Erosion shares water quality blame
The finger of blame for the deteriorating quality of water in New Zealand is frequently pointed at the dairy industry.
However, a recent report produced by Dr Jan Wright, the parliamentary commissioner for the environment, suggests it is both unfair and inaccurate to attribute this problem entirely to dairying.
"There are two nutrients that collectively cause the problem - nitrogen and phosphorus," she says.
"The largest source of nitrogen is urine from livestock; but the largest source of phosphorus is the sediment from ongoing erosion - a legacy of forest clearance and topdressing.
"The two nutrients get into water by largely different routes. Nitrogen occurs in forms that are highly soluble in water and so can travel via groundwater as well as across surfaces. This makes it particularly elusive - preventing it getting into water is a major challenge.
"Most phosphorus, on the other hand, gets into water with soil and if the soil can be stopped from getting into water, so will the phosphorus. Once in water, however, much of the phosphorus is locked up in sediment and can be there for a very long time."
Nitrogen and phosphorus stimulate plant growth, leading to algal blooms (sometimes toxic), oxygen depletion, and ecological damage. Ammonia can kill fish, and elevated nitrate levels can make aquifers undrinkable.
Periphyton build-up in rivers is fuelled by excess nitrogen and phosphorus. Tests show these slimy periphyton mats can be reduced by reducing phosphorus inputs.
But how did these pollutants get into the water in the first place?
Wright says the higher, steeper parts of the catchment are scarred with erosion - a legacy of burning forest to create pasture for sheep in the 1920s.
Sediment has settled - and continues to settle - in some of the river bends and, over time, some of the phosphorus it contains dissolves out and becomes available for plant growth.
She presents a hypothetical scenario of a pipe upstream discharging wastewater from a town treatment plant and several dairy farms spraying shed effluent onto land. In a few cases, stock have direct access to water and heavy rainfall sometimes washes manure into the river and its tributaries.
While high flows from winter storms scour the periphyton off the rocks and wash some of the sediment downstream, the volume of water falls in summer and the flow rate falls, allowing the periphyton to return.
The report asks what can be done about the causes of this water quality problem - and Wright offers some answers.
"Erodible land in the hills can be planted - most effectively with poplars that will grow from poles hammered into the ground and develop extensive root systems that will hold soil. The clearance of any remaining native vegetation in gullies should cease," she says.
"One option for the town wastewater is building bigger storage tanks and not discharging into the river when flows are low. Another is spraying the wastewater onto land, possibly fertilising a forest.
"Stock should be restricted from direct access to water, though this is much more important for cattle than sheep. Because the limiting nutrient is phosphorus, relatively simple riparian strips of an area of rank grass between an electric fence and the river should have a significant effect.
"The relative size of the different sources of phosphorus and the effectiveness of the different interventions need to be key considerations."
While clear, clean, cool streams full of life flowing through forests still exist in remote parts of the country, it is not realistic to return all fresh water to this pristine state, Wright concedes.
"But nor can we afford not to act," she declares. "The quality of our fresh water is one of the biggest environmental challenges that we will face in this clean, green country of ours."