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Outfall Pollution Assessment

Outfall Pollution Assessment, Stafford Gate

Outfall Impact Assessment for Stafford Gate Development M6 Junction 14

Location

M6 Junction 14, Stafford

Service

Drainage and Water Quality Assessment

Date

July 2025

Status
  • Completed

Project Overview

We were commissioned to assess the potential water quality and environmental impacts of the proposed Stafford Gate development near Junction 14 of the M6 motorway. The development would introduce a small extension of local highway (Creswell Grove) into the existing National Highways drainage system.

Challenges & Solutions

Challenge

The additional road area—totalling just 0.23 hectares—would increase the impermeable catchment draining to outfall SJ9025_1527c. Our objective was to determine whether this change would affect surface water quality, spillage risk, or groundwater conditions, particularly given the nearby Doxey and Tillington Marshes SSSI.

Solution

Using the Highways England Water Risk Assessment Tool (HEWRAT) and a precautionary modelling approach, we assessed both the baseline (existing) and proposed drainage configurations. We applied a highly conservative traffic load across the new catchment area to ensure robustness.

Key assessments included:

  • HEWRAT modelling for soluble and sediment-bound pollutants.

  • Spillage risk calculations using LA 113 methodology.

  • Groundwater risk profiling based on geological and hydrological data.

Outcome

  • The outfall required mitigation under both pre- and post-development conditions—indicating no additional water quality burden from the development.
  • Spillage and groundwater risks remained unchanged, with all assessments falling below required environmental thresholds.
  • The proposed development was deemed environmentally neutral, with no increased burden on the receiving watercourse or nearby protected sites.

Conclusion

Our findings supported the integration of the Stafford Gate scheme into the National Highways drainage network without additional mitigation needs beyond existing requirements. This case exemplifies our expertise in delivering precise, policy-compliant drainage impact assessments for complex infrastructure interfaces.