²²²Rn effective to monitioring groundwater intrusion into sewage systm

Abstract

This study systematically validated radon-222 (²²²Rn) tracing technology as an effective monitoring method for groundwater intrusion (GWI) into sewage networks. This experiment addressed the challenge of leakage in the aging drainage network of the coastal city of Shenzhen. In collaboration with the Shenzhen Water Group, after discussions, Yinhu Road (structural valley) and Yantian District (coastal zone) were selected as typical study areas. Three rounds of field sampling were conducted, with a total of 63 samples collected. Real-time radon detection was performed using the RAD7 device, combined with laboratory ion chromatography (IC) analysis and spatial correlation tests, along with data analysis and visualization methods. The study found that radon concentrations in wastewater exhibit significant spatial heterogeneity—the range of samples from Yinhu Road reached 21,320.59 Bq/m³ (2.72–21,323.31 Bq/m³), with a coefficient of variation (CV=175%) far exceeding that of ionic indicators (Na⁺ CV=41%). Among these, the high value of 21,323 Bq/m³ at the community drainage outlet YH2-12 site closely matches the local groundwater background value (GW3-4: 28,431 Bq/m³), precisely indicating leakage from the fault zone; In Yantian District, a strong correlation between radon and chloride ions (r = 0.89, p < 0.01) was observed, such as at the YT1-11 point (Rn: 1,228 Bq/m³, Cl⁻: 718 mg/L), revealing a synergistic mechanism of seawater reverse intrusion through the aquifer. Methodologically, an innovative sub-catchment division strategy was introduced, validated through repeated sampling of 12 residential units to ensure technical transferability (R² = 0.700), and the peristaltic pump closed-loop sampling process was optimized, improving radon concentration detection rates by 57% compared to traditional methods. Empirical evidence shows that radon gas, due to its rapid dissipation characteristics in surface water (Henry's constant 0.25), can avoid interference from domestic emissions and specifically identify GWI signals. Based on this, a hierarchical pipeline repair framework is proposed: when radon levels exceed 10,000 Bq/m³, initiate CCTV inspection and resin sealing within 48 hours; Implement monthly re-testing and hydraulic model optimization for values between 1,000 and 10,000 Bq/m³. The study further developed a "binary method" for leak localization (main pipeline screening → branch pipeline sampling in 50-meter increments), which reduced detection costs by 70% in the Yinhu Road case. The conclusion confirms that ²²²Rn possesses high sensitivity, strong interference resistance, and spatial resolution, making it an "ideal tracer" for diagnosing groundwater intrusion. It is recommended to incorporate radon monitoring into the "Technical Specifications for Urban Drainage Monitoring" (CJJ/T 271) and to establish a smart water management platform integrating radon concentration maps, pipeline GIS data, and ground subsidence data to enhance the climate resilience of drainage systems in coastal cities.