The Role Of Trees in the Bioremediation of Drinking WaterPresentation Transcript
The Role of Trees in the Bioremediation of Drinking Water A Research experiment in Nawakkaduwa, Kalpitiya, Sri Lanka Kamal Melvani, Kanchana Chandrasekera and Rahul Mudannayake, Sri Lanka
The land use planning promulgated by conventional agriculture includes among many other facets, the intensive use of agrochemicals, the monoculture of one or two crop species, the removal of all other vegetation, the introduction of new crop varieties and pressure on the land to support three cropping cycles.
The leaching of these agrochemicals into the ground water table has had a significant impact on ground water quality.
The Kalpitiya Peninsula that is located on the west coast of Sri Lanka is a good case in point.
Map of Sri Lanka showing Kalpitiya It lies within the dry zone and the climate is characterised by high temperatures and a seasonal distribution of rainfall. Most of the rain occurs in the Maha season between October and January. The average rainfall is 800-900 mm annually.
Of special significance is the underlying Gyben-Herzberg lens of fresh water that is present in the beach plains with a flat to undulating topography. Geology Sandy regosols occupy the central portion and a greater part of the elevated beach plain that is adjacent to the lagoon in the Kalpitiya peninsula.
Gyben-Herzberg, Fresh water lens
The fresh water lens is extensively pumped for irrigation and potable water supplies, and the recharge is from direct infiltration from rainfall and return irrigation flows.
Development of the shallow aquifer has altered the natural flow regime and now ground water movement is dominated by the abstraction from shallow irrigation wells.
Ground water quality throughout the peninsula is of the carbon bicarbonate type and has been good until lately where with intensive cultivation, the ground water concentrations of nitrate, nitrite, chloride and potassium are exceptionally high.
There is a clear correlation between ground water quality and land use.
Use of inorganic fertiliser
Farmers in the Kalpitiya peninsula have fairly large land holdings, ranging from 3 to 25 acres.
Annual, cash crops are grown as a monoculture and there is no intercropping.
This type of agriculture is labour intensive and the cost of production is high.
The use of chemical fertilisers and pesticides is high since most farmers grow three crops a year.
Urea, Ammonium sulphate and other nitrogen based mixtures are used including, Triple Super Phosphate and Muriate of Potash.
No organic material is added to the sandy soils except for a little cow dung.
Unused fertiliser lying on soil surface Spraying pesticides every week
Since all farmers grow any one crop at a given point in time, pest and disease spreads easily. Hence the need to use heavy doses of pesticides.
Data gathered by researchers show that the amount of chemical fertiliser and pesticides used are far in excess of those recommended by the Department of Agriculture.
Farmers use up to 75,000 litres of water per acre per day pumped from shallow wells to irrigate their fields. 35% of the cost of production is devoted to the cost of watering. Hence with the rising cost of agrochemicals and that of fuel (for operating water pumps), the cost of production is becoming increasingly unprofitable.
What is even scarier is the impact on human health with the increased leaching of salts through the sandy soils into the ground water
Some of the grave effects of high nitrite intake on human health are the incidence of methaemoglobinaemia and gastro intestinal cancers. Methaemoglobinaemia describes the condition where the oxygen carrying capacity of the blood is reduced. It is a condition mostly restricted to infants and children and commonly referred to as Blue Baby Syndrome. A study done by the University of Ruhuna in 1995 has revealed that over 64% of the infants in Kalpitiya suffered from malnutrition and potential methaemoglobinaemia! In addition the lack of adequate sanitation was evident since the faecal samples taken contained high incidences of bacteria, viruses, protozoan pathogens and helminthes. Impact on human health…..
THE RESTORATION EFFORT
In 2003, the National Water Supply and Drainage Board sought the assistance of the Neo Synthesis Research Centre to resolve the problem of water contamination by nitrates and nitrites through BIOREMEDIATION.
Since the drinking water well in Nawakkaduwa village was the worst affected, initial discussions were held with villagers. They agreed that their present management of the land was causing the contamination of the well and would adopt an alternative, if it proved economically viable.
Objectives of the Research Experiment
The establishment of a demonstration model in bioremediation around the well on which the drinking water well was located.
