JEQ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Related articles in JEQ
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Web of Science (3)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Chung, J.-B.
Right arrow Articles by Lee, Y.-D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Chung, J.-B.
Right arrow Articles by Lee, Y.-D.
Agricola
Right arrow Articles by Chung, J.-B.
Right arrow Articles by Lee, Y.-D.
Related Collections
Right arrow Watershed and Landscape Processes
Right arrow Surface Water Quality
Right arrow Biogeochemical Processes
Right arrow Lysimeter/Rhizosphere Studies
Right arrow Bioremediation and Biodegradation
Right arrow Water Pollution
Published in J. Environ. Qual. 33:1017-1023 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Surface Water Quality

Removal of Organic Matter and Nitrogen from River Water in a Model Floodplain

Jong-Bae Chung*,a, Seung-Hyun Kimc, Byeong-Ryong Jeongb and Young-Deuk Leea

a Department of Agricultural Chemistry, Daegu University, Gyeongsan 712-714, Korea
b Department of Agronomy, Daegu University, Gyeongsan 712-714, Korea
c Department of Environmental Engineering, Yeungnam University, Gyeongsan 712-749, Korea

* Corresponding author (jbchung{at}daegu.ac.kr).

Received for publication April 15, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A significant improvement in river water quality cannot be expected unless nonpoint-source contaminants are treated in addition to the further treatment of point-source contaminants. If river water is sprayed over a floodplain, the consequent water filtration through the sediment profile can simultaneously remove organic matter and nitrogen in the water through aerobic and denitrifying reactions. This hypothesis was tested using lysimeters constructed from polyvinyl chloride (PVC) pipe (150 cm long, 15 cm in diameter) packed with loamy sand floodplain sediment. Water was applied to the top of the lysimeters at three different flow rates (48, 54, and 68 mm d–1). Concentrations of NO3 and dissolved oxygen (DO), chemical oxygen demand (COD), and redox potential (Eh) in the water were measured as functions of depth after the system reached steady states for both water flow and reactions. At the rate of 68.0 mm d–1, a reducing condition for denitrification developed below the 5-cm depth due to the depletion of O2 by organic matter degradation in the surface oxidizing layer; Eh and DO were below 205 mV and 0.4 mg L–1, respectively. At a depth of 70 cm, COD and NO3–N concentration decreased to 5.2 and 3.8 mg L–1 from the respective influent concentrations of 17.1 and 6.2 mg L–1. Most biodegradable organic matter was removed during flow and further removal of NO3 was limited by the lack of an electron donor (i.e., organic matter). These results indicate that the floodplain filtration technique has great promise for treatment of contaminated river water.

Abbreviations: BOD, biochemical oxygen demand • COD, chemical oxygen demand • DO, dissolved oxygen • Eh, redox potential


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ALTHOUGH THERE HAVE been increased efforts in Korea to reduce surface water pollution, these efforts have all concentrated on measures to reduce point-source pollution from industrial and residential sources. Little attention has been paid to nonpoint-source pollution, resulting in unscreened inflow into rivers (Kwun, 1998). As a result, the self-purification of a river is the only treatment of residual contaminants from point- and nonpoint-source contaminants in Korea. Consequently, river waters have not significantly improved in quality and still contain relatively high levels of N, P, and organic matter. These high levels of contamination lead to excessive eutrophication in the water bodies (Chung et al., 1997; Cho et al., 2000).

Rivers in Korea have low flow rates most of the year except for a short period of flooding during the monsoon season; as a result, they are very vulnerable to contamination. A significant improvement in water quality cannot be expected unless nonpoint-source contaminants are treated in addition to the further treatment of point-source contaminants. Although the flow rates of rivers and their first tributaries in Korea are quite low most of the year, they are still too high to be treated by conventional in-plant techniques of wastewater treatment.

