One likely cause is leaching ...

One likely cause is leaching from the inside surface of the pipeline if it is cementitious? If this is the case it is unlikely to change with time to any extent and it could lead to erosion of the pipeline. It may be possible to reduce the buffering capacity of the water or alternatively to consider lining the pipeline with a coating such as an epoxy resin. Or you could draw a polyethylene pipe inside the existing - sliplining. Logistics and size will govern the decision.