Brett’s SLR reply

RE: Response to Hatfield Road Quarry, Consultation Questions and Statements (21 August 2019)​​ 

27th​​ August 2019​​ 

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Brett Aggregates Ltd​​ 

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RE: Response to Hatfield Road Quarry, Consultation Questions and Statements (21 August 2019)​​ 

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DEFINITIONS​​ ​​ 

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UMH - Upper Mineral Horizon​​ 

UMA - Upper Mineral Aquifer​​ 

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LMH - Lower Mineral Horizon​​ 

LMA - Lower Mineral Aquifer​​ 

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LML – Lower Mineral Lagoon​​ 

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RESPONSE TO SPECIFIC QUESTIONS​​ 

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  • What is the difference between 201S/201D?​​ 

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When BH201 was drilled, a small clay lens was identified in the centre of the LMH and a well was installed both above (201S) and below (201D) the clay lens, separated by a bentonite seal.  ​​​​ 

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  • In regard to the readings for U/L/C - how is possible to have 3​​ separate readings from the same borehole?​​ 

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There are 2 boreholes at each triple monitoring zone location. The UMA monitoring well is installed in a separate shallow borehole and the LMA/Chalk monitoring well contains 2 separate riser pipes installed in​​ the single borehole with the levels being separated by a bentonite clay seal.​​ 

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  • The readings are from February 2019. The data does not account for seasonal variations where Bromate concentrations may be affected by the relative height of the water table ​​ 

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The complete bromate data set already issued spans the period August 2013 to February 2019 so there is over 5 years of quarterly data for all the wells which clearly spans all seasons.​​ 

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  • The mineral extraction is very close to the boreholes shown to be​​ contaminated by Bromate, for example, Borehole 201 is 66m and BHB​​ is 150m to the​​ east​​ of the site boundary, both of these boreholes are shown to be contaminated by Bromate​​ 

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The distances referenced above are from these locations to the site boundary. Accurate distances are that BH201 is 240m and BH-B is 320m from the closest proposed mineral excavation in the LMH. The LMH will not be excavated in the location of the LM lagoon.​​ 

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  • They are saying the steady state (Thiem equation) of the lower confined​​ lower aquifer has not been properly assessed. I am told this has the potential to lower the piezometric levels by up to 1m over a 300 to 400m distance away from the borehole well (depending upon the size of the excavation). They​​ assume the effect could be​​ very significant where substantial volumes of sand, gravel and water are removed. They believe the Environment Agency has not properly considered the potential effect on​​ 

piezometric levels or taken into account how close the contamination is to the proposed mineral working​​ – The Thiem equation is a theoretical tool for water only and can demonstrate the drawdown effect over distance. The amount removed from the Well head is relevant to the curve at monitoring points away from the centre.​​ The size and travel rate through the aquifer is also part of the equation. A further complication is the aquifer – confined or unconfined. However we feel that removing some million tonnes of sand, gravel, clay and water must have some effect on drawdown from the adjacent contaminated areas.

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The hydraulic properties of the LMA were assessed in 2018. ​​ The drawdown referenced in the statement above is a substantial exaggeration and especially so when the 4 points referenced in our statements to Qu. 6 are factored in.​​ 

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A Groundwater Management Plan (GWMP) has been prepared that provides a detailed description of the mineral excavation and groundwater management process. The method of operation and GWMP do not align with the statements made in the above paragraph.​​ 

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The LMH will be worked ‘wet’ on a campaign basis allowing its extraction to take place during the lowest point of the LMA natural annual cycle. A small working area of approximately 100m x 30m will be excavated and then backfilled with site won inert material on a rolling basis thus restricting the open area within the excavation cell.​​ 

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Affinity Water plc has reviewed the GWMP and we have agreed a precautionary approach, real time water level data sharing, advanced notification and agreement of pumping, monthly sampling, agreed water target and trigger levels and regular meetings. ​​ The Environment Agency has also reviewed the GWMP and believes the management of groundwater on the site is feasible.​​ 

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6.​​ They argue contaminated groundwater could be drawn in​​ to the LMH lagoon from the surrounding confined lower aquifer. This will result in the LMH lagoon becoming contaminated by Bromate. They say during conditions where groundwater levels are particularly high the design infiltration rates will not occur, therefore the storage volume in the LMH lagoon will be exceeded and contaminated groundwater will be discharged in to the Ellenbrook either directly from the back-drain or as a result of groundwater flooding.​​ 

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Migration of the plume westwards due to our operations will not occur for the following reasons:​​ 

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  • No mineral will be extracted from the LMH within the LMA Infiltration Lagoon;​​ 

  • The LMA infiltration lagoon will create a local recharge mound that will serve as a hydraulic barrier to the lateral movement of the plume;​​ 

  • The capture radius created by a pumping well preferentially extends hydraulically up gradient and, in this case, not in a side gradient direction towards the plume;​​ 

  • The LMH within all phases will be backfilled with site won clay forming a​​ low permeability flow barrier between the lagoons and the rest of the mineral site, thereby the risk of lateral movement of the plume is further reduced;​​ 

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Infiltration rates will decrease with high water levels, but the location of the LMA Lagoon has been deliberately selected because LMA groundwater levels have never been high enough to confine the LMA and there will always be a significant downward head in this location. ​​ 

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With respect to the stated risk of contamination of the Ellenbrook, the GWMP describes strict water level controls such that groundwater in the LML never exceeds the water level in the UMA Lagoon and furthermore, pumping will be simply stopped before any critical over flow threshold is ever reached. LMA groundwater will therefore not be discharged into surface water.​​ 

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