Fortunately, the waterways near Korean rivers have been well-organized for rice (Oryza sativa L.) paddies and as they travel downstream they converge to become tributaries for larger rivers. This organizational structure makes the nonpoint-source contaminants from rural areas treatable before they reach the larger rivers at the tributary level. Most rivers and streams in Korea have wide floodplains in between the levees and shores that become inundated during flooding. These floodplains are needed to accommodate seasonal flooding, which takes place only a few times a year and lasts only a few weeks. The floodplains are composed mainly of permeable materials, mostly sandy alluvial sediments, and usually remain uncultivated and weedy. The vegetation covering the floodplains supplies the top sediment with organic materials and oxygen, resulting in the development of a rhizosphere that serves as an excellent habitat for microbes and worm-like fauna (Vance et al., 1994). These facts suggest that the process of spraying contaminated stream water over the floodplains may allow the contaminants (biochemical oxygen demand [BOD] and nitrogen) to be removed by microbes and worms during infiltration through the floodplain sediment profile. The close relationship that exists between oxygen supply rate and soil water content indicates that the amount of oxygen present in the soil and the depth of the aerobic zone can be controlled by the rate of water flow through the soil (Collin and Rasmuson, 1988). A denitrifying zone develops beneath the aerobic zone due to the depletion of oxygen in this region. By using an appropriate spray rate, the contaminated stream water may infiltrate through the aerobic and denitrifying zones, removing both BOD and nitrogen from the stream water. Having passed through the aerobic and denitrifying zones and entered the ground water, the resulting treated filtrate is expected to flow back into the stream since all streams in Korea are gaining streams during the dry period (Korea Water Resources Corporation, 1996). This process can be classified as a type of land application and has been named in this study as the "floodplain filtration technique" (Fig. 1) . In this case, filtration means biological removal of contaminants including organic matter and nitrogen during the infiltration of river water through the floodplain sediment profile.



View larger version (29K):
[in this window]
[in a new window]
  		Fig. 1. Schematic diagram of floodplain filtration. If contaminated river water is sprayed over the floodplain, organic matter in the water is expected to be decomposed by microbes using O2 as an electron acceptor and a reducing layer is expected to develop beneath the oxidizing surface layer due to the depletion of oxygen in the surface layer. Ammonium in the river water is expected to be converted to NO3 in the oxidizing surface layer and then the NO3 leached down in the reducing layer can be removed by denitrification with the further degradation of organic matter.

 
This floodplain filtration technique, which is engineered to maximize the exploitation of natural conditions, can be compared with natural processes that remove nonpoint-source pollutants in river waters. An old method called riverbank filtration is increasingly being used because it is a relatively inexpensive and sustainable means to improve the quality of surface water (Tufenkji et al., 2002). As water infiltrates through the riverbank into the aquifer, it experiences chemical changes described by four general types of reactions: electron transfer, weathering, ion exchange, and gas exchange (Jacobs et al., 1988). The most significant chemical changes are related to microbial activity, such as degradation of organic matter occurring in the early stages of infiltration (Bourg and Bertin, 1993; Jacobs et al., 1988). When this intense microbial activity in the riverbed sediment consumes more oxygen than is supplied by the infiltrating river water, a reducing zone develops where the microbial activity of denitrifying and sulfate-reducing bacteria further decreases the redox potential of the system. This reducing zone is characterized by oxygen depletion and NO3 concentration reductions ranging from 40 to 100% as compared with river water (Bourg and Bertin, 1993; Tsushima et al., 2002; von Gunten and Kull, 1986).

Water exchange between river channels and unconfined aquifers in natural systems such as shallow aquifers (DeSimone and Howes, 1998), floodplains (Haycock and Burt, 1993; Tsushima et al., 2002), riparian soil (Cooper, 1990; Grimaldi and Chaplot, 2000; Magg et al., 1997), estuarine headwater (Thompson et al., 2000), and vegetated grass and forest filter strips (Groffman et al., 1991) are now generally accepted as important sinks for organic matter and nitrogen through the biogeochemical processes in subsurface ground water.

