B-1667 4-03 Irrigation Water Quality Standards and Salinity Management Strategies Irrigation Water Quality Standards and Salinity Management Guy Fipps* Nearly all waters contain dis - Generally, “salt” is thought of as Water Analysis: solved salts and trace elements, ordinary table salt (sodium chlo - many of which result from the ride). How-ever, many types of Units, Terms and natural weathering of the earth’s salts exist and are commonly Sampling surface. In addition, drainage found in Texas waters (Table 1). waters from irrigated lands and Most salinity problems in agricul - Numerous parameters are used to effluent from city sewage and ture result directly from the salts define irrigation water quality, to industrial waste water can impact carried in the irrigation water. assess salinity hazards, and to water quality. In most irrigation The process at work is illustrated determine appropriate management situations, the primary water qual - in Figure 1, which shows a strategies. A complete water quali - ity concern is salinity levels, since beaker of water containing a salt ty analysis will include the deter - salts can affect both the soil struc - concentration of 1 percent. As mination of: ture and crop yield. However, a water evaporates, the dissolved number of trace elements are salts remain, resulting in a solu - 1) the total concentration of solu - found in water which can limit its tion with a higher concentration ble salts, use for irrigation. of salt. The same process occurs 2) the relative proportion of sodi - in soils. Salts as well as other dis - um to the other cations, solved substances begin to accu - 3) the bicarbonate concentration as *Associate Professor and Extension mulate as water evaporates from Agricultural Engineer, Department of related to the concentration of Agricultural Engineering, The Texas A&M the surface and as crops withdraw calcium and magnesium, and System, College Station, Texas 77843-2117. water. Table 1. Kinds of salts normally found in irrigation waters, with chemical symbols and approxi- mate proportions of each salt. 1 (Longenecker and Lyerly, 1994) Chemical name Chemical symbol Approximate proportion of total salt content Sodium chloride NaCl Moderate to large Sodium sulfate Na 2SO 4 Moderate to large Calcium chloride CaCl 2 Moderate Calcium sulfate (gypsum) CaSO 4 2H 2O Moderate to small Magnesium chloride MgCl 2 Moderate Magnesium sulfate MgS0 4 Moderate to small Potassium chloride KCl Small Potassium sulfate K2SO 4 Small Sodium bicarbonate NaHCO 3 Small Calcium carbonate CaCO 3 Very Small Sodium carbonate Na 2CO 3 Trace to none Borates BO -3 Trace to none Nitrates NO -3 Small to none 1Waters vary greatly in amounts and kinds of dissolved salts. This water typifies many used for irrigation in Texas. 3 Figure 1. Effect of water evaporation on the concentration of salts in solution. A liter is 1.057 quarts. Ten grams is .035 ounces or about 1 teaspoonful. 4) the concentrations of specific Two Types of Salt the TDS (total dissolved solids) or elements and compounds. Problems the EC (electric conductivity). The amounts and combinations of TDS is sometimes referred to as these substances define the suit - the total salinity and is measured ability of water for irrigation and Two types of salt problems exist or expressed in parts per million the potential for plant toxicity. which are very different: those (ppm) or in the equivalent units of Table 2 defines common parame - associated with the total salinity milligrams per liter (mg/L). and those associated with sodium. ters for analyzing the suitability of EC is actually a measurement of Soils may be affected only by water for irrigation and provides electric current and is reported in salinity or by a combination of some useful conversions. one of three possible units as both salinity and sodium. When taking water samples for given in Table 2. Subscripts are laboratory analysis, keep in mind Salinity Hazard used with the symbol EC to iden - that water from the same source tify the source of the sample. can vary in quality with time. Water with high salinity is toxic EC iw is the electric conductivity Therefore, samples should be test - to plants and poses a salinity haz - of the irrigation water. EC e is the ed at intervals throughout the ard . Soils with high levels of electric conductivity of the soil as year, particularly during the total salinity are call saline soils . measured in a soil sample (satu - potential irrigation period. The High concentrations of salt in the rated extract) taken from the root Soil and Water Testing Lab at soil can result in a “physiologi - zone. EC d is the soil salinity of Texas A&M University