Comparing roles of multiple contamination indicators in tracing groundwater pollution nearby a typical Municipal Solid Waste (MSW) Landfill

Abstract

Groundwater pollution resulting from leachate leakage at landfill sites has garnered significant attention. Investigating the migration of pollutants from these landfills to adjacent groundwater is crucial for understanding the diffusion patterns and extent of contamination. It is imperative to develop cost-effective yet highly efficient tracer techniques to aid landfill operators in monitoring groundwater contamination stemming from their operations. The primary objective of this research was to compare the roles of conservative tracers sodium (Na+) and chloride (Cl-), and conventional pollutants permanganate oxidation (CODMn), ammonium nitrogen (NH4+-N), lead (Pb), and zinc(Zn)in assessing pollution levels from municipal solid waste landfills to groundwater. For this purpose, a typical municipal solid landfill was selected to investigate the origin of Cl-, groundwater quality, and spatiotemporal variations of multiple contaminations. Geochemistry analyses revealed that Na-Cl and Ca-HCO3 were the dominant groundwater type in this study and landfill was the primary source of Cl- in groundwater, with an average contribution of 78%. Groundwater in proximity to the landfill (5#, 2#, 22#, 23#) exhibited elevated concentrations of Na+ (15.6∼914.0 mg/L), Cl- (8.9∼1352.0 mg/L), CODMn (0.54∼95.9 mg/L), and NH4+-N (0.33∼49.0 mg/L), yet demonstrated reduced levels of Pb (0.2∼391.0 μg/L) and Zn (2.0∼112.8 μg/L). In contrast, groundwater located at a considerable distance from the landfill (13#, 18#, 15#, 26#) displayed the inverse trend, with relatively low concentration of Na+ (3.2∼8.5 mg/L), Cl- (0.1∼0.7 mg/L), CODMn (0.28∼4.78 mg/L), and NH4+-N (0.03∼0.52 mg/L), but increased levels of Pb (1.2∼483.0 μg/L) and Zn (1.6∼357.0 μg/L). The primary determinant of groundwater quality near the landfill was NH4+-N, with the highest pollution index (Pi) of 492.85, whereas Pb was the predominant factor affecting water quality in areas distant from the landfill, with the highest pollution index (Pi) of 10.9. While no discernible seasonal variation was detected for all pollutants, spatial variation can be observed that pollution levels decreased progressively with increasing distance from the landfill, a trend particularly corroborated by the conservative Cl- and Na+ measurements. This research suggests that conservative ions, such as Cl- and Na+, exhibit superior efficacy in tracing the pollution range from municipal solid landfills to groundwater. Therefore, monitoring these conservative ions in groundwater can yield a more precise understanding of the extent of groundwater contamination originated from landfills.