Reality: At small concentrations, nitrate can cause effects in freshwater that impact on the use of that water for humans and aquatic life.
Nitrogen itself is only harmful to humans at very high concentrations (11.3mg/l). Concentrations higher than this are thought to be a risk to young children and pregnant women, and are thought to cause blue baby syndrome which affects babies and toddlers. Therefore, drinking water with high concentrations of nitrate may be dangerous for at-risk groups.
These levels are often not reached in New Zealand’s rivers and streams but are reached in some groundwater areas, which could be used for drinking. This is not thought to be a problem in public drinking water but if households obtain their own bore water, problems may arise, particularly in intensive dairying and horticultural areas (where nitrogen levels are more likely to exceed safe drinking-water standards). Water with high levels of nitrate may need to be treated to remove the nitrate, which can be costly. If nitrate in groundwater continues to increase, public water supplies may exceed drinking standards and need to be treated. This will be a large cost to the New Zealand public .
The range of leaching from dairy land is 20 – 200 kg of nitrate per hectare. The average in Canterbury is 70 kg for both dairy platform and dairy support land (OVERSEER). Each kilogram of nitrate that reaches water pollutes 88 cubic metres of water from pure to the MAV (Maximum Allowable Value, for drinking water 11.3ppm). Using the average dairy leaching rate of 28 kg N/ha/yr (from OVERSEER), the volume of water estimated to reach nitrate levels for drinking water standards (from zero nitrate levels) from dairying land (2.4 million ha) is 5,947 million cubic metres (Mm3).
To reduce nitrate concentrations by 85 to 95 per cent in water costs at a minimum between $0.30 and $1.80 per 1000 litres ; however, costs vary considerably and can be much higher. Removal of this nitrate is estimated to cost between $1.78 and $10.7 billion . In reality, all of the contaminated water would not be used for drinking. Nevertheless, it represents a degraded natural resource and an externality. Additionally, estimates of nitrate contamination are based on water initially containing no nitrate; however, many groundwater reservoirs presently contain nitrate with some areas exceeding drinking water standards .
Nitrate pollution is already becoming a big problem in Canterbury. In 2012, 11 per cent of tested wells in Canterbury exceeded levels for drinking water, increasing from seven per cent in 2011 . This could have health implications for infants .
Nitrate has ecosystem consequences at much lower levels (Figure 1) than when it becomes unsafe to drink. Ecosystem effects can start to occur at levels below 1 mg/l in water.
Figure 1: Nitrogen is toxic to fish at far lower levels than it is toxic to humans.
Too many nutrients (mainly nitrogen and phosphorus) in rivers and lakes cause unwanted plants (weeds and algae) to grow in excessive amounts which form into mats of slimy growth and soupy, green, smelly water (Figure 2 and 3).
Figure 2: Excess nutrients cause growths of algal mats like this from the Oroua River.
Figure 3: Algal mats in the Matakana River.
Like all plants, the algae photosynthesise: they take up oxygen at night and produce it during the day as they respire. So an overabundance of weed growth in water can cause oxygen levels in the water to fluctuate between very high and very low levels from day to night (diurnal fluctuations). This is shown in a number of Auckland streams in Figure 4. Large differences between high and low oxygen levels can be detrimental to fish and cause death.
Figure 4: Dissolved oxygen levels in some Auckland streams.
Even if these fluctuations only happen once a month, or in the summer months when weed growth is higher, fish cannot escape them. These problems are often overlooked because people testing water quality generally only measure things once a day/week/month. Things may look like they are not changing much but in fact oxygen levels may be fluctuating quite a bit between night and day. By averaging the data (only measuring once a day and taking an average of all the points), we fail to see these fluctuations. It is these high and low times that matter.
In contrast, a healthy stream has constant oxygen levels. As streams become more enriched with nutrients, and the algal/plant life blooms, the more the oxygen fluctuates. High fluctuations in oxygen levels can cause high rates of gross primary productivity (GPP), which is bad for biodiversity.
These changes are harmful and eventually lethal for river ecology, making it impossible for fish and insects to live. Additionally, the bed substrate becomes coated with the algal mats, restricting food and habitat availability for stream life. The changes caused by the excess nutrient-driven growth makes the stream unattractive for bathers and fishermen as well.
Figure 5: Black felt-like mats of cyano-bacteria in the Manawatu River bed near Palmerston North.
Excess nitrogen leaking from agricultural systems is not just a New Zealand issue. It is a huge global environmental problem, labelled by some as the ‘nitrogen bomb’ . In the early 20th century, a process was discovered to allow for the creation of nitrogen from fossil fuels (Habor-Bosch process) . Previously, nitrogen was fixed from the atmosphere by plants and microbes. Because of nitrogen fixation using fossil fuels, humans have changed the natural cycle immensely; we now produce more nitrogen artificially than all natural processes combined . Only a small amount of this nitrogen makes its way into the food we are producing; most ends up in waterways causing many problems including massive dead zones like that off the coast of the Mississippi River .
To remedy the effects of nitrogen in water or remove existing nitrogen in lakes can be very expensive. It is far cheaper to stop the inputs in the first place (read more here), but this requires a reduction in farming intensification – which is not so popular within the agricultural industry.
 Jensen, V.B., Darby, J., Seidel, C., & Gorman, C. (2012). Drinking water treatment for nitrate. Technical Report 6 in addressing nitrate in California’s drinking water with a focus on Tulare Lake Basin and Salinas Valley Groundwater (Prepared for California State Water Resources Control Board). Centre for Watershed Sciences, University of California, Davis.