can do a cal” drought condition. That is, the saturated extract taken from complete salinity analysis of irri - even though the field appears to below the root zone. EC d is used gation water and soil samples, and have plenty of moisture, the to determine the salinity of the will provide a detailed computer plants wilt because the roots are drainage water which leaches printout on the interpretation of unable to absorb the water. Water below the root zone. the results. Contact your county salinity is usually measured by Extension agent for forms and information or contact the Lab at Types of Salinity Problems (979) 845-4816. affects can lead to salinity plants saline soil hazard condition affects can lead to sodium soils sodic soil condition 4 Table 2. Terms, units, and useful conversions for understanding ingly impervious to water pene - water quality analysis reports. tration. Fine textured soils, espe - cially those high in clay, are most Symbol Meaning Units subject to this action. Certain Total Salinity amendments may be required to a. EC electric conductivity mmhos/cm maintain soils under high SARs. µmhos/cm Calcium and magnesium, if pre - dS/m sent in the soil in large enough b. TDS total dissolved solids mg/L quantities, will counter the effects ppm of the sodium and help maintain Sodium Hazard good soil properties. a. SAR sodium adsorption ratio — Soluble sodium per cent (SSP) is b. ESP exchangeable sodium percentage — also used to evaluate sodium haz - ard. SSP is defined as the ration Determination Symbol Unit of measure Atomic weight of sodium in epm (equivalents per Constituents million) to the total cation epm (1) cations 3 multiplied by 100. A water with a calcium Ca mol/m 40.1 SSP greater than 60 per cent may magnesium Mg mol/m 3 24.3 3 result in sodium accumulations sodium Na mol/m 23.0 that will cause a breakdown in the potassium K mol/m 3 39.1 soil’s physical properties. (2) anions bicarbonate HCO mol/m 3 61.0 3 3 Ions, Trace Elements sulphate SO 4 mol/m 96.1 chloride Cl mol/m 3 35.5 and Other Problems 3 carbonate CO 3 mol/m 60.0 nitrate NO mg/L 62.0 A number of other substances 3 may be found in irrigation water Trace Elements and can cause toxic reactions in boron B mg/L 10.8 plants (Table 3). After sodium, Conversions chloride and boron are of most 1 dS/m = 1 mmhos/cm = 1000 µmhos/cm concern. In certain areas of Texas, 1 mg/L = 1 ppm boron concentrations are exces - TDS (mg/L) ≈ EC (dS/m) x 640 for EC < 5 dS/m sively high and render water TDS (mg/L ≈ EC (dS/m) x 800 for EC > 5 dS/m unsuitable for irrigations. Boron TDS (lbs/ac-ft) ≈ TDS (mg/L) x 2.72 can also accumulate in the soil. Concentration (ppm) = Concentration (mol/m3) times the atomic weight Crops grown on soils having an Sum of cations/anions imbalance of calcium and magne - (meq/L) ≈ EC (dS/m) x 10 sium may also exhibit toxic Key symptoms. Sulfate salts affect mg/L = milligrams per liter sensitive crops by limiting the ppm = parts per million uptake of calcium and increasing dS/m = deci Siemens per meter at 25° C the adsorption of sodium and potassium, resulting in a distur - Sodium Hazard effects of sodium. For waters con - bance in the cationic balance taining significant amounts of within the plant. The bicarbonate Irrigation water containing large bicarbonate, the adjusted sodium ion in soil solution harms the amounts of sodium is of special adsorption ratio (SAR adj ) is some - mineral nutrition of the plant concern due to sodium’s effects times used. through its effects on the uptake on the soil and poses a sodium Continued use of water having a and metabolism of nutrients. High hazard . Sodium hazard is usually concentrations of potassium may expressed in terms of SAR or the high SAR leads to a breakdown in the physical structure of the introduce a magnesium deficiency sodium adsorption ratio . SAR is and iron chlorosis. An imbalance calculated from the ratio of sodi - soil. Sodium is adsorbed and becomes attached to soil particles. of magnesium and potassium may um to calcium and magnesium. be toxic, but the effects of both The latter two ions are important The soil then becomes hard and compact when dry and increas - can be reduced by high calcium since they tend to counter the levels. 5 Table 3. Recommended limits for constituents in reclaimed water for irrigation. (Adapted from Rowe and Abdel-Magid, 1995) Constituent Long-term Short-term Remarks use (mg/L) use (mg/L) Aluminum (Al) 5.0 20 Can cause nonproductivity in acid soils, but soils at pH 5.5 to 8.0 will precipitate the ion and eliminate toxicity. Arsenic (As) 0.10 2.0 Toxicity to plants varies widely, ranging from 12 mg/L for Sudan grass to less than 0.05 mg/L for rice.