SPATIOTEMPORAL TRENDS IN WATER QUALITY ALONG THE TASHKENT URBAN REACH OF THE CHIRCHIK RIVER: A PAIRED MONITORING ANALYSIS FOR 2017–2024
DOI:
https://doi.org/10.5281/zenodo.23359486Abstract
This study evaluates spatial and temporal changes in water quality along the Tashkent urban reach of the Chirchik River based on paired upstream and downstream monitoring observations conducted during 2017–2024. Annual mean values derived from quarterly observations were analyzed for total suspended solids (TSS), chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), ammonium nitrogen (NH₄–N), nitrite nitrogen (NO₂–N), nitrate nitrogen (NO₃–N), and phosphate. Spatial enrichment was assessed from the difference between downstream and upstream mean values, while temporal trends were evaluated using the Mann–Kendall statistic and Sen’s slope. The strongest spatial signals were observed for COD and phosphate: mean downstream COD was approximately 59% higher than upstream, while phosphate concentrations were about 5.2 times higher. Downstream NO₂–N showed a significant increasing trend (Sen’s slope ≈ +0.0109 mg/L/year; Kendall τ ≈ 0.79; p ≈ 0.006), and phosphate also increased significantly (≈ +0.0149 mg/L/year; τ ≈ 0.69; p ≈ 0.018). COD exhibited a positive downstream slope of about +0.75 mg/L/year, although it did not reach statistical significance over the eightyear period. TSS showed an episodic pattern: a significant increase was observed upstream, whereas the 2022 downstream maximum reflected a short-term sediment event rather than a sustained monotonic trend. The results indicate that organic matter and phosphorus provide the clearest signals of enrichment within the urban reach, while nitrogen species exhibit contrasting transformation pathways. Therefore, paired-site trend analysis provides more diagnostic information than evaluating individual concentrations or isolated exceedances in particular years.Keywords
Chirchik River, water quality, urban river, COD, nutrients, phosphate, nitrite, Mann–Kendall, Sen’s slopeReferences
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