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Increasing salinity is one of the most significant forms of groundwater and/or stream-flow pollution and certainly the most widespread. The most important causes are an increase in the salinity of groundwater from the effects of irrigation, and the intrusion of saline water (seawater), mainly in basins of internal drainage, islands, and coastal areas.
In arid climates, infiltration from rainfall may be negligible and leaching is not effective in diluting soil salt solutions enriched by evaporation. Any infiltration that does reach the groundwater table will be relatively highly mineralized. In poor drainage areas, particu larly basins of internal drainage which are groundwater discharge areas, evaporation can produce significant increases in salinity.
Changes in the salinity of rivers along their courses and with time are mainly the result of return flows from subsurface drainage water. The effect of irrigation returns on river salinity has been experienced in various arid regions in advanced countries, including the upper Rio Grande in Texas and the Colorado River in Arizona, both in the United States of America, and the Murray River in Australia (Meybeck et al. 1989).
As a case study in the control of river salinity problems, this section discusses the world's largest desalting facility, built to salvage about 72.4 million gallons (274,000 m³) of brackish water per day from irrigation drainage in the Colorado River valley in the state of Arizona. The reverseosmosis desalination project, located at Yuma in the south-western corner of Arizona, is intended to control the quality of the Colorado River where it crosses the border between the United States and Mexico.
The Colorado River is one of the world's most regulated rivers. But the regulation necessary to ensure a sufficient quantity of water for users has also exacted a price in the quality of the water available. As the south-western United States was being developed during the early part of this century, the big question was whether there would be enough water. Today people also ask how good the available water will be. Under a 1944 treaty with the United States, Mexico has a guaranteed allotment of 1.85 x 109 m³ of water per year. Between 1945 and 1961 there were no major problems resulting from the treaty, as the salinity of the water crossing the border into Mexico was generally within 400 mg/l at Imperial dam, the last major diversion for users in the United States.
Regulation of the Colorado by a series of large dams (fig. 2.30) has substantially increased stream salinity by two processes: the tremendously increased evaporation surface, and contaminated irrigation return flows. The stream salinity at the Mexican border has been doubled, from 400 mg of total dissolved solids (TDS) per litre in the early 1900s to 800 mg in the 1950s. In 1961 Mexico began complaining that the increased salinity was harming crops in the Mexicali valley. In 1973 the United States agreed, in Minute No. 242 of the International Boundary and Water Commission, to a salinity level for water being delivered to Mexico at Morelos dam.
Fig. 2.30 The Colorado River basin
This agreed-upon salinity level has had to be achieved by constructing a massive desalination plant. Enough of the salts have to be removed from irrigation return flows to make the water acceptable for discharge into the river and later delivery to Mexico. The plant, completed in 1992, is the world's largest reverse-osmosis desalting facility with an installed capacity of 274,000 m³ per day. It salvages most of the irrigation return flows of 98 million m³ per year which were formerly diverted to the Gulf of California (Applegate 1986).
2.6.2 The river basin
The Colorado River is an international drainage system that drains an area of approximately 583,000 km² and flows through seven states of the United States and the Republic of Mexico.
The average annual natural flow of the river at Lees Ferry, Arizona, the dividing point between the upper and lower river basins, has been estimated at about 18 x 109 m³ per year, which also approximates the present consumptive use within the basin plus deliveries to Mexico. The total annual salt load at Lees Ferry is 7.4 million metric tons, of which irrigated agriculture is estimated to contribute a further 1.8-3 million metric tons. Eighty-eight per cent of the total salt load from irrigated agriculture in the entire basin is estimated to originate in the upper basin (Worthington 1977).
2.6.3 Salinity problems of the Colorado River
Salinity is a naturally occurring phenomenon in almost all rivers in the arid zone. The salinity of the Colorado River water at its headwaters in the Rocky Mountains is about 50 mg of TDS per litre, but where the river crosses the border into Mexico, it was already about 400 mg/l in the early 1900s. Owing to the tremendously increased evaporative surfaces in over 20 reservoirs and numerous irrigation systems in arid terrain, the salinity of the river water at the border reached an unacceptable level.
