Shallow-to-Profile Transferability of Heavy-Metal Source Fingerprints: A Two-Stage PMF Study at a Contaminated Site

Source apportionment at contaminated sites often relies on shallow samples, whereas remediation decisions concern a three-dimensional contamination body. We tested the within-site shallow-to-profile transferability of source-associated heavy-metal fingerprints using 186 soil samples from 43 boreholes reaching approximately 8 m at a former phosphate-fertilizer and leather-production site. Stage 1 positive matrix factorization (PMF) used 37 shallowest source-proximal samples to define four factors; leave-one-borehole-out (LOBO) and separately designed five-strip spatial holdouts evaluated their reconstruction of unseen samples. A five-factor Stage 2 model characterized the full-profile receptor domain. All eight metal concentrations decreased in site-wide depth trends, whereas held-out reconstruction error showed no detected systematic increase with depth (LOBO slope of log10(Qi), 0.012 m−1; 95% CI, −0.022 to 0.049). Cr-, Cu–Ni–Zn-, and Hg-associated factor families remained recognizable across stages. The shallow As–Cd–Pb-associated structure had a composite correspondence (cosine similarity 0.970) with two full-profile factors enriched in Cd and As–Pb; this statistical resolution was not independently corroborated as element-specific transport or physical-source separation. At approximately 8 m, factor median contributions ranged from 0.507 to 0.863 of their shallow medians. Local deep Cr maxima at S35, S42, and S43 coexisted with relatively small fixed-basis Cr residuals. These findings separate fingerprint reconstruction fidelity, contribution persistence, and localized enrichment, supporting a conditional—not universal—use of shallow factor bases when prioritizing deep sampling and independent source verification.