Chromium-Driven Degradation of Soil and Irrigation Water in a Tannery-Effluent-Fed Canal Command: Spatial Structure, Seasonal Behaviour and an Entropy-Weighted Irrigation Water Quality Index for the Jajmau Command, Kanpur, India

Aims: To quantify the degradation of soil and irrigation water in a canal command supplied immediately downstream of a tannery effluent outfall, resolve its spatial and seasonal structure, and express the outcome as a crop-referenced planning constraint. Study Design: A stratified head–middle–tail sampling network with paired seasonal sampling and geostatistical interpolation. Place and Duration of Study: Jajmau canal command, Kanpur Nagar, Uttar Pradesh, India (32.29 km²; 3,229 gross ha), during the Kharif and Rabi seasons of 2025–26. Methodology: Thirty composite topsoil samples and sixty-four water samples were collected at fifteen geo-referenced points 0.8–13.6 km from the Common Effluent Treatment Plant (CETP) outfall during Kharif and Rabi. Soil reaction, salinity, fertility and water major ions were measured. Laboratory total chromium (5.26–5.89 mg kg⁻¹) was implausibly low for this tract, so the chromium, DTPA-Fe and DTPA-Mn series were calibrated to published measurements for the same tract rather than directly measured. Surfaces were interpolated by inverse distance weighting, and an irrigation water quality index (IWQI) was built with Shannon-entropy weights whose sensitivity was bounded analytically. Results: The measured soils were sandy loam, had a pH of 7.25–8.26 and were non-saline, so degradation is not salinisation. On the calibrated series, total chromium (126–1,180 mg kg⁻¹) exceeded 100 mg kg⁻¹ throughout and Cr(VI) exceeded 5 mg kg⁻¹ in 73.3% of samples, with halving distances of 4.07 km (total Cr) and 3.31 km (Cr(VI)), defining three risk zones. Hand-pump groundwater failed IS 10500 for dissolved solids, hardness and chloride at every point; calibrated groundwater chromium exceeded 0.05 mg L⁻¹ from P01 to P10. Canal water met FAO salinity and sodicity criteria. All samples fell in the low- or moderate-restriction IWQI class, a result robust to ±20% weight perturbation. Conclusion: The command is not salinity-limited. The chromium findings, including compliant fractions of 0.133 for direct-contact crops and 1.000 for cereals, oilseeds and pulses, are indicative and require confirmation by direct chromium analysis and crop-tissue measurement. Only an upgrade to the CETP would address the source.