Depth-specific transport of bacteriophages MS2 and ΦX174 in intact soils

Soil geochemical and structural heterogeneities are coupled in influencing the fate and transport of viruses in soil profiles. Only the soil layer at a certain depth, where viruses have least mobility, determines the total flux and distance of virus transport. However, few studies have addressed the dependence of virus transport on soil depths. This dependence is mostly due to the gradients of geochemistry, soil structure, and solution chemistry along the soil profile. We employed intact soil columns (5 cm in length, 4.5 cm in inner diameter) from three depths (5–10 cm, 35–40 cm, and 65–70 cm) to investigate the transport of two very different model viruses: 1) MS2; a hydrophobic, negatively charged, less aggregated phage of E. coli, and 2) ΦX174; a hydrophilic, positively charged (pH=6.58 ± 0.21), highly aggregated phage of E. coli as a function of depth. The breakthrough of both viruses increased with soil depth in both 10 mM and 40 mM NaCl solutions. The mobility of MS2 was greater than ΦX174 at 5–10 cm and 35–40 cm depths, but the mobility of ΦX174 increased much faster than MS2 with soil depth and surpassed MS2 mobility in the soil of 65–70 cm depth. The effect of ionic strength on the mobility of nanosized MS2 decreased with soil depth because of decreased soil organic matter (SOM) content. The decreased SOM lessened ionic strength sensitivity via a hydrophobicity-reduced Lewis acid-base attraction mechanism. In comparison, ΦX174 viruses showed an overall weaker response to ionic strength change since they were aggregated into microsized particles in the experimental solutions and thereby subjected to mechanical straining in the intact soils. The elevated straining effect counteracted the electrostatically facilitated transport of ΦX174. These findings suggest that soil amendments with organic matter might be low cost, effective measures for reducing virus transport through soil profiles.