NITRATE CONTAMINATION IN THE MEKONG DELTA.
Mai Thanh Truyet Ph.D.
The Mekong Delta is characterized by its dual monsoon climate, irrigated by crisscrossing river tributaries and canals draining into the South China Sea and into the Gulf of Thailand. The Mekong River Delta is also the greatest rice-producing region in Vietnam and consequently, an area of ever increasing population growth. This development naturally comes with pressing problems in city planning, communications, education, food supply and above all environmental pollution. An important issue of water pullution is Nitrate. The excessive use of Nitrate-containing fertilizers to boost agricultural production, the traditional (no sewer system) human and animal wastes disposal, the slow oxidation of underwater organic material in inundated areas, are the main sources of Nitrate pollution in the Delta Basin. Nitrate under the form of its water soluble basic salt K, Na, Mg, Ca readily infiltrates soils and underground water and when ingested by humans, especially infants, can be harmful. The maximum allowable amount of Nitrate in water is 45 mg/L, the water biochemical oxygen demand BOD should not exceed 25 mg/L and the water total dissolved solids TDS not reach the value of 20. The transformation of Nitrate into Nitrite in the human body by bacteria leads to the rapid combination of Nitrite with blood Hemoglobin and the formation of Methemoglobin, a substance devoid of Oxygen-carrying capacity, thereby reducing the oxygen supply to body tissues. According to USEPA, Nitrate water pollution has a profound impact on pregnant women with damage to fetal nervous and cardiovascular system, prematurity and low birth-weight.. The prevention and treatment of nitrate-polluted water in the Mekong River Delta with its demographic explosion are more than ever urgent. Through this paper, the author raises the public awareness and concern on the current and future impacts of this problem. The author outlines the method involving an ion-exchange Chlorination and the injection of vegetable oil into the Nitrate polluted water bodies, underground water included to promote the transformation of Nitrate into Nitrogen. A series of preventive measures such as the adequate storage of fertilizers, the location of water supply sources away from Nitrate-containing materials, the remodeling of water wells and latrines, associated with the sanitation education of the population, have also been proposed.
The Mekong Delta, the largest granary of Vietnam, with an area estimated at 3,964,000 hectares, occupies 12% of the country’s total area. With its peninsula shape surrounded by the sea on 3 sides, it has special climate features, influenced by the tidal events of the Gulf of Thailand in the West and of the South China sea in the East. Furthermore, it is characteristically criss-crossed by multiple communicating waterways draining mainly in 2 directions toward the 2 bordering above-mentioned body of waters. The Mekong Delta is also under the influence of the Southwest monsoon from May to November (rainy season); and of the Northeasterm winds from December to April (dry season). The yearly average rainfall is 2,400 mm on its western part and 1,600 mm on its eastern part. Fortunately, the Delta region is not much affected by the tropical typhoons, and consequently, has a more stable climate with an average yearly 2,400-2,800 hours of bright sunlight, which benefit agriculture and natural animal husbandry.
This perhaps accounts for the rapid demographic increase from 2.9 to 15.5 millions in 1995 and for a certain number of problems in addressing the vital necessities of the population, such as urban planning, transportation, education, nutrition and mostly the downgrading of the contaminated environment, with priority focused on clean freshwater requirements. Thus, the contamination of freshwater has never been most urgent, particularly the water contamination by Nitrate.
Nitrate contamination of water raised a grave concern, especially in the rice-producing and food-manufacturing regions such as the Mekong Delta where there always is a need to utilize a lot of fertilizers to increase production. Basically Nitrate (NO3-) is not by itself harmful to human beings and animals, but once it is ingested by the organism, it could degrade into Nitrite (NO2-), a substance known to cause fatalities to children under the age of 6, at a period when their digestive system is not mature enough to detoxify it.
