Leaky Sewers Hydraulically Disconnect from Groundwater: A Proof-of-Concept

Abstract

Leakage from aging sewer and stormwater pipes into the subsurface poses significant environmental risks and threatens the integrity of urban infrastructure, primarily through the degradation of groundwater quality and the alteration of urban water balances. While the presence of defects within pipe networks is well-documented, accurately quantifying volumetric exchange fluxes remains a challenge due to the complex, nonlinear interactions between the pipe, the surrounding variably saturated soil, and the fluctuating groundwater level. Current modeling approaches often overlook the threshold behaviors of these systems, leading to potential inaccuracies in leakage estimation. In this study, we show for the first time that leaky pipes can become hydraulically disconnected from the underlying groundwater, a phenomenon analogous to well-known river–groundwater interactions that include disconnection. In a sewer–groundwater context, hydraulic disconnection is restricted to point sources and is strongly influenced by colmation/clogging. Through numerical modeling of a hypothetical case study, we show that the leakage flux from the pipe (in absolute terms) initially increases with declining groundwater levels, until a critical depth below the leaky pipe is reached. After this point, the hydraulic communication from the groundwater to the leaky pipe stops and the leakage flux can be considered constant. In a sensitivity analysis, we demonstrate the impact of the individual hydraulic parameters of the leaky-pipe-groundwater system on the hydraulic disconnection. We further modify the properties of the aquifer material, resulting in a hydraulic disconnection depth of 0.89 m, 1.77 m and 4.00 m below pipe for sand, loamy sand and sandy loam aquifers, respectively. This insight has important implications for leakage modeling: once hydraulic disconnection occurs, the leakage flux becomes independent of groundwater dynamics. The present study provides a proof-of-concept for the mechanism by which leaky sewers hydraulically disconnect from groundwater.