Nenthead Mine Water Treatment Scheme - Preventing Pollution in the Nent and South Tyne

Overview

There are two types of pollution that need to be addressed to improve the water quality of the Rivers Nent and South Tyne.  We need to tackle:

• point sources of pollution which involves removing dissolved metals from mine water before it reaches the watercourses and,
• diffuse sources of pollution, which includes preventing metal contaminated materials, such as from old spoil heaps, washing or eroding into rivers

Tackling point source pollution

Monitoring of the River Nent and the mine water discharges by the Environment Agency shows that it is point mine water sources, such as the Caplecleugh, Rampgill and Haggs Levels that cause the most severe environmental harm because these discharges contain very high concentrations of metals. They flow all year round and under lower river flows, provide most of the water in the river so there is little dilution of the high metal concentrations. Zinc concentrations just downstream of the Caplecleugh Level can be more than 200 times the Environmental Quality Standard (EQS) which is used by Government to assess whether rivers are polluted. This causes severe impacts on river wildlife like river flies and fish.

The graph below shows how the level of zinc pollution varies with river flows in the River Nent.
  

Figure 1 Zinc pollution levels in the River Nent. (see here for sample point locations.) N1 = Nent upstream of Caplecleugh; N4 = Nent upstream of sewage treatment works; N8 = Nent upstream of Nentsberry Bridge; N9 = Nent upstream of Haggs; N6 = Nent at Nenthall; N10 = Nent at Foreshield Bridge; N7 = Nent at Alston) 

Pollution caused by the Caplecleugh and Rampgill Levels mine water

On average about 4.5 to 5 tonnes of zinc enter the River Nent each year from the Caplecleugh and Rampgill Levels as well as a smaller amount of cadmium. The flows emerging from the adits vary, increasing after rain but never drying up. The average flow from Caplecleugh is around 13 litres per second (l/s) and normally ranges between 9 and 35 l/s. The average flow from Rampgill is around 12 litres per second (l/s) and normally ranges between 7 and 30 l/s. The concentration of metals in the mine waters also vary with flows, with higher concentrations when there is less mine water being discharged.

The proposed scheme will remove approximately 4.5 tonnes of zinc as well as a smaller amount of cadmium each year. Figure 3 below shows the predicted change in zinc concentrations in the River Nent at Alston for two treatment scenarios. These are plotted against river flows to show that the decrease in metal concentrations (and therefore environmental harm) is greater at lower river flows. The two treatment scenarios are: 

1. CC12: treat 12 litres per second of the Caplecleugh mine water
2. Combination 20 l/s: treat 20 l/s of the combined Caplecleugh and Rampgill mine waters

 Figure 3: Predicted change in zinc concentrations with pollution intervention scenarios

We have used these predictions to recommend how big the mine water treatment scheme needs to be to deliver the best possible environmental outcomes in this sensitive location. 

The current treatment scheme is being proposed following a lengthy process to identify a suitable location. Find out more about how we located a suitable site here.

Tackling diffuse pollution

We are also developing measures to reduce diffuse pollution from the mine wastes. This will mean less metal is transported down the river system and to accumulate in the Tyne estuary sediments. These measures include helping calaminarian (metal tolerant) vegetation to flourish which we are successfully doing at several sites in the area, including Carrshield and Coalcleugh.

Find out more about our work on diffuse pollution interventions here.

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