The problem of nitrate contamination deserve more scrutiny in daily life, as Nitrate are present under its basic form (K and Ca), Aluminium, Iron, and under certain other heavy metal forms. Nitrate is readily water-soluble, easily permeates the ground and contaminates groundwater. According to the 1995 safety standards, the permissible levels of Nitrate in potable water is 45mg/L. In Nitrate contaminated ares, water has a level of Biochemical Oxygen Demand – BOD, variable with the concentration of Nitrate in it. If the BOD is >25 mg/L, the health of human beings and animals are likely to be affected. The presence of Nitrate also raised the levels of Total Dissolved Solids – TDS. If the TDS level exceeds 20, water would be unusable because of its potential dangers.
In the Mekong Delta, the population is concentrated on the banks and islands of the Tien and Bassac River, where water freely circulates among the intricate network of canals and waterways, connecting together 11 provinces. In the North of the Tien river, are the provinces of Ðồng Tháp, Long An, Tiền Giang; between the 2 main branches of the Mekong are situated the provinces of Bến tre, Trà vinh, Vĩnh long and Southwest of the Bassac lie the provinces of An giang, Cần thơ, Sóc trăng, Kiên giang and Minh hải. The density of the water network is on the average 0.4-0.6 km/km2, with a height of 0.0 m – 1.5 m above sea level. In the center of the Delta, there are areas below sea level with poor water drainage. The Delta population is primarily devoted to farming and the rice production is the main resource.
Nature and humans are the main causes of Nitrate contamination with the following characteristics:
The main source of Nitrate resides in the agricultural fertilizers, which, if unabsorbed by the vegetation, remain on the ground surface, and gradually infiltrate groundwater. To have an estimate of the amount of fertilizers that have been used in the Delta, suffice it to consult a 1965 statistics which showed that 63 kg/hectares/crop of fertilizers have been utilized by farmers, and this amount has raised to 74 kg/hectares/crop in 1990. Although accurate data about rice production between these 2 periods are not available, the average rice production of 8 tons/hectares/crop in 1965, has not increased much in 1990. As a comparison, the corn production in the state of Iowa, USA, from 1960 to 1980 has only increased 10% for a corresponding increase of 80% in the usage of fertilizers during the same period. According to Le huy Ba, the most Nitrate contaminated areas are in the vivinity of the irrigation canals of Cai San, Tam Ngan and in the Long Xuyen quadrangle (1).
The natural sources of Nitrate are excessive rainfall and the oxidation of organic compounds, including daily human wastes. In the rainy season the Nitrate concentration could attain the level of 80 mg/L.
Human and animal wastes are an important source of Nitrate, since in the Mekong Delta, the majority of the population dispose of their wastes openly on the waterways, in the fields or in the aquacultural ponds. With the flooding season, the favorable conditions for widespread Nitrate contamination is created.
Nitrate is created by slow oxidation of organic materials such as decaying plants, and other permanently inundated ground sediments. Thus, humans and Nature have contributed to Nitrate contamination in the Mekong Delta. In the US, according to an estimate of Geological Survey, humans are responsible for 50% and other causes of Nitrate contamination, including natural events contributed for another 50%. In the Delta humans with the demographic explosion and the deficient sanitation, might play a greater role in the contamination.
UNICEF has warned that nitrate in grounwater is a risk to the health of humans and animals, especially to children under 6-year-old, creating the Blue baby syndrome, due to methemoglobinemia. Nitrate once ingested, has been converted by bacteria present in the organism, into Nitrite, which subsequently reacts with blood Hemoglobin to form Methemoglobin, a substance devoid of the life support oxygen-carrying capacity. Deprived of oxygen, the children skin turns blue, hence the name of the syndrome.
Children are more prone to Nitrate intoxication than adults. In children above the age of 6, the stomach has started to secrete HCl which can destroy digestive bacteria, and thus prevent the transformation of Nitrate into Nitrite. Mortality due to the Blue baby syndrome are relatively rare, but a certain number of digestive cancer are found to be due to long-term Nitrate contamination. (2). US EPA also has warned the public about the danger caused by the presence in daily usage water of Bacteria and Nitrate. Pregnant women who uses Nitrate contaminated water could put their unborn babies at risk for the following: disturbance in nervous system development, potential development of cancer, heart trouble, prematurity and low-weight birth.
