Many water providers add chlorine to drinking water to keep it clean for human consumption. Chlorine prevents bacterial growth in water distribution systems. Many residents use chlorinated water to irrigate their lawn and garden. If chlorine is added to drinking water to kill bacteria, what impact does it have on beneficial soil microorganisms? Does it kill beneficial organisms in compost piles, too? Researchers have found that chlorinated drinking water may kill a number of microorganisms in soil or a compost pile. However, their reproduction rate is so rapid that populations rebound in a short time. Under normal conditions, chlorinated water will not threaten microorganism populations. 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.
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.