Therefore, downstream river water quality in Korea may be improved by spraying the contaminated stream water from tributaries over the surrounding floodplains. In this study, the simultaneous removal of both organic matter and nitrogen from river water by use of the floodplain filtration technique was examined in a model field system.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Laboratory lysimeters constructed and instrumented according to the schematic diagram of Fig. 2 were used for evaluating the simulated removal of organic matter and nitrogen in river water. The lysimeters were made from PVC pipe (150 cm long, 15-cm i.d.). The bottom 10 cm of each pipe was packed with 1- to 2-cm-diameter pebbles and then 130 cm of loamy sand floodplain sediment was added above the pebble layer. The sediment used in this experiment was collected from the top 100 cm of floodplain from the Nakdong River. The Nakdong River passes through the southeastern part of Korea. Particle size distribution of the loamy-sand alluvial sediment in the floodplain was slightly different through the profile, but the collected sediment was thoroughly mixed in the laboratory before filling the lysimeters. Although particle size distribution of the floodplain sediment in the lysimeters was slightly different from the field conditions, the effect of this difference in particle size distribution would be minimal in the examination of biogeochemical processes in the model system. Particle separates of sand, silt, and clay in the sediment were 83.3, 16.4, and 0.3%, respectively, and organic matter content was 4.5 g kg–1. Solution and gas samplers were installed while the sediment was being packed into the lysimeters. Rhizon soil water samplers (Eijkelkamp, Giesbeek, the Netherlands) were installed at depths of 5, 10, 15, 20, 25, 30, 40, 50, 70, 90, and 120 cm. Gas samplers, consisting of a 15-cm length of nylon tubing (0.2-cm i.d.) with glass wool wrapped around the open end within the lysimeter and a rubber septum placed at the end outside the lysimeter, were installed at depths of 10, 20, 30, 50, and 70 cm. The bottoms of the lysimeters were closed and a 1-cm-diameter opening was made 5 cm from the bottom. This opening was connected with spiral tubing to prevent the entry of air into the lysimeters. The tops of the lysimeters were closed with a Plexiglas plate and an inlet for river water and an inlet and outlet for air were installed in the plate.



View larger version (39K):
[in this window]
[in a new window]
  		Fig. 2. Schematic diagram of the experimental model floodplain filtration system.

 
The lysimeters were saturated with tap water by wetting them from the bottom and maintained for 3 d to obtain a physical stabilization of the sediment. After stabilization, tap water was applied from the top water inlet at three different flow rates, 40.8 (Flow Rate I), 54.4 (Flow Rate II), and 68.0 mm d–1 (Flow Rate III), using peristaltic pumps to obtain steady state water flow. The flow rates were selected to compare the efficiency of simultaneous removal of organic matter and nitrogen by oxidative degradation and denitrification, respectively, during the flow of river water through the lysimeter. An oxidizing surface sediment layer is needed for the oxidation of organic matter and NH4 and oxygen in the infiltrate can be depleted during this process. Below this oxidizing surface layer, a reducing condition can develop and removal of nitrogen via denitrification is possible. The water flow rate and organic matter content in the water can control the development of oxidizing and reducing sediment layers. The 68.0 mm d–1 flow rate was slightly lower than the value that could cause partial ponding of the lysimeter surface around the dripping point. Steady state water flow was obtained 3 wk after the application of tap water. At this time the lysimeter sediment had a bulk density of 1.53 to 1.62 g cm–3, porosity of 39 to 42%, and average saturated hydraulic conductivity of 9.68 x 10–4 cm s–1. Volumetric water contents in the lysimeters at Flow Rates I, II, and III were 31, 33, and 34%, respectively. Although water content was higher in the higher flow rate lysimeter, all the lysimeters were free from saturation except below the 100-cm depth. The lysimeters were installed and the following experiments were conducted in a constant temperature room (20 ± 1°C).

After obtaining steady state water flow in the lysimeters, river water collected from the Omogcheon River, which passes through the outskirts of Gyeongsan, was applied at the three different flow rates. Air passing through 500 mL of water contained in a glass bottle was injected through the air inlet at a rate of 10 mL min–1 to facilitate the removal of denitrification gases evolved from the lysimeter surface. The river water was filtered through a 250-mesh sieve (61-µm opening) to remove suspended solid particles, and the concentration of NO3 in the filtered water was adjusted to around 6 mg N L–1 with reagent KNO3 before application to the lysimeters. The average properties of filtered water are shown in Table 1. pH was measured using a Metrohm (Heriasu, Switzerland) 690 pH/Ion meter, and COD and BOD were measured using the methods of the American Public Health Association (1998). Ammonium, NO3, and NO2 ions were measured using a flow injection autoanalyzer FIA-5000 system (Foss Tecator, Höganäs, Sweden). The properties shown in Table 1 fluctuated depending on the collection time. Chemical oxygen demand and BOD of the waters after filtration through a 250-mesh sieve were in the ranges of 16.5 to 18.1 and 10.3 to 16.4 mg L–1, respectively. Therefore, organic matter corresponding to around 70% of COD may be eventually available to microorganisms. About 70% of the total inorganic nitrogen was NO3 and it was in the range of 1.89 to 3.02 mg N L–1. Ammonium and NO2 were about 24 and 4% of the total measured inorganic nitrogen, respectively.


