Rain Barrels: Easy Strategies to Reduce Chemical Load

In the interest of conserving water and keeping the water bill low, my husband Patrick and I were excited to set up a rain barrel for the vegetable garden we share with his parents. We connected the shiny new plastic barrel (pictured below) to the gutter of their home, and have been collecting rainwater from the roof for a few years. A screen reduces the amount of debris that enters the barrel (and hopefully the ability of mosquitos to enter and reproduce). 

There are benefits to collecting rainwater on top of the water-saving perks.  According to the Center for Rainwater Harvesting, rainwater is safe to drink anywhere in the world--even in areas with acid rain. They deem it the "cleanest source of water that is available in nature today" (2006).

However, they tack a major "But" onto this clean water claim:

The problem we face with rainwater harvesting is keeping it that way - all the way to our spigot.  As soon as the rain hits the roof it is subject to contamination by pollutants that were deposited on the roof by wind, animals, insects, or by the leaching and dissolving of the material that the roof is made of (2006).

 In the time it takes a falling raindrop to find its way into a rain barrel, chemical contamination may happen through several avenues. Air pollutants are captured by raindrops before they hit the roof. Chemicals from the roofing material may be dissolved by rainwater, which is naturally acidic even in the absence of acid rain due to the formation of carbonic acid when raindrops dissolve carbon dioxide (EPA, 2019).  Roofs are a depository for airborne agricultural chemicals, industrial pollutants, and more--and rainwater picks those substances up in its journey. Debris in gutters is a haven for contaminants washed off the roof, which rainwater then passes through. Finally, the material and location of the rain barrel may contribute to the water's chemical load.

Unlike public water systems, water collected in rain barrels isn't tested for safe levels of contaminants. In my mind, this isn't a reason to shy away from rain catchment to water our gardens. It's simply a nudge to make a few adjustments to our system that can significantly increase the safety of our rain barrel water. 

Easy Strategies for Improving Rain Barrel Water Quality

Regardless of which material your rain barrel or roof is made out of, the following practices can make a big difference in reducing the synthetic chemical load of the water entering your garden:

1. Ditch the first few gallons of rainwater collected during a rain event.  

 The highest levels of rainwater contamination occur at the beginning of rainwater runoff  (Kim et al. 2005b).  Flushing out the first few gallons before collecting the rest of the rainwater "results in a dramatic increase in water quality" (Lye). 

According to the Center for Rainwater Harvesting, it is generally recommended to ditch the first 10 gallons of water per 1000 square feet (41 liters per 100 square meters) of roof.  It takes about 1/64 of an inch of rain (0.41mm) to collect this volume of water. Some research suggests that the roof isn’t very clean until after the first 1mm to 2mm of rainfall, which would make the 10-gallon recommendation a bit of an underestimate (2006).

2. Clean Your Gutters

This article from the Center for Rainwater Harvesting compares the quality of water collected from a roof with clean vs. dirty gutters (they hadn't been cleaned in around two years). Though not every variable was controlled, the data suggests that keeping your gutters clean can significantly reduce rainwater's chemical load by removing the contaminants held by gutter debris.

3.  Locate your Rain Barrel in the Shade

According to the Alternative Technology Association, the FDA (and other governing bodies around the world) limit how much UV inhibitor and other chemicals can be added to barrels safe for drinking water. Even when UV inhibitors are added to the plastic, polyethylene tanks will generally degrade over time (ATA).

The sun's heat degrades plastics more quickly, increasing how much contamination is entering your water (ATA). Locating your barrel in the shade can slow the rate of degradation and the release of synthetic chemicals into the rainwater. This has the added benefit of reducing the growth of algae, which loves light and warm conditions (EPA, 2010).

4. Collect Rainwater from the North- or East-Facing Side of the Roof

Rooftops exposed to direct sunlight will exhibit significantly higher surface temperatures than surrounding materials. Higher rooftop temperatures may accelerate chemical reactions of material on the roof, increasing the chemical load of the water (Lye).

5. Water the Soil, Not Your Food

As discussed in other A Nourishing Harvest articles, quality compost and soil rich in organic matter can reduce the bioavailability of some contaminants, and/or help to break them down. (Read more here and here). As stormwater seeps into the soil, the microorganisms and particles there help to "clean" it. Watering the soil instead of pouring runoff directly onto veggies and herbs allows the soil to work some purifying magic before that water is taken up by the plants (Sightline Institute).

If you'd like to learn more about reducing the chemical load of rain barrel water, stay tuned for the next A Nourishing Harvest article, where I'll explore rain barrel and roofing materials and their effects on water safety.

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Sources:

 Alternative Technology Association (ATA). "Water Self-sufficiency A Rainwater Tank Buyers Guide." ReNew: Technology for a Sustainable Future, no. 145 (2018): 57-67. Accessed January 19, 2020. doi:10.2307/90025497. 

Center for Rainwater Harvesting. "Roof and Gutters: Safe Materials." 2006.  http://www.thecenterforrainwaterharvesting.org/2_roof_gutters2.htm 

Lye, Dennis. "Rooftop Runoff as a Source of Contamination: A Review."  United States Environmental Protection Agency. Available at: https://rainwaterresources.com/articles/rooftop-runoff-source-contamination-review/ 

The Sightline Institute. "Rain Garden Edibles." Viewed Jan 20, 2020.  http://sightline.wpengine.netdna-cdn.com/wp-content/uploads/2015/02/rain-garden-trifold.pdf 

 United States Environmental Protection Agency (USEPA). "The Importance of Operation and Maintenance for the Long-Term Success of Green Infrastructure." pg. A10. March 2013. https://nepis.epa.gov/Exe/ZyPDF.cgi/P100MF66.PDF?Dockey=P100MF66.PDF  

United States Environmental Protection Agency (USEPA). "What is Acid Rain?" Updated Dec 20, 2019.  https://www.epa.gov/acidrain/what-acid-rain