Using sea water to "save" the floods of the Mekong Delta is not feasible? | Finance – Business


After Thanh Nien post Use sea water to save the Mekong Delta from flooding proposed by Prof. Dr. Nguyen Van Dat, former Rector of the Department of Civil Engineering, Ho Chi Minh City University of Technology (Public School, Phu Tho Technical Center, Saigon before 1975), proposed, Professional civil engineer (professional civil engineer) the state of Florida and California Nguyen Minh Quang provided feedback and proposed solutions to flood the land known as the "rice bowl" of the entire country.

Subsidence is irreversible

The phenomenon of landslide due to the uncontrolled exploitation of groundwater occurs from urban to rural areas in many parts of the world. For example, in the San Joaquin Valley, California; Long Beach, California; City of Orleans Orleans, Louisiana; Mexico City, Mexico; City of Jakarta, city of Indonesia and Bangkok, Thailand. But the most famous is probably the San Joaquin Valley with a photograph of a light pole marking the height of the ground in 1925, 1955 and 1977.

Groundwater is water in pore spaces between grains of grain or sand in soil. The water is located in layers of sand or sand, covered up and down by layers of fine sand or clay, called closed aquifer (unconfined aquifer) and often has very high pressure. If the aquifer does not have superior coverage, it will be called an open aquifer (unconfined aquifer) and will be subjected to atmospheric pressure plus the above water column. In addition to gravel and sand, aquifers also have fine sand or clay in the middle and are the cause of soil subsidence when underground aquifers are overexploited. Layers of fine sand and clay are composed of long, flat particles that are "confused" when deposited, so there is enough room to store water.

When water is pumped out of the aquifers, the pressure of the closed aquifer or water column above the open aquifer will decrease, the layers of fine sand or clay will be compressed because the particles are rearranged in a "yes" manner. The soil subsidence due to overextraction of groundwater is permanent and irreversible, soil elevation can not be restored as before, even if the aquifers are full again. effective way that can prevent the subsidence of land due to groundwater extraction is to reduce exploitation.

Paying for seawater will cause underground salt

To save the Mekong Delta from floods, it is necessary to avoid subsidence of land caused by the exploitation of groundwater, Prof. Dr. Nguyen Van Dat proposed two "very new" solutions. It is taking advantage of seawater to fill natural groundwater deposits (returning seawater to underground floors) and sweetening seawater as an alternative to groundwater.

But the solution to returning aquifers to seawater is not technically feasible because the aquifers being exploited are generally very deep underground aquifers with very high water pressure and are covered by very thick layers of clay. Therefore, sea water in the soil can not flow or penetrate into deep underground aquifers, but must use a pump with high pushability. Not to mention that using seawater to pay for aquifers will cause saltwater intrusion and will help saltwater penetrate deeper into these aquifers.

The cost of refreshing sea water is very high

The seawater purification solution is also not economically viable due to the high cost of using reverse osmosis technology. Currently, the most popular technique in the world to "sweeten sea water" is reverse osmosis, where pressure is used to push liquids through a semipermeable membrane in the direction in contrast to natural osmosis (the liquid passes from low concentrations for high concentrations). As a result, the solute (salt) is retained on the high concentration side and the pure liquid (water) moves to the lower concentration side on the other side of the semipermeable screen.

If you are using reverse osmosis technology, the "seawater" solution is probably not economically feasible because the cost of building and operating a reverse osmosis seawater refinery is very high. For example, the seawater refinery has a capacity of 50 million gallons / day (190,000 m3/ day) in Carlsbad, California, completed in December 2015 at a total cost of about $ 1 billion. The cost of finished products varies from 1.63 to 1.83 USD / m3 (2,014 to 2,257 USD / acre-foot) in the 2012 season. For comparison, the cost of surface water in the reservoir ranges from 0.82 to 0.85 USD / m3 (1,014 to 1,057 USD / acre-foot) and the average cost of groundwater extraction is USD 0.02 / m3.

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Fine sand or clay is compressed, causing the soil to sink

Photo: USGS

Reduce the exploitation of groundwater to avoid subsidence

As mentioned above, soil subsidence is caused by the overextraction of groundwater. This phenomenon of subsidence is permanent and irreversible, because soil elevation can not be restored as before, even if the aquifers are full again. The only effective solution that can prevent soil subsidence is to reduce the extraction of groundwater. The key point here is how to reduce groundwater extraction to avoid negative development impacts, since groundwater is an important and significant resource of the Mekong Delta.
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Return water from the sea to the aquifer

Depending on the natural hydrological conditions of each region, the exploitation of groundwater in the Mekong Delta can be established as follows: First, groundwater is not harvested for rice cultivation throughout the Mekong Delta. Secondly, in areas where salt water is not invaded, the exploitation of groundwater should be limited to the maximum. Using surface water sources for domestic water supply and cultivation. Third, in areas with seasonal saline intrusion, use surface water during the rainy season. The use of groundwater only for domestic water supply and crop cultivation requires less water during the dry season. Fourth, in areas where salt water interferes throughout the year, groundwater is only exploited to provide domestic water to cities and urban areas. Fifth, establish a monitoring system to monitor the subsidence and level of groundwater exploitation throughout the Mekong Delta.

Mr. Nguyen Minh Quang is a professional civil engineer (professional civil engineer) from Florida and California. Mr. Nguyen Minh Quang graduated from the Faculty of Civil Engineering at Phu Tho National Technical Center, Saigon in 1972. Head of Planning Department, National Water Resources Commission, Ministry of Public Works and Transportation, Saigon until April 1975. Graduated in civil engineering (1983) and postgraduate in irrigation (1985) by University of Nebraska, USA. Hydrlogist of Hydraulics Administration, Broward County, Florida, until 1989. From 1990 to 2015, Senior Supervising Engineer of Stetson Engineers Inc., a consulting firm on Irrigation and Water Pollution, founded in 1957 in Los Angeles. Engineer Nguyen Minh Quang retired in 2016.

"The idea of ​​injecting seawater into underground aquifers is useless because it will cause the intrusion of salt water from the precious freshwater layer into the soil, negatively affecting the environment. If it is sweetened by seawater with a large volume of water Freshly pumped to the ground is extremely expensive, not feasible because of the energy.

The cost of turning seawater into freshwater is very expensive: from 0.5 to 3 dollars / cubic meter. In the Middle East they have to do it because there is no fresh water, the cost is $ 2, but it can be done because they have cheap energy. Vietnam is a power outage. But if you make drinking water from the sea water, you will give people to eat and live, but what should be pumped to the ground to worry about subsidence?

Please note that the water has been drained, the soil has collapsed and the natural infiltration will not cause the soil to grow again. If the pump is forced to increase the pressure, how will the pump be able to lift an entire stratum? And how to maintain that pressure forever to keep the stratum in a protruding position? The only thing is that the pressure in the pressure tread is extremely high is very crap. If drilling this water is sprayed, it is extremely high pressure.

The most effective measure is the plan to minimize the exploitation of groundwater, considering that groundwater is also a valuable resource in combination with measures to protect and create additional areas of water retention and water filtering agriculture and people's lives .. . "

For Van Truong

Independent expert in water resources and environment


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