pH Increase in Desalinated Water in Water Transmission Pipeline

Does anyone know the reason why the pH of water increases after it travels through a water transmission pipeline of 20 km? 

The pH of desalinated water changes from 7.1 to 9.8-10.01 after 20 km before reaching a storage reservoir. 

To normalize the pH, we are using the citric acid at end-point of the storage reservoir, before the water is distributed. 

What could be the reason for the change in water pH and how can we regulate this?