Abstract
This study presents long-term background concentration estimates for hydrochemical parameters in the Shu River at the Kaynar transboundary section for 2002–2025 and assesses chemical mass transport across this section. Annual mean values for 31 hydrochemical parameters were analyzed; 533 annual values were processed, and 498 remained after statistical extremes were excluded from background-concentration calculations. Background concentrations were estimated as one-sided confidence limits of the mean at a 0.95 confidence level. The Shu River water is characterized by a bicarbonate-sulfate composition and moderately elevated mineralization. Background-level exceedances of the reference maximum permissible concentration (MPC) values were observed for sulfate, nitrite nitrogen (NO2-N), and copper. Suspended solids were assessed separately because the applicable criterion is expressed relative to the background concentration rather than as a fixed MPC value. The comprehensive water pollution index (CWPI) indicated moderate pollution in most years, with high pollution levels in 2002, 2003, 2006–2008, and 2016. Annual mass transport of ionic constituents increased from 555.2 thousand metric tons in 2021 to 899.6 thousand metric tons in 2023, while nutrient mass transport ranged from 3.14 to 3.55 thousand metric tons per year. Nitrate nitrogen (NO3-N) made the largest contribution to nutrient mass transport. The results support regional water-quality assessment and water-protection planning in arid transboundary river basins.