Long-term change of stream water quality as a consequence of watersheddevelopment and management
T. Kinouchi1, K. Musiake1
Department of Environment System Management, Fukushima University, Japankinouchi@sss.fukushima-u.ac.jp
1. INTRODUCTION
The Abukuma River Watershed, one of major watersheds in Japan, is a basis of social,economical and cultural activities in the southern Tohoku region. Also, the watershed isabundant in natural environment and river landscape. Thus, the comprehensive watershedmanagement encompassing flood control, water resources and water quality conservationis recognized as quite significant for the sustainable development of the region.
The mainstream of the Abukuma watershed runs about 240 km from the south to thenorth, which makes the runoff concentrate against northward moving typhoons. The
spatially-averaged mean annual precipitation is about 1,400mm, which is lower than thenational average. The mainstream collects flows from a great number of sub-basins,
where wastewater is discharged from various sources such as industry, farmland, stock
farming and domestic sectors. Due to these geographical, climatic and social conditions,
the watershed has been experienced devastating floods, shortage of water resources andwater quality degradation for a long time.
The Abukuma River is recognized as one of the worst quality stream among majorwatersheds located in the Tohoku region. 24 municipalities are responsible for the water
quality conservation, which can be achieved by identifying the cause of stream
degradation and coordinated source control. To project the future stream water quality,
this study attempts to analyze the long-term change and current status in relation to thewatershed development and quality management based on historical data and fieldmeasurement.
2. STUDY AREA
The Abukuma Watershed (Figure 1) has an area of 5,600km2 and the population of 1.5
million. The population is dense along the west side of the mainstream. 69 sub-basins join
the mainstream. Coniferous and broadleaf forests, the dominant land use in uplands,
cover 60% of the entire watershed. Paddy field and cropland are the significant land use
in areas between the upland forest and the mainstream, each comprising 15% of thewatershed. Two dams and a weir are located in the mainstream reaches H-I and C-D,
respectively, for the hydropower generation. Many dams are located in sub-basins for
agriculture, drinking water and flood control. A biggest one has the net storage volume of
148 million m3. Five wastewater treatment plants (hereafter WWTPs) directly discharge
into the mainstream at W1 ∼ W5 (Figure 1).
3. LONG-TERM WATER QUALITY CHANGE
The long-term trend of stream water quality can be captured by the monthly data
monitored at multiple stations for the period of 1961–2005. Figure 2 shows monthly
water quality histories and running averages over 6 months at stations E, F, I and K

(Figure 1). It is found that the longterm change of BOD can bedivided into three stages dependingon the periods. The water quality
level in 1960’s and 70’s was quitepoor, which can be attributed to theeconomic growth accompaniedwith urban development andindustrialization. During these
periods,
heavilypolluted
wastewater from domestic andindustrial sectors was discharged
into streams without proper
regulations and treatments. Fromaround 1975 until the end of 90’s,
BOD has been greatly reduced on
annual average but still exhibited
higher values during irrigationseasons. This improvement is
explained by the enforcement ofwater quality related laws andstandards
during1967∼1970.
Under the law, the Environmental
Quality Standards (EQS) were set
for the stream water BOD andother indices such as DO and SS.
Thequalityofwastewater
Figure 1. Abukuma River Watershed with thedischarged into stream frommainstream, sub-basins and tributaries.
specific
facilities
wasalso
regulated by the water pollution control law. On the other hand, non-point sources ofnutrient from farmlands and livestock have not been subjected to the regulation, and thus
becoming the major cause of water quality degradation during this period. It can be seenthat BOD concentration and its seasonal variation were reduced since late 1990’s. This
could be explained by the improvement of the sewerage system and dissemination of
septic tanks. The percentage of people provided with the sewerage system was increased
from 27% in 1989 to 40% in 2003. However, it seems that the change in the mainstream
water quality is not sensitive to the improved quality of wastewater from the domestic
sector during the past few decades (Figure 3). It must be noted that the annual rainfall andstreamflow has a slightly increasing trend over the watershed for the same period, which
denies the possibility of decreasing natural freshwater components.
10

