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Effect of Waste Water Irrigation and Leaching Percentage on Soil and Ground Water Chemistry¹

ABSTRACT

Irrigation with sewage waste water for six seasons produced greater salinity under alfalfa (Medicago sativa L.) than under reed canary grass (Phalaris arundinacea L.) or fallow. Irrigation rates were designed to produce high and low leaching percentages (LP). The high irrigation rate (I₂) was double the low irrigation rate (I₁). Leaching percentages were estimated from the soluble Na concentration at the 90- to 150-cm depth of the soil profile. At the 90- to 150-cm depth of the soil profile, Na reached a steady state for the conditions of the irrigation season by the fall of each year in the first 2 crop years and by the end of the second irrigation season in the last 4 crop years. At the water table depth (2–4 m), Cl in the ground water was close to an annual steady-state concentration by the end of each irrigation season in the first 2 crop years and by the end of the second irrigation season in the last 4 crop years. Sodium and Ca + Mg approached equilibrium during the final 3 crop years.

High LPs (>34%) in the first 2 years reduced original salinity levels [electrical conductivity (EC) 3.99 to <1.39 mmhos/cm]. When irrigation rates were reduced in the last 4 years, salinity increased, but the salt levels were well below those (EC <2.13 mmhos/cm) that would significantly reduce crop yield.

Ground water calculations at low LP (<10%) showed that EC and sodium adsorption ratio (SAR) values were high, but CaCO₃ precipitation was balanced by gypsum dissolution. By comparison, at high LPs (>15%) the EC and SAR values were low, but there was a potential danger of a large net transfer of salt to ground water because of gypsum dissolution.

Key Words: salt • gypsum • calcium carbonate • alfalfa • reed canary grass

¹ Contribution of the Research Station, Agriculture Canada, Lethbridge, Alberta, Canada T1J 4B1.

² Research Scientists.

Received for publication August 1, 1980.