#05 | What are nitrates and why are they so damaging to water?

Author: Damián Sánchez (Cetaqua)

In this new blog post of the LIFE NIRVANA project we are going to try to explain what nitrates are and why every so often we read in the newspapers and hear on the radio that they are a harm or, if you prefer, a hazard, for the environment and, in particular, for our rivers and aquifers. But… let’s take it one step at a time.

¿What is nitrate?

Nitrate is a chemical compound that is dissolved in water just like calcium, sodium or bicarbonate, which I am sure you are more familiar with. From a chemical point of view, nitrate is made up of nitrogen and oxygen in a ratio of 1 to 3, in other words, there is one atom of nitrogen for every 3 of oxygen, which is why we represent it graphically in this way: NO3. Yes, I know, I have put a dash over the 3 and you are wondering what it is. Well, that represents a negative charge, which is why we say that nitrate is an anion (if it had a positive charge it would be a cation).

Like cooking salt, when nitrate comes into contact with water it dissolves completely and therefore moves with the water, that is, wherever the water goes, the nitrate will go.

How does nitrate get into the water of rivers and aquifers?

We will differentiate between two possible origins: one natural, and the other due to contamination (for example, from human activity).

1. Natural origin: The ‘natural’ soil, meaning the soil of our fields, forests and, in general, of any non-urbanized area, contains nitrogen. When rainwater reaches the soil, part of this nitrogen is transformed into nitrate (remember that in the previous section we saw that nitrate is composed of nitrogen and oxygen). This rainwater, with the nitrate dissolved in it, flows into rivers or infiltrates into underground aquifers. Is this nitrate present in large quantities? Well, not really… For reference, we will give a figure of about 5 milligrams per liter (very little!).

2. Pollution origin: Nitrates can also come from several human activities, among which I will highlight the three most important: agriculture, farming and wastewater.

a. Agriculture: To grow, plants need mainly nitrogen, phosphorus and potassium (if you don’t believe me, look at the label of any garden fertilizer you have at home). What happens if a plant needs 3 grams of these components and I apply 5 grams? Well, the remaining 2 grams will not be taken by the plant and will remain in the soil. Nitrogen, as it happens in the ‘natural’ soil, is transformed into nitrate, dissolves in water, and moves with it wherever it goes. If the water ends up infiltrating into an aquifer, then that’s where the nitrate goes too. And if anyone wonders what happens to the other two plant nutrients, I will tell you that phosphorus and potassium are retained in the soil, so they do not dissolve in the water.


b. Farming: Animal excrement (as well as our own, mind you!) contains significant amounts of nitrogen. This nitrogen, as before, is transformed into nitrate in contact with water. When farming is extensive, when there is a lot of space per animal (you can imagine a meadow with a few cows grazing), there is no problem because the amount of nitrogen is very small. But when farming is intensive, for example a pig or chicken farm with thousands of animals in a small area, problems begin to appear, because the amount of nitrogen discharged into the environment is very large and in a very small space.

c. Wastewater: Wastewater is the water that leaves our homes after we have used it (sink, toilet, kitchen, etc.). This water contains excrement and therefore nitrogen. If this water is discharged into a river, once again the nitrogen is transformed into nitrate. It is the same case as above, except that it is not the animals that are responsible, but we ourselves.


What is the nitrate limit and what happens if it is exceeded?

For several decades, in all European Union countries, including Spain, the maximum amount of nitrate allowed in water is 50 milligrams per liter. Water with 51 milligrams per liter is not drinkable and would not be in a good ‘ecological’ state. Remember that above I said that the amount of nitrate in the soil is about 5 milligrams per liter, therefore, when a water has more than 50 milligrams per liter it means that there is some human activity responsible.

That’s all fine and good, but what happens if I drink water with more than 50 milligrams per liter of nitrate? Well, if you drink it for a day…or two, then absolutely nothing is going to happen to you, but if you drink it regularly you could be putting your health at risk because nitrate reduces the ability of red blood cells to carry oxygen. This especially affects breastfeeding babies, who can develop a serious illness due to lack of oxygen called methemoglobinemia or “blue baby syndrome.”

What measures are there to reduce the amount of nitrate in our waters?

These measures can be classified into two categories: preventive and corrective. Preventive measures are those aimed at avoiding the entry of nitrate into the natural environment, such as water treatment plants or advising farmers to apply the exact amount of fertilizer needed by the plant.

Corrective measures are aimed at solving the problem when it already exists, in other words, they reduce the amount of nitrate in an already polluted water.

The main preventive measure is a European directive approved by the European Union that aims to reduce the excess of nitrate in our waters. Do you know when this directive was published? In 1991! In other words, more than 30 years ago, and still today nitrate is the main responsible for the poor quality of our aquifers.

The solution, therefore, must come from a combination of both preventive and corrective measures. Among the latter is precisely the one we are validating within the framework of the LIFE NIRVANA Project, which is based on the elimination of nitrate present in our aquifers by using iron nanoparticles.