertical distance (positive upward) [L] |
| Y | = | Yield coefficient, relates substrate utilization to growth [−] |
Greek Letters
| α | = | Sticking efficiency |
| αmim | = | Rate of exchange between mobile and immobile liquid regions [T−1] |
| ap | = | Width of particle [L] |
| β | = | Parameter quantifying deposition depth-dependency [−] |
| Γmim | = | Particle transfer rate between mobile and immobile regions [NL−3T−1] |
| ΓW | = | Water transfer rate between matrix and fast flow domain [T−1] |
| ζ | = | Zeta |
| η0 | = | Single-collector contact efficiency [−] |
| θ | = | Volumetric moisture content [−] |
| θA | = | Volumetric air content [−] |
| θC | = | Colloid accessible water content [−] |
| θcrit | = | Critical moisture content [−] |
| = | Critical moisture content in compartment i [−] | |
| θim | = | Fraction of immobile colloids [−] |
| θm | = | Fraction of mobile colloids [−] |
| θr | = | Residual volumetric moisture content [−] |
| θs | = | Saturated volumetric moisture content [−] |
| κ | = | Parameter quantifying colloidal release [−] |
| Λatt | = | Attachment reaction rate source-sink term |
| Λfate | = | Total particle fate reaction rate source-sink term |
| Λstr | = | Straining reaction rate source-sink term |
| Λsur | = | Survival reaction source-sink term in water |
| λ | = | Dispersivity [L] |
| μ | = | Parameter relating the straining rate to the saturation [−] |
| ρb | = | Filter media bulk density [ML−3] |
| ψ | = | Depth- or time dependent straining function |
| ω | = | Exposure or residence time [T] |
Figure Colors
 | = | Particle |
 | = | Gas |
 | = | Liquid |
 | = | Solid |