The Approach for Improving the Water Quality in the Sanaru Lake
○Tatsuya Tobe, Toshinari Morikawa, Toshiharu Okada, Kouji Fukunaga, Taku Watanabe, Yoshiki Wada

1. Introduction & Water Quality in the Sanaru Lake

3. Water Quality Simulation Model
Water Quality Simulation ModelLegendJapan

Water Quality Model

Hydraulic Model

Paddy
Dry Field
Fruit Farm
Urban Area
ForestWater Area

Objective Area

Pacific Ocean

Equation for Continuity

Equation for Salinity Balance

Equation for theConservation of Momentum

Equation for Tempreture
BalanceEquation for Shear Stressat the bottom layer andthe water surface by wind

Equation for EcologicalModel (Chl-a, COD, DO,Dissolved or Particulate N, P)

Divided Mesh Distribution
Altitude [T.P. m]+ This NSC original model
can calculate the complicatedfeature of the water quality
with accuracy.
Hamana Lake

Sanaru Lake

Sanaru Lake

Downstream River
Hamana Lake &Pacific Ocean

Downstream River

Three-Dimensional
3次元メッシュモデル
Model

鉛直2次元メッシュモデル
Vertical Two-Dimensional
Model

Shin River+ The salt water flow into the Sanaru
Lake through the downstream river.

Pacific Ocean+ The water quality had deteriorated from the 1960s, because of therapid urbanization and industrialization in the watershed.
Yealy Average COD [mg/L]

Sanaru Lake

Downstream River

Sanaru Lake

Evaluation of the Model Reliability
COD(mg/l)
30

Observation
COD観測値
Value of COD

佐鳴湖 湖心 H19年

Observation
D-COD観測値
Value of D-COD

Calculation
COD計算値
Value of COD

Calculation
D-COD計算値
Value of D-COD

計算値（日変動幅）
Cal.
Value (daily range)

25201520105151050
H19.4.1

2007.4.1

Target of the Local Committee 8.0mg/L

2007.5.1
H19.5.1

2007.6.1
H19.6.1

2007.7.1

H19.7.1

2007.8.1
H19.8.1

2007.9.1

H19.9.1

2007.10.1

2007.11.1 2007.12.1
H19.10.1 H19.11.1
H19.12.1

2008.1.1
H20.1.1

2008.2.1
H20.2.1

2008.3.1
H20.3.1

4. Evaluation of the Effect of Water Purification

Enviromental Standard 5.0mg/L

Purification Measures

01975

1980

1985

1990

1995

2000

2005

Purpose of Study : To figure out the feature of water pollution, andevaluate the effect of the purification measures by developing andapplying the simulation model based on the feature in the Sanaru lake.

2. Feature of Water Pollution by Monitoring+ The mechanism of water pollution was figured out by carrying out
various monitoring and analyzing the monitoring data.
Sanaru Lake

Downstream RiverUpstream River① Nutrients Inflow
from Up and Downstream

Nitrogen Phosphorus

Nutrients such as nitrogen or phosphorus
included in the domestic and industrial drainage
flow into the lake from up or downstream river.
PPO2

In the Watershed

Dredging in the Downstream River

Improvement of the Sewage System

Facility for the Water Purification
Directly in the Lake

Connection to the Sewer regarding
Domestic and Industrial Drainage

Facility for the Nitrogen Removal
in the Upper Stream of the Lake

Installation of the Joint Wastewater
Treatment Tank

Effect of the all Purification Measures by Applying the Model
Before Measures

After Measures

Yearly Average
COD

Yearly Average
COD

9.3 mg/L

8.0 mg/L

PPN

Retention

Nitrogen Phosphorus

In the River or LakeP
Decrease of O2

NRelease of N, PO2O2
Nitrogen Phosphorus

PP② Release of Nutrients from Lake Sediments③ Increase of Phytoplankton
Makes N or P Nourishment

Nutrients such as N or P are easy to be released from thelake sediments, when the salt water flow and stay in thebottom of the lake by tidal variation and oxygen decreases.

Phytoplankton increases when the nutrients such as N or P
are abundant. As a result, the water quality such as COD
value in the lake deteriorates more and more.

Conclusion :+ Yearly average COD value in the Sanaru lake became under 8.0mg/L
in 2009 by carrying out the all purification measures.
+ The approach of Monitoring-Modeling-Evaluation is important tocarry out the water purification effectively.
Acknowledgement : Special thanks to Shizuoka Prefecture, Hamamatsu
City, and Local Committee of Water Renaissance in the Snaru Lake.