Mitsuyo Saito¹, Shin-ichi Onodera² , Misa Sawano²

1 GSPS research fellow, Graduate School of Biosphere Sciences, Hiroshima University, 1-7-1, Kagamiyama, Higashi-Hiroshima, 739-8521, JAPAN, Tel＆Fax: +81-824-24-6496, E-Mail: misaito@hiroshima-u.ac.jp

2 Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1, Kagamiyama, Higashi-Hiroshima, 739-8521, JAPAN,

Tel＆Fax: +81-824-24-6496, E-Mail: sonodera@hiroshima-u.ac.jp

 

Abstract:The authors conducted this study to demonstrate nitrate-nitrogen (NO₃^--N) attenuation process in the coastal aquifer of a small mountainous catchment with intensive agricultural area. in the study area, no₃^--n attenuation along the flow path was confirmed in both shallower groundwater with 2~5m depth and deeper groundwater with 20~30m depth.the decrease of no₃^--n and so₄^2--s concentrations and increase of hco₃^--c concentration in the downstream area suggest the denitrification and sulfate reduction process in the groundwater.δ¹⁵n suggests the isotope enrichment by the denitrification process in the deeper groundwater. on the other hand, such trend has not confirmed in the shallow groundwater. this result implies the mixing of lowδ¹⁵n water such as rainwater with the shallower groundwater as well as denitrification process.

Keywords: nitrate, attenuation process, coastal aquifer, nutrient, nitrogen stable isotope

 

Introduction

Nitrate (NO₃^-) is a widespread pollutant derived from human activities. Many studies have confirmed that agricultural practices such as fertilizer application have resulted in nitrate contamination of groundwater (Burt et al., 1993; Mueller et al., 1995; Böhlke, 2002; Tase, 2006).

On the other hand, previous studies have shown the occurrence of nitrate attenuation by denitrification (microbial reduction of NO₃^- to N₂) in groundwater of the riparian wetlands (Hill et al., 2000; Böhlke et al., 2002), floodplain (Fustec et al., 1991; Tesoriero et al., 2000) or coastal sand dune (Uchiyama et al., 2000). However, the detailed process of nitrate attenuation is little known in these studies.

The objective of this study is to demonstrate the nitrate attenuation process in the coastal aquifer with intensive agricultural area.

2 Materials and Methods

2-1 Site description

The study catchment is located in the coastal area of Ikuchijima-Island within the Seto Inland Sea, southern Japan (Fig.1a, b). Seto Inland Sea is Japan’s biggest inland sea while eutrophication of seawater is also regarded as one of the main environmental issue in Japan. The coastal area of this sea is characterized by temperate, marine climate and wide expanses of orange groves.

In the study area, annual mean precipitation and temperature are 1100mm and 15.6℃ respectively. The area of study catchment is 44ha underlain by granite (Fig.1c). Orange groves cover 42% of the catchment area. This catchment is also characterized by relatively steep topography with relief ratio of 0.24 and alluvial fan deposits in the midstream and the downstream area.

2-2 Field observation

In April 2006, shallower groundwater samples with 2~5m depth and deeper groundwater samples with 20~30m depth were collected from 9 dug wells and 5 pumping wells located in the midstream and the downstream area of the study catchment (Fig.1c). Also river water sample was collected near the river mouth (Fig.1c).

Electric conductivity, pH and Oxidation-reduction potential (ORP) of water samples were measured in the field using portable meter.

2-3 Water analysis

 

 

 

 

 

 

 

 

					(b)
	(a)
							(c)
			(a)
			(b)
			(c)

	Experimental profile line

 

(b)

(b)

Water samples were analyzed for chemical and isotopic components in the laboratory. HCO₃^- concentration was determined by H₂SO₄ (0.01N) titration. NO₃^- and SO₄^2- concentrations were measured using ion chromatography after filtered samples by 0.20μm cellulose ester filter. Total phosphorous (TP) concentrations were determined by colorimetric method using spectrophotometer. Nitrogen stable isotope was measured in aqueous NO₃^- of water samples by mass spectrometer.

3 Results and Discussion

3-1 Nitrate attenuation along the groundwater flow path

(a)

In the study area, groundwater flows from the mountainside to the ocean side (Saito et al., 2005). Fig.2 shows the variations of NO₃^--N and TP concentrations and ORP along the flow path in shallower groundwater collected from the wells located on the experimental profile line in Fig.1c. In the midstream area, NO₃^--N concentrations were higher than the criteria of drinking water quality (NO₃^--N = 0.7meq L^-1), while it decreased in the downstream area with values of less than 0.03meqL^-1 (Fig.2a). Also same trend is confirmed in the deeper groundwater. TP concentrations slightly increased in the downstream area (Fig.2b). ORP of groundwater in the downstream area were relatively low (less than +300mV) compared to that in the midstream area with the value of nearly +400mV (Fig.2c).

y = -0.5x

(a)

 

y = -0.8x

 

These results imply that reductions of NO₃^--N and elution of phosphorous occurred in the groundwater of the downstream area which is relatively reductive condition.

