Wednesday, 25 November 2015

The importance of leaving 1 metre...

It is extraordinary that the issue of leaving 1 metre of unquarried material above the maximum water table to protect drinking water supplies and prevent flooding is still a matter of discussion between Aggregate Industries, the Environment Agency and Devon County Council. This matter should be non-negotiable.

Perhaps someone might explain to those around Straitgate currently reliant on springs and wells for their drinking water supplies with no mains replacement, why the matter appeared non-negotiable at Hanson's Town Farm Quarry at Burlescombe: same geology, same planned restoration to farmland, but far fewer people reliant on private water. Unlike AI, Hanson was straightforward and upfront:
After all, a 'freeboard' of at least 1m is typical where drinking water supplies are at risk. Is it to be that in Devon we have one rule for one operator and one rule for another? 

In contrast to the SPZ running across Straitgate, the salient points for Town Farm were:
There are no groundwater protection zones shown on the Environment Agency web site in the vicinity of the site… The nearest water supply is [50m] to the west of the proposed quarry extension. It is understood this is a potable supply is supplemented by mains water should it be required. The elevation of the water level and the geology indicate that the well is fed by the Lower Marls and not directly from the Pebble Beds. C2.6
Why was it necessary to leave 1m above the water table unquarried? Hanson recognised that:
The unsaturated zone above the water table affords protection of the aquifer from surface pollution, allowing adsorption, attenuation and degradation of contaminants prior to reaching the water table. Removal of lower permeability clay layers from within the Pebble Beds could also remove some protection from the groundwater. During the operation of the site pollution may arise from the extraction and restoration activities. The pollution may be in the form of fuel, lubricants and other fluids associated with the operator’s machinery. C3.1
You would think that all this was pretty standard. Elsewhere, a hydrogeological expert advised:
Maintaining a 1 metre separation between the base of the excavation and groundwater provides additional time to clean up any spills, provides additional capacity of the soils and unsaturated zone to attenuate some contaminants, and reduces the possibility that any fill used post-quarrying may be inundated, with the increased risk of leaching on contaminants that this would entail. 3.4
Future land uses, after site closure, will be limited by the available unsaturated zone thickness. 3.8
Post-restoration, due to the decreased depth to groundwater and more limited ability of the ground to attenuate contaminants, it is appropriate to impose restrictions on land use. Although nitrate and bacteria, as might result from intensive agriculture, are not contaminants of concern for the quarry and cleanfill operation, these may pose a risk to groundwater quality for downgradient users if the post-rehabilitation land use causes discharges of these contaminants (including diffuse discharges). 6.1(g) 
Through quarrying the exposure pathway for any contaminants has been modified. This means that there may be rapid access to the groundwater system for any contaminants, including pathogens. 6.2
When soil is first reinstated, its ability to attenuate contaminants will be lower than for a well-established soil. In a well-developed soil, the top soil grades into the underlying gravels, allowing for further attenuation of nutrients and pathogens. The soil condition would improve with time, but depends on soil management practices. 6.5
So, it’s not just contamination from quarrying that we have to worry about, it’s contamination from future land use, farming or otherwise - be they nitrates, chemicals or worse. Amec's Hydrogeological Assessment is silent on this matter. Again, you would think it's all common sense; the more sand and gravel left, the more subsequent protection against pollution in drinking water sources. 

The same expert also stated that:
Determining the highest groundwater level with confidence at a site is difficult, and the interpreted highest groundwater may not be accurate to within several metres, because almost invariably, there is not a shallow well with a long term record of water levels at the site. Because of this, a degree of conservatism is often required when using interpreted highest groundwater levels. 3.6 
...there is uncertainty about how smooth the [seasonal groundwater elevation] transition is because there is no piezometer in the centre of the Site and there is the possibility for steps in the water table related to faulting 2.4 ...unmapped local faulting... 3.1 ...the two [maximum water table grids] therefore represent just two of the many possible interpretations of the data which themselves are based on an incomplete parameterization of the detailed groundwater dynamics of the site 4.2 Groundwater levels do not fluctuate evenly across the site... 4.2 Mineral extraction down to this [maximum groundwater level] surface would be dry for the vast majority of time as on average, groundwater levels are expected to be below this level6
In any case, it is highly unlikely that groundwater monitoring over the last 2 years or so will have recorded the maximum possible water table elevation.

On the subject of runoff - a concern to downstream communities around flood-prone Ottery St Mary - Hanson acknowledged:
During the operational phase, the excavation of the quarry will change the response to rainfall with the potential for greater and more rapid surface water runoff than would occur naturally over a vegetated surface C3.2
By maintaining an unsaturated zone thickness of at least 1 m (which increases to 5 m or more in some locations during summer months), the ability of water to infiltrate into the ground is not expected to change as the hydraulic conductivity and infiltration capacity remains the same. All that may change is the ability of the unsaturated zone to “store” water after intensive rainfall events... Bearing in mind that a certain proportion of this rainfall would have naturally ended up as runoff and evaporation then a 1m unsaturated zone should be enough to accommodate this intense rainfall event... The removal of the unsaturated zone down to a level of 1 m above that defined by the maximum winter water level will mean that any change in the recharge / runoff split, if it occurs, may be seen in the winter months and during very high rainfall events... It is not possible to quantify this possible change [in runoff] with any great accuracy. ...the assumption has been made that there may be a 20% increase in runoff. 5.1
It is proposed that no excavations be conducted beneath the water table, this includes up to the 1% AEP extreme high water table level, and that an unsaturated buffer is also maintained between the water table and the surface. Groundwater ingress is consequently not considered a risk at the Site. 3.5.3
It would seem utterly perverse if AI now claimed this 1m buffer was not needed; the EA says peak rainfall intensity could increase by 20% in 2055-2085 and 30% in 2085-2115.

Leaving 1m unquarried above the maximum water table should be a precautionary given; 1m allows for a margin of error and a margin of safety, because the maximum water table is not known with accuracy; faulting across the site is not known with accuracy; excavators would not dig with accuracy; future climate is not known with accuracy; future land use pollutants are not known with accuracy.

At Town Farm, there were two properties reliant on private water. Here's who relies on Straitgate's water: