REUSE OF POWER PLANT COOLING WATER FOR IRRIGATION
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VOL. 16, NO. 5 WATER RESOURCES BULLETIN AMERICAN WATER RESOURCES ASSOCIATION OCTOBER 1980 REUSE OF POWER PLANT COOLING WATER FOR IRRIGATION' W. A. Jury, H. J. Vmx, Jr. , and L. H. Stolzyz ABSTRACT: Current water quality policies in California require dis- posal of saline blowdown waters from power plants in sealed evapora- tion ponds to avoid degradation of ground waters. This policy high- lights the conflict between increased energy demands, increasing scarcity of water, and environmental priorities. Saline blowdown waters can be used for the irrigation of salt tolerant crops, albeit with some reduction in yields. The results of experiments intended to specify these yield reductions are reported. If such irrigation is carefully managed, the soil profile can be used to store residual salts and ground water degradation will be avoided, provided that irrigation ceases before the salts are leached to the ground water. An analysis of discharge below a carefully managed irrigation project shows that the downward movement of salts below the root zone is no worse than with conventional methods of dis- posal. Thus, irrigation reuse with blowdown water is shown to be a viable means of saline water disposal while maintaining existing stand- ards of ground water quality protection. Further analysis demonstrates the economic feasibility of such irrigation reuse by showing that it is significantly less costly than the evaporation pond alternative. (KEY TERMS: blowdown water; saline irrigation water; evaporation pond.) INTRODUCTION Electric power demand in the United States has been con- tinually rising during the last century, and though conserva- tion measures have slowed the rate of increase, it shows no sign of leveling off (ERCDC-CAL, 1977). To meet anticipated future power demands without sharp increases in price, new electric generating facilities will have to be constructed. How- ever, the construction of new plants is currently being ham- pered by a combination of resource shortages and environ- mental restrictions, which have caused the growth of energy supply to be slower than the growth of energy demand. Nu- clear power, once thought capable of carrying the burden of increasing energy demands, is now being discouraged because of fears of reactor accidents and waste disposal problems. Oil and natural gas will become increasingly expensive as supplies grow scarcer. Coal presents environmental hazards through land damage and air pollution, which have caused delays in coal fired power plant construction. Along with the difficulties caused by shortages or environ- mental drawbacks associated with the alternative primary fuels (i.e., oil, natural gas, coal, nuclear) water supplies for cooling, particularly in the arid western states, have become an addi- tional barrier to the construction of new power plants. Until recently, the majority of power plants in California were built along the coastline and used sea water for cooling. However, when studies suggested that the thermal discharge from the cooling system was endangering certain marine life near the power plants, the California Water Resources Control Board, already concerned about air pollution discharges and earth- quake vulnerability, ruled that future plants would have to be constructed at inland sites and use a closed system for cooling (CSWRCB, 1975b). Since a large lGWe (lo9 watt output) fossil fueled plant requires about 6.1 x lo4 m3/day (18,000 acre ft/yr) of water for closed system cooling (DWR-CA, 1977) even a single plant could tax the fresh water resources of a water scarce desert region. Furthermore, a closed cooling sys- tem, such as a wet cooling tower, requires the continuous re- moval of a portion of the condenser flow to prevent excessive salt buildup in the cooling system. This residual water, called blowdown, is typically around 20 percent or 1.4 x lo4 m3/ day (3600 acre ft/yr) for a lGWe power plant, and is very high in dissolved salts (DWR-CA, 1977). The California State Water Resources Control Board has set forth a policy stipu- lating that this water must be disposed of in lined evaporation ponds to prevent its return to the ground water (CSWRCB, 1975a). The dilemmas posed by conflicts between