There are a number of aspects of the site in this specific setting that mean that the controls on groundwater below the site are quite complex. The geology under the site is a layer of peat, then some sands and gravels, and then quite a thick layer of clay, and then below that are the Coal Measures. The particular area where they want to extend the landfill has been mined and there are two shafts on the site, so there are direct connections down to the coal measures, which are very heavily fractured and faulted, because of the mining and natural fracturing.
The shafts go from the surface, down though the layers of peat, sands and gravels and down into the Coal Measures, and they go some distance into the coal measures.
What does that mean may happen to the site?
The Coal Measures are an aquifer and carry water. They tend to carry water in fractures and fissures and within the rock itself where there are sandstone layers within the Coal Measures. The landfill is going to be excavated down through the peat, sand and mud into the Coal Measures, and at that point, if all you did was dig a big hole into the Coal Measures, the groundwater, which is under pressure, would come up above the level of the coal measures, and the level would settle somewhere quite close to ground level. So it’s going to require active pumping to keep the groundwater below the base of the landfill.
Is that unusual for a hazardous waste landfill? Don’t they all require active pumping?
It’s a particular method that they call hydraulic containment. If you site a landfill in a setting like this, where the groundwater levels are naturally higher than the base of the landfill, the theory is that there will be an inwards pressure from the groundwater, and you can control that by pumping from the landfill, and that contains any potential contamination because water is coming into the landfill and you’re pumping it out, and it’s not migrating out into the aquifer.
So that’s the theory behind it.
But the worry with this particular setting is that it’s not at all clear how much water will need to be pumped to keep control of the system. While the landfill is being filled, groundwater levels have to be kept below the base of the landfill. There’s a concern that the pumping volumes needed to control the water levels in this setting have been very much underestimated. And there’s no good data to support what they’re pumping currently from the existing landfill. As the extension goes into this area that’s been actively mined with the shafts, you’d expect that there’d be quite a lot more fracturing, quite apart from old mine workings. So you’d expect that there could be quite a lot more groundwater inflow than from the existing landfill. And this hasn’t been quantified. It’s likely that the volumes of groundwater might be quite a lot more than the planned infrastructure for the site can cope with.
What’s the worst case scenario?
The worst case scenario is that they wouldn’t be able to handle the pumping volumes, which means that the integrity of the landfill liner might be compromised, or the facility might be inundated with groundwater, which leads to a risk of pollution. The quality of the water in the coal measures itself is not particularly good, plus you’ve got whatever’s in the landfill to contend with as well.
The real concern is that this groundwater will require treatment before it goes offsite because it’s not of a suitable quality to be discharged into the surface water system.
Is that a practical option?
It hasn’t been examined in any detail. And I think it might be cost-prohibitive. There’s a real lack of detail in the planning application. There’s no sensible estimate of how much water they are going to have to deal with. Much of the planning and infrastructure of the site depends on knowing what that volume is.
What other concerns do you have about the suitability of the site for taking hazardous waste?
There are two key things that haven’t been looked at, in my view. The first is that in this particular setting the groundwater levels will come up above the coal measures, so they are exerting this upward pressure. So while you’re filling the landfill you need to pump the groundwater to keep the level below the base. Otherwise it will interfere with your operations. Then when you’ve filled the landfill to a certain level, you allow the groundwater levels to come back up. But it’s absolutely necessary that the weight of waste above that is enough to counteract the pressure of the water pushing up. And if it’s not, you get what’s called basal heave.
These hazardous waste landfills are lined with clay and a synthetic liner, which are designed to prevent pollution from the waste mass moving into the surrounding aquifer. So if you have a situation where the waste mass is not sufficiently heavy to counteract the pressure of the groundwater pushing up, the liner can rupture due to basal heave.
We’ve done some calculations and it’s not clear that there will be sufficient waste thickness to counteract basal heave, and it’s not clear at what point they’ll be able to know there’s a sufficient thickness if they’re not monitoring the density of the waste that’s landfilled. Detecting whether or not this is an issue once you’ve got tens of metres of waste is a difficult thing. I feel there’s not been a thorough enough examination.
This relates to the second factor: there’s uncertainty about whether there’s sufficient demand for this facility. They’ve not filled the existing void in the time that they expected. If there’s not sufficient waste to fill the void, you’re in a situation where you have to pump groundwater out forever, which won’t be financially sustainable.
Residents are concerned about the possibility that there’s not enough waste to fill the tip extension from the point of view that if it’s not possible to fill the void in the timescale stated, the site may never close. But you’re saying there’s also a risk to the environment. If there’s not enough waste to counteract basal heave, what might be the consequences?
If they don’t have enough waste to fill it, they’ll have to keep pumping. And if they don’t keep pumping, for example, if the company goes bust, then several years into the future when there’s no money left, if the pump turned off and there was basal heave, then you have a situation where the containment that’s designed into this system is completely compromised, and hazardous pollutants can leak into the Coal Measures aquifer.
What are the potential consequences of that?
There are some local groundwater abstractions in the coal measures. The linkage between the site and some water features to the south and a particular groundwater abstraction to the south has not been examined in enough detail, in my view. So I think there’s a plausible linkage between the site and where the coal measures subcrop to the south that’s not been recognized in the application.
So there’s a potential linkage between the site and water features – would this be sources of drinking water or a lake for wildlife – what would it be?
This is a private abstraction – a borehole. It’s not a public drinking water supply but there would be a household that might drink it, or it might be used to water animals. Plus surface water: small ponds, streams and ditches might be affected by pollution.
Might the River Tawd be affected?
It’s less likely that basal heave will cause pollution of the river. If there is an issue with not being able to control their discharge volumes and needing to pump a lot of water to somewhere, then potentially that might impact on the river.
What happens after the site closes, if it ever does close?
Most landfill companies have to put by a certain amount of money for post-closure, so that there is money available for ongoing operations to control environmental issues. The timescales tend to be 25-50 years.
This leads to another deficiency in the application documents. When you close the landfill, for a certain period after the landfill is complete, you allow groundwater levels to recover, but you keep pumping leachate