EVALUATING THE EFFECTS OF GRAIN SIZE AND DIVALENT CATION CONCENTRATION ON THE ATTENUATION OF VIRUSES AND MICROSPHERES THROUGH CRUSHED SILICA SAND

Over the last decade in North America, an increasing number of microbiological drinking water regulations have been used to manage groundwater resources that are potentially influenced by surface water. Groundwater Under Direct Influence studies (GUDI or GWUDI in USA) are conducted to assess the need to treat well water at an extraction point. Currently, there is a lack of knowledge regarding factors that affect the transport of pathogens through porous media at the surface water-groundwater interface. Such information is required to supply sufficient quantities of drinking water in a cost effective and safe manner.

This study examines the effects of ionic strength, grain size and influent virus concentrations on pathogen transport in porous media. 14 column tests were conducted using the bacteriophage MS2 and 1.5 um microspheres. Two size distributions of crushed silica sand, with median grain diameters of 0.7 and 0.34 mm, and two ionic strengths of 8 and 95 mmol/L were used.

The results show that complete breakthrough of both viruses and microspheres occurred in medium sand at low ionic strength. It was found that increasing ionic strength by Calcium addition precluded breakthrough of MS2 in both the medium and fine sands. This represents a greater than 8 log reduction in peak effluent concentration and essentially complete attenuation.

In fine sand, with low ionic strength water, a 5 log reduction in peak MS2 concentrations was observed. In the same sand at high ionic strength, no MS2 broke through the column, corresponding to a greater than 8 log removal. Since complete attenuation occurred in both grain sizes at high ionic strength, the effect of higher ionic strength in the fine sand was indistinguishable from the effect observed from raising the ionic strength in the medium sand.

Microsphere transport was essentially unaffected by increasing ionic strength. A 1 log reduction in peak concentration was observed in the high ionic strength water in the medium sand. Grain size had a profound effect on the attenuation of microspheres. There was no evidence of microsphere breakthrough in any of the fine sand columns at the low or high ionic strengths, yielding a minimum greater than 5 log reduction in microsphere concentration due to grain size alone.