energy demands, environmental priorities, and resource shortages are real and all too frequent. Often, efforts to resolve such conflicts focus almost exclusively on the relative weights or emphasis to be given to increasing energy supplies, protecting the environ- ment, and alleviating resource shortages. The result is that once the relative weights are determined, little importance is attached to achieving that mix of goals in a fashion that mini- mizes costs. In this paper, we explore a case in point by analyzing the environmental and economic impacts associated with the use of power plant cooling tower blowdown water for irrigating crops. The analysis indicates that utilization of blowdown water for crop irrigation makes productive use of 'Paper No. 80022 of the Water Resources Bulletin. Discussions are open until June 1, 1981. 'Respectively, Associate Professor of Soil Physics, Associate Professor of Resource Economics, and Professor of Soil Physics, Department of Soil and Environmental Sciences, University of California, Riverside, California 92521. 8 30 Jury, Vaux, and Stolzy water that would not otherwise be used while maintaining stipulated environmental priorities at costs which are signifi- cantly lower than those associated with the more conventional practice of disposing of blowdown water in evaporation ponds. COMPOSITION OF BLOWDOWN WATER A closed wet tower cooling system consists of a continuous cycle of cooling water that picks up waste heat at the con- denser and then removes this heat by convection and evapora- tion at the tower as the cooling water cascades down the cool- ing tower fins (Figure 1). A portion of the water (B), called blowdown, is removed during each cycle to avoid excessive salt buildup. Makeup water (M) is added at a rate sufficient to replace the evaporation and drift losses (E) as well as the blowdown (B). The ratio of makeup to blowdown water de- fines the number of cycles ( C ) of the system, and determines the concentration of the blowdown. When the makeup water is of good quality five or six cycles may be achieved before the water causes significant scaling or corrosion. The only sub- stantial alteration of the water other than by concentration arises from the addition of sulfuric acid to lower scaling poten- tial. Table 1 summarizes the principal ions present in blowdown samples taken from Southern California Edisori plants at Bar- stow, California, and Etiwanda, California, as well as the com- position of several synthesized treatments we have been using in our experiments (Jury, et al., 1978~) . The concentration varies with power output during the year, but all samples are characterized by high concentration of SO:- from the sulfuric acid treatment. Each of these samples is near saturation with gypsum (CaS04.2H20) and lime (CaC03). USE OF BLOWDOWN FOR IRRIGATION The salinity level of the blowdown water rules out its use for many, but by no means all, crops. Table 2, adapted from Ayers and Westcot (1976) summarizes the allowable Electrical Conductivity of the saturation extract (EC,) and of the irriga- tion water (ECi) for a number of salt tolerant crops under 0, 10, 25, and 50 percent yield reduction. These figures suggest that blowdown waters with salt concentrations similar to those reported in Table 1 could be concentrated several times within a root zone without seriously reducing yields of many crops. STACK LOSSES 265MW EVAPORATION IOOOMW FUEL 2650MH - LOSSES I l8MW Figure 1. Water and Energy Balances for a 1 GWe Power Plant With Closed Wet Tower Cooling. Over the last three years this concept has been tested on 28 field lysimeters (1.22 m diameter) alternately growing wheat and sorghum while receiving synthesized blowdown water of 2.1, 4.2, or 7.1 mmho/cm EC (Jury, et al., 1978). Table 3 summarizes the grain yields of the three irrigation treatments normalized to a per hectare basis. This experiment was carried out with almost no drainage, so that root zone salinity was very high, and the reduction of yield in later crops was sub- stantial. The leaching fraction LF (drainage volume/irrigation TABLE 1. Composition of Power Plant Cooling Water Samples. Location and Time EC CI SOT2 HC03 Mg+2 Na+ K+ SAR TDS of Sampling (mmhoicm) PH (meq/i) (meq/l) (meq/l) (meq/l) (meq/i) (meq/l) (meq/i) (meq/# (mg/l) Southern