Monday, 3 July 2017

Seasonal working scheme for Straitgate can't work as AI describes

Aggregate Industries says that any "excavation [to the Maximum Winter Water Table at Straitgate Farm] will always have at least 1.0m of unsaturated gravels beneath".

For large areas of the site, however, this is impossible.

It throws yet another question mark over AI's plans for Straitgate. The company's Head of Geological Services will apparently refer back to water consultants Amec Foster Wheeler, after the matter was pointed out to him at a meeting last week.

Just to recap, AI’s seasonal working scheme, unlike other local quarry schemes that leave 1m unquarried above the MWWT where groundwater receptors are at risk, relies on groundwater levels falling by at least 1m during the summer months to allow extraction down to the MWWT; DCC wanted to know if such a scheme had even been tried before. AI's Supporting Statement explained:
The resource declared assumes a working base that coincides with, and never drops below the Maximum Winter Water Table (MWWT) modelled by hydrogeological specialists AMEC Foster Wheeler following extensive monitoring and analysis since January 2013. Moreover, the working method ensures that the floor of the excavation will always have at least 1.0m of unsaturated gravels beneath. 2.4.7 
But such a working method could ensure no such thing.

Firstly, the MWWT was derived from just a handful of 'maximum' water levels, which may or may not actually be maximums. As Dr Helen Rutter said in her report:
This surface is only a model of reality, and may not represent actual groundwater levels across the site...
The steep hydraulic gradient combined with limited monitoring, in my opinion, is likely to result in errors in the actual depth to maximum groundwater across the site.
The accuracy of the maximum winter water level grid may benefit from additional piezometers...
It will, however, be 12 months or more before any useful information can be drawn from these.

But, crucially, groundwater levels at three of the older piezometers do not fall by 1m over the year. In fact, the difference between the maximum and minimum values over the various years of monitoring is just 24cm for PZ01, 42cm for PZ05 and 40cm for PZ2016/001. It is therefore impossible for quarrying down to the MWWT in these areas to "always have at least 1.0m of unsaturated gravels beneath".

Again, this point was picked up by Dr Helen Rutter, who said:
Groundwater levels may fluctuate by less than a metre across the intended deepening area, raising questions as to the practicality of the proposal in terms of maintaining a one metre separation between excavation and the water table.
All of this obviously affects the amount of resource available. 

AI produced a document for last year's Minerals Plan Examination - XD28 Straitgate Farm Resource Statement May 2016 - to convince the Planning Inspector that the company really could lay its hand on 1.2 million tonnes. It again claimed:

The resource declared assumes a working base that coincides with, and never drops below the maximum recorded winter water table... these levels will only be progressed during summer months when the water table is at least 1m below the said modelled surface thus maintaining a minimum 1m buffer zone.
But the Inspector was misled. The document boasted that "Calculations have been undertaken by Chartered Geologists", and yet a 1m buffer zone could obviously not be maintained "below the said modelled surface".

AI would therefore be unable to extract 1.2 million tonnes, even by using this unorthodox scheme. PZ05 and PZ2016/001 are towards the middle of the site - shown on the map below. Losing say 60cm depth of resource across 25ha would equate to some 240,000 tonnes (0.6m x 25ha x 10,000m x 2/m3 less 20% waste) - reducing the size of any benefit that must be balanced against the proposal's considerable harm.

Of course, it may be that AI knew this this all along, and had no intention to "always have at least 1.0m of unsaturated gravels beneath" where it quarried. After all, who else would know?

Who else would know - until drinking water supplies became impaired or properties became flooded?