Contaminated soil and groundwater management for wastewater pipeline upgrades

Challenge

Gracefield is a suburb of Hutt City developed since the 1920s for commercial/industrial landuse. The suburb is located in close proximity to Wellington Harbour and overlies the regionally important Waiwhetu Aquifer which is abstracted for drinking water.

Wellington Water owns the wastewater infrastructure serving Gracefield. They require sections of the ageing existing wastewater pipeline to be replaced or relined to accommodate current and future wastewater volumes, whilst managing the environmental risks from hazardous substances in the soil and groundwater during the upgrade works. The pipeline route crosses many separate land parcels comprising approximately 60 selected land use register (SLUR) and 20 hazardous activities and industries list (HAIL) category sites, including the bulk storage of hazardous substances, metals recycling, hazardous waste treatment and metal plating.

An assessment of project risks was required to inform the resource consent requirements and the construction management strategy to replace sections of the wastewater infrastructure.

Solution

Our Environmental Services team provided specialist contaminated land expertise and produced the reports required to seek resource consents for the construction works.

We carried out an initial Preliminary Site Investigation (PSI), conducted concurrently with a pipe condition survey undertaken by the client. The PSI documented the contamination status of the land to be subject to soil disturbance during the works and the likely contaminants of potential concern (CoPC) in soil and groundwater for various pipeline zones. The PSI and pipe condition survey results were used to inform the scope of the pre-works contaminated land Detailed Site Investigation (DSI).The DSI works comprised soil and groundwater sampling from 20 shallow window sample boreholes and from three deeper groundwater monitoring wells.

Analysis of the soil and groundwater results considered national and regional environmental guideline values, to develop management pathways for residuals management during earthworks, including options for the management of groundwater arisings from dewatering and soil reuse where appropriate.

These were documented in a Contaminated Site Management Plan (CSMP) including an Asbestos in Soil Management Plan (ASMP), which were finalised following discussions and agreement of construction methodologies and desired consenting pathways.

Our reports were used to obtain resource consents for the construction works and to inform the terms of reference for the construction contract.

Impact

Completion of the project will facilitate the appropriate management of wastewater generated in this environmentally sensitive industrialized area of the Wellington Region.