Mining may stop, but the legacy (waste) it leaves behind often keeps moving.
In rivers affected by acid mine drainage (AMD), metal(loid)s like arsenic, copper and zinc don’t just disappear once mining ends. Instead, they attach themselves to sediments and are carried downstream, sometimes for decades.
My research looked at how this plays out in the Carnon River in Cornwall; a river impacted by centuries of mining and ongoing AMD inputs.
Contamination lives in the riverbed
We often focus on contaminated water, but much of the contamination is actually stored in river sediments.
In the Carnon River, sediment metal(loid) concentrations were extremely high not only near mine inputs, but all the way downstream into the estuary. That’s unusual. In many AMD-affected rivers, contamination decreases with distance as metal(loid)s dilute or deposit. Here, it didn’t; showing that they are continuously transported, stored, and reworked within the river system, leading to very limited natural attenuation.
To understand why this is happening, we need to look more closely at the sediments themselves.
Two sediment characteristics, two contamination pathways
What controls this comes down to particle size and mineralogy. What we found were two concentrated phases: 1 – fine sediments (<0.063 mm) and 2 – coarse sediments (>2 mm).
- Fine sediments
These particles are rich in iron oxides formed when AMD mixes with less acidic river water. Those iron minerals act like metal magnets, especially for arsenic.
But they’re not permanent sinks. Changes in pH, redox conditions, or river flow (like floods) can destabilise them and release metals back into the system.
- Coarse sediments
These are pieces of historic mine waste (sulfide minerals such as chalcopyrite and sphalerite that still contain copper and zinc).
They behave differently: instead of adsorbing metal(loid)s, they are the source. As they slowly weather/break down, they continue to release contamination long after mining has stopped.
Why the estuary matters
Downstream, the estuary turned out to be a major sink. Fine, metal(loid)‑rich particles accumulate there under changing salinity and redox conditions, storing large amounts of contamination into the sediment. This means that the estuary has the potential to be a prolonged source of secondary contamination.
The take‑home message
Mining contamination doesn’t just stay near mine sites; it’s stored, moved and recycled by river sediments.
In the Carnon River, both fine iron‑rich particles and coarse mine‑waste fragments are keeping arsenic, copper and zinc in the system decades after mining ended. Under the right conditions, that contamination can be remobilised again.
Cleaning up rivers like this means thinking beyond water quality alone. We need to understand what’s in the sediments, where it came from, and how stable it really is; because in mining‑affected rivers, the past is still very much part of the present.
If you’re curious to dive deeper, check out the full paper in Applied Geochemistry (https://doi.org/10.1016/j.apgeochem.2026.106859).