ABSTRACT Nitrate-nitrogen (NO3-N) contamination in groundwater has been worldwide concern, and especially it is a critical issue for the region depending on groundwater for water supply. Kumamoto city depends on only groundwater for all of water supply, however, lowering groundwater level and increasing NO3-N concentration have been revealed recently. In this study, the current situation of NO3-N contamination in the groundwater of Kumamoto city was investigated. Nitrate-nitrogen was detected in the range from 0.2 to 5.8 mg/l in the groundwater of deep wells at 22 points out of total 30 sampling points including drinking water resource. The NO3-N concentration was closely related to the location of wells and groundwater flow. The upward tendency in NO3-N concentration was recognized at a number of wells for recent 20 years, and it was considered due to the agricultural activity in the corresponding area and their upstream areas.
Trang 1Present Status and Feature of Groundwater
Contamination by Nitrate-nitrogen in Kumamoto City
Kazuo TOMIIE*, Yasuhiro IWASA**, Kaori MAEDA**, Michiko OTSUZUKI**,
Tsutomu YUNOUE**, Ryuji KAKIMOTO*, Yasunori KAWAGOSHI*
*Department of Civil and Environmental Engineering, Kumamoto University, Kurokami 2-39-1
Kumamoto, 860-8555, Japan
**Kumamoto City Waterworks Bureau, Suizenji 6-2-45, Kumamoto, 862-8620, Japan
ABSTRACT
Nitrate-nitrogen (NO 3 -N) contamination in groundwater has been worldwide concern, and especially it is a critical issue for the region depending on groundwater for water supply Kumamoto city depends on only groundwater for all of water supply, however, lowering groundwater level and increasing NO 3 -N concentration have been revealed recently In this study, the current situation of NO 3 -N contamination in the groundwater of Kumamoto city was investigated Nitrate-nitrogen was detected in the range from 0.2 to 5.8 mg/l in the groundwater
of deep wells at 22 points out of total 30 sampling points including drinking water resource The
NO 3 -N concentration was closely related to the location of wells and groundwater flow The upward tendency in NO 3 -N concentration was recognized at a number of wells for recent 20 years, and it was considered due to the agricultural activity in the corresponding area and their upstream areas
Keywords: groundwater, contamination, nitrate-nitrogen (NO3 -N), water quality, drinking water
INTRODUCTION
Kumamoto city is an exceptional midsize city, in that 100 % of water supply for about
700 thousands people depends on only groundwater In addition, water quality of groundwater in Kumamoto city is quite excellent and renowned as good-tasting water in Japan (Hashimoto, 1989) However, decline in the water amount caused by the decrease
of a recharge area has been addressed in recent years, and also water quality aggravation such as nitrate contamination is currently identified (Kumamoto City Water Works, 2006; Tsuru et al, 2006) Nitrate contamination of groundwater is a worldwide problem (Tayfur et al, 2008; Shomar et al, 2008; Hu et al, 2005; Nas and Berktay, 2006; Showers
et al, 2008; Kumazawa, 2002), especially in agriculture area caused by over fertilizing and insufficient control of livestock-waste
Kumamoto city and the surrounding districts are also agricultural areas, and the gradual elevation of nitrate concentration has become obvious in the groundwater of these areas (Kumamoto City Water Works, 2006) The drinking water standard and ground water quality standard of nitrate nitrogen (NO3-N) in Japan were both determined below 10 mg/l according to the recommendation by World Health Organization (WHO) based on the relationship between nitrate intake and methemoglobinemia in infants (WHO, 1970) Since groundwater is the only drinking water resource for Kumamoto city, nitrate contamination of the groundwater will be a serious issue for the future, thus the urgent study on present status and countermeasures must be required In addition, knowledge obtained from the study on nitrate contamination in Kumamoto city-groundwater might
Address correspondence to Yasunori Kawagoshi, Department of Civil and Environmental Engineering,
Trang 2be valuable for water environmental administration of other districts where groundwater
is important water resource In this study, the present status and long-term change of nitrate-nitrogen concentration in Kumamoto city-groundwater are investigated and discussed
MATERIALS AND METHODS
Subsurface geological structure and groundwater flow in Kumamoto city area
Kumamoto prefecture is located at center of Kyushu area in southern part of Japan (Fig 1) Kumamoto city is the prefectural capital with a 670,000 population and the 267
km2 area Kumamoto city depends on 100 % of water supply with only groundwater
Kumamoto prefecture
Kumamoto City
Kyushu area
Fig 1 - Location of Kumamoto city in Japan