Miyota (E)BOD (mg/l)

BOD (mg/l)

8
6
4
2
0
61/04

66/10

72/04

83/03

88/09

94/03

99/08

BOD (mg/l)

8
6
4
2
0
61/04

05/02

Year/Month
Kuroiwa (I)10
BOD (mg/l)

77/10

14
12
10
8
6
4
2
0
61/04

66/10

72/04

77/10
83/03
Year/Month

88/09

94/03

99/08

05/02

Akutsu (F)

66/10

72/04

61/04

77/10

83/03

88/09

94/03

99/08

05/02

83/03
88/09
Year/Month

94/03

99/08

05/02

Year/Month
Fushiguro (K)

14
12
10
8
6
4
2
0
66/10

72/04

77/10

Figure 2. Monthly BOD time series for the period from 1961 to 2005.

4

3
2
1981-1985
2000-2004

1

2
1981-1985
2000-2004

0
2

3

4

5

6 7 8
Month

9 10 11 12

1

4

TN (mg/l)

32
1981-1985
2000-2004

1

2

4

5

6 7 8
Month

2

3

4

5

6 7 8
Month

9 10 11 12

0.1
1981-1985
2000-2004
1

2
1981-1985
2000-2004

2

3

4

5

6 7 8
Month

0.3

3

0

1

0.2

9 10 11 12

Takada-Bashi (H)

1

0

3

4

Takada-Bashi (H)Miyota (E)0

TP (mg/l)

1

BOD (mg/l)

3

1

0

0.3

Miyota (E)TP (mg/l)

Miyota (E)TN (mg/l)

BOD (mg/l)

49 10 11 12

Takada-Bashi (H)0.2
1981-1985
2000-2004

0.1
0

1

2

3

4

5

6 7 8
Month

9 10 11 12

1

2

3

4

5

6 7 8
Month

9 10 11 12

Figure 3. Mean monthly BOD, TN and TP for 1981-1985 and 2000-2004.
4. WATER QUALITY AT PRESENT
4.1 Field measurement
The main sources and their locations causing mainstream pollution have not yet beenclarified, although it is quite significant for appropriate and efficient water quality
management. We conducted an intensive measurement in March 2006 to reveal thesource, transport and fate of pollutants in a non-irrigated season, when the hydrologicaleffect of irrigation is negligible. We collaborated with authorities in charge of the routine
water quality measurement, i.e., the local office of MLIT, Fukushima Prefecture,
Fukushima City and Kooriyama City. This measurement covered 57 stream locations (12in the mainstream and 35 in the tributaries) and effluents from 2 WWTPs (W2 and W3).
Streamflow was measured on site or converted by the relationship between water leveland flow rate. Water was sampled for the chemical analysis of BOD, DO, pH, SS, total
nitrogen (TN), total phosphorus (TP), NO3-N, NO2-N, NH4-N, PO4-P, Cl-, chlorophyll-aand stable isotope ratio of nitrogen (δ15N).
4.2 Characteristics of stream water quality
The measured concentration of tributaries and WWTPs is presented for BOD, TN and TP
in Figure 4. BOD ranges from less than 0.5 up to more than 20 mg/l. TN is higher at themost downstream location in each sub-basin. TP at a few sites is extraordinary high, andstrongly affects mainstream TP concentrations.
In order to detect the sources of pollutant in tributaries (point or non-point, human or
natural, urban or agriculture), the relation between land use and pollutant concentrationand load was investigated for BOD, TN, NO3-N, NH4-N, particulate N, TP, PO4-P andparticulate P. Except for two plots of high BOD level, the concentration and load of BOD
are moderately related to the percentage of built area in a sub-basin (Figure 5), whichsuggest dominance of both point and non-point sources. The inverse relationship between
the BOD concentration and the percentage of forested area is more correlated. NO3-N isstrongly related to the percentage of cropland, except for one site (open circle) possibly
influenced by the upstream point source (manufacturing factories). NO3-N load per unit
basin area (NO3-N × Discharge /Area) showed little correlation to the percentage ofcropland because the flow discharge depends on the area and the presence or absence ofthe reservoir operation and snowmelt. Similarly, NH4-N and particulate N were related tothe percentage of built area and forested area with some exception, respectively.
Phosphorus was mostly influenced by a variety of human impacts, i.e., wastewater frommanufacturing industry and fish-raising industry, sediment runoff due to construction
works, and low-quality effluents from septic tanks and treatment plants. A relationshipbetween TP and the percentage of built area (Figure 5) was selected to give moderate
estimate of TP loadings. In the subsequent study of mass balance in the mainstream, thoserelationships were applied to calculate loads of BOD and nutrients from sub-basins where
we have no measurement data.

