The Parliamentary Commissioner for the Environment's 2012 report points to diffuse dairy runoff as a major contributor. Dairy cows are the greatest source of nitrate in many catchments.
They produce about 14 times the effluent of a human, says Talstra, and are responsible for the equivalent excretions of 100 million people onto the land every day.
Clearing land for farming is also leading to increased phosphate pollution.
But dairying is the nation's cash cow, creating nearly three per cent of GDP and - says Statistics New Zealand - both the industry and its intensification are growing.
In 2012, 6.4 million dairy cows roamed our pastures, up 23 per cent from 2007.
Fish and Game's recently published survey showed that 37 per cent of Kiwis think that the economy is too heavily dependent on the industry, and 31 per cent believe that intensification has gone too far.
So, too, does Talstra. On October 2, lacing up his tramping boots and donning a 25-kilo backpack filled with porridge, protein bars and dehydrated peas, he set off from Ninety Mile Beach.
Meeting with scientists, farmers, water quality advocates, country-dwellers, politicians and Federated Farmers' Colin English, Talstra found that all were unanimous in their desire for healthy waterways that are swimmable and harvestable.
In Talstra's opinion, this could be achieved by an immediate polluter pays scheme. "At the moment it costs more to have methods of mitigation on your farm than it does to let it go into the waterways and face a possible fine."
Diffuse pollution, he adds, needs to be monitored and controlled by regional councils.
"Farming is a very leaky business. But if you are not conforming to the regulations you need to be seriously reprimanded."
Talstra can finally rest his blistered feet this week after more than six months on the road. His next adventure will be to work on a book about his epic journey.
Lance's top spots
1. South Canterbury's Luke Pukaki: "It's like a full length mirror for Mount Cook."
2. Nelson Lakes National Park
3. Nelson's Richmond Ranges
4. Pureora Forest: This fantastic forest is filled with huge trees saved by campaigners in the 1970s.
5. The Canterbury high country
Lance's worst spots
1. Mangatawhiri stream (North Waikato). Riddled with nitrogen pollution, the stream takes a turn for the worse after a mere 10 kilometres traversing farms.
2. The Waikato river
3. The Waipa River (Waikato) which joins the Waikato and Ngaruawahia
4. Lake Horowhenua (near Levin): It's a hypertrophic lake (constantly plagued by algal blooms due to high nitrogen levels) and source of historic pollution.
5. The Foxton River Loop (near Levin in the Manawatu): It boasts a "no fishing, no swimming, no human contact" sign.
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