California Edison Etiwanda Plant 4/1/77 6.85 7.50 20.07 67.69 0.75 31.12 22.78 38.17 1.22 8.75 5840 1/25/78 4.73 7.50 16.08 50.39 2.49 25.38 15.00 29.19 0.56 6.50 4525 Southern California Edison Barstow-Dagget Plant UC Riverside Synthesized Blowdown Waters 2/2/76 3.70 6.50 10.50 34.50 0.50 20.50 7.00 13.60 - - 3.67 2865 High 7.10 8.10 28.00 67.00 5.00 28.90 21.10 50.00 - - 10.46 6500 7.98 3200 Medium 4.20 8.30 11.40 32.70 4.10 14.50 7.40 26.30 - - LOW 2.10 8.00 6.30 13.30 3.80 8.00 4.30 11.10 - - 4.56 1560 83 1 Reuse of Power Plant Cooling Water for Irrigation volume) ranged from 0.02 to 0.10, which is much smaller than usual field practices. As a result, the average salinity in the root zone was higher than that assumed in the guidelines in Table 2. Furthermore, many of the crops listed in Table 2 are more salt tolerant than wheat or sorghum and have the poten- tial to produce higher relative yields with blowdown water. TABLE 2. Salinity Tolerance of Various Crops. Percent Reduction in Yield Barley 8.0 5.3 10.0 6.7 13.0 8.7 18.0 12.0 28.0 Cotton 7.7 5.1 9.6 6.4 13.0 8.4 17.0 12.0 27.0 Sugarbeet 7.0 4.7 8.7 5.8 11.0 7.5 15.0 10.0 24.0 Wheat 6.0 4.0 7.4 3.9 9.5 6.4 13.0 8.7 20.0 Sorghum 4.0 2.7 5.1 3.4 7.2 4.8 11.0 7.2 18.0 Corn 1.7 1.1 2.5 1.7 3.8 2.5 5.9 3.9 10.0 Date Palm 4.0 2.7 6.8 4.5 10.9 7.2 17.0 12.0 32.0 Broccoli 2.8 1.9 3.9 2.6 5.5 3.7 8.2 5.5 13.5 Tomato 2.5 1.7 3.5 2.3 5.0 3.4 7.6 5.0 12.5 Tall Wheat Grass 7.5 5.0 9.9 6.6 13.3 9.0 19.4 13.0 31.5 BermudaGrass 6.9 4.6 8.5 5.7 10.8 7.2 14.7 9.8 22.5 Alfalfa 2.0 1.3 3.4 2.2 5.4 3.6 8.8 5.9 15.5 *ECe = Elect. cond. of saturation extract (mmholcm). **ECI = Elect. cond. of irrigation water (mmholcm) assuming 15-20 percent leaching fraction. TABLE 3. Crop Grain Yields From Lysimeter Blowdown Experiments (metric tonstha). Crop Date LOW Medium High Wheat 12/15-6176 8.04 8.27 8.64 Sorghum 7176-12176 7.23 6.77 5.24 Wheat 12/76-6177 6.1 1 5.92 4.65 Sorghum 7177-1 1/77 3.83 3.67 2.59 Wheat 12/17-6178 4.93 4.73 3.85 ENVIRONMENTAL IMPACT OF PROJECT Reuse of blowdown water will eventually return some salts to the ground water because of the need for some drainage in an irrigation operation to avoid excessive salt buildup. Gyp- sum and lime will precipitate from solution as the water passes through the root zone because of the high concentration of SO$- and Ca2+ in the blowdown water. Analyses of this water, as Jell as the conclusions of numerous experiments conducted with other waters (Rhoades, et al., 1973, 1974) have shown that reducing the leaching fraction will maximize the /amount of precipitation and thus minimize the salt burden of drainage waters. Table 4 shows the results of precipitation calculations for the high salinity blowdown water. The first column shows steady state drainage concentrations and the second column steady state annual salt loads for different leaching fractions. The salt load of the drainage water de- creases as leaching fraction decreases because of increased pre- cipitation, whereas drainage concentration increases with de- creasing leaching fraction. Furthermore, the fractional preci- pitation is enhanced up to 70 percent over steady state values during the transient phase of irrigation wheq exchange inter- actions are still important (Jury, etal., 1978b). TABLE 4. Predicted High Salinity Blowdown Drainage Concentrations and Salt Fluxes for Different Leaching Fractions (ET = 150 cmlyr). Percent of Applied Salt Percent Precipitated Drainage Salt Flow of Applied During First Leaching Concentration in Drainage Salt Year of Fraction (mgll) (mT/ha/year) Precipitated Irrigation 0.05 95190 15.2 26.8 46.0 0.10 48220 80.3 25.6 43.7 0.20 24760 92.8 22.8 38.3 0.30 16960 109.0 19.8 32.2 0.40 13070 130.7 16.8 26.0 Our three-year lysimeter experiments have demonstrated that salt precipitation on the order of 50 percent of applied salt will occur during the initial stages of irrigation with blow- down water (Jury, et al., 1978~) . This stage may last for many years, particularly if the cation exchange capacity of the soil between the surface and the ground water table is high. Ulti- mately when steady state conditions are present between the surface and the ground water, precipitation will occur at an intensity which may be calculated by assuming chemical equilibrium conditions in the soil water. A crucial point to bear in mind when assessing the environ- mental impact of an irrigation operation is that a root zone may be thought of as an evaporation pond. In fact, all eva- poration ponds have a certain amount of seepage loss (Kays, 1977). Relationships between pond lining costs and seepage are plotted in Figure 2. Shown for comparison are the seepage losses from an irrigated project in a desert (ET = 150 cmlyr) with LF of 0.2. The seepage losses from irrigation are actually significantly smaller than the losses from many commercial ponds, including those lined with compacted clay, a common sealant for blowdown ponds. In fact, the compacted clay- lined pond with an annual seepage of 91 cm would be equiva- lent to an irrigation operation with a leaching fraction of 0.38 in an area with an annual potential evaporation of 150 cm/yr. Using the precipitation estimates from Table 4 we can esti- mate the salt load of various projects using the 5.1 x lo6 m3/ yr blowdown output from a 1GWe power plant. Table 5 shows the salt load and concentration of drainage water from a clay sealed evaporation pond and two irrigation projects with LF = 0.1 and 0.2. These calculations assume that a given annual volume of blowdown must be disposed of, so that land operations with less drainage per unit area require larger land 832 Jury, Vaux, and Stolzy areas for disposal. The relation between blowdown volume B, evaporation E, seepage loss S, and area A required is TABLE 6. Projected Travel Time to Ground Water and Transition Time to Steady State Precipitation Assuming Depth to Ground Water H = 50 m, Water Content = 0.1, ET = 150 m/yr. B E + S A = __ r t .EXPOSED ASPHALT PANEL ACONCRETE A ASPHALT cO?mlETE ACOUPbCTED CLAY DRAINAGE RATE FROU IRRIGATION SYSTEM WITH L.F. -0.2 I I I it , 1 1 1 1 I 10 100 loo0 SEEPAGE RATE cm/yr Figure 2. Capital Costs and Seepage Rates of Evaporation Pond Linings. Although the drainage concentrations of the two irrigation projects are higher than from the evaporation pond, the salt loads carried with the drainage water are 6.4 percent and 10.6 percent less for the 0.2 and 0.1 LF irrigation projects, respec- tively. Furthermore, the travel time of the drainage water to the ground water is greatly increased as the drainage volume is decreased. Table 6 contains calculations of the arrival time, called travel time, of the first salt from the project to under- lying ground water assuming a 50 m depth to the water table and a volumetric water content 6 = 0.1 using the piston flow approximation t = Le/D where D is drainage volume and L is depth to ground water. Also reported is the estimate of the equilibration time for sodium prior to which precipitation will occur at a level characteristic of the transient state (Table 4) causing less salt loading of the ground water. This estimate was calculated by the method of Jury, et al. (1978b). The transient state precipitation influences on salt burden are given in parentheses in Table 5. The transient precipitation results in salt loads that are 14 and 22 percent less than steady state for the 0.2 and 0.1 LF operations, respectively. Travel Time Transition Time Operation 0.r) W Evaporation Pond, LF = 0.38 5.5 10.5 Irrigation, LF = 0.2 13.3 25.0 Irrigation, LF = 0.1 30.0 46.0 In assessing the effect of a salt input on ground water it is important to distinguish between high concentrations and high mass emissions. If a well is located immediately adjacent to an input of high concentration then the water taken up by the well will be more impaired by the high concentration/low mass emission input than by the low concentrationlhigh mass emis- sion input. If the well is located some distance from the input, then the reverse will be true. The velocity of the ground water and the ground water geometry will also have to be taken into account in making this assessment, as they will influence the mixing potential of the saturated zone. In all cases in Table 5 , the concentrations are far in excessof ground water quality standards. However, for the irrigation operations the travel times are sufficiently long (13.3 and 30 yr) that an operation could be undertaken and subsequently relocated prior to the arrival of the salt. In desert areas with potential evaporation far in excess of rainfall, this action would diminish significantly the