Aims:  To investigate the impact of iron particles in groundwater on the inactivation of two model viruses, bacteriophages MS2 and T4, by 254-nm ultraviolet (UV) light.

Methods and Results:  One-litre samples of groundwater with high iron content (from the Indianapolis Water Company, mean dissolved iron concentration 1·3 mg l−1) were stirred vigorously while exposed to air, which oxidized and precipitated the dissolved iron. In parallel samples, ethylenediaminetetra-acetic acid (EDTA) was added to chelate the iron and prevent formation of iron precipitate. The average turbidity in the samples without EDTA (called the ‘raw’ samples) after 210 min of stirring was 2·7 ± 0·1 NTU while the average turbidity of the samples containing EDTA (called the ‘preserved’ samples) was 1·0 ± 0·1 NTU. ‘Raw’ and ‘preserved’ samples containing bacteriophage MS2 were exposed to 254-nm UV light at doses of 20, 40, or 60 mJ (cm2)−1, while samples containing bacteriophage T4 were exposed to 2 or 5 mJ (cm2)−1, using a low pressure UV collimated beam. The UV inactivation of both phages in the ‘raw’ groundwater was lower than in the EDTA-‘preserved’ groundwater to a statistically significant degree (α = 0·05), due to the association of phage with the UV-absorbing iron precipitate particles. A phage elution technique confirmed that a large fraction of the phage that survived the UV exposures were particle-associated.

Conclusions:  Phages that are associated with iron oxide particles in groundwater are shielded from UV light to a measurable and statistically significant degree at a turbidity level of 2·7 NTU when the phage particle association is induced under experimental conditions.

Significance and Impact of the Study:  While the particle association of the phage in this study was induced experimentally, the findings provide further evidence that certain particles in natural waters and wastewaters (e.g. iron oxide particles) may have the potential to shield viruses from UV light.