View this table:
[in this window]
[in a new window]
  		Table 1. Characteristics of the Omogcheon River water measured after filtration using a 250-mesh sieve.{dagger}

 
During the application of river water, COD and NO3 concentrations in water sampled at lysimeter depths of 10, 20, 40, and 70 cm were periodically monitored, and steady state reactions in the lysimeters were confirmed 60 d after the application of river water. After the system reached steady state water flow and reactions, water and gas samples were collected seven times at various depths during the next 4 wk. Dissolved oxygen and Eh were measured using a Checkmate 90 DO meter (Corning Co., Corning, NY) and a Metrohm 690 pH/Ion meter in the water sample collection system connected to the lysimeter before the water was exposed to the outer atmosphere (Fig. 2). The Eh was measured with an Orion (Beverly, MA) combination Pt electrode with an Ag-AgCl reference electrode. The Eh was recorded in mV after a stable reading was obtained, and true Eh values were calculated by adding 242 mV to the recorded Eh values to correct for the potential of the reference electrode. Chemical oxygen demand and concentrations of NH4, NO3, and NO2 were also measured as described above using the same water samples used to measure DO and Eh.

Concentrations of N2 and N2O in the gas samples collected from the headspace and at various depths of the lysimeters were determined by gas–solid chromatography. A Hewlett-Packard (Palo Alto, CA) 4890 gas chromatograph equipped with a thermal conductivity detector was used for N2 analysis. The analytical column was a stainless-steel column (200 cm long, 0.32-cm o.d.) packed with a 5A molecular sieve (45–60 mesh). The detection limit of N2 was 3000 µL L–1 at a signal-to-noise ratio of >10, and no interference with O2, Ar, and CO2 was found during the analysis. Nitrous oxide was analyzed using another Hewlett-Packard 5890 Series II gas chromatograph equipped with an electron capture detector and a glass column (200 cm long, 0.2-cm i.d.) packed with Porapak QS (100–120 mesh). Nitrous oxide was detected up to 0.04 µL L–1 at a signal-to-noise ratio of >10, and the high specificity of the detector ensured no interference with other gaseous compounds.

This lysimeter experiment was not replicated and the presented results are a mean of repeated measurements with standard deviation.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Under steady state water flow and biological reactions, removal of organic matter and nitrogen from river water applied to the lysimeters was estimated by measuring COD and concentrations of NH4, NO3, and NO2 in water samples collected from various depths.

Organic matter removal during river water flow through the sediment profile was similar in all three lysimeters (48, 54, and 68 mm d–1 flow rates) as shown in Fig. 3 . Most of the organic matter degradation occurred in the top 40 cm, and after flowing through 120 cm of sediment profile about 70% of the COD in the river water applied was removed in all three of the lysimeters. Since BOD in the river water applied to the lysimeters was 70% of COD, the results shown in Fig. 3 indicate that most of the biodegradable organic matter was removed in the top 40 cm during the flow through the lysimeters. Although different amounts of organic matter were introduced in the lysimeter using the different water flow rates, removal efficiency of organic matter was not different among the three flow rates. This result indicates that the effect of retention time on the organic matter degradation is minimal in all three lysimeters. After its partial disappearance due to microbial activity, the COD remained constant even though O2 and other electron acceptors (NO3 and NO2) were still present (Table 2 and Fig. 4) . The remaining COD must be mainly due to its nonbiodegradable nature. Also, a decrease in microbial activity or population along the depth of the lysimeter could be another reason for the negligible organic matter degradation at depths below 40 cm.



View larger version (22K):
[in this window]
[in a new window]
  		Fig. 3. Removal of chemical oxygen demand (COD) at various flow rates of river water in the model floodplain filtration system. Flow Rates I, II, and III were 40.8, 54.4, and 68.0 mm d–1, respectively. Error bars indicate standard deviation.