With a population of 16.5 millions in 1995, a density double of the total density of the country’s population, a birth rate of 2.2% and the prospect of reaching 18 millions in the year 2000, the danger of Nitrate contamination in the Mekong Delta and its effect on the population should be addressed with measures aiming at reducing the level of nitrate in usage water.
With the many difficulties encountered in accessing regional data and in on-site investigation, the objective of this writing is to ring an alarm bell to the Mekong Delta about the problem of Nitrate contamination, in the light of successes achieved by developed countries in the remediation of Nitrate, in spite of high costs, and complicated modern technologies such as distillation, ion transfer and reverse odmosis.
The distillation and flocculation methods requiring high energy consumption and wide distillation network apparatus are not practical in the Mekong Delta.
The method of ion transfer by chlorination of nitrate contaminated water in which nitrate ion are replaced by Chlor ion has 3 advantages: bactericidal, neutralization of nitrate and decreasing TDS. This method could be put into practice because of the availability of Chlor production in the Delta and its simple introduction in the current water network.
Reverse osmosis consisting in forcing the movement of water through expensive pressurized equipment and filtering membranes is only cited here more as a reference than a proposed method to eliminate nitrate in Vietnam.
Recently, the US Department of Energy has published information about a new method of neutralizing nitrate by vegetal oil in groundwater. By pumping vegetal oil into the ground around aquifers layers, wells, ponds, backwater creeks, accumulation of oil in different layers occurs depending on the soil structure. Nitrate contaminated water in contact with the oil layers will have their nitrate transformed in nitrogen by micro-organisms the growth of which is enhanced by Carbon contained in oil. The released harmless nitrogen is then absorbed back into the groundwater.
With this method, it is hoped that concentrations of nitrate above 800 mg/L could be brought down to its normal 45mg/L value. This method is practical for the Mekong Delta, because vegetal oil is devoid of toxicity, inexpensive, and regionally available. Furthermore, certain other organic substance such as, gasoline, petroleum, pesticide solvents, herbicides could also be likewise treated.
Considering the fact that preventive or mitigating measures against contamination should be given priority, and that feasibility conditions is readily achievable, these are the measures which could be taken to address the problem of water Nitrate contamination:
· the sources of usage water, namely ponds, lakes, reservoirs should be located on high ground, at least 50 meters far away from animal husbandry and natural fertilizers storage sites.
· Chemical fertilizers should be adequately stored to avoid their contact with groundwater.
· Wells should be constructed with surrounding solid walls to avoid spilling of surface water into the well.
· Latrines should be located far from water sources and sanitarily built.
· Information about the dangerous habit of openly defecating in the creeks or fisheries ponds should be provided to villagers ans farmers.
· Guidance about the correct utilization of fertilizers, insecticides, pesticides, herbicides... with the knowledge that the excessive use of fertilizers is not proportional to agricultural production and that it is the cause for chemical contamination of the soil.
· Good farming practices combined with the variation of crop planting can improve the soil productivity more than the use of fertilizer.
With the application of these simple measures, and the serious concern of everybody in the Delta, it is hopeful that the water nitrate contamination will be prevented and/or reduced to its acceptable levels.
1. Lê Huy Bá - The Environment – Science & Technology Publishing 1997 - VN.
2. Kross B.C. - Am. J. Public Health 83:270-272-1993.
Mai Thanh Truyet
Orange, CA 10/97 For suggestions please contact: Thank you.
The author, Mai Thanh Truyet Ph.D. is currently:
· QA Manager at Weck Laboratories Inc., Industry, CA.
· Laboratory Manager and Leachate Treatment Plant Manager at BKK Laboratories, West Covina, CA.
· Specialist in Toxic and Hazardous Waste Management and Air Monitoring.