Our community is debating a proposal to quarry sand and gravel from a field in Bengeo that contains boreholes supplying six million litres of water to Hertford each day. Here Dr Bryan Lovell considers three main questions:
- How might quarrying affect the quality of water in the boreholes?
- Does the project make sense commercially, when the cost of adequate protection of the water supply is taken into account?
- Is it really necessary to extract this particular sand and gravel, so close to those boreholes?
You can’t argue with a rock
It is useful to start with the geology when tackling the three main questions. The rocks are what they are: you can’t argue with a rock. We should aim to understand the geology of that field before we move on to matters over which we can exercise some control, such as economics, engineering and environmental protection.
The field itself lies on the western flank of the valley of the River Rib, and slopes down towards the east. The highest point, at the Sacombe Road, is 72 metres above sea level, 25 metres above the Wadesmill Road to the east. The sand and gravel it is proposed to extract are at or near the surface of the field, handy for quarrying. They are underlain at a shallow depth by Cretaceous chalk, which is a regional aquifer.
The Wadesmill Road water-supply pumping station lies on the eastern edge of the field. Borehole Hertford number 1 at the pumping station is 137 metres deep. The supply of water comes from the chalk aquifer penetrated by the borehole: the top of the chalk is 10 metres below the top of the borehole. Obviously enough, as water is pumped out of a borehole it is replaced by inflow from the surrounding rocks. It is clearly crucial to avoid pollution of the water flowing into the Wadesmill Road boreholes from those surrounding rocks.
What is the risk of pollution of water supply?
For analysis of the risk to the water supply from the Wadesmill Road boreholes, we can turn to a July 2014 report commissioned by the applicants themselves. The consultant hydrogeologists at Hafren Water state in that report:
“Groundwater movement in the Chalk is predominantly by fracture flow. Therefore, any polluted material that reaches the chalk has the potential to be rapidly transported in the direction of flow. In the case of Ware Park, this could mean contamination reaching the Wadesmill Road PWS in a very short period.”
Contamination would migrate rapidly through fractures in the chalk, but pollution would also seep into less permeable parts of the aquifer. Removing pollution from the chalk aquifer as a whole would be a lengthy process. Figures quoted in the Hafren Water report indicate just how disruptive damage to the Wadesmill Road facility could be to local supply. Daily flow from Wadesmill Road pumping station is 5.9 million litres a day, compared with 2.5 million and 1.8 million a day from the two nearby Hertford pumping stations at Port Hill and Molewood.
What form might contamination take? Possible pollution by both mud and hydrocarbons is identified by Hafren Water. I quote the report again at length here, because it describes so clearly the risks involved:
“Groundwater quality
(a) Turbidity
There is a risk to groundwater quality from increased turbidity if fine materials are mobilised in the workings and transported into the Chalk aquifer. This could then be rapidly transported, via fracture flow, to the nearby public water supply abstraction. The risk of this occurring is reduced by the fact that the sand and gravel lies above the water table and there is a 10 m zone of unsaturated Chalk beneath the gravel. Without any mitigation the impact of this is considered to be ‘medium’ with a significance of impact of ‘major’.
(b) Hydrocarbons
There is a risk of contamination of the sand and gravel, and therefore the Chalk aquifer, as a result of accidental spillage of oil and fuel from mobile plant operating in the quarry. This may result in contamination of groundwater in the Chalk aquifer and the nearby public water supply abstraction. As the site is dry, mobile plant will be crossing the base of the workings where the thickness of sand and gravel will be reduced, removing part of any potential attenuation layer. The use of a mobile dry screening plant is also proposed, which will be located in the working area. This equipment will be diesel powered and will require refuelling to be undertaken within the quarry. Without mitigation measures, the potential for impact on groundwater from hydrocarbon spills is considered to be ‘high’ with a significance of impact of ‘major’.”
As stated by Hafren Water, any pollution by mud or hydrocarbons could reach the chalk aquifer rapidly. This is a setting in which it is essential to prevent pollution from occurring at all, rather than concentrating on monitoring to see if pollution is taking place. By the time monitoring shows a potential problem, lasting damage may well have been done. Pollution is prevented by good engineering and by good staff: there needs to be constant vigilance on site by the operator.
Submissions by Affinity Water, the operator of the water-supply pumping station on Wadesmill Road, and by the Environment Agency, describe what would have to be done to reduce the risk of pollution to an acceptable level. For example, the observation borehole 1A at the Wadesmill Road water-pumping station is in disrepair and is a potential pathway for pollution. Obviously repair and maintenance of boreholes would be a priority. Another example is the need to ensure that any spillages in the area to be used for refuelling equipment on site are entirely contained within that refuelling area. Such precautions must be taken, and require significant investment of capital. There is a further requirement for investment, to recruit and train staff to operate safely on the site.
Is the project commercial in itself?
Geology and engineering have to be set in a commercial context before the worth of a project to extract natural resources can be assessed. A geologist working in one of the resource industries would be closely involved in that assessment, commonly in the role of exploration manager.
Major factors involved in the assessment of commerciality are the size of the reserve and forecast price of the product during the life of the project, set against capital and operating costs. In the particular case of the Bengeo quarry, both capital and operating costs would be significantly increased by the essential requirement to prevent pollution of the chalk aquifer. These costs would include decommissioning of facilities and clearance from the site following completion of operations. In such circumstances, the authorities granting permission for the operation need to be satisfied that the operator has the financial resources to comply with the stringent environmental requirements, and that the project still makes commercial sense in the light of those requirements.
In projects such as the proposed Bengeo quarry, where environmental protection is so important, initial capital expenditure adds significantly to costs that can only be recovered once production begins. For example, establishing refuelling facilities in the lower part of the quarry that meet exacting environmental standards would not come cheap, nor would training of staff to operate those facilities properly.
In some extractive projects, there is an additional commercial benefit beyond the value of the product. For example, sand and gravel might be produced and sold during an operation to create a reservoir for agricultural use. More lucratively, the location of the extraction itself, or nearby areas, might become available for housing development once operations cease.
The present application for quarrying describes restoration of the quarry itself to farmland at a lower level, so expenditure but no gain there. There is also mention of prospective development of housing at the southern edge of the proposed area of extraction. The Ware Park Trust has from an early stage been quite open on the issue of “minerals sterilisation” of housing development in this area. A submission to Herts District Council on 19 May 2014 on behalf of the Trust says:
“This confirms that a planning application for the extraction of the mineral resource is being prepared…This will ensure that the minerals sterilisation issue is dealt with.”
From a geological perspective, if no other, we can feel uneasy about quarrying, in an environmentally sensitive site, that is to any significant degree linked commercially to removal of a planning restriction on development of housing. In a competition for use of both the surface and the subsurface of the land, the crucial resource has to be fully protected. In the case of this field in Bengeo, the most valuable resource to the community is supply of water from the chalk aquifer.
Why quarry this sand and this gravel?
There is one thing that would clearly justify quarrying at Bengeo, in the face of the costs and risks involved. If that sand and gravel had to be recovered to meet an essential local or national need, then costly precautions against pollution would just have to be taken and quarrying would need to take place.
But there is no local or national imperative for supply of sand and gravel from Bengeo. The local need can be met from elsewhere, if necessary from within the county itself. Nationally, the British Geological Survey reports that 176 million tonnes of construction minerals were extracted from the UK landmass in 2014. Forecast annual production from the proposed Bengeo quarry is 0.2-0.25 million tonnes, giving up to 2.6 million tonnes in total by the time operations cease. Bengeo quarry would therefore contribute less than 0.2% of the national total during the life of the quarry.
Is Bengeo’s backyard a special case?
Conventionally, issues such as the proposed Bengeo quarry involve an applicant whose main motivation is commercial, opposed by those who live nearby who wish to preserve the amenities of the landscape as it is at present. The Bengeo case is rather different, in that it also involves a judgement about potential jeopardy to water-supply, the security of which is clearly a major priority for all citizens.
By means of uncommonly large capital expenditure, and through exceptional recruitment and training of vigilant operating staff, the risks to water supply identified by the applicant’s own consultants can in principle be reduced to quite low levels. But there are alternative sources of supply of the small quantities of sand and gravel it is planned to recover from Bengeo.
The balance of the geological argument has to be in favour of leaving that commonplace sand and gravel where it is. Let’s just keep pumping that precious water safely from the chalk and leave that field to the skylarks.
Bryan Lovell
1 August 2016
For the last 20 years Dr Bryan Lovell, OBE, CGeol, has been Senior Research Fellow in Earth Sciences at the University of Cambridge, and is a geological consultant. In the sixties he studied geology at Oxford and Harvard, in the seventies he was Lecturer in Geology at the University of Edinburgh, and in the eighties and nineties he was with BP Exploration. From 2010 to 2012 he was President of the Geological Society of London.