3-2 Reduction processes of nitrate and sulfate in the groundwater

Some previous studies have shown that decrease of NO₃^--N concentrations in groundwater can be attributed to biochemical denitrification process (Hill et al., 2000; Böhlke et al., 2002).The reaction formula of this process is presented as follows (Böhlke, 2002):

4NO₃^- + 5C + 3H₂O = 2N₂ +5HCO₃^- +H⁺           (1)

y = -0.5x

Denitrification is the biochemical nitrate redution which occurres only in the reductive condition with sufficient organic compounds. Fig.3a shows the relation between HCO₃^--C and NO₃^--N concentrations in the shallower and deeper groundwater on the experimental profile line in Fig.1c. The line equation of y = -0.8x in this figure indicates the relationship of HCO₃^--C and NO₃^--N concentrations (NO₃^--N : HCO₃^--C = 4 : 5) in the denitrification process represented by Eq. (1). The relation between the midstream and the downstream groundwater suggests that reductions of NO₃^--N concentrations in  the downstream area can be attributed by denitrification process.

Fig.3b shows the relation between HCO₃^--C and SO₄^2--S concentrations same as Fig.3a. Groundwater in the downstream area is characterized by lower concentrations of SO₄^2--S than that in the midstream area (Fig.3b). Sulfate reduction by organic matter is presented by following formula (Appelo and Postma, 2005).

			y = -0.5x

 

2CH₂O+ SO₄^2- → 2HCO₃^- + H₂S            (2)

Generally this process occurs under more reductive condition than that in the denitrification process (Appelo and Postma, 2005). The line equation of y = -0.5x in this figure indicates the relationship of HCO₃^--C and SO₄^2--S concentrations (SO₄^2--S : HCO₃^--C = 1 : 2) in the sulfate reduction process shown by Eq. (2). This result suggests the occurrence of sulfate reduction in the groundwater of the downstream area. Also this implies the reductive condition and the possibility of denitrification process in the downstream area.

3-3 Isotopic examination for denitrification in shallower and deeper groundwater

Fig.4 shows the relation between NO₃^--N concentrations and nitrogen stable isotope ratio (δ¹⁵N) in the shallower and deeper groundwater. δ¹⁵N in the deep groundwater is approximately 6.0‰ in the midstream area, approximately 10~15‰ in the downstream area, respectively. This result suggests the isotope enrichment caused by the denitrification process in the downstream area. On the other hand, such trend has not confirmed in the shallow groundwater of the downstream area, in spite of the decrease of nitrate concentration. This result implies the mixing of lowδ¹⁵N water such as rainwater with the shallow groundwater as well as denitrification process.

4 Concluding Remarks

In order to confirm the nitrate-nitrogen (NO₃^--N) attenuation process, we investigated the variations of chemical components and nitrogen stable isotope ratio (δ¹⁵N) along the groundwater flow path.

The results show the occurrence of NO₃^--N attenuation along the flow path in both shallower and deeper groundwater.

The variations of NO₃^--N, SO₄^2--S and HCO₃^--C concentrations along the groundwater flow suggest the denitrification and sulfate reduction process in the downstream area.

δ¹⁵N variation from the midstream to the downstream area suggests the isotope enrichment by the denitrification process in the deeper groundwater. However, there is little change of δ¹⁵N in the shallower groundwater. This result implies the effect of lowδ¹⁵N water mixing such as rainwater with the shallower groundwater as well as denitrification process.

 

References

[1]      Appelo, C. A. J. and Postma, D. 2005. Geochemistry, groundwater and pollution. A. A. Balkema, Leiden. 649pp.

[2]      Böhlke, J. K. 2002. Groundwater recharge and agricultural contamination. Hydrogeol. J. 10:153-179.

[3]      Böhlke, J. K., Wanty, R., Tuttle, M., Delin, G. and Ladon, M. 2002. Denitrification in the recharge area and discharge area of a transient agricultural nitrate plume in a glacial outwash sand aquifer, Minnesota.Water Resour. Res. 38: 10-1-10-26.

[4]      Burt, T. P., Heathwaite, A. L. and Trudgill, S. T. 1993. Nitrate; Processes, Patterns and Management. John Wiley & Sons, New York. 444pp.

[5]      Fustec, E., Mariotti, A., Grillo, X. and Sajus, J. 1991. Nitrate removal by denitrification in alluvial groundwater: role of former channel. J. Hydrol. 123:337-354.

[6]      Hill, A. R., Devito, K. J., Campagnolo, S. and Sanmugadas, K. 2000. Subsurface denitrification in a forest riparian zone: Interactions between hydrology and supplies of nitrate and organic carbon. Biogeochemistry,51:193-223.

[7]      Mueller, D. K., Hamilton, P.A., Helsel, D. R., Hitt, K. J. and Ruddy, B. C. 1995. Nutrients in ground water and surface water of the United States－An analysis of data through 1992. U.S. Geol. Surv. Water Resour. Invest. Rep. 95-4031.

[8]      Saito, M., Onodera, S. and Takei, T. 2005. Nitrate transport process in a small coastal alluvial fan catchment. Japanese J. Limnol. 66:1-10 (in Japanese).

[9]      Tase, N. 2006. Groundwaer pollution by nitrate-nitrogen. Groundwater technology. 48: 31-44 (in Japanese).

[10]  Tesorieto, A. J., Liebscher, H. and Cox, S. E. 2000. Mechanism and rate of denitrification in an agricultural watershed: electron and mass balance along groundwater flow path. Water Resour. Res. 36: 1545-1559.

[11]  Uchiyama, Y., Nadaoka, K., Rölke, P., Adachi, K. and Yagi, H. 2000. Submarine groundwater discharge into the sea and associated nutrient transport in a sandy beach. Water Resour. Res. 36: 1467-1479.