Groundwater impacts of underground infrastructure in London

1. Groundwater impacts of underground infrastructure in London Jane Dottridge, Megan Durrant, Victoria Price and Dino Giordanelli, Mott MacDonald 24 September 2019

2. Outline Setting the scene • London’s infrastructure • Geological setting Risks to aquifers • Barriers to flow • Disturbance and dewatering • Mobilisation of contaminants • New sources of contamination Benefits • Extensive investigation and monitoring Risks and mitigation Conclusions

3. London’s expanding subsurface infrastructure • Transport • London underground – 1863, 400 km 270 stations • High Speed 1 Rail Link - 2007 • Crossrail - 42 km, 2020/21 • HS2 - 2026 • Water • Thames Water Ring Main – 80km, 1994 • Lee Tunnel (sewer) – 7km, 2016 • Tideway Tunnel (sewer) – 25km, 2024 • Energy and communications

4. London’s current infrastructure is very dense Future major infrastructure extends further from city centre

5. London’s geological structure Geological section from Royse et al, 2011 after Sumbler 1996

6. Simplified cross section shows main risks to aquifers in East

7. Aquifer classification and transmissivity

8. Potential Risks to Chalk aquifer

9. Case Study - Thames Tideway and Lee Tunnels Abbey Mills PS Beckton STW Existing sewer connections Lee Tunnel Acton Thames Tideway Shafts and connections Source: Talk Tunnel

10. Made Ground / Alluvium Potentially affected aquifers London Clay Piezometricsurface PS STW Lambeth Group 0 Shallow River Terrace Deposits Harwich Formation Principal ‘Deep’ Aquifer Thanet Sands Depth (mbgl) White Chalk 100 Aquifer

11. Lee Tunnel Groundwater monitoring network 52 boreholes Lambeth Group River Terrace Gravels Harwich Formation Thanet Sand Chalk Crossrail Groundwater flow in: Chalk Shallow Boreholes monitored during construction phase

12. Benefits • Extensive investigation • Long term monitoring • Improved understanding of 3-D geology, detailed structure, aquifer properties and background water quality

13. Diversion of flow is local and insignificant

14. Dewatering risk assessment shows potential impacts Public Water abstractions Private abstractions Shaft R. Thames GW flow

15. Dewatering for shaft construction Original approach With mitigation • Pumped: 50 – 60 l/s for 80m drawdown • Widespread drawdown, ~2m at closest abstractions • Water quality deteriorated • Rapid response to mitigate impacts • Soil mixing at shaft site to immobilise contamination • Diaphragm Wall around shaft • Pressure relief wells, passive flow <3 l/s • Monitoring but flow pattern still modified

16. Modelled drawdown after 21 months dewatering From Golder Associates, 2019

17. New contaminant pathways Concerns Linear projects create pathways Vertical shafts connect upper and lower aquifers • Mitigation by • Design • Construction methods • Remediation • Monitoring of impacts • Emergency action if things go wrong Tunnel leakage

18. New contaminant pathways Example of 70m deep shaft at site with historical contamination Dissolved hydrocarbons, PAHs and local DNAPL at 12-15m depth Main risk from free phase DNAPL, main sources of dissolved contaminants are off-site Deeper free phase – reduce to as low as practicable by physical removal Shaft installation with mitigation as dewatering Groundwater decontamination prior to discharge Monitoring of water levels and quality before, during and after construction Tunnel leakage

19. But dewatering increases contaminant capture zone From Golder Associates, 2019

20. New sources of contamination Tunnel conveys raw sewage 60-90 times/year, surcharged < 8 times/year for 0.3-0.8 days • Leakage rates from tunnel with cracks calculated using cubic law >84 m3/d • Flow in Chalk (K<2 m/d) during surcharging assuming radial flow < 0.13 m3/d • Outflows controlled by aquifer properties (not cracks), • confirmed by observations during commissioning of tunnel Tunnel leakage

21. Figure 4: Predicted change in ammonia concentration: 50 m from the tunnel, 19 hours in duration New sources of contamination Impacts on groundwater quality of surcharging leakage based on 25mg/l of Ammonium • 1-D model of contaminant transport used to calculate increase in ammonium • Maximum modelled increase in ammonium 0.17 mg/l. • Baseline concentration of ammonium is 0.76 mg/l, >DWS • So leakage does not change the Water Framework Directive quality status of poor (deteriorating). Tunnel leakage

22. Measuring potential impacts Alert Levels to identify changes in groundwater quality Exceeds highest concentration recorded in baseline monitoring Uses statistical analysis taking into account the highest concentration and standard deviation Where 3 consecutive Alert Level 2’s are recorded Alert Levels 1 2 3

23. Concluding remarks • Risks to groundwater are inevitable where underground infrastructure is constructed in aquifers • Impacts can be mitigated through good design, construction methods and monitoring • Thorough investigation and baseline monitoring are essential to understand risks • Monitoring during and after construction is required to identify any impacts • Emergency action to manage impacts is rarely needed but must be rapid

24. Acknowledgements CVB, Golder Associates, MVB, Thames Water, Tideway and colleagues at Mott MacDonald