Remediation of polluted sites in the typical area of karst underground river based on "Three-Source Model": A case study in the Pingqiao underground river system, Zunyi, China

Karst groundwater is an important water resource in karst areas of China and even the world, playing a crucial role in residents' daily life and industrial and agricultural production. The soil layer in karst areas is generally thin, with a double-layer structure between the surface and underground. Pollutants can directly enter the underground aquifer through thin soil layers, sinkholes, skylights, and karst cracks, making the karst groundwater extremely susceptible to pollution. In recent years, human's industrial and agricultural activities and domestic pollution have caused a large number of various types of organic and inorganic pollutants to continuously enter the underground river, posing a huge threat to the safety of drinking water and ecological agriculture development of local residents. Therefore, pollution remediation of underground rivers is of great significance for the protection of karst underground water resources. Focusing on water sources and pollution sources, this study conducts a systematic analysis of the investigation, evaluation, and remediation process of underground river pollution. It establishes a pollution remediation and control model of karst underground rivers—the "Three Source Model". This model mainly includes dual-source investigation, source tracking, and source control.

As a typical dendritic underground river in the exposed karst area of Southwest China with a pollution history of nearly 20 years, the Pingqiao underground river in Zunyi City, was selected as the study area for remediation and control of the pollution in the underground river with the use of "Three-Source Model". The results show: (1) There are 25 water points in the study area, mainly boreholes, karst springs and underground river outlets, and the characteristic pollutants are mainly NH$_4^{+}$, ${\rm{NO}}_3^{-}$, ${\rm{SO}}_4^{2-}$, Mn²⁺, and Se²⁺. Fifteen types of pollution source points are distributed, mainly including industrial waste disposal sites in the area of Pingqiao Industrial Park downstream of the underground river system. The characteristic pollutants are also mainly NH$_4^{+}$, ${\rm{NO}}_3^{-}$, ${\rm{SO}}_4^{2-}$, Mn²⁺, and Se²⁺. (2) There are mainly three groundwater pollution channels within the scope of the underground river system: PQE019 (sinkhole)-PQG025 (skylight)-PQS009 (underground river outlet); the flood discharge shaft of 2^# waste residue disposal site-CK8 (borehole)-J02 (monitoring well)-CK6 & CK11 (borehole)-JC04 (monitoring well) & ZK2 (borehole)-PQG025 (skylight)-PQS009 (underground river outlet); 1^# waste disposal site-PQG025 (skylight)-PQS009 (underground river outlet). The above pollution channels are distributed between the outlet of the underground river and Z1 waste disposal site in Pingqiao Industrial Park. (3) By curtain engineering, remediation was conducted in the upstream section of the polluted channel from the flood discharge shaft of 2^# waste disposal site to the outlet of the underground river, and then the groundwater level upstream of the curtain steadily increased from 815.68 m to 823.35 m, but there was no significant change in the groundwater level downstream of the curtain. The characteristic pollutants in the polluted water extracted from the upstream drainage wells (PWZK1 and PWZK2) of the curtain had a maximum NH$_4^{+}$ content of 469 mg·L⁻¹ (exceeding 938 times of a permitted level) and a maximum Mn²⁺content of 254 mg·L⁻¹ (exceeding 2,540 times of a permitted level). During the stage when the wells (PWZK1 and PWZK2) were not pumped, NH$_4^{+}$ content at the outlet of the underground river was 76-143.6 mg·L⁻¹, and Mn²⁺content 14.206-21.31 mg·L⁻¹, with the outlet flow rate of 5.0-295.349 L·s⁻¹. In the stage of intermittent pumping in these two wells, concentrations of NH$_4^{+}$ and Mn²⁺ at the outlet of the underground river showed a downward trend during the pumping period, while concentrations of NH$_4^{+}$ and Mn²⁺ at the same place showed a significant upward trend during the cessation of pumping. During the stage of continuous pumping, concentrations of NH$_4^{+}$ and Mn²⁺ at the outlet of the underground river remained relatively low, with NH$_4^{+}$ content generally ranging from 14.4 to 58.6 mg·L⁻¹ and Mn²⁺content from 2.38 to 6.39 mg·L⁻¹. Concentrations of NH$_4^{+}$ and Mn²⁺ decreased by 66% -78%, compared to concentrations in the stage when the wells were not pumped. The calculation results of reducing pollutant emissions in the treatment project show that the underground river outlet can reduce wastewater emissions by 47,244 m³·a⁻¹, with a minimum reduction of 16,250 kg·a⁻¹ for NH$_4^{+}$ and 10,960 kg·a⁻¹ for Mn²⁺. The research results can provide reference for the remediation of pollution in underground river systems in similar regions.