Tuesday, 8 January 2013

DCC is overstating the 'recoverable' resource at Straitgate by more than 50%

We have long argued that the resource figure for Straitgate with which DCC went out to Public Consultation, up to 3.6 million tonnes (Mt) giving an expected lifetime for the site of about 10 years, was inaccurate and misleading. English China Clays' "Report on the reserves of Pebble Beds at Straitgate Farm", December 1990, and appendix, substantiates this claim.

DCC would have long had a copy of this report. We however have only just seen it, kindly supplied by Aggregate Industries (see note below). The report analyses results from 24 boreholes drilled across the site in 1990. Only limited geological information existed beforehand, and subsequent resource calculations are based upon this data. 

From the report it is clear that 2.311Mt is the maximum resource figure at Straitgate that DCC should have consulted upon, as this is the net tonnage recoverable above the water table from "sub area A" (section 5.3). There was no reason for DCC to ever think Straitgate could be quarried below the water table, particularly considering airport safeguarding, and in fact at Blackhill "to protect the hydrology of the area it [was] proposed to cease excavation at 1 metre above the maximum level of the water table". Would the hydrology and the receptors around Straitgate Farm deserve any less protection?

And even 2.3Mt is based on a number of assumptions, some of which are now unrealistic:

A "working margin of 5 metres behind the tree screen and 15 metres around Straitgate Farmhouse and access" [BGS recommends 250m stand-offs]; A "working margin of 15 metres" from the A30 [unrealistic]; "Water levels are based on July readings" [the water table is up to 1.4m higher in winter]; "No allowances have been made for face angles or haul roads".

Reducing the depth of deposit by say 1m for the seasonal movement in the water table could cut the resource by c.0.4Mt. Restricting excavation to "1 metre above the maximum level of the water table" as at Blackhill loses another c.0.4Mt. Sensible stand-offs from properties, another c.0.2Mt. So even 2.3Mt can quickly become c.1.3Mt. Of that, gravel (the component of value) makes up about 70%.

And remember, DCC was originally talking about 8Mt at Straitgate, even though it had been known for many years that the eastern half of the site was "geologically more complex". Here, the Pebble Beds are not only covered with a deep outcrop of sandstone, but a large portion of them is also underwater.

So if we are going to talk about destroying a farm and violating the East Devon landscape, creating something like the photograph above, let's consult on the correct figures so that local people know exactly what's involved and are able to make an informed response. We should then consider how little economically recoverable sand and gravel there actually is at Straitgate Farm, and debate whether that's a price worth paying for the loss of so much.

Note: In relation to ECC's report, AI wishes to point out that "it covers the first detailed borehole exploration and resource analysis in 1990 and that the estimates quoted therein do not reflect modern planning conditions or economic expectations - a finite, modern resource calculation will be undertaken following receipt of all expert opinions into hydro-geology, landscape, noise, ecology etc etc probably this time next year (ie as most studies need a full set of seasons to be valid)."