Watershed Woes

Lincolnshire County Council’s Policy DM16 deals with ‘Water Resources’ and states “Planning permission will be granted for minerals and waste developments where they would not have an unacceptable impact on surface or ground waters and due regard is given to water conservation and efficiency.”

“The quantity, and physical and chemical quality, of surface waters and groundwaters may be affected by quarrying activities; flows can be increased or decreased and may be contaminated by runoff or dust from the quarry. The removal of topsoil, overburden and aggregates may affect the quality of water recharging of an aquifer, and excavation below the water table may lead to de-watering of adjacent watercourses and wells.”

One of the biggest problems that Breedon face with the proposed mining sites are the complex hydrology issues of the area which they wish to excavate. The fluvial sands and gravels upon which we sit are moderate-high permeability deposits and as such any dewatering activities especially adjacent to a river may induce very high rates of inflow into the quarries and away from the rivers. Whilst pumping the water from an aquifer may have widespread drawdown effects increasing this flow.

Breedon’s proposals for mining MS29, SG17 and SG11 are to excavate the sand and gravel which sits some 1-2metres below the surface of the ground under the topsoil and extends down to a depth of 7-8 metres below the surface. The normal water table level for the area sits at around 2-3metres beneath ground level.

One of the problems that Breedon have to overcome is how to excavate the sand and gravel whilst the minerals that they seek are under water.

They can do this in one of two ways –

1. By dredging the minerals out of the ground (“Wet working” of sand and gravel enables aggregates to be dredged from below the water-table) This also helps to preserve the water table of the surrounding area.

2. By dewatering (pumping the groundwater away so that they can dry-excavate). They much prefer this option as it is much cheaper and easier albeit not as sustainably friendly.

The proposed quarry sites MS29, SG17 and SG11 are bisected by a main river - the mill stream of the River Welland and they are bordered on their southern boundary by another main river, the River Welland. As the mill stream nears Molecey Mill, the river has been puddled with clay to provide a lining to the riverbed, however upstream of this and in the section where the river bisects the quarry a more fragile natural balance exists between the riverbed and the surrounding ground. The same fragile balance exists along the southern boundary with the River Welland.

The Impacts of Dewatering

This dewatering of the mine has dramatic effects on the hydrology of the surrounding land and has far reaching effects on the water table. Effects can include -

 Lowering the groundwater level within the sand and gravel surrounding the extraction, which is in hydraulic continuity with the rivers and streams in the area.

1. A loss of groundwater storage - the consequence of this is extensive in moderate and high permeability strata.

2. In moderate to high permeability strata the volume of water can be severely affected

3. A derogation of groundwater fed watercourses

4. Contamination of watercourses and damage to associated flora and fauna;

5. Alterations to the groundwater flow regime

6. Groundwater contamination due to induced saline intrusion or drawing in of contaminated groundwater to the workings;

7. Inadequate control of groundwater rebound leading to ground instability, flooding etc

8. Restoration of sub-water table quarries, dry restorations requiring indefinite pumping are likely to be least sustainable, where as wet restorations result in less impact

9. It is accepted that geologically and hydrogeologically, quarries in low permeability strata (not us) are generally more environmentally sustainable than those in high permeability strata (us)

10. Long term complex water management systems (such as indefinite pumping) are likely to place an impractical burden on future generations,

11. method of extraction, wet extraction is more environmentally sustainable than dry, however, there may be planning constraints that override the water resources issues

There is much evidence to suggest that the proposed excavations would affect the main rivers and the surrounding lands as far as 500m from the quarry and yet Breedon are proposing a 30m stand off from the rivers. This is not enough and there is a substantial risk that the mill stream of the River Welland and the River Welland itself will be severely affected by the excavations.

Our own independent Hydrogeology Report came to these conclusions –

 The extraction proposals are currently at the mineral planning stage and, should these enter planning or operation, it is the detail of these proposed operations that will determine the detailed risks to the Molecey Estates Site (MES). Factors influencing this would include, amongst others, whether the extraction is to be worked wet or dry, the phasing of extraction and restoration proposals. Notwithstanding this, several risks which are considered to have potential for a major impact on the viability of MES are outlined below.

The loss of river base flow (and groundwater level).

Dewatering would be required if the deposits were to be worked dry. The method for dewatering would need to be determined. Dewatering will reduce groundwater levels locally and hence potentially impact baseflow and water levels in adjacent water courses. This could impact on the operation of the historic water mill, an integral part of MES, as well as other groundwater and surface water features upstream of, and within the footprint of MES. Such impacts would have a material effect on MES.

1. Subsidence of buildings. Dewatering of unconsolidated deposits may result in a reduced volume of materials beneath buildings and other land and infrastructure through the removal of groundwater. This has the potential to cause compaction of the sands and gravels and in turn subsidence. In particular, the historic nature of buildings and associated foundations is such that the potential for damage through settlement may be particularly significant.

2. Adverse changes to the river flow which could result in an increased flood risk. Dewatering, together with management of surface water runoff which will enter the void, is likely to require a significant discharge to surface water. Surface water courses run through the MES site and discharges of surface water upstream, adjacent to, or even downstream of the extractive operations could result in higher surface water levels and/or increased flows. Locally, land is low lying and changes to surface water flow and levels has the potential to increase local flood risk and impact MES.

3. SuDS and groundwater flooding. Whilst it may be possible to mitigate some discharge to surface water through the return of dewatered groundwater to ground, the use of infiltration SuDS may be extremely limited where groundwater is shallow. Shallow groundwater would potentially prohibit infiltration both in terms of capacity and the need for the required 1 m unsaturated zone to treat the infiltrated water. Notwithstanding a potential risk from increased surface water flow, infiltration of significant volumes of water in a concentrated area could have an impact on local groundwater levels and increase the localised risk of groundwater flooding and/or rainfall recharge rejection. In addition to high groundwater levels, where rainfall infiltration is rejected, this could exacerbate the risks from surface water flooding locally through increased runoff.

4. Contamination. There is the potential for impacts on water quality from the operations. For instance, the working method could mobilise suspended solids and, due to the groundwater/surface water hydraulic connection, result in both a visual and ecological impact on surface water courses. There is also the potential for more general contamination from the operations.

5. Restoration. Finally, the extraction will require future restoration once extraction is complete. Depending on the nature of the restoration this could have an adverse impact on the local environment, especially if this is to be completed using importation of materials. Impacts could take the form of groundwater and surface water contamination in addition to those of aesthetics and odour.

In Conclusion

The effects of lowering the water table or creating a barrier to groundwater flow must be evaluated in the short term during quarry development, in the medium term during quarry operation, and in the long term for final after use.

Any solution or restoration requiring a pump running 24/7 in perpetuity cannot be justified as sustainable. This is what we are left with in the quarries north of the A1175.

We have to be very careful that any mitigation proposed by Breedon is indeed what they say it is because evidence suggests that the problems they face cannot be rectified with mitigation and we will see severe effects on our lovely local watercourses.