Aggregate Industries' application for Straitgate talks about the extraction area containing "in the order of 1.2Mt of saleable sand and gravel" with further saleable mineral "anticipated" in the overburden 3.8; "the proposed ‘dry working option’ would avoid the need to excavate below the groundwater table" 3.37.
After the Environment Agency confirmed that "Aggregate Industries have proposed to stop quarrying a metre above the water-table", a revised resource figure would have been expected, but a company spokesperson has claimed that this metre had already been factored in to its calculations and therefore it still has 1.2 million tonnes.
After the Environment Agency confirmed that "Aggregate Industries have proposed to stop quarrying a metre above the water-table", a revised resource figure would have been expected, but a company spokesperson has claimed that this metre had already been factored in to its calculations and therefore it still has 1.2 million tonnes.
Whatever you believe, nowhere in AI’s application did it talk about leaving 1m of sand and gravel to safeguard groundwater. And AI would surely have broadcast the matter of charitably forsaking 500k tonnes gross, 400k saleable, to protect water supplies for people and ancient woodland; after all, it found space to talk about cutting back its own trees to mitigate 1.2 million HGV miles! Besides, if AI had factored in leaving 1m above the water table, it’s a mystery why statements like this were made:
Mineral extraction to the high groundwater level surface contoured from maximum groundwater levels from all data would be dry for the vast majority of time… 4.2
Fortunately, because we have the 1990 borehole data on which AI’s resource calculations are based, and because "an area of uniform shallow simple geology [covers] the western part of the site" 4.1, it’s relatively easy to estimate the available resource.
By combining AI’s geological map and AMEC’s groundwater map, with surface elevations (brown contours) overlaying maximum groundwater levels (blue contours), and taking a series of differences at 55 equally-spaced points across the site, the average depth over maximum groundwater was calculated to be 5.53m. Subtract from this, 2.28m (the average depth of topsoil/overburden - AI's figure) and 1m (the depth above the water table to be maintained), multiply the result across the 25.6ha (256,000m2 - AI's figure), and, assuming a density of 1.9 tonnes/m3 and 20% waste (AI's figures), we are left with about 875,000 tonnes, before any of the unproven "workable overburden".
The map of the combined contours is below; here’s the calculation:
(5.53-2.28-1.0)m x 256,000m2 x 1.9tonnes/m3 x 80% = 875,000 tonnes (saleable)
Even assuming a margin of error of +/- 0.5m on each of the 55 differences, the resource figure falls in the range of +/-195k tonnes, or 680-1070k; i.e. still short of AI's '1.2 million'.
And of course, there won’t be 25.6ha by the time face angles, proper standoffs from the A30 - see Highways England’s response - and listed buildings are factored in; an area of 23ha, for example, could yield less than 800k tonnes. Bear in mind too, this submission in 2012 to the Competition Commission by Wardell Armstrong, ‘one of the largest teams of Chartered Minerals Surveyors in the UK’:
... no aggregates operator would consider (for example) trying to develop a sand and gravel deposit of less than one million tonnes. We have clients who have sites which have been turned down on this basis.
In 2012, DCC went to public consultation with an incorrect resource figure of 3.6 million tonnes for Straitgate Farm. This week, DCC's DMC committee will be asked to endorse the Pre-submission Draft of the new Devon Minerals Plan, before the start of a 12 week public consultation. This plan seeks to identify Straitgate Farm as a "specific site" for the future supply of sand and gravel, because there is "1.2 million tonnes of proven resource"; yet again, that so-called 'proven' figure looks incorrect before consultation even starts.
Given the ever declining and consistently overestimated resource, it’s time for AI to show exactly how it has calculated the recoverable resource at Straitgate, and exactly what assumptions have been made, so that we all know exactly what the damage to an East Devon farm and a site of European importance to nature is all for.