The efficacy of the experiment in bioremediation be assessed by periodically investigating the levels of the contaminants in the water.
Natural forests are integral for environmental stability since they at all stages fix and cycle energy or carbon, regulate hydraulic flows and conserve nutrients.
Forests provide man with food, medicine, fuelwood, timber, fodder and fibre while also providing habitat for biodiversity.
Forests or tree dominant vegetation, play an important role in the bioremediation of water.
Dietz and Schnoor (2001) state that “one of the most promising frontiers of environmental research is bioremediation; the use of natural means to breakdown or degrade hazardous substances into less toxic compounds and thus repair environmental damage. Underneath the broad umbrella of bioremediation there are two main categories: microbial remediation and phytoremediation. Microbial remediation , the use of fungi or bacteria to break down pollutants, is a proven method for environmental remediation. Phytoremediation , or the use of plants to repair and restore damaged sites, is a relatively new technique that promises to be sustainable, low maintenance, and versatile.
“ When selected plants are grown in a contaminated substrate, the root system of those plants function as a highly dispersed, fibrous uptake system to form giant underground networks that function as solar-driven pumps that extract and concentrate essential elements and compounds from soil and water. Absorbed substances are used to support reproductive function and carbon fixation within shoots.”
By planting trees in close proximity over a contaminated aquifer, or as a border around a site, a barrier to the movement of contaminated ground water can be effected.
Description of the work done
a). Establishment of the demonstration model
The landscape design of the demonstration model would incorporate the silvicultural technique of Analog forestry.
Analog forestry seeks to establish a tree dominant ecosystem that is analogous or similar in architectural structure and ecological function to the original climax or sub climax vegetation community or natural forest.
The data gathered from a small forest patch located in Daluwa provided the basis for the landscape design.
The design used native and exotic species that could provide the land owner with an alternative to the present land management practices he engaged in; an alternative that would generate an income, provide clean food, water and habitat for biodiversity.
The landscape design of the model well area involved three main aspects of planting:
1. The watershed of the water source or the immediate area around the well is referred to as the buffer zone, where several deep rooted, mostly native trees that had long and short growth cycles, were planted in a dense manner so as to form a ‘root mat’ below the surface. The main objective was to facilitate the uptake of the contaminants by the roots of the trees.
Some of the species used were: Kumbuk (Terminalia arjuna), Mee (Madhuca longifolia ), Palu (Manilkara hexandra), Thimbiri (Diospyros malabarica), Halmilla (Berrya cordifolia), Wetakeiiya (Pandanus kaiida), Karanda (Pongamia pinnata), Surya (Thespepsea populnea), Murunga (Moringa oleifera). 2002 2003 2004 2006 2006
2. The fence area of the demonstration model was developed using several species that could withstand the salt laden sea breeze and serve as wind breaks. Trees like Mudilla ( Barringtonia speciosa ), Kasa ( Casuarina equisetifolia ), Watha Bhanga (Pisonia alba ), Murunga ( Moringa oleifera ) and Kotta ( Ceiba pentandra ) were planted. 3. The surrounding area was developed as a production area where both perennial and annual crops were grown organically. Many fruit plants like Papaya, Banana, Mango, Guava, Pomegranate, Sapodilla, Anona, Lime, Lemon, Mandarin, Orange, Avocado, Jak, Grape, Passion, and new varieties like Acerola were planted. Further, plants that had medicinal value like Nelli ( Phyllanthus embelicus ), Bulu ( Terminalia bellerica ) and shrubs like Katukarosana ( Picrorhiza kurrooa ), Tippili ( Piper longum ) and Aloe ( Aloe vera ) were promoted. Several vegetables were grown organically that included Snake gourd, Bitter Gourd, Ridge gourd, Amaranthus sp., Maize, Spinach, Ladies Fingers, Cucumber, Pumpkin, Long Bean, Tomato, Spring Onions, Winged Bean, and varieties of chillies. The sandy soil was periodically treated with a special soil mix containing several carbon rich inputs like Coconut peat and straw to facilitate humification provide conditions for ‘denitrification’.