The Wellton-Mohawk Irrigation and Drainage District in southwestern Arizona, east of Yuma (fig. 2.30), established in the early 1950s, was one of the last districts to be developed. The project included a system of drainage wells, the discharge from which included a substantial amount of highly saline groundwater that had been concentrated through re-use during the previous 50 years. Initially it had a salinity of 6,000 mg/l This resulted in a sharp increase in the salinity of the water crossing the border into Mexico, from around 850 mg/l in 1960 to more than 1,500 mg/l in 1962. At about the same time, releases into Mexico were greatly reduced in anticipation of storage behind the newly constructed Glen Canyon dam. This loss of dilution water is illustrated by the fact that from 1951 to 1960 the average delivery to Mexico was 5.2 x 109 m³ per year, while from 1961 to 1970, the flow averaged only 1.9 x 109 m³ per year. Mexico raised strenuous objections (Worthington 1977).
As a result of Minute 242 of 1973, the salinity of water as it enters Mexico at Morelos dam now averages no more than 115 mg/l plus or minus 30 mg/l over the average annual salinity of waters arriving at Imperial dam (Worthington 1977).
Fig. 2.31 Colorado River basin salinity-control project (Source: US Bureau of Reclamation 1980)
2.6.4 Countermeasures to control river salinity
To comply with Minute 242, the United States has been undertaking the following works (fig. 2.31):
All of the costs in money or water to satisfy Minute 242 are to be borne by the United States, at a cost of several hundred million dollars annually. Both the United States and Mexico will receive tangible benefits. The US Bureau of Reclamation estimates that an increase of one mg/l in salinity at Imperial dam results in a cost of US$240,000 per year to water users in Arizona, California, and Nevada. In the absence of any measures to control salinity, the total impact of salinity increases on users in the three lower-basin states was predicted to be about US$80 million per year by the year 2000 (Worthington 1977). The dollar values of detriments to users in Mexico would be additional, but have not been estimated.
Authorization to begin the salinity control work was provided by the Colorado River Basin Salinity Control Act, passed by Congress in June 1974. This legislation was in two parts: one for salinity-control measures downstream of the Imperial dam, and one for salinity-control measures in the seven Colorado River basin states upstream of Imperial dam.
2.6.5 Salinity control by the world's largest RO desalting facilities
The agreed-upon salinity level is being achieved by desalination. Enough of the salts are removed from irrigation return flows to make the water acceptable for discharge into the river and later delivery to Mexico. While the desalination plant was being completed, drainage water from the farmlands east of Yuma were bypassed around Mexico's diversion point at Morelos dam and carried in a concrete-lined drain to the Santa Clara slough at the Gulf of California (Wagner 1989). At the same time, these bypassed flows were replaced by water from upstream storage to fully meet the quantity of 1.5 million acrefeet (1.85 x 109 m³) of water owed to Mexico. When the desalting plant was completed, the irrigation return flows that were being diverted could be salvaged.
The Yuma desalting plant provides for salinity-control measures downstream of Imperial dam. Approximately 100 million gallons of saline irrigation drainage per day (138 million m³ per year) from the WelltonMohawk farmland is delivered to the plant via an existing concrete-lined drain. The plant has an installed design capacity of about 72 million gallons (274,000 m³) per day, which can be expanded to 96 million gallons (365,000 m³) per day. A flow diagram of the treatment system is shown in fig. 2.32.
Fig. 2.32 Flow diagram of water treatment and reverse osmosis (Source: Buros et al. 1993, from original diagram by Wagener)
PRE-TREATMENT. Before being desalted, the water passes through three pretreatment steps to remove all solids that would quickly clog the expensive desalting membranes if not removed. Pre-treating the water will ensure a membrane life of three to five years.
As the water flows into the plant, chlorine is added to prevent the growth of algae and other organisms. The water first goes through a grit sedimentation basin to remove heavy grit, sediment, and sand suspended in the water. The water is also softened by removing some of the calcium. Lime and ferric sulphate are both used in solid contact reactors. The last step in the pretreatment process is dual media filters, which remove any fine particles or organisms remaining in the water.
