A Stable Isotope Approach for the Quantification of Sewer Infiltration

Abstract

Extraneous flows in wastewater collection systems are conventionally evaluated solely on the consideration of discharge hydrographs, which often involves a great degree of subjectivity and oversimplification. To obtain reliable information on the material fluxes within the urban environment, the use of intrinsic tracers can be the optimal choice. We demonstrate the successful use of naturally occurring stable isotopes of water (18O/16O and D/H) to accurately quantify extraneous discharge of groundwater in a combined sewer network. Fresh water supply from a distant hydrological regime provided usable isotopic separations between drinking water (proxy for real foul sewage) and local groundwater (proxy for sewer infiltration) of 1.8 per thousand in delta18O and 11.7 per thousand in 62H. Diurnal variation of wastewater isotopic composition reflected both the varying rates of foul sewage production and irregular dispersion effects in the pipe network. The latter suggest the existence of larger cumulative backwater zone volumes, which have not been attended to yet. Infiltrating groundwater contributed 39% (95% confidence interval = +/- 2.5%) of the total daily dry weather wastewater discharge. This paper discusses all relevant aspects for practical application of the method. It presents a comprehensive framework for uncertainty analysis and details on the detection and discrimination of possibly interfering effects.

Innovative tracer methods for sewer infiltration monitoring

“…This can result in an obscured tracer signal, when parts of a catchment receive drinking water from differing sources during the course of an experiment. An infiltration study based on isotopic tracers finally comprises the following three steps (Kracht et al 2007):…”

“…Thus, all relevant groundwater exclusively originates from local precipitation formed at a moderate altitude. In contrast, Ru¨mlang depends for a major part of its water supply on lake water obtained from Lake Zurich, which to a large extent stems from precipitation in the Alps (average altitude of the drainage basin % 1220 m asl, for further details see Kracht et al (2007)). Zurich lake water is about 1.8% (d 18 O) and 11.7% (d 2 H) lighter compared to the local groundwater (local terrain % 450 m asl).…”

“…the corresponding number and spatial distribution of defects). In order to account for this effect, an additional simulation step was incorporated into an extended bootstrap based MCS scheme (Kracht et al 2007). Herein, we hypothesised 50 equally sized groundwater ingresses to be randomly distributed over the entire sewer system, which is considered to reflect a realistic value for our catchment.…”

“…From our basic understanding of the process, the actual infiltration flux at the groundwater to sewer boundary should remain constant during a diurnal cycle. Such fluctuations may therefore indicate irregular dispersion effects, probably caused by larger cumulative backwater zones (a detailed discussion on these observations is given in the supporting information of Kracht et al (2007)). However, such phenomena do not seriously affect the 24 h-based quantifications (Figure 2).…”

Quantifizierungen von Grundwassereintritten in städtische Abwassersysteme mittels stabiler Isotope

“…Unter der Annahme, dass Grundwasser die Abwassermessstelle 3 komplett beeinflusst, bietet das Wasser aus Genderkingen hierzu eine Gelegenheit für eine Massenbilanz. Ähnliche Untersuchungen in Lyon (Frankreich) und in Rümlang (Schweiz) erzielten, aufgrund größerer Isotopenunterschiede zwischen Abwasser und Grundwasser, eindeutigere Resultate (De Bénédittis & BertrandKrajewski 2005b;Kracht et al 2007 …”