downward movement of water and salt, thus using the unsaturated zone as a salt storage reservoir. These results show, then, that in areas of low rainfall, blow- down water can be used to irrigate crops if water applications are carefully managed. Such a disposal scheme would have an impact on ground water quality no worse than existing methods of disposal and would make productive use of water that would otherwise be lost to the atmosphere from evapora- tion ponds. COST ANALYSIS The economic feasibility of using blowdown water to grow cash crops depends upon a number of different factors. At the outset, it should be recognized that the purpose of any such irrigation project is to dispose of blowdown water and not to TABLE 5. Comparisons of Salt Emissions From Evaporation Pond and Two Irrigation Operations. Land Drainage Drainage Salt Emission* Area Conc. Volume (metric tons/yr) Percent Precipitation* Operation (ha) (mg/L) (m31yr) x 106 From Operation (precip/input x 100) Evaporation Pond (clay sealed), LF = 0.38 211 14100 1.94 27300 (23,900) 17.5 (28) Irrigation, LF = 0.2 272 25100 1.02 25600 (20,500) 22.8 (38) Irrigation, LF = 0.1 306 47900 0.51 24450 (18,500) 26.2 (44) *Transient emissions given in parentheses. 833 Reuse of Power Plant Cooling Water for Irrigation compete effectively in agricultural markets. Accordingly, the test of economic feasibility of a blowdown irrigation project is not whether it generates significant positive net returns but whether it is less costly than the least cost alternative means of disposing of the blowdown water, given the environmental constraint on ground water quality. For purposes of this analysis, the appropriate least cost alternative involves dispo- sal of the blowdown water in a lined evaporation pond. The assumption is consistent with prevailing policies governing the handling of power plant blowdown water in California. The data presented in Figure 2 suggest that the total cost of evaporation ponds is crucially influenced by the material that is used to line the pond. The nature of the pond lining ma- terial also indirectly influences pond costs inasmuch as the op- timal size of an evaporation pond is inversely related to the quantity of water lost to seepage, when a given amount of water must be disposed of (Equation 1). Thus, the area to be lined and, consequently, the total lining costs are functions of the type of lining used. The total annual costs of evaporation ponds with different types of linings are presented in Table 7. These costs, reported in constant 1967 dollars, are for an evaporation facility re- quired to handle the blowdown water from a lGWe power plant. The calculations include appropriate adjustments for optimal size dictated by the type of lining to be used. The total annual cost figures include both an annual increment of capital cost (computed over 20 years at 8 percent interest) and operation and maintenance costs. With the exception of ponds lined with exposed asphalt panels, the costs of blow- down evaporation ponds increase as the desired seepage rate is decreased. These cost estimates form the base against which the costs of disposing of blowdown water through irrigation reuse must be compared. TABLE 7. Total Annual Costs and Approximate Seepage Rates of Alternative Evaporation Ponds (in 1967 dollars). evaporation pond (i.e., a pond lined with exposed synthetic membrane). The yields of wheat and sorghum are assumed to be depressed proportionately from the average yield obtained by growers in the southern California desert region. These average yields are 5.6 MT/ha/yr for wheat and sorghum, respectively (California Cooperative Extension, 1979). The proportion of yield depression has been computed from the data presented in Table 3. For the wheat crop, the yield is depressed proportionately to the level of the medium salinity treatment of the third crop which is 43 percent. This depresses the average yield to 3.2 MT/ha/yr. For the sorghum, the yield depression is proportionate to the level of the medium salinity treatment for the second crop which is 46 percent. This de- presses the average yield to 3.15 MT/ha/yr. I t is assumed that only the grain is marketed. Table 8 shows the costs and revenues per hectare associated with the production and harvesting of the wheat and sorghum crops with two