How can I remediate rainwater captured from my asphalt-shingled roof?

I would like to capture rainwater from my roof in a 55-gallon rain barrel. I plan to use the water on my vegetable and herb garden, but I am concerned about contaminates in the water.

The Seattle Public Utilities warns against using rainwater from asphalt-shingled roofs: "...general practice is to avoid watering vegetables and other edible plants, such as herbs you plan to use in cooking, with rain barrel water collected from asphalt-shingle roofs. These kinds of roofs may leach various complex hydrocarbon compounds, so most people avoid using water from asphalt-shingle roofs or flat tar roofs on plants meant for human consumption. To date there is no definitive research on the amounts and types of hydrocarbon compounds which may leach from such roofs..." (http://www.ci.seattle.wa.us/util/static/COS_004351.pdf)

What is the best way to "clean" the water of these hydrocarbons?

Many thanks in advance!

-Fred

Here is a link that might be useful: The Seattle Rain Barrel