<![CDATA[Large-scale copper-gold mining in Mimika generates tailings that interact with the Ajkwa River system and may influence aquatic geochemistry. This study integrated paired water-sediment sampling, laboratory metal(loid) analysis, censor-aware interpretation of non-detect water data, and spatial modeling to characterize environmental patterns along the Ajkwa River corridor. Twenty paired surface-water and sediment samples were collected at identical coordinates. In-situ pH and dissolved oxygen (DO) were measured, while Cu, As, and Hg in water were analyzed by atomic absorption spectrometry (AAS), and sediment Cu and As by X-ray fluorescence (XRF). Water pH ranged from 7.49 to 7.78, and DO from 5.41 to 6.59 mg L−1. All water-phase Cu, As, and Hg concentrations were below laboratory reporting limits, which did not exceed the relevant Class III/IV river-water criteria of Indonesian Government Regulation No. 22/2021. However, non-detect results should not be interpreted as zero concentrations. Sediment Cu ranged from 805.74 to 1,644.79 mg kg−1, while As ranged from 104.62 to 471.55 mg kg−1, with peaks at S07 and S09, respectively. Leave-one-out cross-validation of inverse distance weighting produced R² values of 0.391 for Cu and 0.965 for As, indicating greater spatial stability for As. All sediment Cu and As concentrations exceeded consensus probable-effect concentrations, although these benchmarks are non-regulatory. Overall, the findings reveal a strong contrast between low or non-quantifiable water-phase concentrations and pronounced sediment geochemical signals, supporting sediment-focused monitoring and validated spatial assessment in the Ajkwa River Basin.]]>
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