Assessment of Small-Settlement Wastewater Discharges on the Irtysh River Using Tracer-Based Mixing Diagnostics and Regularized Predictive Models

An integrated field–analytical framework was applied to quantify the impact of two small-settlement treatment facilities (TF1 and TF2) on the Irtysh River (East Kazakhstan). The main objective of this study is to quantify effluent-driven dilution and non-conservative changes in key water-quality indicators downstream of TF1 and TF2 and to evaluate parsimonious models for predicting effluent-outlet BOD and COD from upstream measurements. Paired upstream–downstream control sections are sampled in 2024–2025 for 22 indicators, and plant influent–effluent records are compiled for key wastewater variables. Chloride-based conservative mixing indicated very strong dilution (approximately D≈2.0×103 for TF1 and D≈4.2×102 for TF2). Deviations from the mixing line were summarized using a transformation diagnostic θ. At TF1, several constituents exceeded mixing expectations (θ≈13 for COD, θ≈42 for ammonium, and θ≈6 for phosphates), while nitrate shows net attenuation θ