This is from a study done at Rutgers University, that I found in the Journal of the NACAA (National Association of County Agricultural Agents): Results from this study showed heavy metals were well below the EPA irrigation thresholds for reclaimed water and posed minimal risk for irrigating a vegetable garden. Results also showed the majority of water samples to be below recommended irrigation guidelines for E. coli. Regardless, the researchers recommend chemical treatment of rain barrel water before irrigating a garden grown for consumption, in addition to cultural practices, to reduce the risk of exposure to harmful pathogens. Rain barrel users should make sure to clean the barrel with a 3% bleach solution before collecting water to irrigate a vegetable garden. Household, unscented bleach with a 5-6% chlorine solution can be added at the rate of 1/8 teaspoon per gallon (8 drops). Prior to irrigating a vegetable garden, water in a typical 55 gallon rain barrel should be treated with approximately 1 ounce of bleach. Wait approximately 24 hours after the addition of bleach to allow the chlorine to dissipate before using the water. It should be noted that household bleach is not labeled for use in water treatment by the Food and Drug Administration, although it is frequently recommended for emergency disinfection of drinking water (USEPA, 2006b).
Implementing good cultural practices is also important for reducing the pathogen contamination. Harvested rainwater from a rain barrel should only be used for non-foliar applications on vegetable gardens possibly through drip irrigation. Water should be applied in the morning. Harvesting should not take place right after watering in order to benefit from leaf drying and ultraviolet light disinfection. I hope that helps!
