Analytical and numerical analysis of utility trench water levels in an aquitard

ABSTRACT: Sewer networks are widely recognized as having a significant effect on urban groundwater systems. Utility trenches are an integral part of a sewer system; the coarse granular material that comprises the bedding of a sewer can form a preferential flow zone, particularly when the native geomaterials comprise an aquitard. Information on groundwater conditions in utility trenches may be obtained from utility trench wells (UTWs) that are constructed with a screen located in the highly permeable bedding material. The present paper considers UTW water levels, their recession characteristics and the resultant information they provide on the hydrologic functioning of the utility trench network in an aquitard. Data are considered from a district-wide investigation in Mississauga, which represents to our best knowledge the most detailed field characterization to date of the interaction between a storm sewer system, the associated utility trench and an aquitard system. The UTW hydrographs from this investigation are characterized by a sawtooth shape with notable linear recessions that are a response to widespread storm sewer exfiltration. The recessions have been examined using analytical results based on Rorabaugh's 1960 foundational recession analysis with the 1-D Boussinesq equation as well as 1-D and 2-D numerical models of utility trenches. The results indicate that post exfiltration the water levels are strongly influenced by 'up-catchment' trench desaturation and 'down-catchment' convergence of flow through the connected utility trench network. A utility trench network could be considered an artificial analog for a natural groundwater system with preferential flow paths. Unlike natural systems, the geometry and connectivity of the utility trenches are relatively well known. The results from UTW analysis may provide some useful insights for hydrograph analysis in more natural groundwater systems.