Leaking pipes and the urban karst: a pipe scale numerical investigation on water leaks flow paths in the subsurface

Millions of cubic meters of water are lost every year from water utilities, with detrimental consequences on the environment and on society. However, this large amount of water is not necessarily lost. Although leaking water utilities have the potential to recharge urban aquifers, very little is known about the fate of leaked water and its interaction with subsurface anthropogenic features, such as the high permeability trenches surrounding utility pipelines. As these features are deemed to create preferential pathways, it is crucial to understand if these can reduce urban aquifers recharge by interception and diversion of water leaked. Through a series of transient 3D numerical simulations under a variety of conditions (i.e., different native soil properties, slopes of the utility trench, depths of the groundwater table, and leak rates), this study provides evidence that utility trenches can potentially alter the migration pathway of water leaked from utility pipelines located above the groundwater table. Therefore, the urban karst effect has the potential to occur directly at the utility trench. Indeed, from roughly 55% up to 73% of the water leaked is retained within the utility trench and does not reach the underlying aquifer in one third of the scenarios modeled. This occurs because of the combined effect of soil retention properties, initial effective water saturation, relative permeability, and intrinsic permeability. Although this analysis is still preliminary, these results potentially challenge the traditional concept of leaking pipes recharging groundwater.