5

20

20

4

15
10
5

NO3-N (mg/l)

25

BOD (mg/l)

BOD (mg/l)

Figure 4. BOD, TN and TP concentration of tributaries and WWTPs effluents.
25

15
10
5

0
10

20

30

40

50

0

0

20

Percentage of built area (%)

40

60

80

100

0

1

4

2.0

0.8

1.5

0.6

2
1

T-P (mg/l)

2.5

3

1.0
0.5

0
20

30

40

Percentage of built area (%)

50

30

40

0.4
0.2

0.0

10

20

Percentage of cropland cover (%)

5

0

10

Percentage of forest area (%)

Particulate N (mg/l)

NH4-N (mg/l)

2
1

0
0

3

0
0

20

40

60

80

Percentage of forest area (%)

100

0

10

20

30

40

50

Percentage of built area (%)

Figure 5. Relationship between land use and concentration of BOD and nutrients. Opencircle was excluded when derive a regression curve that was used for further analysis.

4.3 Water quality change along mainstream
The longitudinal variations of BOD, TN and TP are due to the load inflowing fromtributaries, WWTPs (wastewater treatment plants) along the mainstream, and chemical,
biological and physical processes in the stream. If the flux of BOD, TN and TP decreases
as water flows down between 2 adjacent locations, it is an evidence of net removal ofpollutants by those processes.
Figure 6 shows streamflow, concentration and flux of BOD, nitrogen and phosphorus at
12 mainstream locations. Locations of wastewater discharge and main tributaries arereferred to Figure 1 and 7. Different behaviors are found for the concentration of BOD,
nitrogen and phosphorus along the mainstream. BOD increases considerably in reaches
between B-C and F-G-H. The former is owing to the inflow from agricultural tributaries,
while the latter is due to the inflowing loads from urbanized streams and WWTPs. BothBOD concentration and flux decrease in reaches C-D and H-I, which is attributed to thesettlement of particulate components while flowing through a stagnated zone near themainstream dams and weir. Similar to BOD, TN and NO3-N concentrations significantly
increase in reaches B-C and F-G-H. However, the flux through the dams and weir does
not decrease, probably due to the high ratio of dissolved inorganic components. It isclearly seen that NH4-N turns into NO3-N by nitrification in the reach H-I.
2.5

80
Flux 10 (kg/yr), Flow rate (m/s)

BOD
3

1.5
1.0

5

Concentration (mg/l)

2.0

0.5
0.0

AB

0

C D

500

1000

1500

FG

H

I

J

2000
2500
3000
2
Catchment area (km )

K

3500

30
20
10
0

AB

500

5

1.5
1.0

C D

1000

E

1500

FG

H

I

FG

H

I

J

FG

H

I

J

2000
2500
3000
2
Catchment area (km )

J

K

3500

L

4000

TN
NO3-N
NH4-N
PN

50
40
30
20
10

0.5
AB

0

500

C D

1000

E

FG

H

I

1500
2000
2500
3000
2
Catchment area (km )

J

K

3500

0

L

0

4000

AB

500

C D

1000

E

1500

2000
2500
3000
2
Catchment area (km )

1500

2000
2500
3000
2
Catchment area (km )

K

3500

L

4000

3

0.20
TP
PO4-P
PP

0.15

Mass flux 10 (kg/yr)

0.10

TP
PO4-P
PP
2

5

Concentration (mg/l)

40

60

2.0

0.0

50

4000

Mass flux 10 (kg/yr)

2.5

60

0

L

TN
NO3-N
NH4-N
PN

3.0

Concentration (mg/l)

E

BOD
Flow rate

70

0.05

0.00
0

AB

500

C D

1000

E

1500

FG

H

I

2000
2500
3000
2
Catchment area (km )

J

K

3500

L

4000

1

0

AB

0

500

C D

1000

E

Figure 6. Streamflow and water quality at 12 mainstream locations.

