How polluted is the River Derwent? Can you eat the fish from it and take a dip?

Curious Hobart has been given the task of finding out the state of play.

You could write a book on the subject — people have — and it's hard to distil the issue into a few paragraphs.

In a nutshell, the legacy issues of the zinc works, increasing development, ageing infrastructure and nutrient spikes are still major challenges for the Derwent.

So what is currently going on under the water and how did it get to this?

Like many rivers in the early days, the Derwent was used as place to dump waste, and in 1824 its water was deemed undrinkable because of pollution.

Crayfish can be found under the Tasman Bridge but are off limits because of heavy metal pollution. ( Supplied: Millie Banner )

The advent of heavy industry, the growing population, urban and agricultural development and fish farming all have, and continue to have, an impact on the waterway.

But heavy metal contamination remains the biggest issue.

The legacy of the Lutana zinc works is still felt, and means you can't eat certain fish, or plant backyard vegetable gardens in some suburbs.

The zinc works, now owned by Nyrstar, operates very differently to how it did in its early days, but its lasting impact means more than 100 tonnes of zinc was extracted from groundwater at the factory site last year alone.

The latest assessment of the river's overall health, tallied by the Derwent Estuary Program (DEP) in 2016, is sober reading.

The river contains:

Heavy metals, which are toxic to aquatic plants, animals, and accumulate in seafood

Heavy metals, which are toxic to aquatic plants, animals, and accumulate in seafood Excessive nutrients, which can smother fish habitats and release heavy metals from sediments

Excessive nutrients, which can smother fish habitats and release heavy metals from sediments Pathogens from human sewage

Pathogens from human sewage Sediments, which reduce light to aquatic plants

Sediments, which reduce light to aquatic plants Litter, particularly floating plastics

Sorry, this video has expired Fly over Hobart's River Derwent

DEP chief executive officer, Ursula Taylor, pulls no punches when describing the legacy issues of the zinc works.

"The Derwent sediments are highly contaminated and are probably some of the most contaminated sediments you can find anywhere," she said.

But there is some good news.

Pylons in the Derwent attract plants and fish. ( Supplied: Millie Banner )

"That level has actually been going down. We find that most contaminated sediments are 10 to 20 centimetres below the surface, which shows that as time goes by cleaner fill comes down the catchment and is covering those historically contaminated sediments," Ms Taylor said.

"They key is to leave them in situ at the moment. The concerns are if they're disturbed too much it could mobilise the heavy metals in the sediments."

The problem at Nyrstar remains zinc leaching into groundwater.

"They have installed horizontal bores to extract that groundwater and reprocess the zinc they find in it, and last year they extracted 104 tonnes of zinc out of groundwater," Ms Taylor said.

"It's from historical sources … they have capped a lot of sources that cause that contamination in the water. But there's certainly a lot there and over time that amount will drop as they extract and process it. "

The zinc works has been operating at Lutana for more than 100 years. ( Chris Parks: Nyrstar )

Nyrstar's Todd Milne said in the early days run-off from outdoor stockpiles contaminated groundwater, but stockpiles were now all undercover.

"[The material] was stored out in the open and when it rained, rain hit those surfaces and material has been able to percolate down into the ground, so that's accumulated over 100 years," he said.

It's affected the factory site as well as some suburbs, prompting health warnings.

A study 10 years ago showed that any land where the soil had not been disturbed for a long time was a health risk, and the advice about vegetable growing still applies today.

"The study indicated that anywhere where there had been any sign of significant gardening or anything like that, or any real disturbance of the land, that wasn't showing particularly high levels," Mr Milne said.

"It was areas that had been undisturbed for a long time that still showed some elevated levels of metals.

"Some precautionary advice applies to a narrow band of properties in Lutana and some properties on the foreshore at Lindisfarne and Geilston Bay.

"For those particular properties there is advice around avoiding contact with bare soil with small children and also a preference for growing veggies in raised beds with clean soil."

He said information about testing and monitoring are available on the company's website and the Environment Protection Authority's site.

"For the past 15 years we have been aggressively pursuing a groundwater remediation strategy," he said.

"We are really absolutely completely open with all of the monitoring results from the site and that will include all of the monitoring of our estuarine outfall, the groundwater levels — we publish all of it."

Who looks after the health of the river?

Plinio Taurian is worried about the ongoing health of the Derwent. ( Supplied: Plinio Taurian )

The DEP was set up as a partnership between industry, local government and state government to assess and do something about the river's health.

It is a non-statutory, not-for-profit, voluntary group working "in a unique partnership with industry and government", which Ms Taylor says is a positive way to approach the environmental problems.

"It is a big communication happening as complex as the river itself," she said.

"We have this unique arrangement where we work positively with industry and government to make a difference."

But responsibility for monitoring and assessing water quality also falls on authorities such as the Department of Primary Industries, Parks Water and Environment (DPIPWE), TasWater and the Environment Protection Authority.

Fisherman Plinio Taurian is worried not enough is being done. He's not a fan of the DEP.

"The story coming from the Derwent Estuary Program is everything is rosy, everything is getting better, and I just don't see that is the case," he said.

Mr Taurian is an avid fisherman who is passionate about the waterway and spends an "unnatural amount of time … chasing black bream and sea run brown trout."

Algae growing on seagrasses can smother fish habitats. ( Supplied: Plinio Taurian )

He has a lot of concerns, but mainly about the changes in weed beds and seagrass.

A recent two-year study confirmed high nutrient levels some caused by salmon hatcheries upstream were causing an algal growth to smother sea grass, making it uninhabitable for fish.

