The salinity and cation composition of water and soil were documented in a large (98 km(2)) wastewater-irrigated area (WIA) downstream of Hyderabad, India. The wastewater, which flows in a river that passes through the city, had a high to very high salinity hazard (EC = 1.1-3.0 dS m(-1)) that increased with distance from the city. The EC of soil irrigated by wastewater sampled within 8 km of the city was 6.2 to 8.4 times the EC of soil irrigated by uncontaminated groundwater. Between 57 to 100% of soil samples in the upper 10 cm within 8 km of the city exceeded the salinity tolerance of rice (Oryza sativa L.). Soil salinity fell rapidly after 8 km downstream and changed most in the upper 0 to 5 cm of the soil, indicating retention of cations in the upper soil horizon. The effect of wastewater irrigation on soil exchangeable cations was most evident for Na(+) (Exch-Na) near the city (<8 km downstream), where Exch-Na averaged 20 to 22 times the Exch-Na in soils irrigated by groundwater outside the WIA. Exchangeable Mg(+) and K(+) correlated with clay percentage, though both still had higher concentrations near the city controlling for clay content. Near the city, where salinity and Exch-Na concentrations were highest, farmers had replaced rice with para grass [Brachiaria mutica (Forsk.)], which has higher salinity tolerance and expanding demand as a fodder crop. Salinity may constrain rice production in wastewater-irrigated areas of India and elsewhere.