 

View this table:
[in this window]
[in a new window]
  		Table 2. Summary of redox potential (Eh) and concentrations of dissolved oxygen (DO), NH4–N, and NO2–N in the water at various depths in the model floodplain systems of different water flow rates.{dagger}

 


View larger version (24K):
[in this window]
[in a new window]
  		Fig. 4. Removal of NO3–N at various flow rates of river water in the model floodplain filtration system. Flow Rates I, II, and III were 40.8, 54.4, and 68.0 mm d–1, respectively. Error bars indicate standard deviation.

 
Together with the intensive degradation of organic matter at the top 5 cm of the lysimeter sediment, the concentration of DO was remarkably decreased at this depth and Eh was lowered to around 200 mV, where denitrification can be induced (Table 2). In all of the lysimeters, concentrations of DO in the water collected from the sediment profile were lower than the atmospheric equilibrium concentration in the applied river water. Nearly 90 to 93% of DO in the river water was consumed at this depth. Concentration of DO in the sediment profiles of the different flow rates was higher where the flow rate was lower. Such higher concentrations of DO at the lower flow rates might be due to less organic matter degradation and higher inflow of gaseous O2 under less saturation of the sediments at lower water flow rates. Although the average Eh of river water applied to the lysimeters was 294.8 mV, Eh values measured at a depth of 5 cm were 205 to 225 mV for all three lysimeters. Through the sediment profile below 25 cm at Flow Rates I and II and below 5 cm at Flow Rate III, Eh levels were lowered to around 200 mV, where denitrification can occur. In the lysimeters with Flow Rates I and II, the Eh of the top 25 cm of sediment was in the range of 205 to 225 mV (Table 2). Such values indicate that the sediments in lysimeters at Flow Rates I and II are under more oxidizing or less reducing conditions than the sediment in the lysimeter with Flow Rate III. This result could also be due to the higher inflow of gaseous O2 under reduced sediment saturation conditions caused by lower water flow rates.

As opposed to microbial degradation, physical filtration of organic matter could account for the removal of COD found along the lysimeter depth. In our experiment, about 50% of the COD removal occurred in top-5-cm depth as shown in Fig. 3. Also, most of the dissolved oxygen removal occurred at this same depth (Table 2). Removal of COD and DO at this depth were about 6 and 5 mg L–1, respectively. Considering the possibility of some additional oxygen supply from the atmosphere, most of the organic matter removal from river water can be attributed to microbial degradation. Although we cannot completely exclude the possibility of physical filtration of organic matter in the lysimeters, that would be minimal.

Oxidative degradation of organic matter at the surface layer of the lysimeter consumed most of the available O2 and biogeochemical conditions favorable for denitrification were developed in the depth below the oxidizing surface layer. Changes of NO3 concentration during the river water flow in all three lysimeters are shown in Fig. 4. Nitrate removal at the top-25-cm depth was negligible for all three flow rates. At this depth, since O2 could be preferentially used as an electron acceptor during the organic matter degradation, any large removal of NO3 by denitrification was not accomplished. The removal of NO3 at Flow Rate I was negligible and not efficient at the 120-cm depth of the lysimeter. At Flow Rate II, the removal of NO3 was also not efficient, and the concentration of NO3 was lowered by only 1 mg N L–1 after flowing through 120 cm of the sediment profile. Comparing the concentrations of NO3 in the inflow and outflow waters, only 17% of NO3 was removed at Flow Rate II. But in the lysimeter with Flow Rate III, even though the retention time was shorter at the faster flow rate, removal of NO3 was much higher than those in the lysimeters with Flow Rates I and II. Most of the NO3 removal at Flow Rate III occurred at a depth of 25 to 50 cm, just below the oxidizing surface layer, and further continuous removal of NO3 was found at lower sediment profiles. After flowing through 120 cm of the sediment profile, about 40% of NO3 introduced in the lysimeter had been removed at Flow Rate III.

