CEMP - NZ Transport Agency

NZ Transport Agency
CSM2 & MSRFL
2.3.7 Groundwater management
Potential Impacts
Existing groundwater levels have been a key influence in the design of the Project, as it has dictated
the vertical level for the road and preventing the placement of the Project into a cutting. While
groundwater levels can change over time, the risk of groundwater inundation is considered to be
low given the dispersed nature of the disposal system, meaning that failure of any individual
soakage device is unlikely to cause inundation of the road.
In general terms the groundwater is currently at sufficient depth not to impact on the construction
of the Project. It is not envisaged that large scale dewatering systems will be required during the
construction phase, although some localised dewatering during deeper trenching or excavations
may be needed on a site by site basis. There is however a possibility that during construction
groundwater will be encountered.
Project-Specific Issues and Requirements
The proposed stormwater treatment measures and implementation of the CEMP will avoid
contamination of the Christchurch Groundwater Protection Zone. However ECan have advised that
during a recent subdivision near to the project that groundwater was encountered and that there
needs to be a process for capping and confining any springs or other groundwater encountered. In
response to this a specific and targeted management plan has been developed entitled “Accidental
Aquifer Interception Management Plan” (SEMP 006). This outlines practices to be undertaken should
the aquifer be accidentally intercepted during construction including methods to cap and confine
groundwater encountered.
CEMP Expectations for the appointed Principal Contractor
The guidance and expectations set out in the Accidental Aquifer Interception Management Plan
(SEMP 006 within Appendix F of this CEMP), of this CEMP should be followed and where necessary
be reviewed and updated once a detailed construction programme and build methodology has been
agreed with the NZTA by the Principal Contractor and the Construction Environmental Manager.
2.3.8 Hydrology & Stock race management
Potential Impacts
The key stormwater issues are the collection and disposal of stormwater generated within the
Project, the passage of stock-water race flows beneath the Project and the passage of overland
flows generated in the upstream catchment beneath the Project.
The majority of the catchment crossed by the Project does not directly contribute to any natural
watercourse. Surface water typically ponds in local depressions on the catchment surface and soaks
to land or evaporates. In larger events overland flows have the potential to flow along surface flow
paths. These overland flow paths are often intercepted by field drains, irrigation channels and the
existing stock-water race network, which either eventually discharges to the Halswell River or
discharges to land via engineered soak pits.
Draft 31 CEMP