TEGA-NUMA (LAKE TEGA)

Little stationary nets in the lake


Photo: A. Kurata

A. LOCATION

• Chiba Prefecture, Japan.


• 35:50N, 140:03E.

B. DESCRIPTION

Tega-numa derived from a valley eroded into the deluvial plateau during the last glacial period about 20,000 years ago, which became invaded by the sea in the hypsithermal period (5,000 years B. P.) and further left as an isolated freshwater body owing to the recession of the sea and the blocking by river washouts.
The lake is regarded as a great asset for the residents of Chiba Prefecture, since it provides an annual amount of 1.7 million cubic meters for irrigation, industrial and city water, besides serving as the fields for fisheries and recreation. The influx of immigrating population from the Tokyo metropolitan area, however, resulted in the rapid eutrophication of the lake since the early 1970's, though the water quality tended to remain more or less the same during the last several years. The eutrophication caused frequent blooms of blue-green algae (Microcystis) and reduced fish catch. Rice plants irrigated with the nutrient-rich lake water became liable to fall on the ground at harvest time due to excessive vegetative growth.
Only 34% of the total population of 400,000 in the catchment area (150 km2) are provided with sewerage facilities, and unregulated discharge of household gray-water became a major source of pollutant loads carried into the lake. The quality of effluents from industrial plants is regulated by setting limitation standards. Sewerage systems are being extended, while other measures for improving inflowing water quality are promoted, e.g. the purification of stream water by gravel filters, removal of nutrients by water hyacinth cultivation and contact filters, introduction of kitchen drain filters for reducing organic waste load discharged from individual households, etc.

C. PHYSICAL DIMENSIONS (Q)

Surface area [km2]	7
Volume [km3]	0.0056
Maximum depth [m]	3.8
Mean depth [m]	0.9
Water level	Regulated
Length of shoreline [km]	38.0
Residence time [yr]	0.1
Catchment area [km2]	150

D. PHYSIOGRAPHIC FEATURES (Q)

D1 GEOGRAPHICAL

• Bathymetric map: Fig. ASI-28-01.


• Names of main islands: None.


• Number of outflowing rivers and channels (name): 1 (Tega R.).

D2 CLIMATIC

• Climatic data at Abiko, 1981-1985

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Ann.
Mean temp. [deg C]	2.6	3.4	6.6	12.5	17.5	19.6	24.1	26.5	21.5	16.0	10.4	5.3	13.8
Precipitation [mm]	23	68	110	122	90	192	102	105	167	149	78	22	1,228

• Number of hours of bright sunshine: 2,370 hr yr-1.

Fig. ASI-28-01
Bathymetric map (1).

• Water temperature [deg C]
  Lake centre

Depth [m]	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.2	3.7	5.4	8.2	16.8	19.6	22.3	26.2	28.4	25.1	19.9	12.0	6.6

• Mixing type: Polymictic.


• Notes on water mixing and thermocline formation
  Thermocline formation is not observed.

E. LAKE WATER QUALITY (Q)

E2 pH

Lake centre, 1981-1985
Depth [m]	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.2	9.2	9.0	9.0	9.4	8.8	8.4	8.9	9.0	9.1	9.0	9.1	9.2

E3 SS [mg l-1]

Lake centre, 1981-1985
Depth [m]	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.2	43	40	48	53	66	59	42	56	73	50	52	53

E4 DO [mg l-1]

Lake centre, 1981-1985
Depth [m]	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.2	16	17	16	14	9.6	8.6	11	9.3	11	12	14	14

E5 COD [mg l-1]

Determined by KMnO4 method.

Lake centre, 1981-1985
Depth [m]	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.2	24	26	25	23	22	21	18	23	26	19	20	22

E6 CHLOROPHYLL CONCENTRATION [micro l-1]

Lake centre, 1984-1985
Depth [m]	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.2	430	370	270	400	470	450	200	220	450	340	440	390

E7 NITROGEN CONCENTRATION

• Total-N [mg l-1]

Lake centre, 1981-1985
Depth [m]	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.2	7.50	8.34	7.70	5.73	5.52	5.33	3.96	3.49	5.17	4.79	5.74	5.90

E8 PHOSPHORUS CONCENTRATION

• Total-P [mg l-1]
  

Lake centre, 1981-1985
Depth [m]	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.2	0.74	0.93	0.62	0.52	0.53	0.48	0.37	0.68	0.71	0.44	0.35	0.46

E10 PAST TRENDS : Fig. ASI-28-02, 03 and 04.

Fig. ASI-28-02
Trend of COD at the lake centre.

Fig. ASI-28-03
Trend of chlorophyll a at the lake centre.

Fig. ASI-28-04
Trend of nutrients at the lake centre.

F. BIOLOGICAL FEATURES (Q)

F1 FLORA

• Emerged macrophytes
  Phragmites communis, Zizania latifolia, Typha angustifolia, Nelumbo nucifera.
• Floating macrophytes: Trapa natans var. japonica.