In the lysimeters with Flow Rates I and II, the reason for the negligible removal of NO3 seems to be the sufficient availability of O2 as an electron acceptor during the degradation of most of the biodegradable organic matter. When O2 is used preferentially as an electron acceptor instead of NO3 during the degradation of organic matter, removal of NO3 by denitrification, together with the removal of organic matter, is not possible (Dendooven et al., 1994). As mentioned earlier, at Flow Rates I and II, the lysimeters were less saturated than at Flow Rate III, and inflow of O2 by diffusion through the pores would be relatively more extensive. As shown in Table 2, DO at Flow Rates I and II was not depleted as much as in Flow Rate III. The O2 supply rate from the atmosphere is closely related to the air content of the sediment and can be controlled by the water application rate (Ouyang and Boersma, 1992). At Flow Rate II, although O2 was the main electron acceptor during the organic matter degradation in the top 50 cm of the sediment profile, some NO3 could be used as an electron acceptor at sediment depths below 50 cm.

The greater depletion of O2 in the surface layer could be the main reason for the remarkable removal of NO3 in Lysimeter III. Such depletion of O2 could be due to the greater degradation of organic matter introduced by the higher water flow rate and also due to the effective limiting of O2 supply from the surface atmosphere under more saturated conditions in Lysimeter III.

Although DO was lower in the higher flow rate lysimeter, the Eh values in Table 2 show similar changes at the sediment profile below 50 cm for all three flow rates. These similar Eh values could be due to the fact that the potential does not decrease much until a chemical species controlling the redox potential is nearly exhausted. Thus, at all of the flow rates, redox potential was lowered to a level where NO3 can be reduced. However, DO was still preferentially used as an electron acceptor in the lower flow rate lysimeters, but more NO3 was consumed as an electron acceptor in the highest flow rate lysimeter, where DO was more depleted.

In the lysimeter with Flow Rate III, no further intensive removal of NO3 by denitrification was found in the lower sediment profile below 50 cm. The reason for such negligible denitrification activity was a lack of any electron donor (i.e., biodegradable organic matter). Degradation of organic matter in sediment profiles below 50 cm was negligible (Fig. 3). When dissolved organic carbon concentration in ground water is low, the denitrification rate is limited by dissolved organic carbon as an electron donor (Tsushima et al., 2002). DeSimone and Howes (1998) and Smith and Duff (1988) also found that denitrification rates are significantly correlated with dissolved organic carbon concentrations in a shallow aquifer receiving wastewater discharge or treated sewage. Therefore, organic matter could be a limiting factor in the removal of NO3 in this model floodplain system, especially in the lysimeter with Flow Rate III.

Concentrations of NH4 and NO2 ions measured at various depths in the sediment profile are shown in Table 2. In all three of the lysimeters, about 70% of the NH4 introduced was removed in the top 5 cm, where redox conditions were favorable for the oxidation of NH4. Therefore, the NH4 could be oxidized to NO3 at the surface layer, and then leached down to deeper layers and denitrified. Concentrations of NH4 in further deeper sediment profiles of the three lysimeters showed little change. The measured concentrations of NH4 at various depths would reflect the balance of organic matter degradation, leached NH4 contained in river water, and oxidation of NH4 to NO3 by nitrification. At deeper sediment profiles (below 15 cm), NH4 concentration remained relatively unchanged, probably due to the negligible degradation of organic matter and redox conditions unfavorable for the oxidation of NH4. Most of the NO2 ion introduced with river water was also removed in the surface sediment layer (Table 2) and NO2 was not detected at the 5- to 15-cm depth. But in the lysimeters with Flow Rates II and III, some NO2 was found at lower sediment profile depths. This finding indicates that the denitrification process would occur continuously at sediment profiles beyond a 15-cm depth, but reduction of NO2 ion to gaseous N2O or N2 was not completed with less availability of degradable organic matter as an electron donor (Cho et al., 1997).

During denitrification, NO3 is converted mostly to N2 and/or N2O and removed to the atmosphere. Concentrations of N2 in Table 3 show the difference between N2 concentration in the lysimeter gas and in the atmosphere. However, the increase of absolute concentration of N2 in the lysimeters above ambient levels cannot be inferred as an indication of denitrification because differences in the rate of O2 consumption and CO2 production as well as CO2 dissolution in water would cause elevated N2 concentrations (Rolston et al., 1976). In this experiment, however, organic matter degradations were nearly the same in all three lysimeters as shown in Fig. 3, and we can imagine that the elevated concentration of N2 is proportional to the transformation of NO3 by denitrification (Fig. 4).