The bed rock in Kumamoto city and its surrounding areas consist mainly of the Pre-Aso volcanic rock, and Aso-pyroclastic flow deposits, which are called Aso1, Aso2, Aso3 and Aso4, cover it in chronological order The most important groundwater from the perspective of water resource is the second aquifer which flows mainly in the Aso3-lava bed There are some impermeable beds called Futa-bed and Hanabusa-bed between Aso-3 bed and Aso-4 bed in places, however, all of these areas are not always covered with impermeable beds Therefore the influent water from a ground surface can easily reach the second aquifer through the first aquifer (Aso-4 bed) in some places where there is no impermeable bed Figure 2 shows partial subsurface geological structure and groundwater flow in the second aquifer in Kumamoto city (Nagai et al, 1983; Kumamoto prefecture and Kumamoto city, 2005) with sampling points In Kumamoto city, the areas except western part are covered with the Aso-1~4 lava beds, and the middle area from northeast to the central part lacks the impermeable bed between Aso-3 bed and Aso-4 bed The northwestern area has Kimbo volcano where the geological condition is different from those of Aso-lava beds and the third aquifer flows in this area The southwestern part is alluvial area covered with muddy sediment, thus there is not any Aso-lava beds in this part
Groundwater flow in the second aquifer in Kumamoto city is roughly indicated by the arrows in Fig 2 The main water flow is from the western foot of Mt Aso to Ariake Sea through the middle-southern part of the city The area called “Shira river-midstream basin” which extends across Ozu town and Kikuyo town located between Mt Aso and Kumamoto city is also considered as an important groundwater recharge zone In addition, the groundwater flows from Kikuchi plateau (north east) and Ueki plateau (north west) contribute the water resource for Kumamoto city
Trang 3Mountain or volcano Area with impermeable bed Alluvial area
Area without impermeable bed
Kinbo volcano
Mt Aso
Shira
River
Ariake
Sea
Ueki Plateau 1 Kikuchi Plateau
5 3
6
7
8 9 10 11
12
13
14
20 21
18 23
24
25 26
27
19 28
22
29 30
15
1
5 3
6
7
8 9 10 11
12
13
14
20 21
18 23
24
25 26
27
19 28
22
29 30
15
Lake Ezu
Fig 2 - Partial subsurface geological structure and groundwater flow in the second
aquifer (the third aquifer in Kinbo volcano area) in Kumamoto city with sampling points The allows indicate groundwater flow
Location and grouping of sampling points based on groundwater quality characteristics
Figure 3 shows the locations of groundwater sampling points and the regional groups where water-quality characterization analysis was performed previously (Tsuru et al, 1998; Kawagoshi et al, in submission) A total 40 groundwater samples at 30 sampling points were collected and investigated during high-water season (September to November) in 2006 The depth and purpose of the wells are summarized in Table 1 The regional groups characterized by water quality are almost depending on their location and groundwater flow The distinctive water quality characteristics of each group is as follows: Group 1: dissolved components (DCs) amount is low and soluble silicate concentration is high, Group 2: concentrations of F ion and boron are high, Group 3: DCs amount is high, Group 4: sulfuric ion concentration is high, Group 5: F ion is involved, Group 6: Cl ion concentration is high and reductive atmosphere, Group 7: DCs amount is quite low (main groundwater flows in the third aquifer and water quality characterization was conducted using the third aquifer-groundwater)
Nitrate-nitrogen analysis
The NO3-N concentration in the groundwater was determined by using ion exchange chromatography according to Japanese Standard Methods for Examination of Water (Japan Water Works, 2001)
Trang 4Table -1 List of the wells for survey in this study
Mt Aso
Ariake
Sea
Ueki Plateau 1 Kikuchi Plateau
5
7
8 9 10 11
12
13
14
20 21
18 23
24
25 26
27
19 28
22
29 30
15
1
5
7
8 9 10 11
12
13
14
20 21
18 23
24
25 26
27
19 28
22
29 30
15
Group 7
Group 1
Group 3
Group 4 Group 5
Group 6
Group 2
Group 7
Group 1
Group 3
Group 4 Group 5
Group 6
Group 2
Fig 3 - Location of sampling points and grouping characterized by the water quality
of groundwater
Trang 5RESULTS AND DISCUSSION
Overall situation of groundwater contamination by NO 3 -N in Kumamoto city
Figure 4 shows NO3-N concentration in the groundwater at each sampling point in autumn 2006 Only data for deep well are shown for the sampling points of nos 2, 16,
18, 19, 21, 23, 25-27 which have both deep well and shallow well Nitrate-nitrogen was principally detected at 22 sampling points located in the area except middle- and southwestern-part The NO3-N concentration range was 0.2~5.8 mg/l, and average and median were 3.0 and 3.1 mg/l, respectively
Figure 5 shows the NO3-N concentrations in the deep well and shallow well at nos 2-27 sampling points mentioned before Nitrate-nitrogen was not detected even in shallow wells in the southwestern part (nos 23, 25-27), but detected in shallow wells in the middle part such as nos 19 and 21 In other areas (nos 2, 17-19), NO3-N concentration
