Aggregate Industries and consultants Wood, previously Amec Foster Wheeler, tell us that Straitgate:
...is underlain by Budleigh Salterton Pebble Beds (BSPB) deposits which form part of the Sherwood Sandstone Principal Aquifer.
On that we can all agree. It’s the rest that’s a problem. The above consultants have previously argued:
...recharge reaches the water table in the BSPB through unsaturated thicknesses of between approximately 3 and 10 m within between 1 and 3 days. This is consistent with the conceptual model for relatively rapid recharge occurring in the BSPB (i.e. days rather than weeks or months).
The problem? It's not days. It’s years. The British Geological Survey Permeability Index says:
Even in the Sherwood Sandstone... recharge can take several years to reach the water table
According to Wang et al (2012) the unsaturated zone velocity for the Sherwood Sandstone Group falls in the range 0.6 – 2.3 m/year with a mean of 1.06 m/year (these values were summarized by Chilton and Foster (1991)). The values for unsaturated zone flow rates are several orders of magnitude (three to five) lower than flow velocities in the saturated zone.
Scientific papers echo this – that for Sherwood Sandstone the flow rate of water percolating through the unsaturated layer is measured in metres per year (m/a), not metres per day:
Isn't this embarrassing? This is something hydrogeologists are supposed to learn about at university as undergraduates, that the water flow in the unsaturated layer above the water table can be many times slower than in the saturated layer below the water table, that percolating water moves more easily through the underlying strata when all of the pores are filled with water (saturated) than when some of the pores are filled with air (unsaturated). In other words, according to BGS again:
vertical flow under unsaturated conditions will be slower than under similar saturated conditions (often considerably so)
The ‘unsaturated hydraulic conductivity’ is not a constant but rather a function of the volumetric water content. Hence at low volumetric water contents (e.g. late summer and early autumn) the hydraulic characteristics and behaviour of contrasting lithologies in the unsaturated zone may be quite different to their behaviour in the saturated zone.
Click on the drawing below for a lesson in water flow in unsaturated soils from the University of California, Davis, or this course from a university in Syria.
What difference does all this make? It means that Aggregate Industries' conceptual model of how the groundwater behaves at Straitgate Farm is fundamentally flawed. Removing the unsaturated layer – which is what the company wants to do – would in time permanently change the chemistry of the groundwater, making it forever more acidic for those that have long been dependent on it – be they people, a Grade I listed house with mediaeval fishponds, livestock farms, or wetland habitats in ancient woodland. In relation to the Cadhay spring for example, Prof Brassington says:
The water takes almost 15 years to make this journey and if all but 1 m of the unsaturated zone is removed this reduces some 50% of the total travel time.
This reduced period will mean less time for rock/water interaction and will result in a less chemically mature groundwater that is more acidic.
The impact on the travel time also means that the deterioration of the groundwater chemistry will be a permanent change and it will not be possible to reverse it.
The concern for the 100 people or so who obtain their water supply from these springs is that the water make up will substantially change and a reduction in the pH will mean that the water is too acidic for drinking without treatment and it will dissolve metal pipework and storage tanks.
So, why have Aggregate Industries’ consultants got something so fundamental so very wrong? Did they make a simple blunder one day and found it difficult to backtrack and say sorry? Was it to stack the argument in favour of their paymaster? Or was it because they need to go back to the classroom?
