Is Rain Water Safe to Use in Organic Gardening
Using collected rain water is often thought to be the most sustainable way to water your garden. Some of you however have expressed concerns that rain water might contain contaminants and using it in the garden could harm your plants and soil.
On this month’s installment of the Testing Garden Assumptions Series we are going to take a look at both the potential risks and benefit of using collected rain water in your garden.
Today’s hypothesis is that rain water is not harmful to use in the garden but does not have any added benefit either.
It is important to note today’s video is only addressing the use of rain water in your garden not for drinking water. If you are interested to see if Tap water is harmful in the garden make sure to check out the link at the end of this video or in the description below.
Samples / Research
The simplest method to test out hypothesis is to analyze samples from my rain water collection system. The analysis will let us know if the rain water I have collected has acquired any contaminants from coming in contact with the building materials like asphalt shingles, aluminium troughs or plastic rain barrel.
Another source of potential contaminants is the air the clouds and rain passes through on the way to ground. In my area recent news coverage has expressed concerns over the air quality in my province. 
Depending on where you live and the different materials in your rain catchment system the results may vary.
Air pollutants and the building materials may result in contamination sources like Heavy metals, Hydrocarbons, Bacteria or in the addition of Nutrients. I submitted samples from my rain barrel to Maxxam Analytics.
The only withdrawal from my rain barrel prior to the sample was for future testing garden assumptions experiments. The water for all tests was taken from the same reservoir having not new water added during the sample period. The water at the time of sampling had been collecting and sitting for a minimum of 6 weeks.
As we have spoken about many times in the past there are elements that are essential and beneficial for plant growth. That said these elements if in high enough concentrations such as phosphorus can actually become toxic. There are also some elements that are not useful for plants and at a minimal concentration are toxic. Generally speaking these are referred to as heavy metals.
The analysis provided by Maxxam Analytics represents the total concentration of both available and unavailable elements.
The results showed there were no exceedances of any element when comparted to the most stringent environmental criteria.
With no metals contaminants let’s move on to Hydrocarbons.
Hydrocarbon contamination we would expect to mostly see as a result of the waters contact with the asphalt shingles. Hydrocarbons are organic molecules mostly made of Hydrogen and Carbon. They can be danger to human health as they are chemically active. If a person is exposed to hydrocarbons a large portion of them can cancer causing. 
Our lab results showed that there were no hits at all. In fact all results were below the very sensitive detection limits of the equipment running the analysis.
With no heavy metals and Hydrocarbons let’s move on to the bacterial counts.
Bacteria are not necessarily a bad thing. In fact bacteria in soil can be helpful, neutral and in a few cases potentially harmful if given the right conditions.
That said I felt it was important to know if there was bacteria in a rain collection system as a baseline for a future episode evaluating the effectiveness of compost tea.
Interestingly enough the sample of rain water we tested had well above the detection parameters of bacteria with over 12,000 colony forming units per milliliter of water.
Based on these results I do not recommend using a system like mine to collect rain water drinking purposes. General back yard systems like my own are not safe enough to prevent the colonization of bacteria in the water.
There are reports of other systems that help avoid colonization however I recommend doing research and contacting your local environmental regulator before using rain water as a drinking water source. If done correctly many scientists conclude that rain water can be a valuable source of drinking water.
Application of rain water to the garden is likely not going to have any positive or negative effect. First off we don’t know which bacteria are being applied if they are beneficial, harmful or neutral. The bacteria once added to the soil will likely die and are not likely to colonize as most healthy garden soils already have a resident population.
With limited potentially harmful things in the next let’s take a look at the nutrients it may have. The results don’t show a whole lot of nutrients being added to the rain water as most are below the detection limit. That said there are trace amounts of organic nitrogen, chloride, calcium sodium and sulphur.
This result is not surprising in light of the result showing high numbers of bacteria. In order to run the analysis the samples are treated with strong acids breaking down anything that is in the water. The bacteria would not survive this process releasing their internal components resulting in these nutrients being picked up in the analysis. Essentially bacterial death is how a healthy nutrient cycle releases nutrients to the plants in the soil.
Peer reviewed research
There are many research papers looking at rain water collections systems and if they contain contaminants. Generally speaking it appears the research supports the observation that newer collection systems in rural settings are less likely to contain contaminants in the water whereas older structures in urban areas are much more likely. That said most researcher is looking at rainwater and its contaminants for the purposes of drinking water. Criteria are much more stringent than for the use in gardens.
The reason for the different criteria is that contaminants are much more dangerous if consumed then if they simply touch your skin. There is limited evidence to suggest that any of these contaminants are able to make their way into our garden crops tissues.
Recap of the Hypothesis and conclusion
In conclusion rain water based on today’s evidence is safe to use with negligible benefits aside form a sustainable water source.
I will continue to use water conservation techniques in collaboration with collected rain water to make sure my garden is as sustainable as possible. If you have missed any of the testing garden assumptions videos such as if tap water is harmful to use in the garden check out the playlist on screen now.
Does the Chlorine in Tap Water Harm Beneficial Bacteria?
How I Conserve Water While Gardening Through a Drought.
References: Air Quality Article CBC:
 CCME description of F1-F4
 Rainwater harvesting, quality assessment and utilization in Kefalonia Island Greece
 Contamination of potable roof-collected rainwater in Auckland, New Zealand
 World Health Organization Known Carcinogens
 CCME water quality guidelines
 Alberta Environment and Parks Water Quality Guidelines