Nitrate Contamination in Harvested Rainwater: What You Need to Know

What Are Nitrates?

Nitrates (NO₃⁻) are naturally occurring compounds made up of nitrogen and oxygen. While they are essential nutrients for plants, high levels of nitrates in drinking water can pose serious health risks, especially for infants and pregnant women. Ingesting too much nitrate can interfere with the blood’s ability to carry oxygen—a condition known as methemoglobinemia, or “blue baby syndrome”.

Rainwater is a valuable and renewable water source, but it is not automatically safe to drink, especially if untreated. Nitrate contamination is invisible and tasteless, so relying solely on visual inspection or taste is not enough.

How Do Nitrates End Up in Rainwater?

While rainwater itself begins as distilled water during evaporation, it becomes vulnerable to contamination as it travels through the atmosphere and when it lands on surfaces such as roofs or collection tanks. Here are some common pathways for nitrate contamination in harvested rainwater:

1. Atmospheric Deposition

Nitrates can be present in the atmosphere due to vehicle emissions, industrial activities, and agricultural fertilizers that release nitrogen oxides (NOx). These gases dissolve into rainwater droplets and fall to the ground as acid rain or nitrate-rich precipitation.

2. Roof Runoff and Bird Droppings

If your catchment surface (roof) is not clean, organic waste such as bird droppings or decaying plant material can introduce nitrates and other nitrogen compounds into the harvested water. These materials decompose over time, releasing nitrates as a by-product.

3. Nearby Agricultural Activity

In rural or farming areas, nitrates from fertilizers and animal manure can seep into the atmosphere or be blown as dust onto roofs. These nitrates wash into rainwater tanks during collection.

4. Contaminated Storage Tanks

Poorly maintained tanks can allow organic matter to decompose inside the system, increasing nitrate levels. Algal growth and biofilm can also lead to nitrogen compound formation over time.

Why Nitrates in Rainwater Are a Concern

South African National Standards (SANS 241:2015) specify that the nitrate concentration in drinking water should not exceed 11 mg/l as nitrogen (N). Exceeding this level makes water unsafe for consumption.

High nitrate levels:

• Are especially dangerous for babies under 6 months old.

• May contribute to long-term health problems such as thyroid issues and potential links to some cancers.

• Indicate broader contamination issues, other microbial or chemical pollutants may also be present.

How to Test for Nitrates in Rainwater

It’s important to test your harvested rainwater regularly, especially if you use it for drinking, cooking, or bathing. A basic water test should include nitrates, pH, turbidity, TDS, and bacterial counts (like E.coli or total coliforms). Use SANAS accredited water labs for reliable results.

How to Remove or Reduce Nitrates

If your water tests positive for nitrates, several treatment options are available:

• Reverse Osmosis Systems: Highly effective at removing nitrates, especially for point-of-use (under-sink) purification.

• Ion Exchange Systems: Often used in larger systems for treating nitrate heavy water.

• Proper Pre-Filtration and Maintenance: Regularly clean gutters, first-flush diverters, and tanks to minimize organic contamination.