The Impact of Water Hardness on Calcite Deposition and Geomorphological Changes in the Shatt al-Hillah Channel: A Comparative Study between Autumn 2024 and Spring 2025

Abstract

This study aims to evaluate the effect of chemical hardness—represented by the concentration of calcium (⁺Ca²) and magnesium (⁺Mg²) ions—on the deposition of calcium carbonate (Calcite) and the formation of armored layers in the Shatt al-Hillah River during the fall of 2024 and spring of 2025. Water samples were collected from five stations along the river and subjected to physical and chemical analyses, including pH, EC, TDS, total hardness, Ca hardness, Mg hardness, alkalinity, phosphate, sulfate, chloride, nitrate, and nitrite.

Bottom shear stress (τ_b) and friction velocity (u_*) were calculated using standard hydraulic equations based on depth, slope, and flow velocity measurements. The results showed that τ_b (24.5 - 55.9 Pa) and u_* (0.156 - 0.236 m/s) remained below the critical threshold for destabilizing calcite-covered sediments, indicating relative stability of the riverbed.

The study also showed an increase in calcite precipitation rate from 98 mg/L in autumn to 162.6 mg/L in spring, a relative increase of 66%, resulting in the formation of flow-resistant, laminated calcite layers. Laboratory analysis was supported by geochemical modeling (PHREEQC) and statistical analysis (SPSS).

These results highlight the importance of monitoring chemical hardness and nutrients to ensure riverbed sustainability, and the quality of water used for irrigation.