Wednesday 2 June 2021

Fracture flow? What fracture flow?


Straitgate Farm is underlain by the Budleigh Salterton Pebble Beds deposit, now called the Chester Formation, part of the Sherwood Sandstone aquifer. The deposit can be seen in the cliffs at Budleigh Salterton – as shown above, below and here.


Aggregate Industries and consultants Wood, previously Amec Foster Wheeler, maintain – as we have previously posted – that at Straitgate:  
...recharge reaches the water table in the BSPB through unsaturated thicknesses of between approximately 3 and 10 m within between 1 and 3 days. 
However, there was a time when Aggregate Industries was arguing the reverse – when the debate was about the risk of importing nutrient-laden soils to Woodbury Common. The company was claiming the soils at Straitgate Farm were "slowly permeable and at times poorly drained": 
This would strongly inhibit the movement of any nutrients added to the surface, either as artificial fertilisers or manure, through the soil profile and into the deeper overburden and mineral resource that would be exported off site for processing. 
This is still in fact echoed in the soils report which also talks about "slowly permeable subsoils". The infiltration tests also found that the undisturbed material "did not show any measurable ability to soakaway". At that time, it wasn’t the answer Aggregate Industries was looking for, and so the company concluded – surprise, surprise – that "these results cannot be considered to be representative of the site as a whole." 

Water speeding through the unsaturated zone "within between 1 and 3 days" is in contradiction with what the textbooks say. Professor Brassington explains how movement of water through the unsaturated zone at Straitgate takes years, and that the loss of this unsaturated zone would make water permanently more acidic for all those who depend upon it. 


How do Aggregate Industries’ consultants explain this rapid movement through the unsaturated zone, and therefore the apparent benign effect on groundwater chemistry from its removal? In part because of fractures. In three different documents it is written:
2.7.2 ...recharge to the BSPB was best represented as a single quick release number, independent of unsaturated depth because of its more fractured nature etc 
2.5 ...transit of water through the unsaturated zone is considered to be fairly rapid due to the intergrannular and fractured nature of the BSPB and therefore the thickness of the unsaturated zone may not be as important. 
3.7.2 The proposed quarry at Straitgate Farm is located on the outcrop of the BSPB in which intergranular flow predominates but an element of fracture flow may also be present as evidenced by the quick response, and subsequent decline, of groundwater levels in PZ02 and PZ06 following heavy rainfall events. 
The problem with fracture flow? Prof Brassington says there are no fractures at Straitgate: 
There is no evidence that the unsaturated zone (nor any other part of this aquifer) is fractured. The lack of cement means that the rock cannot be fractured as the adjacent uncemented particles would fall into any fractures present. The measurements carried out that led to this conclusion are not based on any published methodology and I believe them to be misleading.  
 

How can we be sure the deposit is not cemented? Easy. It’s why Aggregate Industries is drawn to the deposit in the first place. As Prof Brassington explains: 
The Chester Formation at Straitgate is not cemented which is why it is attractive to a quarrying company as it can be dug with excavators and does not require any explosives. Because of the lack of cement the presence of fractures or fissures are unlikely in the extreme.
This BGS document explains further: 
3.1.3 In the south of the study area, the Budleigh Salterton Pebble Beds Formation forms the top of a 160 m high ridge of heath land, distinct from the Otter Sandstone Formation by the elevation and vegetation type (see Figure 2.2) (Ussher et al., 1913; Walton, 1982). South of Uffculme the nature of the formation changes considerably. Locally derived limestone clasts are replaced by quartzite, and the formation becomes unconsolidated and uncemented (Allen et al., 1997; Walton, 1982). In the south the formation consists of brown gravel with subordinate beds of sand. The gravel comprises well-rounded pebbles, cobbles and boulders contained within a coarse to fine gravel and a silty sand matrix. Up to 90% of the clasts are metaquartzite with some composed of porphyry, vein quartz, tourmaline and feldspathic conglomerate (Edwards, 1997). There is a lack of carbonate cement, and only localised patches of iron minerals are found. This southern portion of the Budleigh Salterton Pebble Beds Formation has therefore been worked for aggregates over the years. 
And the strange thing? In their response to Prof Brassington in September 2019, Aggregate Industries' consultants don’t dispute that the deposit is uncemented: 
The BSPB comprise largely unconsolidated uncemented gravel and sand. 
Nevertheless, Wood still continue to argue for the presence of fractures/fissures in an attempt to explain why the speed of water percolating through the unsaturated zone is not in the range of 0.6 – 2.3 metres per year put forward by Prof Brassington: 
The range of 1 to 2 metres per year has been given as an ‘average’ and, hence, could be considerably faster at Straitgate. The poorly cemented nature of the BSPB at Straitgate, along with its high content of relatively coarse material and evidence of recharge via fractures/fissures seen in some piezometers on site suggest that the rate of percolation will be higher than this average range. 
No robust evidence has ever been provided to support Wood's claims that it "could be considerably faster." Tests have been done, here and here. None have been conclusive. Here was one attempt: 
2.7.5 As discussed with the EA on 24 May 2017 the opportunity was taken during the drilling of the replacement and new piezometers to undertake some semi-qualitative testing of the unsaturated zone. A version of a falling head test (slug test) was undertaken. The details and results are included in Appendix E. The conclusions are: Seven infiltration tests were carried out in five test boreholes at Straitgate Farm targeting the unsaturated zone of the BSPB formation, below the cryoturbated overburden. Water was introduced to the boreholes via a pipe to the base of the hole and the time it took the water to infiltrate into the BSPB was recorded. The majority of boreholes were affected by collapse to varying degrees throughout the tests, indicating that the BSPB is unconsolidated and loosely cemented which often suggests a high degree of permeability… Although these results can only be considered as semi-quantitative the results are consistent with the conceptual model for relatively rapid recharge occurring in the BSPB (i.e. days rather than weeks or months). 
But whether the tests were semi-qualitative or semi-quantitative, whether the hosepipe was green or orange, Prof Brassington dismisses them: 
They also used boreholes to make tests that were unique, and although I have searched, I cannot find any reference to this method in the literature. In my view, these tests would not provide any quantified description of the unsaturated zone flow regime.
Indeed, the fact that the "majority of boreholes were affected by collapse" would seem to back up Prof Brassington’s assertion that there would be no fracture flow because "uncemented particles would fall into any fractures present." 


Fortunately we know what the resource at Straitgate Farm looks like. Trial pits some 6-7m deep were dug in 2012 to analyse the resource. We were there. Both we and Aggregate Industries took photos.
In Amec Foster Wheeler (2017b) a series of photographs are published of the trial pits that were excavated to examine the presence of a clay layer across the site. These pits are to 6-7 m depth and the photographs appear to be good quality. I have examined them most carefully and find no evidence for any fractures or fissures. I conclude that there is no direct physical evidence for the existence of a fracture network at Straitgate Farm; indeed the uncemented strata provides some geological evidence that no such fractures exist, as fractures would depend on the rock through which they penetrate not to collapse so that they remain open. 
And who can disagree with that? Perhaps Aggregate Industries’ consultants Wood haven’t quite made up their minds yet. Having told the Environment Agency and the rest of us about the "fractured nature of the BSPB", Wood has now conceded – at the recent Hydrogeology Meeting – that fractures "can’t be totally discounted". 

So, who to believe? Consultants, bankrolled by a company determined to dig up the aquifer so important for people’s drinking water supplies, who perhaps haven't quite made up their minds yet? Or the Professor who writes textbooks on the subject? The answer should be very easy.
 
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