types of irrigation systems. The table also shows, for purposes of comparison, the costs (in parentheses) and net returns associated with the use of conventional water supplies instead of blowdown water. The estimates of cost are generally reflective of the costs paid by growers in the California desert region but are somewhat conservative in total since land rents paid by power companies would not likely be as high as $11 2/ha. The costs of production using surface sys- tems for irrigation have also been shown for purposes of com- parison. Sorghum and wheat are normally grown using sur- face irrigation in California but surface systems would not be appropriate for applying blowdown water since they do not permit the same flexibility and precision in water management that sprinkler systems do. The gross returns have been calculated conservatively by using the mean per bushel price of sorghum and wheat for the period 1950-1977, expressed in constant 1967 dollars. While the price of sorghum has remained quite stable, the price of wheat has risen rather sharply in recent years and further in- creases are forecast for the near future, Thus, the gross re- Approximate Seepage Rate venues reported here are likely to be lower than those that could be realized in the future. The data in Table 8 suggest that under current cost and long-term revenue conditions, wheat and sorghum could not be grown with blowdown water Exposed Asphalt Panel $1,101,729.61 9 Exposed Synthetic Membrane 802,245.92 1 in quantities sufficient to yield positive net returns. The per Annual Lining 5 P e Cost (cm/year) Compacted Clay Concrete Asphalt Concrete 267,319.48 90 154,944.88 1,000 131,056.97 3 00 Reference: Pound, er al., 1975; Kays, 1977. In order to illustrate the costs and returns from an agricul- tural operation, the following scenario, typical of agricultural practices in the southern California desert region, was analyzed: The blowdown water is used in an irrigation operation to grow one crop of grain sorghum and one crop of winter wheat an- nually. Water application is regulated through the use of sprinkler irrigation systems and the leaching fraction is as- sumed to be 0.2. Such an operation requires 306 hectares of land, roughly 60 hectares more than the most land intensive hectare loss of $67.73 translates into a net cost for the entire irrigation operation of $20,725 annually and they are con- sidered next. There is no a priori reason why the timing of crop demands for water should coincide precisely with the timing of blow- down water availability. Therefore, it appears virtually certain that a waste water irrigation operation would require a storage pond to allow the integration of the power producing and crop producing activities. Indeed, most studies of waste water irri- gation recognize the need for storage facilities if the rate of water application is to be precisely controlled (e.g., Pound, et al., 1975). A storage pond, like an evaporation pond, would undoubtedly have to be lined to meet environmental priorities. At first glance, it may appear that the need for a pond with either operation would make irrigation and crop production 834 Jury, Vaux, and Stolzy virtually superflous. However, this turns out not to be the case. TABLE 8. Sample Costs and Returns From Production of Sorghum and Wheat With Conventional and Blowdown Water Supplies (in 1967 dollars). Cost of Production (per hectare) Sprinkle Surface Irrigation Irrigation provide storage capacity for only a portion of the annual flow of blowdown water from a power plant. In this analysis it was assumed that storage ponds should have the capacity to hold two months flow from a lGWe power plant. The annual costs of an irrigation operation together with the storage pond and associated works are displayed in Table 9. These estimates include an annualized component of capital cost for the storage pond (computed over 20 years at 8 percent interest), the operation and maintenance costs, and the costs of the irrigation operation that were previously discussed. Fertilizer Land Preparation Seed Irrigation Water* Protection Harvesting Transport Land Rent Overhead TOTAL Grain Yield $ 61.33 48.85 20.73 169.07 (73.31) 50.31 47.17 19.64 112.31 49.82 $579.23 ($652.54) $ 61.33 48.85 20.73 137.36 (56.71) 50.31 47.11 19.64 112.31 49.82 $547.52 ($604.23) TABLE 9. Annual Costs of Blowdown Water Disposal Through Crop Irrigation With a Storage Pond and Associated Works (in 1967 dollars). Type of Pond Lining Annual Cost Exposed Asphalt Panels $143,178.03 Exposed Synthetic Membrane 112,180.53 Compacted Clay 74,011.63 Concrete 113,02433 103,237.21 Asphalt Concrete Gross Revenues (oer hectare) Reference: Pound, e t al., 1975; Kays, 1977. ~~ ~ ~~ With Conventional With Blowdown Water Source Water Source $913.01 $511.50 Net Returns (per hectare) With Conventional With Blowdown Water Source Water Source Sprinkle Irrigation Surface Irrigation ~ ~~ $260.47 -$ 67.73 $308.78 -$ 36.02 *The cost of conventional water supplies includes the annualized capital cost (plus interest) on wells and pumps plus the cost of energy needed to lift the water. It is assumed 4 A.F./acre/year is applied to sorghum and 2 A.F./acre/year to wheat. Reference: Guidelines to 1979 Production Costs and Practices: Imperial County Crops, Circular 104, California Cooperative Agri- cultural Extension, El Centro, California, pp. 42-43,48-49. Division of Agricultural Sciences, 1978. Irrigation Costs, Leaflet 2875, University of California, Berkeley, California. Of necessity, the design criteria for evaporation ponds must place heavy emphasis on maximizing the area occupied by the pond to ensure that evaporation requirements are met. Design criteria for storage ponds, on the other hand, emphasize mini- mum surface area so as to minimize evaporation. This dif- ference in area is crucial since the total costs of lining, a major component in the capital cost of any pond, are predominantly a function of the area to be lined. Moreover, under usual cir- cumstances, the reduction in lining costs achieved by reducing the area of the pond is not offset by increases in the costs of construction and embankment protection (Pound, et a l , 1975). Since irrigation in the California desert regions is usually conducted on a year around basis, it is necessary to A comparison of the costs of disposing of blowdown water through evaporation and those associated with an irrigation reuse operation reveals that the irrigation operation is less costly so long as comparable lining material is used. The total annual cost savings that could be realized by disposing of the water through an irrigation operation are arrayed in Table 10. The substantial savings that can be realized where any of the first three types of linings are used are largely attributable to the significantly smaller total surface area required for an opti- mal storage pond. The relatively modest savings for ponds lined with the last two materials stem from the fact that the relative permeability of these liners is so high that the areal difference between an evaporation pond and a storage pond is quite small. TABLE 10. Annual Savings Attributable to Blowdown Disposal Through an Irrigation Operation as Compared to Pond Evaporation (in 1967 dollars). Type of Storage Pond Annual Saving Exposed Asphalt Panel $958,551.58 Exposed Synthetic Membrane 690,065.39 Compact Clay 193,307.85 Asphalt Concrete 27,819.76 Concrete 21,920.05 Table 7 shows that an irrigation project with a leaching fraction of 0.2 will perform somewhat better than a clay lined evaporation pond in controlling the contamination of ground water by power plant blowdown. Additionally, the earlier dis- cussion suggests that an irrigation project will protect ground 835 Reuse of Power Plant Cooling Water for Irrigation water as well as an evaporation pond lined with exposed syn- thetic membrane so long as the irrigation project is abandoned at the end of power plant life. Accordingly, disposal of blow- down water through carefully managed irrigation in areas without shallow ground water tables offers equivalent pro- tection for ground water resources at a cost that is substantially less than the cost of evaporation ponds lined either with com- pact clay or exposed synthetic membranes. Over a 20-year period disposal through irrigation reuse would result in total savings of nearly $14,000,000 when compared to an evapora- tion pond lined with synthetic membranes and in excess of $3,000,000 when compared to a clay lined facility. SUMMARY AND CONCLUSIONS This paper presents evidence which demonstrates both the feasibility and desirability of disposing