The risk of soil and groundwater pollution associated with accidental spills of hydrocarbons used by work machinery is inherent to construction activities. Furthermore, another possible source of pollution of these resources is the storage of chemical products or harmful waste. These two risks were identified as part of the project to construct lines 4, 5 and 6 of the Riyadh metro (Saudi Arabia).

The prevention of these sources of pollution is of significant importance, as they can affect the quality of groundwater and soil, with a subsequent impact on the corresponding habitats.

To minimise these risks, the following activities were carried out:

• A company certified by the environmental agency of the Kingdom of Saudi Arabia (Presidency of Meteorology & Environment) was contracted to provide a system for segregating hazardous and non-hazardous waste.
• Collection of leachate generated by rainfall and subsequent treatment to minimise the risks of handling and disposal.
• To prevent accidental spills, specific areas were designated for storing substances (at a distance from rain runoffs or natural watercourses), equipped with a sun shed to reduce the impact caused by direct sunlight, a perimeter concrete wall and correct signposting in the waste stockpile area. Furthermore, chemical products were organised in this area to prevent incompatibilities.
• Location of diesel operated generators and fuel oil tanks covered by a sun shed, making it possible to temper the changes in extreme climate conditions, insulate them and prevent the runoff of hydrocarbons in the event of an accident.
• Supply of kits for staff to use in the event of a spill and training on how to use them, in addition to training on the correct segregation of chemical products.

The adopted measures have resulted in a reduction in the risk of soil and water pollution due to infiltration, the prevention of pollutants seeping into runoff water that could reach water traps, a reduction in the risk to which the health of ecosystems and humans are exposed and an improvement in the physical and chemical control of pollutants.

Furthermore, contracting an external company led to improvements in the way hazardous waste is handled and their correct environmental management.