[fatbelly]

On the subject of rainwater. I have 2 large Blue water butts on my plot connected by drainpipes to my shed roof.

Last season I didn't have to top them up from the tap all season and they were full to the brim each week . So far this season I have had to fill them up with tap water because of the little rain we have had.

[ConfusedGardener]

Well I can't find any papers that you don't have to pay for, but I did find some good sources for info.

There's actually less effect than I thought that there would be:

Researchers have found that chlorinated drinking water may kill the number of microorganisms in soil or a compost pile. However, their reproduction rate is so rapid populations rebound in a short time. Under normal conditions, chlorinated water will not threaten microorganism populations.

One reason chlorinate water has little impact is that chlorine binds to soil particle surfaces. This immobilizes chlorine and reduces its ability to kill microorganisms. The organisms in the topmost surface of soil or a compost pile may be affected after irrigation but as the water moves downward little chlorine remains. In one study, researchers found that water chlorinated at 5 parts per million killed organisms only in the top half inch of soil. Organisms deeper than one half inch were thriving.

The amount of chlorine in drinking water is quite low. In order to kill soil microorganisms to 6 inch soil depth, water containing 65 parts per million of chlorine was required in one study. Drinking water usually contains much lower chlorine levels. For example, Colorado Springs Utilities water contains between 0.05 to 0.90 parts per million of chlorine, 70 times below the threshold level.

Microorganisms reproduce rapidly. In one study, researchers continuously applied highly chlorinated water to soil for 126 days. Two days after they stopped, the soil microorganism populations reached pre-treatment levels at all depths of soil.

Colorado State University

Even so, I'd rater rather stay away from tap water straight from the tap to avoid even temporary setbacks.

There's no doubt that the microorganisms in your soil play an important part in keeping plants health and well fed:
http://www.soilandplantlaboratory.com/pdf/articles/BeneficialSoilMicrobes.pdf
http://www.academia.edu/333405/Soil_Beneficial_Bacteria_and_Their_Role_In_Plant_Growth_Promotion_a_Review
http://www.cmg.colostate.edu/gardennotes/212.pdf

There is obviously more to it than just avoiding chlorine, but as others have said, each to their own