Water quality limits in Canterbury

Different forms of nitrogen

Nitrogen is measured in different forms. Plan limits are set for levels of nitrate-nitrogen, ammoniacal-nitrogen, and dissolved inorganic nitrogen.

Why too much nitrogen is a problem

Once in groundwater, nitrate-nitrogen can persist for years and travel long distances, including to upwell through springs to contaminate streams, rivers, and lakes. 

While nitrogen is one of the most important nutrients for plant growth, excess levels of different forms of nitrogen can create conditions that make it difficult for aquatic insects and fish to survive and cause excessive growth of aquatic weeds and algae. In drinking water, high concentrations can be harmful to humans and livestock.

Where nitrogen comes from

The most common pathways are leaching and runoff of nitrogen into groundwater, rivers and lakes from agriculture (through animal urine and fertiliser), wastewater treatment plants, fertilised lawns, leaky septic systems and industrial discharges.

Nitrate nitrogen is one of the most common contaminants of groundwater. It is highly water soluble and leaches through soils and into groundwater easily.

Ammoniacal nitrogen enters waterways primarily through raw sewage or dairy effluent.

Dissolved inorganic nitrogen is the sum of nitrite, nitrate and ammonia, and is the most bioavailable form of nitrogen used by plants.

Total nitrogen is simply the sum of all nitrogen types found in a water sample.

Learn more

For more information about nitrogen and its different forms, see this factsheet page from LAWA.

E.coli is a type of bacteria commonly found in the gut of warm-blooded animals (including people and birds) and is excreted in faeces. E. coli in water indicates recent contamination with faecal material and that other pathogens harmful to humans may also be present.

Why E. coli is a problem

E. coli can survive for prolonged periods outside the body in the environment such as soils, freshwater and sediments. In water used for swimming and other recreation, E. coli contamination from faecal material can cause gastroenteritis or infections of ears, eyes, nasal cavity, skin, and the upper respiratory tract.

Drinking water should be completely free of E. coli.

Where E. coli come from

Common sources of E. coli are human wastewater discharges, animal waste, bird droppings and stormwater run-off. If soil is contaminated with E. coli, for example from grazing animals or wastewater discharges, rainwater percolating through the soil can carry E. coli downward into groundwater. Groundwater may not filter out E. coli contamination before it reaches a water supply well. 

Learn more

For more information, see these fact sheets from LAWA.

DRP is a form of dissolved phosphorus in water that is available for aquatic plant and periphyton growth. Periphyton include algae, bacteria, other microorganisms, and non-living organic matter attached to any submerged surface.

Why too much DRP is a problem

Phosphorus attaches to soil particles and is naturally present in water in low concentrations as DRP. In high concentrations, DRP can cause rapid aquatic plant growth or algal blooms which can choke aquatic life, reduce dissolved oxygen levels in water and cause long-term damage to the health of a waterbody. Typically, natural DRP concentrations in groundwater are low. Elevated DRP in groundwater may indicate influences of human and intensive land use activities and can also contribute to the growth of algae in lakes, rivers, and streams.

Where DRP comes from

Phosphorus is in domestic wastewater and animal waste. Runoff from land is the main way phosphorus enters waterways - it can end up in sediment in rivers, streams, and lakes or percolate through the soil into groundwater.

Learn more

For more information, see this fact sheet from LAWA.

Chlorophyll a is the green pigment in plants and algae that is used for photosynthesis. Measuring how much of this pigment is in lake water provides a good indicator of the total amount of algae in a lake.

Why much algae is a problem

It’s okay to have some algae in a lake as this is important for lake ecosystems, just not too much. The more algae present, the poorer the water quality. Lakes with high levels of algae are at risk of changing to a persistent degraded state. 

Large amounts of algae in lakes (called an algal bloom) can decrease water clarity and make it greener, reduce water oxygen levels, produce unpleasant tastes and smells, and contain large amounts of cyanobacteria, some of which produce toxins that can cause illness or death in humans and animals.

Where algae comes from

The amount of phytoplankton in a lake is closely linked with the amount of nutrient enrichment (nitrogen and phosphorus in the lake water) and the biological productivity of the lake ecosystem (called trophic level).  Runoff from land is the main way nutrients such as nitrogen and phosphorus enter lakes. Excess algal growth is often a result of surrounding human and intensive land use activities.

Learn more

For more information, see these factsheets from LAWA.

The Trophic Level Index (TLI) is calculated based on annual average concentrations of Total Nitrogen (TN), Total Phosphorus (TP) and phytoplankton biomass (chlorophyll a). The TLI combines these measured concentrations into one score and makes it easier for us to compare and understand the overall condition of a lake. 

How we get our data

• Our high-country lakes are sampled five times a year, in monthly intervals from December to April. Samples are taken from the top 10 meters of the water column (or less if the lake is shallower), or by deploying a sampling container from a helicopter.

• Te Waihora/Lake Ellesmere and Wainono Lagoon are sampled monthly from a boat at about 0.3 m depth.

• Wairewa/Lake Forsyth, Muriwai/Coopers Lagoon, Mata Kopae/St Anne’s Lagoon, and Lake Rotorua are sampled from shore, either monthly or quarterly.

Learn more

For more information, see this Lake TLI factsheet from LAWA.