in shallow wells was higher than that in deep well
Figure 6 shows the long-term variation in NO3-N concentration in deep well at the sampling points representing each water quality-group The NO3-N concentration remained almost the same level for about recent 10 years at nos 17 and 30 sampling points, but upward tendency was observed in other sampling points
Mountain or volcano Area with impermeable bed Alluvial area
Area without impermeable bed
Mt Aso
Ariake
Sea
Ueki Plateau 1 Kikuchi Plateau
5 3
6
7
8 9 10 11
12
13
14
20 21
18 23
24 25
26 27
19 28 22
29 30
15
0 4 8 10 0 4 8 10
0 4 8 10
0 4 8 10 0
4 8 10
0 4 8 10
0 4 8 10
0 4 8 10
0 4 6 10 0 4 6 10 0 4 8 10
0 4 8 10
0 4 6 10
0 4 8 10
0 4 8 10
0 4 6 10
0
4
8
10
0 4 8 10
0 4 8 10 0
4 8 10 0 4 8 10
0 4 8 10
0 4 8 10
0 4 8 10
0 4 8 10
0 4 8
10 0 4 8 10
0 4 8 10
0 4 8 10
0 4 8 10
Fig 4 - Concentration of NO3-N in groundwater of the second or third aquifer (nos
29 and 30) in Kumamoto city (autumn, 2006)
Trang 6Mountain or volcano Area with impermeable bed Alluvial area
Area without impermeable bed
Mt Aso
Ariake
Sea
Ueki Plateau Kikuchi Plateau
2
21 16 18 23
25 26
27
19
0 2 4 6 8
0 2 4 6 8
0 2 4 6 8
0 2 4 6 8
0 2 4 6 8
0 2 4 6
2 4 6 8
0 2 4 6 8 0 2 4 6 8
Fig 5 - Concentration of NO3-N in groundwater of shallow well (black bar) and deep
well (white bar) at some sampling points in Kumamoto city (autumn, 2006)
Situation at northern part (Group 1 and Group 2 areas)
In this part, NO3-N was detected at any sampling points in a concentration range from 3.2 to 5.4 mg/l It is presumed that the main groundwater flow into the second aquifer of Group 1 area is from Ueki plateau, and that of Group 2 area is from Kikuchi plateau Additionally, the water flow from Kinbo volcano into no.3 point and the water flow from Shira river-midstream basin into nos 6 and 7 points are also presumed The
NO3-N concentration in this area was relatively higher than that in other areas, suggesting the presence of a contamination source of NO3-N in this area or upstream site There is impermeable bed in this area except no 7 point, thus direct NO3-N input from a land surface in this area On the other hand, it was reported that the groundwater
at some places in Ueki plateau area was highly polluted with NO3-N caused by over fertilizing or inappropriate treatment of livestock wastes (Kumamoto prefectural institute of public-health and environmental, 2000) Kikuchi plateau is also known as an active agricultural area, and crops such as melon and watermelon are famous as a brand-name Therefore, high NO3-N concentration in the groundwater of this part is considered mainly due to the groundwater inflow from these upstream areas In Ueki city, various measures have been continued to prevent NO3-N contamination of groundwater such as fertilizer-management for about recent 10 years, however, the
NO3-N concentration is still increasing as shown in Fig 6 Needless to say, continuous monitoring of the groundwater is needed in this area, moreover, further cross-regional efforts to prevent NO3-N pollution in the upstream areas should be required
Trang 7Group 7
Group 1
Group 3
Group 4
Group 5
Group 6
Group 2
0 1 2 3 4 5
95 97 99 01 03 05
0 1 2 3 4
85 88 91 94 97 00 03 06
0 1 2 3 4
95 97 99 01 03 05
0 2 4 6 8
85 88 91 94 97 00 03 06
0 1 2 3
95 97 99 01 03 05
0 1 2 3 4
85 88 91 94 97 00 03 06 0
1
2
85 88 91 94 97 00 03 06
0
1
2
95 97 99 01 03 05
No.7
No.9
No.11
No.15 No.17
No.22
0
0.1
0.2
0.3
0.4
85 88 91 94 97 00 03 06
No.28
No.2
No.30
Fig 6 - Long-term variation in NO3-N concentration of groundwater at the sampling
points representing each water-quality group
Situation at eastern part (Group 3 area) and southeastern part (Group 4 area)
The groundwater in these areas is the most important as the water resource for Kumamoto city There is a huge lava bed called Togawa lava bed lying beneath the Group 4 area, and a large amount of groundwater flow into Togawa lava bed from various directions Nitrate-nitrogen was detected at all sampling points as well as in the northern part, and the highest concentration was detected at no 9 point in the Group 3 area At least, it seems that the groundwater is contaminated before it flows into the Group 4 area because the NO3-N concentrations were high and showed upward tendency in the zone having impermeable bed (nos 13-16 points) At nos 16 and 18 sampling points, the NO3-N concentration in shallow well was higher than in deep well
as shown in Fig 5, indicating that the groundwater in the first aquifer was contaminated
