Tracing contamination sources and geochemical processes in groundwater of industrial complex using multiple isotopes (Zn, Cr, S-O of sulfate, and H-O of water), PCA, and PMF.

A multi-tracer framework combining rank-based principal component analysis (PCA), positive matrix factorization (PMF), and four stable-isotope systems was applied to 26 groundwater samples from three industrial zones and a residential reference area in South Korea. Trace metals were elevated in the industrial zones, with Cr reaching 17,324 and Zn 2026 µg L⁻¹ . PMF resolved four sources: sewage (Factor 1, NO₃⁻-dominant) at residential and Zone 2 wells, industrial SO₄²⁻ (Factor 2) at Zone 3 wells, reduction of Mn-Fe (oxy)hydroxides (Factor 3), and Zn alloy/plating (Factor 4). Three roadside wells showed low deuterium excess (d-excess, down to -1.7‰), suggesting characteristics of sewage. Among the 10 samples analyzed for sulfate isotopes, DS-11 showed increased δ³⁴S-SO₄ indicating bacterial sulfate reduction, with stable Δ¹⁸O(SO₄-H₂O) values suggesting oxygen exchange. δ⁶⁶Zn (n = 26; -0.32-1.04‰) was inversely related to dissolved Zn (R² = 0.60, p < 0.001), suggesting in-aquifer Zn sulfide (ZnS) precipitation. δ⁵³Cr (n = 3; 1.14-1.84‰) exceeded the industrial electroplating range (≈0‰), suggesting Cr(VI) reduction at those wells. The framework thus separated contaminant sources from in-aquifer attenuation. This separation, unattainable from dissolved concentrations alone, supports targeted remediation.