K

3500

L

4000

TP and PO4-P significantly increase between E and F, where a huge amount ofphosphorous is brought into the mainstream from one tributary, which collects
wastewater from fish-raising ponds. PO4-P flux, a dissolved component, decreases inreaches H-I and I-J, which are not likely due to the settlement of particulates. The uptakeof PO4-P by the growth of phytoplankton is a possible reason for the reduction in thereach H-I. The reduction in PO4-P flux between I and J can be attributed to the adsorption
to soil particles and/or flocculation with cations like Fe3+ and Al3+, as the particulate
phosphorous increases in this reach. A tributary runs into the mainstream with relatively
high acidity is the possible source of soil particles and/or cations.
4.4 Mass balance in mainstream
As was seen in previous sections, water quality in the Abukuma Watershed is strongly
dependent on various human impacts, i.e., the wastewater from domestic, industrial andstock farming sectors, and non-point pollutant loads from agricultural lands.
Improvement of the sewerage system and dissemination of septic tanks has been carriedout to reduce pollutant loads from point-sources, but only a small effect is found. It has
not yet clarified how much impact is posed by the stock farming. It should be investigated
how much and where major pollutant loads are generated and flow into mainstream
through tributaries. It is also significant to quantify mass balance of pollutants andestimate the self-purification function in the river system.
Figure 7 presents measured and estimated pollutant loads to the mainstream from thedownstream end of each tributary and WWTP, in which A~L indicate mainstream
measurement locations, T1~T29 tributaries, W1~W5 wastewater treatment plants,
U1~U11 un-gauged tributaries. Pollutant loads from un-gauged tributaries are calculated
from the previously described relationships between BOD, TN (NO3-N, NH4-N,particulate N), TP concentrations and land use, giving a constant streamflow per unit areaof sub-basin (=0.0112m3/s/km2).
As an overall characteristic, BOD, TN and TP loads inflowing from the left hand side of
the mainstream tend to be large. BOD loads from T9 and T21 are larger than any othertributaries. Those tributaries shed from areas that are densely inhabited and less provided
with sewerage system. BOD loads from T4, T8 and T29 are large, because thosewatersheds are large and collect pollutants from a variety of non-point and point sources
and hold large dams upstream (T4 and T8). Tributaries T4, T8, T17, T20, T24 and T29
are major TN input to the mainstream. Croplands in watersheds T4, T8, T17 and T29 arelarger than any other watersheds, which implies that the fertilizer is the main source ofTN. Watersheds of T20 and T24 have lower percentage of farmland, and famous for theimmaculate water upstream. However, they are also popular for hot springs and thus
human impacts are inevitable. The impact of TN from wastewater effluent W2 is larger
than any single tributary, which is due to poorly treated effluents. Tributaries with large
TP load (T7 and T12) are neither classified as major sources of BOD nor TN. It is
supposed that the sources of TP in T7 and T12 are the wastewater from the fish-raisingindustries and the manufacturing industries, respectively.
Changes in streamflow and BOD, TN, TP flux between sites B and K are presented in
Figure 8. In each graph, upper bar indicates flow or flux at site B plus input fromtributaries and WWTPs. Flow 1 and load 1 are the sum of the measured discharge andload, respectively, inflowing from tributaries and WWTPs between B and K. Flow 2 andload 2 are the sum of the estimated discharge and load, respectively, inflowing from un-gauged tributaries. Flow rate and TN are almost the same between upper and lower bars,
which suggests the conserved flow and TN between sites B and K. BOD and TP show
large discrepancies between upper and lower bars. These discrepancies indicate theexistence of some processes that reduces BOD and TP load in the mainstream. Figure 9shows flow and flux changes within a reach between sites F and H, where a great amountof loads is missing. This figure suggests the existence of unidentified flow and loads of
BOD, TN and TP. This is partly explained by the underestimates of discharge and loads
from un-gauged tributaries between F and H. This underestimate may be caused bystreams highly contaminated with nutrients from point and non-point sources such likestock farming sectors or contaminated ground water seepage. Thus, it is clear that thoseundetected loads must be taken into account in order to quantify the removal of nutrientsby chemical, biological and physical processes.
L