b). A study to assess the efficacy of conventional agriculture The economics of conventional agriculture with specific regard to the quantum of water used in cultivation is also an important area for study. Surveys carried out on the 3 most dominant crops namely, chillies, red onions and tobacco in the past year have revealed that the greatest expenditure farmers in Kalpitiya incur is related to watering their crops. They need to water their fields twice a day since there is no moisture retained in the sandy soil that is bereft of mulch. The average water use is almost 75,000 L per acre per day as opposed to 12,600 L used to water our one acre demonstration model that is watered only once a day and has a thick sheet of mulch (NSRC 2004, et. al.)
c). Investigation of the water Investigation on water quality commenced from January, 2004. Hypothesis: The vegetation established around the model well would uptake the nitrates and nitrites and thus reduce the contaminants in the water. 1. Baseline tests were conducted on the water from the model well to determine: Physical properties (colour, odour, taste, turbidity, total Solid at 103-1050, Oil and Grease). Chemical properties (pH at 250 °C, Electrical Conductivity at 250C, Total Alkalinity (as CaCO3), Total Hardness (as CaCO3), Chloride (as Cl), Total Residual Chlorine (as Cl2), Free Ammonia (as NH3), Albuminoid Ammonia (as NH3), Nitrate (as N), Nitrite (As N), Fluoride (as F) at 250 °C, Total Phosphate (as PO4), Total Iron (as Fe), Sulphate (as SO4), Phenolic Compound (as phenolic OH), Calcium (as Ca), Magnesium (as Mg), Copper (as Cu), Manganese (as Mn), Zinc (as Zn), Aluminium (as Al), Arsenic (as As), Cadmium (as Cd), Cyanide (as CN), Lead (as Pb), Mercury (as Hg), Selenium (as Se), Chromium (as Cr). Micro Biology (Total Coliform count and E.coli). These tests were carried out at the Laboratory of the National Water Supply and Drainage Board, Getambe.
2. Monthly tests are carried out on water in the 8 piesometers established around the well, the model well and from the two wells located on either side of the model well, namely that of Chooti and the sea well.
The tests are carried out to determine the standard physical and chemical properties were potable water and include the following:
Physical properties : colour, odour, taste & turbidity and depth.
Chemical properties : pH at 250 °C, Electrical Conductivity at 250 °C, Total Alkalinity (as CaCO3), Total Hardness (as CaCo3), Chloride (as Cl), Free Ammonia (as NH3), Albuminoid Ammonia (as NH3), Nitrate (as N), Nitrite (As N) & Total Iron (as Fe).
These tests are carried out at the Laboratory of the National Water Supply and Drainage Board, Getambe.
3. Rainfall and ambient temperature of the Kalpitiya peninsula, where data is collected from the Metrological Department.
Results a) Establishment of the demonstration model The native trees established around the well displayed fast growth rates. This was seen in the increase in shade conditions in the model, where the canopy closure increased to almost 50%, over the whole area, in the three year period. The same phenomenon was seen in the growth rates recorded for the perennial and annual planting in the production area. Further, The growth of the surface biomass was fast and led to the rapid humification of sandy soils. There was a dramatic increase seen both in the number of birds, butterflies and frogs as well as in the soil macrofauna (earthworms, pill bugs etc). b) A study to assess the efficacy of conventional agriculture The compilation of the data collected has revealed that the greatest expenditure farmers, in Kalpitiya, incurred were related to watering their crops. They need to water their fields twice a day since there is no moisture retained in the sandy soil that is bereft of mulch. The average water use is almost 75,000 L per acre per day as opposed to 12,600 L used to water our one acre demonstration model that is watered only once a day and has a thick sheet of mulch. c). Investigation of the water 1. The results of the baseline tests conducted on physical, chemical and microbiological properties of the well water revealed that:
Physical properties: All the parameters tested were within permissible limits except for the excessive presence of oil and grease. Chemical Properties : The water contained high levels of nitrogen in the form of nitrate, nitrite, free ammonia and albuminoid ammonia. There was also recorded an excess of phenolic acid residue. Microbiological properties: There was recorded both E. coli and coli form bacteria. 2. The results obtained by testing the water in the piesometers from February 2004 to February 2005 show a significant decrease in the levels of nitrates. The concentration of nitrates (N) in the model well water in February 2004 was 58.5 mg/L; in 2005 30.8 mg/L and in 2006 27.2 mg/L. There were decreases seen in the levels of Nitrite (N), and in the parameters tested, but none as dramatic and as consistent as that seen in the levels of nitrates. 3. The rainfall in the Kalpitiya area did appear to have an impact on the physical nature and chemical concentrations of contaminants in the water . Levels of conductivity increased during the monsoon rains and declined with the cessation of the rains. The advent of rain altered the pH levels where the addition of rainwater increased pH value. The alkalinity levels were high during the drought period, and declined during the rains possibly due to dilution. The turbidity levels of the well water increased with the addition of rainfall and decreased during the drought.