PROCESSING. Reverse osmosis is the separation of one component of a solution from another (in this case, salt from the water) by means of pressure exerted on a semi-impermeable plastic membrane. A total of about 9,000 membrane elements, inserted into fibreglass pressure vessels desalt the water. While the pressure tubes are all 6 m (20 feet) long, some membranes have a diameter of 30 cm (12 inches) while the diameter of others is 20 cm (8 inches). The element is made up of a number of sheets rolled into a spiralwound membrane.
Separation of salts from the product water is both a chemical process and a physical diffusion process. The water is forced through the walls of the cellulose acetate membranes by applying pressure at about 30 kg/cm² (about 400 pounds per square inch), allowing only the freshly desalted water to pass through. This process removes about 97% of the salts from the water. The fresh water is forced by the pressure down towards the centre tube.
WATER CONTROL AND MANAGEMENT. After desalination, the product water (with a salinity level of 285 mg of TDS per litre) is collected and combined with untreated drainage water (with salinity around 3,000 mg/l) to achieve the desired salinity level of about 700 mg/1. The salvaged water is then conveyed in a concrete channel to the Colorado River. Brine (with 10,000 mg/l salinity) is piped to the existing bypass drain, where it mixes with excess untreated Wellton-Mohawk drain
Table 2.8 Anticipated performance of Yunma desalting plant
|Constituent||Feed water(mg/l)||Reject water (mg/l)||Product water (mg/l)|
Source: US Bureau of Reclamation, Yuma Desalting Office.. age. The anticipated performance of the reverse-osmosis desalination is shown in table 2.8. This effluent/drainage flow then travels to the Santa Clara slough above the Gulf of California, where it combines harmlessly with 30,000 mg/l salinity ocean water (Applegate 1986). No adverse effect on the water environment in the Gulf of California is foreseen.
COST. The US Bureau of Reclamation estimated the project cost of the Yuma desalting plant in 1975 at US$149,446,000, including:
The annual cost was estimated to be US$8,988,500 in financing costs plus US$11,520,000 for operation and maintenance for a design output of 126.6 million m³ of product water per year with a salinity of 386 mg of TDS per litre. The unit cost of the product water was estimated to be US$0.161m³, based on 1975 prices without interest during construction. A recent cost study of the project estimated the unit cost of the product water with salinity at 285 mg/l and an output of 85 million m³ per year to be US$0.48/m³, with a construction period of three years and an interest rate of 8%. The project cost of the plant based on 1990 prices was estimated to be as follows:
Table 2.9 Unit costs of reclaimed water from venous wflter-processing facilities
|Salinity of source water (TDS mall)||Unit water cost (UN$/m³)|
|Seawater desalination (Kuwait)|
|multi-stage flash evaporationa||45,000||2.70|
|Advanced waste-water treatment(Orange County)||<500||0.17|
a. Using waste heat from steam-driven power plant.
A comparison with the unit water costs of various other projects- desalination of seawater by multi-stage flash evaporation and by reverse osmosis in Kuwait (Darwish and Jawad 1989), and desalination of brackish water and advanced waste-water treatment at a facility in Orange County, Calif., USA-is shown in table 2.9.
The operation and maintenance costs of reverse-0smosis desalination is likely to be reduced in the future by the introduction of low pressure membrane modules.
2.6.6 Remarks on the Colorado River salinity control and water resources management
The product water from the desalting plant is being mixed with raw drainage water to develop a total of 89 million m³ of blended water per year to be delivered to the Colorado River. Salinity control of the river by the desalting facility is not only to protect the water-quality environment but also to sustain arid-land agriculture in both the United States and Mexico. The Colorado River salinity control programme, of which the Yuma desalting plant is a key element, may be a significant development in water-resources management of the river.
Such large-scale reverse-osmosis desalination will be applicable, however, only in countries where plant operational skills are already at a high level. The US Bureau of Reclamation and the National Water Supply Improvement Association jointly held a seminar and workshop on design, operational, legislative, and educational issues impacting large-scale desalting plants, including technical transfer to the developing countries in August 1993 (Burgs et al. 1993).
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