• Phytoplankton
  Spring: green algae (Chlamydomonas). Summer: blue-green algae (Microcystis). Autumn and winter: diatoms (Cyclotella).

F2 FAUNA

• Fish
  Pseudrasbora parva, Rodeus ocellatus ocellatus, Cyprinus carpio*, Carassius auratus, Hypophthalmichthys molitrix, Anguilla japonica*. * Economically important.

F5 FISHERY PRODUCTS

• Annual fish catch [metric tons]
  1985: 440.
• Fishery products other than fish [metric tons]
  1980: 8.

F6 PAST TRENDS : Fig. ASI-28-05.

Fig. ASI-28-05
Trend of annual fish catch.

G. SOCIO-ECONOMIC CONDITIONS (Q)

G1 LAND USE IN THE CATCHMENT AREA

1985
	Area [km2]	[%]
Natural landscape	39.60	26.4
Agricultural land	51.59	34.4
Residential area	58.97	39.3
Total	150.16	100

• Main kinds of crops: Rice.

G2 INDUSTRIES IN THE CATCHMENT AREA AND THE LAKE

1980
	No.of persons engaged	No.of establishments
Primary industry	(7,917)
- Crop production	7,819
- Fisheries	74
- Others	24
Secondary industry	50,069	2,270
Tertiary industry	101,560	9,844

• Numbers of domestic animals in the catchment area
  Cattle 686, swine 3,615, horse 161.

G3 POPULATION IN THE CATCHMENT AREA

1985
	Population	Population density [km-2]	Major cities (population)
Urban	345,610	-	Kashiwa, Abiko, Matsudo, Nagareyama, Kamagaya
Rural	53,440	-
Total	399,050	2,657

H. LAKE UTILIZATION (Q)

H1 LAKE UTILIZATION

Source of water, fisheries, sightseeing and tourism (no. of visitors in 1980: 240,000), and recreation (sport-fishing).

H2 THE LAKE AS WATER RESOURCE

1985
	Use rate [m3 day-1]
Irrigation	138,000

I. DETERIORATION OF LAKE ENVIRONMENTS AND HAZARDS (Q)

I1 ENHANCED SILTATION

• Extent of damage: Serious.


• Supplementary notes
  Heavy siltation occurs at the mouths of Obori River and Otsu River flowing through urban areas.

I3 EUTROPHICATION

• Nuisance caused by eutrophication
  Algal bloom (dominant species of algae): Microcystis spp.
• Nitrogen and phosphorus loadings to the lake [t yr-1]

1985
Sources	Industrial	Domestic	Agricultural	Natural	Total
T-N	158.0	524.1	-	181.0	863.1
T-P	25.4	78.0	-	8.5	111.9

I4 ACIDIFICATION

• Extent of damage: None.

J. WASTEWATER TREATMENTS (Q)

J1 GENERATION OF POLLUTANTS IN THE CATCHMENT AREA

f) Severe pollution with limited wastewater treatment.

J2 APPROXIMATE PERCENTAGE DISTRIBUTION OF POLLUTANT LOADS

	COD [kg day-1]	[%]
None-point sources
- (agricultural, natural and	6,231	94.2
dispersed settlements)
Point sources
- Municipal	142	2.1
- Industrial	242	3.7
Total	6,615	100

J3 SANITARY FACILITIES AND SEWERAGE

• Percentage of municipal population in the catchment area provided with
  adequate sanitary facilities (on-site treatment systems) or public sewerage: 68% (nightsoil treatment for the rest 32%).
• Percentage of rural population with adequate sanitary facilities (on-site
  treatment systems): Nearly 100%.
• Municipal wastewater treatment systems
  No. of secondary treatment systems: 2.
• Industrial wastewater treatment systems
  No. of industrial wastewater treatment systems: 114.

K. IMPROVEMENT WORKS IN THE LAKE (Q)

K1 RESTORATION

Dredging is carried out mainly at the mouths of inflowing rivers, Obori River and Otsu River.

K3 OTHERS

Harvest of Microcystis, planting and recovery of water hyacinth.

M. LEGISLATIVE AND INSTITUTIONAL MEASURES FOR UPGRADING LAKE ENVIRONMENTS

M1 NATIONAL AND LOCAL LAWS CONCERNED

• Names of the laws (the year of legislation
)

1. For national laws see "Biwa-ko".
2. Ordinance for Determining Effluent Standards on the Basis of Water Pollution Control Law (1976)
3. Chiba Prefecture Environmental Pollution Control Ordinance (1971)
• Responsible authorities


1. Chiba Prefectural Government
2. Chiba Prefectural Government
• Main items of control
  
1. BOD, COD
2. BOD, COD

M3 RESEARCH INSTITUTES ENGAGED IN THE LAKE ENVIRONMENT STUDIES

1. Chiba Prefectural Laboratory of Water Pollution, Chiba

N. SOURCES OF DATA

17. Questionnaire filled by Chiba Prefectural Government.
1. Kobayashi, S. (1982) Journal of Water & Waste (Yosui to Haisui), 24: 965-976.*

* Printed in Japanese.