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TECHNICAL REPORTS

Ecological Risk Assessment

Soil Salinity and Exchangeable Cations in a Wastewater Irrigated Area, India

^a Geography Dep., San Diego State Univ., 5500 Campanile Drive, San Diego, CA 92182
^b Program on Energy and Sustainable Development, 616 Serra St., Encina Hall East, 4th floor, Stanford Univ., Stanford, CA 94305-6055

^* Corresponding author (tbiggs{at}mail.sdsu.edu).

Received for publication May 29, 2008. The salinity and cation composition of water and soil were documented in a large (98 km²) 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.

Abbreviations: EC, electrical conductivity • EC_e, electrical conductivity, saturation extract • EC_1:x, electrical conductivity, extract from a 1:x dilution • Exch-Na, exchangeable sodium • Exch-Mg, exchangeable magnesium • Exch-K, exchangeable potassium • SAR, sodium adsorption ratio • WIA, wastewater-irrigated area