You may have noticed a pattern to the previous few posts:
They all relate to Aggregate Industries not knowing with any certainty where the maximum winter water table at Straitgate Farm lies, which is a problem, because it is this surface that AI proposes to dig down to, it is this surface that must be known if environmental impacts are to be understood, it is this surface that must be known before any quarry planning application can be determined.
We've pointed out some of the problems in the posts above, but it’s actually worse than that.
Look at this table of infiltration rates derived from tests undertaken at Straitgate Farm:
AI is having enough trouble determining the maximum winter water table beneath the existing geology, beneath poorly draining clayey material, material that according to the table above has a "negligible infiltration rate".
AI proposes to open up 22.6 hectares of this area, to expose the Budleigh Salterton Pebble Beds for sand and gravel extraction, and reckons the disturbed overburden material, will have a "much enhanced infiltration rate"; i.e. there will be less surface water and more groundwater. Amec confirms:
Not only that, but during the:
What effect would this "rapid infiltration" have on the MWWT? Well, it’s obviously going to increase groundwater levels, but by how much we have not been told.
The Environment Agency recognised the issue, writing earlier this year:
... Table 2.2, shows the disturbed overburden material to have a ‘much enhanced infiltration rate’. The applicant should clarify how the infiltration rate of the proposed backfill material compares to that of the in-situ material, and what effect the change between in-situ and disturbed overburden material might have on groundwater levels beneath the site.
How much might enhanced and rapid infiltration raise groundwater levels? Here's a clue from AI's Hydrogeological Assessment:
Taking the most intense daily rainfall event recorded over the monitoring period from 2013 of 59 mm (23/12/2013), ignoring evaporation and overland flow, and assuming a conservative porosity for the BSPB of say 10% and 15% for the overburden, this rainfall event would result in a rise in the saturated rock of 0.59 m for the BSPB or 0.40 m for the overburden 5.3.9
In other words, as little as 6cm of extra infiltration could lead to a 0.6m increase in groundwater levels; likewise 10cm could lead to 1m, 20cm to 2m.
This is a big deal – particularly as water levels in large areas of the site are already higher than expected.