by NO3-N in this area The groundwater flow in the first aquifer is determined to be almost same as that in the second aquifer in the Group 3 and 4 areas (Kumamoto prefecture and Kumamoto city, 2005), thus it is presumed that the groundwater in the shallow well is also contaminated in upstream area The NO3-N concentration has been flat at no 17 point for recent 10 years, suggesting no further NO3-N pollutant source and the possibility of denitrification activity around here as well as the Group 5 and 6 areas described below While the Group 4 area is regarded as being groundwater storage area, the Group 3 area plays the role of groundwater recharge especially in the place
Trang 8around nos 8 and 9 points because of no impermeable bed there In addition, the Group
3 and further eastward areas are covered with vast upland field as shown in satellite pictures given by Kumamoto city government (Home page of Kumamoto city) and dotted with many livestock farmers Therefore, not a little influence of these agricultural activities on the NO3-N contamination is considered In fact, the wells containing
NO3-N of over 10 mg/l have been found in the Group 3 area (Tsuru et al, 2005; 2006) The recharge amount of groundwater in the Shira river-midstream basin shows downward trend (Kumamoto prefecture and Kumamoto city, 2005; Ichikawa et al, 1995) due to the decrease of paddy fields, thus its effect on the groundwater quality has also been concerned Furthermore, some paddy fields have been turned to upland fields, which may influence the NO3-N contamination of groundwater In any cases, more elaborate study should be conducted as soon as possible to ascertain the source and mechanism of NO3-N contamination of the groundwater in these areas
Situation at middle (Group 5 area) and western parts (Group 6 and Group 7 areas)
In the Group 5 area, NO3-N was detected at only one sampling point of no 19 located near Group 2 area Other two sampling points are located at right in the middle of the city where the impermeable bed is lacking and also are nearby the alluvial area On the other hand, the NO3-N concentration in shallow well was relatively high at no 19 point, and the downward tendency of NO3-N concentration in shallow well of no 19 to no 21 points was observed as shown in Fig 5 In addition, the dissolved oxygen (DO) concentration in the groundwater at no 22 point was 0.6 mg/l (July, 2006) which was obviously lower than that in other areas (7.0~8.4 mg/l), suggesting it is reductive atmosphere in the subsurface environment of Group 5 and Group 6 areas From these results, the following hypothesis was proposed: the groundwater in the first aquifer of this area is contaminated in upstream areas such as Group 1 and 2 areas, and additionally with direct inflow from a ground surface right above this area, however, the
NO3-N is removed by biological denitrification under anaerobic condition (reductive atmosphere) during the water permeate through the soil
This hypothesis will be supported by the situation of NO3-N pollution in the Group 6 area The NO3-N was detected in low concentration level at only nos 24 and 28 points, but not detected at other points even in shallow wells The contribution of biological denitrification activity to nitrate removal in the soil environment was well-known so far (Tayfur et al, 2008; Shomar et al, 2008) The muddy sediment in the alluvial area includes reduced sulfur and organic compounds as an electron donor for NO3-N reduction, suggesting that this area has enough capability of NO3-N removal In addition, low-permeable property of muddy soil might contribute to prolongation of retention time of groundwater, which results in promoting a biochemical reaction However, since the NO3-N concentration at nos 22 and 24 points showed upward tendency for recent 20 years as shown in Fig 6, the future trend will have to be careful even in this area
In the Group 7 area, the NO3-N was detected at all points However, their
concentrations were low and showed flat in recent 10 years as shown in Fig 6, thus it is not considered to be in severe situation at this time
Trang 9CONCLUSIONS
Current situation of NO3-N contamination of groundwater in Kumamoto city was investigated and the following insights were obtained
1) Nitrate nitrogen was detected in the groundwater at 22/30 points in various area of Kumamoto city
2) The NO3-N concentration was in the range from 0.2 to 5.8 mg/l, which was presumed to be related with area-location and groundwater flow
3) Upward tendency in NO3-N concentration was recognized in the groundwater at most of wells that NO3-N was detected
Finally, The NO3-N concentration level in the Kumamoto city groundwater is considered to be not critical situation, however, it still shows continued increase and there is no denying that it will exceed the drinking water quality standard in near future The earliest possible every effort to determine the primary pollutant source and to reduce the pollution should be conducted before it is too late
ACKNOWLEDGEMENTS
The authors thank Mr Katsunobu Onitsuka and Mr Shinichi Kanehira of Kumamoto City Water Preservation Section for kindness and support to carry out this study
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