Salinity Implications of Wastewater Irrigation in the Musi River Catchment in India

As a consequence of increasing urbanization and shortage of good quality water, wastewater irrigation is a growing phenomenon in many arid and semi-arid countries. A common characteristic of wastewater is high salinity, with cities typically adding 200 – 500 mg l of total dissolved solids compared to the source water supplied to the city. Wastewater from the city of Hyderabad in southern India is discharged to the Musi river. Downstream of the city this water, supplemented with groundwater and runoff captured in small reservoirs, is an important source for irrigation. Comparisons between upstream and downstream monitoring sites, over a distance of 39.7 km, revealed changes in the salinity of the river water. A simple mass-balance model was developed to simulate the observed differences. Results indicate that 94% of the salt load originates in the city. Downstream salinity increased by about 9%. In fields irrigated with wastewater, soil salinity increased with time with salt retention of approximately 34 kg ha y. This represents approximately 0.1% of the total salt load applied to the land. In many places the soils have salinity in excess of recommended tolerance levels for rice, once the principal crop, but which is now increasingly being converted to fodder grass.