L

U11
T29

W5
T28
T27
T25
W4
T24
T23
T22
T21
T20

J

T20

I

W2
T13
T12

W4
T24
T23
T22

U9

U7

G

W2

U6

T11

T9
T7

E

T6

T5

D
C

U3

A
4

2

0 0

5

BOD load 10 (kg/yr)

D
C

A
2

410

5

G

T10

T8

E

T7(1.58)
T6
T5

U2

W1

T4

T3

T2

T2

B

U1
T1

U1
T1

A
5 0.8

0 0

U4

D
C

U3

TN load 10 (kg/yr)

U5

T9

U2

5

U6

F

T4

B

U7

T11

U4

T3

T2
U1
T1

H

U5

W1

T4

B

E

U8
T18
T17

T13

T10

T8

T6

U3
U2

W1
T3

6

T7

U4

W2
T12

T9

T8

I

T16
T15
T14

U6

F

U5

T17

U7

G

U9

W3
T19

T18

T11

T10

F

T5

T13
T12

U10

J

T21
T20

U8

H

K

W4
T24
T23
T22

U9

I

T16
T15
T14

T26

T25

U10

W3
T19

T18
T17

H

J

T29

T28
T27

T26

K

T21

U8

W3
T19
T16
T15
T14

T25

U10

U11

W5

T29

T28
T27

T26

K

L

U11

W5

0.6

0.4

0.2

0

0

5

0.2

0.4

TP load 10 (kg/yr)

Figure 7. BOD, TN and TP loads inflowing to mainstream. A ~ L: mainstream
measurement locations (A is the most upstream location), T1 ~ T29: tributaries, W1 ~W5: WWTPs, U1 ~ U11: Ungauged tributaries. Each bar indicates the magnitude andlocation of pollutant load input to the mainstream from tributaries or wastewater
treatment plants.

3

6

Flow rate （m /s）
020
Flow at site B

BOD （10 kg/yr）

40

60

Flow 1

Flow 280 0

1

2Flux at site B

Flow at site K

Load 16

1

4

Load 2Flux at site K

6

TN （10 kg/yr）
03

2

TP （10 kg/yr）

3

4

Load 1

Flux at site B

50

0.1

0.2

Flux atsite B

Load 2Flux at site K

0.3

0.4
Load
2

Load 1

Flux at site K

Figure 8. Flow rate and BOD, TN, TP flux changes between sites B and K.
6

3

BOD （10 kg/yr）

Flow rate （m /s）
0

10

20

40 0

30
Flow 1

Flow at site F

Flow 2

1

6

Flux at site F

3

Load 2Flux at site H
6

TN （10 kg/yr）
1

Load
1

Flux at site F

Flow at site H02

TP （10 kg/yr）

2

3

Load 1

Load 2Flux at site H

40

0.05

0.1

Flux at site F

0.15
Load 1

0.2

0.25

Load 2Flux at site H

Figure 9. Flow rate and BOD, TN, TP flux changes between sites F and H.
5. CONCLUSION
This study investigated long-term stream water quality change and present status in the
Abukuma River Watershed. The nutrient concentration for two periods separated by 15
years showed a little change or even worsened in some months at mainstream locations,despite the improvement of wastewater treatment facilities. Water quality in tributaries
showed strong correspondence to the land use. NO3-N is particularly related to thepercentage of cropland, and BOD, NH4-N and TP to the percentage of built area in each
sub-basin. BOD and nutrient in the mainstream is likely affected by chemical, biologicaland physical processes in the flow such as deposition, adsorption and flocculation, in
addition to the inflow from specific tributaries.