Groundwater Infiltration Potential (GWIP) as an aid to determining the cause of dilution of waste water

Abstract

Groundwater infiltration through leaking sewers represents a considerable fraction of the total amount of wastewater dilution. In search for an easy-to-determine yet acceptably accurate estimation of the likelihood of groundwater infiltration into leaking sewers, the parameter ‘groundwater infiltration potential’ or shortly GWIP was defined. GWIP expresses the extent to which groundwater infiltration could – in contrast to the inflow of surface water – be a cause of dilution of sewage. The GWIP is determined by a comparison between the elevation of the groundwater table with the position of the sewer conduits per geo-spatial aggregation level (GAL). This first order analysis compares sets of three representative figures of the groundwater table’s elevation, i.e. the minimum, the maximum and the average level with sets of two representative values of the pipes’ positions, i.e. average invert and soffit levels.A GWIP map can be set-up indicating per GAL a GWIP score that represents a generic evaluation of the common (i.e. most occurring or representative) situation regarding the presence of the groundwater table versus the elevations of the sewer system. In this way the GWIP map can assist in the determination of the overall most likely origin – either surface water or groundwater – of dilution per GAL. Eventually this facilitates strategic decisions regarding the search for particular locations of dilution, and subsequently for the selection of specific remediation measures. The methodology is developed on a local scale of Flanders, Belgium but is generic and therefore applicable to any other region provided that information on the elevation of the sewer system and groundwater table is available.