The decreasing trend in nitrate levels could be attributed to the process of denitrification that was taking place in the below surface environment of the model well. Denitrification refers to the process that occurs during completion of the nitrogen cycle. This term refers to the reduction of nitrates and nitrites to nitrogen or oxides of nitrogen through microbial activity (biological denitrification) or to the chemical reduction of nitrites and other unstable nitrogen compounds (chemical denitrification). There are two types of biological denitrification: *Assimilative denitrification *Dissimilative denitrification Discussion Assimilative denitrification refers to the actual uptake of nitrates by the roots of plants for protein synthesis and the outputs of nitrogen or ammonia are released through respiration into the atmosphere.
The concentration of oxygen is reduced to a sufficiently low level when the soil air is displaced by water, as after heavy rainfall, irrigation or flooding.
The presence of nitrate, metabolizable carbon compounds and the complete absence of oxygen at the site of reduction. Soil organic matter, plant roots and organic manures provide the metabolizable carbon compounds.
The soil does not need to be devoid of oxygen since denitrification can occur at microsites like soil aggregates in the rhizosphere that are water saturated areas and hence oxygen is restricted; these conditions can then be considered as anaerobic.
The rate of denitrification increases with temperature and is highest at a soil pH between 6 and 8.
The process of denitrification is a stepwise reduction:
Nitrate (NO3)--> Nitrite (NO2)--> X--> Nitrous Oxide (N2O)---> Nitrogen (N2) gas, Where the intermediate X might be nitric oxide.
Dissimilative denitrification requires the following conditions:
Trees, it is clear, play a vital role in bioremediation; be it in the uptake of contaminants or in the interaction tree roots have with microorganisms. The decreasing trend in nitrate levels could be attributed in part, to the process of assimilation taking place in the ground water/root interface. The growth of the canopy cover of the vegetation in the area around the well is concomitant with the growth of the root mat.
The decrease in the levels of nitrate could also be attributed to the process of dissimilative denitrification taking place in the ground water/root interface. Since all the conditions required for dissimilative denitrification are met with in Nawakkaduwa, the assumption that the process was occurring is valid.
Carbon in the form of coconut peat and straw were added to the sandy soil.
The conditions that exist between the tree roots and the ground water (micro-sites) could possibly be anaerobic because of exudates (ex., tannins) given out by the roots. The conditions could alternatively be such that the bacteria performing the function of nitrate conversion use the extra electron of oxygen in the nitrate by reduction.
The Kalpitya Peninsula is located in the arid zone and hence is subject to high temperatures, low rainfall and low relative humidity. The pH value of the water was also between 6 and 7. Hence with the combination of factors favorable for dissimilative denitrification in place, that probability is very high.
Conclusions and Recommendations The roles played by tree roots, soil organic matter and denitrifying bacteria are a subject worthy of further research and discussion. However, the collection of data over an extended period of time will enable the possibility of statistical testing for enhanced verification. In addition, there will be the need to monitor the movement of water and the contaminant in the various wells and piesometers when the process of mass extraction takes place. It is then that the efficacy of this technology can really be gauged. Having said that it must be remembered that the growth of the root mat vis a vis the growth of tree cover should also be taking place in synchrony with the reduction of the contaminant in the aquifer. This is our conclusion and after the third year of experimenting, it seems like more than just a distinct possibility.
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