Monday, 1 August 2022

It’s all about the unsaturated zone – the independent expert hydrogeologists will say

What happens to rain when it falls on the ground? The simple answer is that it either evaporates back into the atmosphere, flows to surface water bodies such as streams, or infiltrates into the soil.


Some of the water infiltrating into the soil will be taken up by vegetation. The remaining water will percolate down through the unsaturated or vadose zone to the water table and the saturated zone, where it becomes recharge to the groundwater system. In the Straitgate area, less than half of the rain falling onto the ground will add to the groundwater system as hydrologically effective rainfall.


The rain falling onto the ground is slightly acidic. After evapotranspiration – the loss of water from the soil both by evaporation from the soil surface and by transpiration from the leaves of the plants growing on it – the substances dissolved in that water remain behind, and the water in the soil becomes more acidic. Carbon dioxide in the soil – from microbial decay of organic matter and respiration of plant roots – will dissolve into the water making it even more acidic. 

The table below – from Baseline groundwater chemistry: the Sherwood Sandstone of Devon and Somerset – shows the chemical composition of rainfall at North Wyke, 45 km west of Ottery St Mary.

The rain falling on Straitgate Farm will have much the same composition, which means that the groundwater – so important for many people's drinking water supplies – starts life with a pH of 4, or less.

Water with a pH of 4 is ten times more acidic than water with a pH of 5 and 100 times more acidic than a pH of 6. The Water Supply (Water Quality) Regulations 2016 state that tap water should have a pH between 6.5 and 9.5. A pH of 7 is neutral. 

At Straitgate, the weakly acidic solution travels slowly through the unsaturated zone over a period of years and, after reacting with carbonates in the rocks, loses most of the acidity and becomes drinkable.

Straitgate has no doubt provided a valuable source of drinking water for local inhabitants for hundreds, if not thousands of years.

Why is the unsaturated zone important? The rock/water interactions to control acidity are dependant on the time the water spends in contact with the underlying geology. The flow rate in the unsaturated zone – where the soil pores are drier and their ability to conduct water drastically decreases – can be many times slower than in the saturated zone below the water table, as this video explains. 


It is the unsaturated zone at Straitgate that Aggregate Industries wants to remove.

As you might expect, the company claims this would make no difference. Its Statement of Case says:
6.25 The appellant has considered each private water supply and it has been concluded that there will be no significant impact on these current private water supplies 
6.16 The hydrological impact assessment has resulted in the findings that the dry working option will not affect the groundwater and surface water quantities feeding into the four streams flowing off the site, including the two flowing into Cadhay Wood and Cadhay Bog. The fish ponds at Cadhay House will similarly not be affected.
However, Aggregate Industries' conceptual groundwater model is based on a BIG mistake. The company's consultants have argued that groundwater moves through the unsaturated zone in a matter of days. The science – here and here for example – says it takes years, that for Sherwood Sandstone – the geology at Straitgate Farm – the flow rate of water percolating through the unsaturated layer is measured in metres per year (m/a), not metres per day.


The difference between the current situation and the one that will pertain once 5 m of the unsaturated zone has been removed will be a loss of some 32% of the flow time taken for water to reach the Cadhay Spring from the ground surface at Straitgate Farm. This is a very large loss and will result in a significant reduction in the pH of the local spring waters. In my opinion, the acidity level in the spring waters in the area could fall to pH 5 or even less, a value that is very much lower than that today and, contrary to the EA’s understanding is well below the lowest level set in the Drinking Water Standards.
The Public Inquiry in October will undoubtedly tackle the effect on drinking water supplies of taking away virtually all of the unsaturated zone at Straitgate. To this end, Devon County Council and Straitgate Action Group have instructed independent expert hydrogeologists to act as witnesses. The Statement of Case from Devon County Council – the MPA – says:
6.12 ... the MPA has instructed an independent expert hydrogeologist to consider the evidence and materials produced on all sides to date, by Wood for the Appellant, by Professor Brassington for SAG, and by the EA. Having done so, their clear view is that Professor Brassington’s position that the Straitgate Proposals will result in a lowering of the pH of water reaching Cadhay is to be preferred (and has not been countered). The best available information is Professor Brassington’s analytical model. This predicts a halving of the average travel time of the water after it lands as rainwater. The water starts, due to evaporation and plant take up, as acidic. At present, its journey through the unsaturated zone sees it mineralise and the acidity “buffered” (its pH increases). Even then, at the moment (in baseline conditions), when water reaches Cadhay it is mildly acidic. The removal of the unsaturated zone by reason of the Straitgate Proposals would reduce the opportunity for the water to mineralise and so its pH would not be raised even to the extent it is presently.  
6.13 The result would be more acidic water at the fishponds than at present, and the likelihood is therefore that the ponds would be harmed by reason of a change in their ecology. The ponds themselves are part of a designated heritage asset (Grade II), as well as integral to the setting of Cadhay itself (Grade I). There may even be impacts to ecology outside the ponds themselves
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