Nitrates in groundwater
Nitrate is the most widespread groundwater contaminant in the world, contributing to algal blooms and posing a human health hazard to drinking water. Nitrate can also threaten the ecological health of surface water bodies like streams, rivers and lakes. If groundwater has high nitrate-nitrogen concentrations, any surface water fed by groundwater springs will also have high concentrations.
How much is too much?
Ecological health: Ecological studies have shown that above 1.0 mg/L the environmental quality of stream habitats is affected and biodiversity begins to decline. Levels above 1.0 mg/L cause excess plant and algae growth and can be toxic to fish or aquatic invertebrates.
Drinking water: The Maximum Acceptable Value (MAV) has been set at a nitrate-nitrogen concentration of 11.3 mg/L. Some studies have suggested that there are human health risks at concentrations much lower than the MAV. A level between 0 and 1 mg/L is the optimum result for drinking water.
How does nitrate enter groundwater?
Nitrate is highly soluble in water. As it forms in soil from the decay or oxidation of other nitrogen compounds, nitrate dissolves readily into the water in the soil pores. From there, plants can draw the nitrate in through their roots and use it to build proteins and amino acids that they need to grow. However, if there is more nitrate present than the plants can take up, the excess remains in the soil. Then, when it rains, that nitrate is washed, or leached, downward through the soil and into the underlying groundwater.
Therefore, nitrate contamination of groundwater occurs as a result of excess nitrate in the soil. This might be a result of excess application of fertiliser or farm effluent. It can also result from animal urine, human wastewater, or soil cultivation.
Does nitrate occur naturally in groundwater?
Nitrate is a natural chemical and it does occur naturally in groundwater, but generally at low concentrations. Estimates of natural nitrate-nitrogen concentrations in New Zealand groundwater range from as low as 0.25 mg/L to as high as 3.5 mg/L.
Click here to read about the ecological baseline survey of this spring
