The impact on surrounding private drinking water supplies – be they wells, springs or boreholes – is one of the most contentious aspects of the proposal by Aggregate Industries to quarry Straitgate Farm.
Over the years, we have posted about how the maximum winter water table – the MWWT, the base of any quarry at Straitgate – can not be predicted accurately. The elevation of the MWWT put forward by Aggregate Industries’ consultants Amec (now Wood) is just a model, a prediction, a guesstimate of what might be happening across some 55 acres, based on water levels recorded over a limited time from just 6 piezometers across that area. The accuracy of this model would matter less if Aggregate Industries were planning to leave a safety margin, an unquarried buffer, above this surface to protect surrounding drinking water supplies – but it is not. After all this time, AI still won’t come clean on the accuracy of the MWWT – which for most people would be warning enough.
Last May, Professor Rick Brassington Eur Geol Eur Ing BSc MSc CGeol FGS CEng MICE FCIWEM, recent winner of the Whitaker Medal "in recognition of an outstanding contribution to hydrogeology", produced an extensive and damning report which warned that "any proposals to quarry at Straitgate Farm will impact on the fragile groundwater system and cause the flows of springs to decrease and the quality of the water also to deteriorate".
As readers may remember, this report was largely ignored by the Environment Agency, which in its wisdom decided early this year that it "will not be providing further responses" in relation to Aggregate Industries’ application – before then going on to do just that, as we posted in Alternative water supplies: EA’s remit is 'objectives and outcomes, not solutions', EA "consolidates and revises" conditions – but still fails to justify position, and Another response from the EA – and a swipe at SAG.
In his report, Prof Brassington recommended that "an unquarried buffer of at least 3 m is left above the maximum water table to minimise the negative impacts", in part because "the MWWT surface cannot be modelled accurately".
Why so? There are many reasons – as we have posted about in the past – but there’s another reason too, which we have not previously highlighted, and which has obviously not been picked up by the EA either. It’s based on work by this renowned scientist almost 80 years ago:
Prof Brassington outlined the relevance of M.K.Hubbert’s work with regards to Straitgate in his report:
3.16 Hubbert (1940) produced a ground-breaking demonstration of how groundwater flows through aquifer systems based on Darcy’s law. In essence, he showed that groundwater flow is three-dimensional with a vertical component of flow. Both the depth of a borehole and its location within an aquifer have a significant influence on the water level within it. Figure 3.5 below shows how these factors impact on groundwater levels.
3.17 The figure shows groundwater flowing through an idealised aquifer from recharge areas to discharge areas in valley bottoms. Flow lines are shown as solid lines, while equipotential lines (contours of equal pressure) are hatched. The effect of this flow system on the water levels in wells gives decreasing elevations with depth in recharge areas and increasing elevations with depth in discharge areas. As a consequence, the water level in adjacent boreholes will not be the same if the depths are different.
3.18 As a result of the above, it is important to take account of the depth of the monitoring borehole or piezometer and its location in respect of the groundwater flow pattern when interpreting the elevation of a water table. This will not always be significant when looking at regional flow over an aquifer. However, in the small area of an aquifer such as being considered in this report these differences are likely to be significant. For example, fully penetrating boreholes in recharge areas will have a lower groundwater level than the local water table.
Our emphasis. Wikipedia confirms:
In an unconfined aquifer, the water level in the piezometer would not be exactly coincident with the water table, especially when the vertical component of flow velocity is significant.
How much is not exactly? According to this paper:
In the case where groundwater shows a significant vertical flow component (Fig. 4), the water level measured in a well can vary considerably (from a few centimetres to decimetres, and locally to a few metres) from the depth to the top of the saturated zone of the aquifer. This difference depends on the vertical hydraulic gradient, thus on the location of the observation well along a flow line, but also on the depth and length of the piezometer screen.
A number of boreholes have been drilled across the Straitgate site over the years. Eighteen have now been installed with piezometers. As Wikipedia also confirms, a piezometer – like the ones at Straitgate – will typically have:
a solid casing down to the depth of interest, and a slotted or screened casing within the zone where water pressure is being measured
Back in April 2015, Aggregate Industries' Hydrogeological Assessment was telling us:
To date, 11 piezometers have been installed by AI to monitor groundwater levels in the BSPB across the Straitgate site... All piezometers have been drilled into the underlying Aylesbeare Mudstone but are installed with a 3 m screened section in the BSPB... In some cases the screened section is now partially filled with sediment and it is unknown what effect this may have.
In other words, all these boreholes fully penetrate the aquifer.
That was 5 years ago. The latest HA has, for whatever reason, removed the above information.
But to confirm: Straitgate Farm is in a recharge area: rain falling on the ground surface slowly percolates through the sand and gravel to the unconfined aquifer below, to emerge many years later in springs and wells. The land is sloping, as is the water table. Dr Helen Rutter warned:
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.
Here’s Amec’s conceptual cross section of the site:
And here's another representation of Hubbert’s work. It shows that water will flow from regions of higher pressure to regions of lower pressure. In areas of recharge – like at Straitgate – where there is a vertical and horizontal component to the groundwater flow, the lower pressure at the open or screened section of deeper piezometers will only be able to support a column of water (the black columns) in the borehole to an elevation lower than the immediate water table. In the example below, boreholes D and E have been drilled in the same location to different depths. The lower pressure at the bottom of piezometer E supports a column of water to a lower elevation than D. Boreholes C,E and F have been drilled to depths of equal pressure and thereby support columns of water to the same elevation.
It can therefore be seen that using water levels recorded in piezometers in recharge areas would produce a model of the water table at a lower elevation than in reality.
Aggregate Industries’ proposal to quarry down to the MWWT – modelled from groundwater levels in an area of recharge – would therefore in reality be a proposal to quarry below the level of the maximum water table. This would be unlike other operators that maintain a freeboard above the water table, where water sources are at risk.
Not only would quarrying below the water table at Straitgate risk derogating drinking water supplies for more than 100 people, including a Grade I listed manor house, and three livestock farms, it would also be contrary to the Devon Minerals Plan.
Of course, over all these many years, Aggregate Industries’ consultants have not mentioned Hubbert. They have not mentioned that the depth of a borehole and its location in respect of the groundwater flow pattern is important. They have not mentioned it, even though any hydrogeologist worth their pay grade would know about it. These consultants have not mentioned it, because it’s not in their interest, or that of their paymasters.
But why hasn't the EA – given that groundwater is one of our most precious raw materials?
💧Groundwater plays a major role in the water cycle supporting baseflows to #rivers, #wetlands & its also one of our most precious of earth's raw materials used in #drinkingwater supply, #industry & #agriculture in #food production— Geoscience EnvAgency (@GeoscienceEA) March 20, 2020
💧#Groundwater 'It truly is our hidden asset' pic.twitter.com/FFcvhD5bKK
It turns out that the EA – or at least, the one person overseeing this project – would rather side with consultants representing an operator hell-bent on extracting as much material as possible, over the warnings from an independent Professor responsible for writing textbooks on the subject.
