Trace metal enrichment and terminal contaminant accumulation in water-based drilling wastes: a depth-resolved...

Oil and gas drilling operations generate large quantities of water-based drilling waste that can act as reservoirs for trace metals of environmental concern, yet enrichment across waste fractions remains insufficiently characterized. This depth-resolved geochemical case study examines trace metal distributions in drilling mud, drill cuttings, and terminal spent mud from a single water-based drilling operation (0-2000 m). Concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS) following open acid digestion with a 1:3 (v/v) HNO₃-HCl mixture. Contamination behavior was evaluated using the contamination factor (CF), Fe-normalized enrichment factor (EF), geoaccumulation index (Igeo), modified contamination degree (mCd), the Nemerow pollution index, and non-parametric statistics. Pb showed the most pronounced enrichment, increasing from 435 to 623 mg kg^-1 in shallow waste to 2530 mg kg^-1 at 2000 m and 32,677 mg kg^-1 in the terminal spent mud, and correlated positively with depth (ρ = 0.85; ρ = 0.80 after removing the terminal sample). Theil-Sen regression indicated a 12.5-fold increase in Pb per 1000 m. CF and EF values were highest for Pb. Exploratory principal component analysis indicated co-variation among Ni, Co, Cr, and V on PC1 and a Pb-dominated component with As and Cd on PC2. Terminal spent mud may represent a priority fraction for monitoring, classification, and risk assessment. The depth-related pattern is interpreted as a single-well case observation that may reflect both lithogenic input and operational redistribution.