Rapid on-site screening of excessive infiltration and inflow into sewer systems

Excessive infiltration and inflow (I&I) of environmental water into sewer networks renders conventional wastewater treatment ineffective and costly. Physical inspection techniques (e.g., Closed Circuit Television and Sonar) are the go-to methods for diagnosis but are expensive ($5–15 USD/m). Chemical fingerprinting is over an order of magnitude cheaper ($0.5–1.5 USD/m) but the diagnosis is delayed due to reliance on laboratory analysis and is error-prone using deterministic algorithms. To overcome such limitations, sewer samples ( n = 21) were collected from a small (0.5 km²) coastal district of Shenzhen and analyzed initially for 9 tracers including pharmaceutical compounds unique to sewage, but later focusing on 3 tracers (electrical conductivity (EC), radon-222 (²²²Rn), and ammonia nitrogen (NH₃−N)). New data from 31 environmental water samples were critical for constraining endmember compositions. A Bayesian Chemical Mass Balance (CMB) model distinguished and quantified seawater, groundwater, and rainwater intrusion probabilistically. The model using 3 tracers under dry-weather conditions (or 4 when δ¹⁸O was included for rainwater inflow during wet-weather events) yielded sewage proportion estimates comparable to that of 8 or 9 tracers, differing only by 3.0% ± 2.5%, although the mean relative error was twice as high. At four sewage wells, raw sewage accounted for 32.3 ± 13.3%, 12.8 ± 5.3%, 41.9 ± 15.1% and 33.6 ± 13.4% along the flow paths in August 2021. Significant temporal variability was evident at one well, with raw sewage contributions ranging from 21.2 ± 6.7% in August 2021 to 58.8 ± 8.0% in November 2022 and 50.8 ± 37.5% in January 2025.