Analysis of hydrochemistry and pollution characteristics in manganese-related industrial watersheds

GONG Xiaoyu; ZHANG Zhongqian; LI Wei; XIE Weisong; ZHU Ling; XU Lei; YE Liangkai; ZHANG Shaojian; WANG Chengcheng; ZHU Bin; AN Siwei; HUANG Daikuan

doi:10.11932/karst20250302

Abstract：The counties of Xiushan in Chongqing,Songtao in Guizhou,and Huayuan in Hunan are home to the enterprises producing Electrolytic Manganese Metal(EMM)in China and form China's "Manganese Triangle".Although the development of the manganese industry has driven economic growth,it has also caused significant ecological and environmental damage.Statistically,the production of 1 ton of EMM generates 6 tons to 10 tons of electrolytic manganese slag.The leachate from fresh slag is acidic(pH≈5)and contains high concentrations of ammonia nitrogen(565 mg·L-1 to 792 mg·L-1),manganese(936 mg·L-1 to 1,460 mg·L-1),and sulfate(9,250 mg·L-1 to 11,400 mg·L-1).Additionally,the leachate contains high levels of ions such as nitrogen(N),phosphorus(P),potassium(K),and magnesium(Mg).Rainfall and surface water can dissolve these soluble components,facilitating their migration into the environment.However,due to inadequate anti-seepage measures and complex geological conditions,the leachate has not been fully collected and treated,leading to contamination of surrounding soils and surface water bodies.Hence,to better understand the impact of the manganese industry on the environment,this study investigated the hydrochemical characteristics and pollution patterns in the Songtao River Basin.Water samples from potential pollution sources and receptors were collected during both the rainy season(May)and the dry season(September),respectively.Principal Component Analysis(PCA)and Positive Matrix Factorization(PMF)were used to identify the controlling factors affecting the hydrochemical characteristics of surface water in the study area. The findings are as follows:(1)Electrical Conductivity(EC)and Total Dissolved Solids(TDS)exhibited the following decreasing trend:leachate from manganese slag yard＞landfill leachate＞polluted groundwater from two wells＞surface karst spring＞surface water.Mn concentrations generally followed the order:leachate from manganese slag yard＞polluted groundwater from two wells＞landfill leachate＞surface water＞surface karst spring.The nitrogen in surface water and surface karst springs primarily existed in the form of NO3--N,while in other instances,nitrogen mainly appeared as NH4-N.In surface water,NH4 and Mn concentrations exceeded 13％and 9.6％of the permitted levels,respectively. (2)the pH of the landfill leachate had a pH above eight,and the chemical composition included SO4·Cl-Na·K.In contrast,the leachate from the manganese slag yard exhibited a pH value ranging from weakly acidic to neutral(pH 4.64 to 7.17),with a chemical composition of SO4-Mg.The surface water and surface karst spring water were dominated by HCO3-Ca·Mg type water.Interestingly,the polluted groundwater from two wells,although classified as groundwater,exhibited a SO4-Mg·Ca type due to the influence of the leachate from manganese slag. (3)in the rainy season,surface water demonstrated substantial dilution effects,whereas specific indicators within waste leachate exhibited a significant percolation.Meanwhile,chemical indicators in other water bodies exhibited minimal variation between the rainy and dry seasons. (4)both PCA and PMF extracted four main influencing factors,identifying geochemical processes,the manganese industry,and domestic pollution as the primary contributors to the hydrochemical characteristics.Factor 1(F1)in PCA and Factor 4(F4)in PMF exhibited consistent patterns,primarily associated with HCO3-,Ca2+,Mg2+,and TDS.These factors represent the combined effects of geochemical processes,with an average contribution rate of 52.99％.F4 in PCA aligned closely with Factor 3(F3)in PMF,where Mn as the dominant loading variable,indicated pollution from the manganese industry,contributing an average of 5.06％.F3 in PCA and F1 in PMF,characterized by K+,Cl-,and Na+,represented domestic pollution,with an average contribution rate of 16.44％.However,due to the overlapping signatures of pollutants from both manganese industrial emissions and domestic sources,neither method can accurately quantify the individual contribution rates of these two pollution sources to the contamination of the water environment in the study area. By comparing the chemical compositions of surface karst springs,and according to the synchronous trends of manganese and ammonia nitrogen concentrations along the Songtao River,it can be inferred that surface water continues to be affected by both manganese slag storage and domestic pollution.This highlights the need to sustain and strengthen remediation efforts targeting manganese-related industrial activities,slag yards,and domestic pollution in the Songtao River Basin.Additionally,localized monitoring of manganese and ammonia nitrogen should be enhanced to identify pollution pathways and prevent untreated leachate from slag yards and other pollution sources from being directly discharged into surface water.Furthermore,stricter supervision and regulation of domestic pollution discharges are essential to ensure that domestic sewage and pollutants are not released untreated into surface water.