At first, the small purple flowers are hard to spot in the weak May sunshine. Slowly the drifts of delicate mountain pansies, along with the white rosettes of alpine pennycress, begin to jump out, scattered across an area little bigger than a football pitch, on the banks of the River Allen in Northumberland.

This is a pocket of calaminarian grassland, an increasingly rare habitat where specialist plants called metallophytes have adapted to live in soils deeply contaminated by heavy metals, the legacy of more than 1,000 years of lead mining.

“This is absolutely a case of nature responding to pollution caused by humans,” says Geoff Dobbins, estates manager for the Northumberland Wildlife Trust, who is passionate about saving these grasslands.

But as they become cloaked in more thuggish plants such as gorse and broom, and the zinc and lead brought by mine-wash became slowly buried beneath a blanket of humus, there is a growing debate about whether these human-made meadows should be protected or allowed to gently fade away.

The grasslands are named after Viola calaminaria, a rare yellow violet found growing in the metal-rich soils of northern Europe, and itself named after calamine, the old term for zinc ore. About 30% of Europe’s calaminarian grasslands are found in the UK, although they are scarce, covering just 450 hectares (1,100 acres), with pockets in northern England, mid-Wales and the Highlands of Scotland.

The grasslands originally evolved in small patches around rocky upland outcrops, where veins of lead, cadmium and zinc had been exposed by the elements. As these began to be mined, according to Dr Ruth Starr-Keddle, a botanist at the North Pennines National Landscape, a biocrust of lichens and mosses developed that could tolerate toxic wastewater washing over them.

Then, plants such as the spring sandwort – once known as leadwort – and alpine penny-cress began to take hold, along with other tough customers such as sea thrift, bladder campion and kidney vetch, plants often found by the coast or growing in equally inhospitable limestone soils.

Despite their delicate appearance, these specialist plants can live in soils 30 times more toxic than most other species can tolerate. As they grow, metallophytes act as “hyper-accumulators”, cleansing the soils that feed them through a process called phytoremediation. This turns the metals they absorb through their roots into complex organic compounds, which are locked away below the surface once the plants die.

“They don’t get rid of the contaminants – they’re accumulated into the plant,” says Starr-Keddle.

Absorbing the metals also acts as a defence mechanism, making the plants unpalatable to herbivores and insects, while the level of toxicity is also enough to prevent fungal diseases from taking hold.

The barren, rocky uplands of the northern Pennines were first mined by the Romans, but the industry reached its peak in the mid-18th century. Today, the landscape is dotted with abandoned workings and spoil heaps; some high up on the moors, others closer to the rivers and the water the industry needed.

Once the veins of heavy metals were discovered, miners used a process known as hushing to uncover them. Artificial dams were built high above the seams and, once they were opened, would send a torrent of water cascading across the landscape, washing away topsoil and rocks, and exposing the metal ores.

These were then mined and carried by packhorses down to smelting mills, such as Plankey smelt mill at Briarwood Banks, close to where the mountain pansies are in full flower.

The remains of the mill poke out from under a shroud of brambles and hazel trees, near where there would have been a dressing floor where the rocks and ore were unloaded and sorted, says Dobbins. This involved using water to separate the heavier ores from rocks and stones, and fragments of the heavy metals were inevitably washed away into the rivers.

Over the centuries these deposits, together with those carried by mine-wash from the workings farther upstream, have gathered on shingle banks and silt beds along rivers such as the South Tyne and the Allen.

They provided the perfect base for the seeds of calaminarian plants, washed there with the minerals, to take root, and it is these small sites, miles away from their craggy homelands, that have become some of the last outliers of calaminarian grasslands.

The Environment Agency estimates that abandoned metal mines are still polluting about 900 miles (1,500km) of rivers in England, despite the fact that most closed more than 100 years ago. The pollution reaches rivers in two ways; from a “point source”, when contaminated water flows out of the tunnels dug to keep the mines drained; and diffuse sources, when rain washes metals from waste heaps into the watercourse.

According to Starr-Keddle, there is a balancing act between improving water quality and preserving an increasingly rare habitat.

“If you took samples from most of the rivers in the North Pennines, most have got contamination from lead mining in them,” she says. “We’re just trying to reduce it to safer levels.”

By improving the watercourse, she adds: “we are going to naturally impact the development of calaminarian plants.”

In the North Pennines alone, more than 60% of upland calaminarian grasslands have been lost since the 1970s. The grasslands are shifting, “successional” habitats, says Starr-Keddle, and as they move from areas of bare soil to denser, grassier habitat, the grasses out-compete with the flowers and they disappear.

Across northern England, several projects are looking at ways of harnessing the unique properties of metallophytes to help clean up the region’s rivers, while also keeping the grasslands blooming.

The workings at Nenthead, in Cumbria, are part of an industrial landscape where the vast spoil heaps are protected as a scheduled monument by Historic England. Here, areas of topsoil are being stripped back in a process called scraping, which re-exposes the metal rich subsoils and encourages calaminarian species.

Over the border in County Durham, the government’s Water and Abandoned Metal Mines (WAMM) programme, set up to deal with the pollution caused by former mines, is working on a similar scheme with several organisations, including the North Pennines National Landscape and the Rivers Trust.

At three former mining sites, all close to tributaries of the River Tees, the soil around the old mine workings has been stabilised using a mix of fast-growing grasses and coir matting to reduce the runoff. More than 1,500 plugs grown from the seeds of seven key calaminarian species, including non-metallophytes such as wild thyme and mouse-ear hawkweed, have been planted on the edge of the spoil heaps.

The plants, grown in a special mix of compost, stones and metal-rich soil, are acting as a natural barrier to stop metals leaching from the site, says Starr-Keddle, who hopes their seeds will spread across the whole site, so the metallophytes can continue to work their magic.

At Briarwood Banks, and the Northumberland Wildlife Trust’s other riverside calaminarian grasslands, Dobbins has a simpler solution to keep the pansies flowering – a strimmer. Twice a year, he cuts back the grasses, shrubs and perennials that lurk around the margins, waiting for their chance to move in.

But he is realistic, and as the water quality improves, these riverine sites, built on a legacy of pollution, will eventually be lost. “There’s good and bad in that outcome,” he says.

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