Assessment of cyanide contamination in artisanal gold mining: a case study of Busia gold fields, eastern uganda

Abstract

Trench wastewater had the highest free cyanide, reaching 15.50 mg/L.•Metallic pipes drainage exhibited extreme acidity and elevated electrical conductivity.•Sediments accumulated strong acid dissociable cyanide up to 84.26 mg/kg.•Dry and wet seasons analysis showed cyanide levels above regulatory thresholds.•Geochemical controls on cyanide speciation differed significantly between sediments and tailings. Artisanal and small-scale gold mining (ASGM) in Busia, Uganda, relies on cyanide for gold extraction. However, data on its environmental fate remain scarce, with most research focused on mercury. This study presents a systematic assessment of cyanide speciation across multiple environmental matrices at the Busia Gold Fields site. These included seepage water, surface water, waste water through pipes and trenches, sediments, and tailings, which were sampled comprehensively during the dry and wet seasons. Water and solid samples (n = 140) were analyzed for their physicochemical properties. The concentration of free cyanide (CNFree), Weakly Acid Dissociable Cyanide (CNWAD), Strongly Acid Dissociable Cyanide (CNSAD), and Total Cyanide (CNT) was determined using APHA-standard spectrophotometric techniques. Trench wastewater recorded the highest CNFree (15.50 mg/L). Metallic pipe drainage exhibited the most extreme conditions (pH 3.07–3.78; EC up to 11,800 µS/cm). Seepage and surface water yielded CNT of 0.37–0.88 mg/L and 0.16–0.42 mg/L, respectively. Stream sediments accumulated CNSAD up to 84.26 mg/kg, and sand-active tailings CNT up to 79.09 mg/kg. Significant dry-to-wet seasonal reductions in CNFree (34.8–49.5%, p < 0.001) were observed across all compartments but failed to achieve compliance with any applicable threshold. The majority of samples exceeded multiple regulatory thresholds. All 60 water samples surpassed the Uganda NEMA effluent standard and the WHO/FAO irrigation guideline. Out of 60, 55 exceeded the US EPA criterion, and 43 out of 60 exceeded the WHO short-term guideline. Wet-season seepage and surface water predominantly accounted for non-exceedances. Spearman correlation, two-way ANOVA, and principal component analysis confirmed that physicochemical gradients, particularly Eh, pH, and EC, governed cyanide speciation and mobility. Contrasting pH–CNFree relationships between sediments (ρ = −0.954) and tailings (ρ = 0.701) captured the mechanistic duality between Acid Mine Drainage (AMD)-driven accumulation and alkaline cyanidation stabilization. These findings underscore the urgent need for source-specific remediation and integration of multi-fraction cyanide monitoring into Uganda's environmental regulatory framework.