of blowdown water from power plants via irrigation reuse. In addition to being the least cost alternative, reuse of saline power plant cooling water for irrigation can provide a resource on the order of 5.1 x lo6 m3/yr of water which is currently being lost in costly disposal operations. This blowdown water, although high in dissolved salt, could produce substantial yields of salt tolerant crops such as wheat and sorghum, the revenues from which can be used to offset some of the costs of disposal. The major draw- back of cooling water reuse, the potential pollution of the underlying ground water, is shown to be either less than or no worse than the potential associated with an evaporation pond sealed with compacted clay provided that drainage below the root zone is carefully managed. The increasing scarcity of both energy and water resources coupled with environmental protection mandates pose signifi- cant dilemmas that must be resolved if additional power generating facilities are to be developed. Current controversies often focus almost exclusively on the weights to be assigned to conflicting scarcity and environmental quality goals. The con- flicts over which goals are to be favored is often so intense that little attention is paid to achieving the mix of goals that finally emerges in the most efficient fashion. A case in point is the mandate in California that blowdown water be disposed of in evaporation ponds, even in arid desert regions. The scientific and economic evidence reviewed in this paper sug- gests that reuse of power plant blowdown water for irrigation in arid environments is both a realistic and more efficient al- ternative to achieving environmental mandates while utilizing water and energy resources efficiently. Such irrigation reuse offers the potentiality of reducing the already spiralling costs of energy. LITERATURE CITED Ayers, R. S. and D. W. Westcot, 1976. Water Quality for Agriculture. Irrig. and Drainage, Paper 29, FAO, Rome. California Cooperative Agricultural Extension, 1979. Guidelines to Production Costs and Practices: Imperial County Crops. Circular 104, El Centro, California, pp. 42-43,48-49. CSWRCB, 1975a. Water Quality Control Policy on the Use and Disposal of Inland Waters Used for Power Plant Cooling. California State Water Resources Control Board. CSWRCB, 1975b. Water Quality Control Plan for Control of Tempera- ture in the Coastal and Interstate Waters and Enclosed Bays and Estuaries of California. California State Water Resources Control Board. DWR-CA, 1977. Water for Power Plant Cooling. Dept. Water Res. Bull. 204, State of California. Division of Agricultural Sciences, 1978. Irrigation Costs. Leaflet 2875, University of California, Berkeley, California. ERCDC-CAL, 1977. California Energy Trends and Choices, Vol. 11: Electricity Forecasting and Planning. 1977 Biennial Report of the California State Energy Commission, Energy Resources Conserva- tion and Development Commission, State of California, Sacramento, 180 pp. Jury, W. A., H. Frenkel, H. Fliihler, D. Devitt, and L. H. Stolzy, 1978a. Use of Saline Irrigation Water and Minimal Leaching for Crop Pro- duction. Hilgardia 46:169-192. Jury, W. A,, H. Frenkel, and L. H. Stolzy, 1978b. Transient Changes in the Soil Water System From Irrigation With Saline Water. I. Theory. Soil Sci. SOC. Amer. J. 42579-585. Jury, W. A., H. Frenkel, D. Devitt, and L. H. Stolzy, 1978c. Transient Changes in the Soil Water System From Irrigation With Saline Water. 11. Analysis of Experimental Data. Soil Sci. SOC. Amer. J. 42585- 590. Kays, William B., 1977. Construction of Linings for Reservoirs, Tanks, and Pollution Control Facilities. John Wiley and Sons, New York, New York, 379 pp. Pound, Charles E., Ronald Crites, and Douglas A. Griffes, 1975. Costs of Wastewater Treatment by Land Application. EPA, Report No. Rhoades, J. D., R. D. Ingvalson, J. M. Tucker, and M. Clark, 1973. Salts in Irrigation Waters. Soil Sci. SOC. Amer. Proc. 37:770-774. Rhoades, J. D., J. D. Oster, R. D. Ingvalson, J. M. Tucker, and M. Clark, 1974. Minimizing the Salt Burden of Irrigation Drainage Waters. U. Env. Qual. 3:311-316. 43019-75-003. ACKNOWLEDGMENTS The authors would like to thank the Southern California Edison Co. for financial assistance on this project. 836
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