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Experimental And Modelling Of Nitrogen Compounds Fate And Transport In Groundwater, Mekala C & Indumathi M Nambi Research Scholar & Asso Prof, Ewre Division, Civil Department, Iit Madras

Congress: 2015
Author(s): mekala c (chennai, India)


Keyword(s): Sub-theme 2: Surface water and groundwater,
AbstractINTRODUCTION

In order to meet the water demand by population growth and urbanization, many countries started using untreated or partially treated wastewater for irrigation, groundwater recharge, landscaping and other purposes. On the other hand, the use of nitrogen-based fertilizers in agriculture has helped to feed millions of people but it's also disrupted the nitrogen cycle, and resulted in nitrate contamination in groundwater creating various health issues. A scientific investigation on the nitrogen dynamics in the subsurface, its sources, sinks, transformations and transport is essential to device proper wastewater reuse strategies and to ensure safe groundwater. A conceptual model was hyopothesed which includes major nitrogen transformation process in soil like adsorption, chemical and biological transformations namely nitrification and denitrification, plant uptake and others will control the final release of nitrates into the groundwater. Therefore, it is very important to study the fate and transport of nitrogen species from fertilizer and/or wastewater irrigation in order to predict the spatial and temporal distributions of concentrations in the vadose zone and groundwater.

In this aspect, experimental evidences are needed to quantify the individual reaction rates of all the processes by batch and bench scale column studies. The data from batch experiments will be used to propose a mathematical model for simulating the processes of nitrogen dynamics spatially and temporally in real scenario. Finally, a comprehensive study, replicating the field conditions will be helpful in validating the model and thereby applying it for predictions, scenario analysis and guidelines development for wastewater irrigation and fertilizer management.

METHODOLOGY

Experiments

Soil characterization -- Chemical and hydraulic properties
Soil bacterial enumeration and isolation for nitrification and denitrification activity under aerobic and anoxic conditions respectively.

Batch studies

Sorption and desorption equilibrium studies for ammonium, nitrate and acetate ions
Biokinetic studies for aerobic and anoxic mixed consortia of microbes showing nitrification and denitrification activities

Column studies

Tracer study with Bromide
Transport of ammonium, nitrate and acetate without biotransformation
Leaching study for ammonium and nitrate under low and high infiltration rates

Modelling
Model development for transport studies with calibration and validation from experimental data.

RESULTS AND DISCUSSION

Table 1 Soil Physicochemical & hydraulic parameters characteristics

Soil used for column studies were of less than 2mm size taken from IITM and were classified as per ASTM standards(2), as silty sand.Mixed bacterial culture for aerobic nitrification and anoxic denitrification were isolated and confirmed.

From batch sorption studies, it was observed that pseudo equilibrium was attained at 6 hours for ammonium, 3 hours for nitrate and 10 hours for acetate. Based on the equilibrium study it was evident that, ammonium, nitrate and acetate were better modeled by Freundlich isotherms(5) given in table2 which may be due to soil heterogeneity with different functional groups

Table 2. Freundlich constants for ammonium, nitrate and acetate sorption with soil

Batch microcosm studies were done to establish bio kinetics for mixed consortium showing nitrification and denitrification activity under aerobic and anoxic condition for initial concentrations 50,100, 300, 500 mg/l of ammonium and nitrate. From batch biokinetic studies, it was observed that ammonium degradation was almost complete within 20 h when the initial concentration was less than or equal to 100 mg/L, in aerobic conditions, the inhibition effect was observed at high concentration ranging above 200 mg/l of ammonium(3).

In denitrification studies, initial concentration of nitrate was varied from 50 to 500 mg/L, whereas the TOC concentration was kept constant at 3004±96 mg/L, and initial biomass concentration in all the experiments was equal to 100±5 mg/L. Results of these studies shows that the nitrate reduction was complete up to a concentration of 300 mg/L within 70 h. Even at high initial nitrate concentration, there was significant nitrate reduction occurred and no inhibitory effect was observed. From substrate reduction profile, it was inferred that, excess acetate was avaliable at all nitrate concentrations with no substrate inhibition observed.The microbial growth was well fitted by Monod model and their biokinetic parameters(4) were established and given in table 3

Table 3. Monod Biokinetic constants of bacteria under aerobic and anoxic conditions

TRACER STUDY

Based on bromide tracer study, the dispersivity coefficient was established, and was found to be be 0.1 cm/min.

TRANSPORT STUDY
It is essential to understand the transport of ammonium and nitrate without any biotransformation, considering only the adsorption in order to evaluate the role of biotransformation in the containment in aquifers. Moreover, in the absence of bacteria, but in presence of 3000 mg/L of acetate, there was little reduction in inlet concentration of 100mg/L, which can be observed. Breakthrough occurred around 4 hours for ammonium and reached 80 mg/l at the outlet concentration by 20 hours with for rate of 1 ml/min. Almost all the nitrate ions were coming out of column with c/co of 0.9. Break through curves clearly indicates that there is little containment by adsorption alone.

Breakthrough curves of ammonium and nitrate leaching profile at different flow velocities helps us understand how the change in water velocity influences the transport and leaching in column study. The experiments were conducted by varying the water velocity values (4ml/min and 1ml/min). The ammonium concentration reached around 80 mg/l and 95 mg/l for 1 and 4 ml/min respectively. It was observed that about 70 mg/l of ammonium was leached out of column. In case of nitrates, only 10 mg/l was retained in column at 35 cm depth and the remaining is leached away for both the flow rates.(1) This may cause a serious contamination to groundwater. But the presence of plant in this root zone depth may alleviate the problem to some extent which is not considered in this study.

Further these studies will be extended for partially saturated soil system.

CONCLUSIONS

This experimental study highlights the role of sorption, and biotransformation for nitrate transport in groundwater. Batch studies results give an idea of sorption and biokinetic constants which could be a significant input to model. The studies on transport of ammonium and nitrate with and without biotransformation in 1 D soil columns will give an insight into process happening in groundwater. A detailed experimental study and modeling of nitrogen compounds transport and transformation in soil and groundwater will be valuable research contribution for minimizing aquifer contamination by wastewater irrigation and fertilizers 1. Al-Darby and G. Abdel-Nasser , (2006),"Nitrate leaching through unsaturated soil columns: Comparison between numerical and analytical solutions", Journal of Applied Sciences, 6 pp 735-743.

2. BIS Compendium of Indian Standards on Soil Engineering. Part 1, (1989), Bureau of Indian Standards, India.

3. Kim, S.M., Im, S.J., Park, S.W., Lee, J.J., Benham, B.L., Jang, T.I., (2008). Assessment of wastewater reuse effects on nutrient loads from paddy field using field-scale water quality model. Environ. Model. Assess. 13, 305–313.

4. Pradyut K ,Arnab Pramanik, Sayani M, Jayanta Debabrata C, Joydeep M, Somnath M, (2012) Heterotrophic nitrification by Achromobacter xylosoxidans S18 isolated from a small-scale slaughterhouse wastewater, Bioprocess Biosyst Eng 35:721–728.

5. S. Jellali, E. Diamantopoulos, H. Kallali, S. Bennaceur, M. Anane,N. Jedidi (2010), Journal of Environmental Management Volume 91, Issue 4, Pages 897–905

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