Isotopic tracing and quantitative assessment of rainwater in a separate sewer system in Tiszavasvári, Hungary

Abstract

The hydrological processes that commonly occur within the sewer system i.e., rainwater percolation or groundwater infiltration, can be easily described applying water isotopes. This study investigates the potential of tritium and stable isotope composition (δ2H and δ18O) of water for tracing rainwater entering urban sewer and estimating the amount. The work consists of collecting and analyzing wastewater and rainwater samples over a six-day-long period including a 30-min-long rainfall event, and then applying the isotopic results to determine the mixing ratios in the sewer pipes and the sensitivity of the applied tracing methods. Results reveal that the isotopic composition of wastewater reacts to inflowing rainwater sensitively: detectable flow rate increase resulting from rainfall is 0.62 m3h-1 for tritium, 2.49 m3h-1 for δ18O and 1.25 m3h-1 for δ2H. While applying variation of discharge, these values are 12.15 m3h-1 and 18.91 m3h-1 in daytime and nighttime, respectively. Due to the more positive stable isotope values of precipitation in the summer half-year, isotopic difference of rainwater and groundwater (being less enriched) is more notable, subsequently, traceable amount of parasitic rainwater in case of the examined area is lower than in the winter half-year. In case of groundwater recharged under cold climate conditions sampling should be carried out in summer in the night hours, after an at least three-day-long dry period. Though, all three isotopic tracers applied in this study proved to be suitable to detect small amount of parasitic precipitation, tritium came out as the most sensitive one.

Keywords

• Wastewater
• Parasitic rainwater
• Separated sewer system
• Isotopic tracing
• Hungary