Sodium is not limited by primary drinking-water standards, and the sodium content of typical municipal wastewaters is not a significant water-quality concern. A sudden change to high sodium content will adversely affect the biota in an aquatic system, but most systems can acclimate to gradual changes. Sodium and also calcium influence soil alkalinity and salinity, which in turn can affect the vegetation in land treatment systems. The growth of the plant and its ability to absorb moisture from the soil are influenced by salinity. The structure of clay soils can be damaged when there is an excess of sodium with respect to calcium and magnesium in the wastewater. The resulting swelling of some clay particles changes the hydraulic capacity of the soil profile. The sodium adsorption ratio (SAR) as shown by Equation 3.31 defines the relationship among these three elements:
Natural Wastewater Treatment Systems [Na]
Sodium adsorption ratio.
Sodium concentration (mEq/L) = (mg/L in wastewater)/22.99. Calcium concentration (mEq/L) = (mg/L in wastewater)(2)/40.08. Magnesium concentration (mEq/L) = (mg/L in wastewater)(2)/24.32.
The SAR for typical municipal effluents seldom exceeds a value of 5 to 8, so it should not be a problem with most soils in any climate. Soils with up to 15% clay can tolerate a SAR of 10 or less, while soils with little clay or with nonswelling clays can accept SARs up to about 20. Industrial wastewaters can have a high SAR, and periodic soil treatment with gypsum or some other inexpensive source of calcium may be necessary to reduce clay swelling. Soil salinity is managed by adding an excess of water above that required for crop growth to leach the salts from the soil profile. A "rule of thumb" for total water required to prevent salt buildup in arid climates is to apply the crop needs plus about 10% (Pettygrove and Asano, 1985). A report by the USEPA (1984) provides further details.
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