A8.4 - Water quality

Water quality

Water quality is affected by:

• short term impacts during construction such as modifications of river channels, and lake or sea beds causing interruption or change to natural flows and the release of sediment downstream caused by disturbances from engineering works
• permanent modifications of river channels, and lake or sea beds, caused by engineering works, and modifications in ground water levels caused by aquifer penetration and changes in permeability or the shape of the ground surface
• increased discharges resulting from modifications of natural flows caused by faster rates of run-off from paved surfaces and the use of storm water drains and channels
• pollution of surface water and ground water.

Impacts on water quality

Potential impacts include the following:

• surface water pollution from surface run-off or spray. Potential pollutants include suspended solids, lead and other heavy metals, organic materials (such as rubber, bitumen and oil), salt and herbicides or pesticides (from roadside maintenance)
• surface water pollution from accidental spillage which is potentially very damaging
• ground water pollution from either soakaways which discharge directly into ground water or surface waters which find their way into aquifers. Pollution of ground water can also occur when road construction disturbs contaminated ground
• changes to water flows or levels which can increase the risk of flooding, interfere with aquifers, and affect the ecology of surrounding areas.

Mitigation of impacts on water quality

Avoidance and mitigation of some effects is possible through a wide variety of measures including bunding, vacuuming and filtering during construction; stormwater run-off management using marginal strips along roads that provide for infiltration; and emergency management such as sealing of drains and collection of clean-up materials. For more detailed guidance on erosion and sediment control of earthworks refer to the Auckland Regional Council publication Erosion and sediment control guidelines for earthworks, 1992.

The assistance of regional councils shall be sought where appropriate on the water quality and the hydrological regime within the road corridor, and to obtain further advice on the mitigation of impacts.

Measurement of impacts on water quality

All water effects are directly measurable through clarity and volume measurements (sediment), chemical analysis (water pollution), flow measurements (change in run-off rates), physical observation (some surface pollutants), and ground water level measurements. Appropriate measurement techniques are well established, and should be applied to determine the effects of road projects (Kingett, Mitchell and Associates 1992).

Prediction of impacts on water quality

If the impacts on water quality are significant reference shall be made to an appropriate design manual, eg, UK Manual Design manual for roads and bridges, volume 11, Environmental assessment, part 10 - Water quality and drainage or an equivalent.

Reporting of impacts on water quality

The expected short term construction effects and permanent effects of projects on water quality shall be reported. This reporting shall include effects on ground water and natural water courses and levels, and the pollution effects of surface water run-off and potential accidental spillage.

Where projects incorporate measures to mitigate the effects on water quality, the incremental costs and benefits of these measures shall be reported.