"Algal growth can form a big green mat and smothers sea grass and they are important habitats for fish breeding important for birds," Ms Taylor said

"It's also like a canary in the coal mine as an indicator of estuarine health."

Ms Taylor said the data would be shared with stakeholders for them to act on.

"We've shared results that we've found in the river system with hatcheries as well and they will take on board."

"In winter nutrients from agricultural sources tend to dominate in the river system and in the summer fish hatcheries contribute nutrients as well, but it's multiple sources of contamination I guess.

"Now we have this information it's a starting point for how can we best manage. That's something we need to talk to our stakeholders about. That will be the next step."

The focus must be on the future and improving the river's health, Ms Taylor says. ( ABC News: David Robertson )

It's another issue which is not going away.

"As the population increases there's more demand for land and building and also more contributions to wastewater treatment plants, which are another source of nutrients, so that's why we work quite closely with TasWater, and councils," she said.

Mr Taurian is not convinced the Derwent is being looked after as well as it could be and wants industry and other stakeholders to take more action after they are given water sampling data.

"We're hearing a lot from the Derwent Estuary Program when it comes to media but essentially they are not actually responsible for any of it," he said.

"The main inputs are increased developments, more intensive farming and salmon hatcheries on the upper system, growth at Norske Skog and then there's the base block of the pollution at Nyrstar.

"There's increasing negative factors coming at it from all directions.

The stormwater systems can become overloaded after heavy rain. ( ABC News: David Robertson )

"The whole issue needs much more attention. I want to make people aware what is happening under the water surface.



"We all live off the back of the value of our houses these days. Our houses and suburbs line the Derwent. If we end up with a river where the health balance gets tipped into the negative then we're in trouble."

He wants more action from governments — local and state.

"Councils at times are pumping untreated effluent into the river at times of flood and at times where the capacity is not up to speed," he said.

"We are just living in the dark ages with our effluent inputs into the river. We can do more, we should do more."

"Is there a disconnect there with who is responsible and funding for those services. Councils, TasWater, State Government should be doing more."

So can you eat the fish in the Derwent?

While the heavy metal issue remains, so do the warnings about seafood.

Shellfish are a definite no-no and bream are also off the menu because they live longer and accumulate heavy metals.

Plinio Taurian holds a Derwent bream, a fish which can be caught but not eaten. ( Supplied: Plinio Taurian )

"We also look at heavy metals levels in fish and shellfish," Ms Taylor said.

"We are slowly starting to see a decline in heavy metal levels in most species."

The health advice for flathead is to limit consumption to two meals per week, but young children and pregnant women should not have any.

"Flathead have been measured by Nyrstar for about 25 years for mercury levels, and I think the reason it was chosen is because it is a popular table fish," Ms Taylor said.

"They are finding the levels are at or just above health guidelines for mercury levels in fish."

When do you avoid swimming?

Swimming for some is a year-round activity despite the temperature, and Hobart is on the map now for nude solstice swimming thanks to the Dark Mofo festival.

The Dark Mofo nude dip in the Derwent is a mid-winter ritual but the Derwent is not always safe for swimmers. ( ABC News: Gregor Salmon )

But it pays to heed warnings after heavy rain when some nasties can be washed through the stormwater system.

"Water runs off roads and creeks and that's not treated, and it picks up pollutants along the way from garden, animal faeces — sometimes sewers can overflow, if there has been a big storm it can overflow," Ms Taylor said.

There's also a growing awareness about plastics and rubbish — thanks to the War on Waste program.

Some time ago a monitoring program estimated 24 tonnes of rubbish was found in the Derwent each year.

"It's top of mind for community and I really do think it's important we all contribute so reducing our waste consumption so it doesn't end up in our waterways, and we do see a lot of litter," Ms Taylor said.

Will the Derwent ever get a clean report card?

Perhaps only time will tell if the Derwent will ever be fully restored.

Map The Derwent estuary extends from the rural river town of New Norfolk south to the Iron Pot lighthouse

"There's no point shying away from the situation and what has happened in the past but the key is to look to the future now and what we can do about it, and which we're doing in partnership," Ms Taylor said.

"It's a very vibrant system which supports a wide range of species.

"We are keeping an eye on things to make sure things don't decline or get worse.

"We can make improvements and identify sources of nutrients and having the conversation about how they can be managed."

In a statement, EPA Tasmania said it undertakes monthly water sampling in the Derwent River below the Tasman Bridge "under the DEP monitoring program and partnership, and regular water sampling between New Norfolk and the Tasman Bridge is undertaken by Nyrstar and Norske Skog".

Heavy rain washing street pollution into the Derwent makes it a no-go zone for swimmers for several days. ( ABC News: David Robertson )

The EPA said both it and the DEP "use the Nyrstar data to monitor changes in the river".

It said there had been "periodic independent testing of the heavy metal contamination in the Derwent by the Derwent Estuary Program and its research partners".

The EPA said routine monitoring of water quality was undertaken "at a number of locations in the river, however above New Norfolk it is generally undertaken by TasWater".

Asked about the response when a breach occurs, the EPA said that would "depend on the location of the discharge, who is responsible for it and the characteristics of the exceedance".

The EPA said salmon farming generally did not contribute to nutrient load in the Derwent, with the nutrient levels reported in the research generally above New Norfolk.

"The EPA regulates a number of entities that release treated waste water into the Derwent River, and from time to time directs that discharges need to be managed more appropriately to reduce nutrient loading," it said.