View this table:
[in this window]
[in a new window]
  		Table 3. Production of N2O and N2 at various depths in the model floodplain systems of different water flow rates.{dagger}

 
Among the three lysimeters, the elevated N2 concentration was highest at Flow Rate III. Also, the elevated N2 concentrations at Flow Rate II were slightly higher than those at Flow Rate I. These results correspond well to the removal of NO3 in the three lysimeters as shown in Fig. 4. The distribution of N2 concentration in the sediment profiles of all three lysimeters indicates that denitrification occurs from the surface down to deeper sediment layers.

Concentrations of N2O in the gas samples were much higher than the values in the atmosphere (0.03–0.04 µL L–1). This result again indicates that denitrification occurs in all of the lysimeters. Production of N2O was relatively higher in the lysimeter with Flow Rate III, and this result corresponds well to the removal of NO3 as shown in Fig. 4. The higher concentrations of N2O at Flow Rate III could be also due to the lack of an electron donor in the deeper sediment layer where organic matter degradation was minimal.

Considering N2 and N2O production in the lysimeters, although the removal of NO3 was negligible at Flow Rates I and II, the denitrification process was still in progress. At Flow Rate I the production of NO3 during organic matter degradation could be nearly identical to the amount of NO3 removed by denitrification under steady water flow and reactions rates.

Since N2O is not the only end product of denitrification, it can be further reduced to N2 where degradation of organic matter actively occurred. Denitrification may reduce the potential of NO3 contamination in ground and surface waters, but gaseous emissions of NOx are of concern from an environmental standpoint (Cicerone, 1987; Yung et al., 1976). As mentioned earlier, available organic matter was a limiting factor for denitrification in the lysimeters, where the organic matter introduced with the river water could supply most of the electrons consumed in denitrification. If this floodplain filtration technique were applied in a native floodplain, we expect that N2O production would be further reduced with enough organic matter supplied from the rhizosphere of vegetation.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Considering the above results and taking into account the characteristics of floodplains, the floodplain filtration technique shows great promise in the simultaneous removal of organic matter and nitrogen from contaminated river water. By using an appropriate water application rate, a denitrifying zone develops beneath the aerobic zone due to the depletion of O2 in this region and the contaminated river water may infiltrate through the aerobic and denitrifying zones, removing both organic matter and nitrogen from the river water. In this model experiment, most of the biodegradable organic matter was removed, but only about 40% of NO3 was removed and the most important limiting factor in the removal of nitrogen by denitrification was the availability of biodegradable organic carbon.

In the model floodplain system used in this experiment, vegetation was not included on the top of the sediment profile. However, natural floodplains contain a rhizosphere of vegetation at the top and are expected to be more effective in the removal of NO3. This floodplain can convert NO3 into plant biomass or into N2 and N2O via denitrification. In the rhizosphere, organic matter content would be higher and O2 consumption would be more rapid, thus creating more favorable conditions for denitrification. Therefore, if the floodplain filtration technique is applied to a natural floodplain, we expect a more significant removal of nitrogen than that estimated in the model system.

When this river water treatment technique is applied under field conditions, the characteristics of river water and floodplain sediment profile should be considered to determine an appropriate water application rate for simultaneous removal of organic matter and nitrogen. In addition, environmental conditions, including vegetation and temperature that would affect water balance during the infiltration, should be further considered.


   ACKNOWLEDGMENTS
 
This work was supported by Korea Research Foundation Grant KRF-99-042-E00089.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


Related articles in JEQ:

This Issue in Journal of Environmental Quality

JEQ 2004 33: 799-804. [Full Text]  




This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Related articles in JEQ
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Web of Science (3)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Chung, J.-B.
Right arrow Articles by Lee, Y.-D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Chung, J.-B.
Right arrow Articles by Lee, Y.-D.
Agricola
Right arrow Articles by Chung, J.-B.
Right arrow Articles by Lee, Y.-D.
Related Collections
Right arrow Watershed and Landscape Processes
Right arrow Surface Water Quality
Right arrow Biogeochemical Processes
Right arrow Lysimeter/Rhizosphere Studies
Right arrow Bioremediation and Biodegradation
Right arrow Water Pollution

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome