A numerical simulation to investigate the relation ofrivers and lake to groundwater flow systems inL. Kasumigaura watershed: Research planning

2011.10.13FS 研究勉強会

Graduate School of Life and Environmental SciencesUniversity of Tsukuba

Wang Shiqin

Research Backgrounds

• In L. Kasumigaura region, though many policies have been designed to prevent environmental degradation in the catchment area, the quality of the lake has not recovered greatly.

• Nitrogen concentrations in river water are two or three times higher than those in the lake. And nitrogen concentrations in groundwater are one order of magnitude higher than those in the lake.

Interaction between groundwater and surface water is important to understand the water quality change of a lake system.

Groundwater is important for understanding lake systems because it can influence surface water budget, nutrient budget.

Previous studies in L. Kasumigaura areaResults of interaction between surface water and groundwater: 1. River and aquifers: surface water receive groundwater inflow. 2. Lake and aquifers: predominant by the inflow of groundwater; there are

outflow in the south. ( 村岡・細見 ,1981;山本 ,1992; 内藤 ,2008; 中山・渡辺 ,2008; )

(Nakayama and Watanabe, 2008)：湖水に流出入を繰り返す地点図．平均動水勾配の分布傾向

( 内藤 ,2008)

Darcy’s law NICE –LAKE model

Flow (G/R) 1%, >1% 10%Nitrate load (G/R)

1 ～ 4.5% 30%Spatial scale Around lake

boundary 3-D

Time scale Determined period

Transient

The exchange of water and solutes between groundwater and lakes is complex and there is still a challenge in understanding the temporal and spatial variability across different scales.

To understanding the interaction between surface water and groundwater from a view of groundwater flow system

(J.Toth, 1963) (大井信三 ,国土地理院）

Exchange flow between lake water and groundwater is defined by the local and regional groundwater flow system.

10m

-10

-20

0

BsAcAsdtLmNsYcYsYg

LakeDejima

local

regional

• To set up a numerical model simulating the interaction between surface water and groundwater based on the understanding of the groundwater flow system.

• To recognize the source of nitrogen in surface water and groundwater and to study the mechanism of solute transport (nitrate) from groundwater to the lake.

• To quantity the temporal and spatial variation of the flow and Nitrogen load between aquifers and the lake.

Objectives of this research

data collection

3-D Geology Model

3-D Geology Model

Groundwater flow system

Groundwater flow system

Concept modelConcept modelField

experiment

Water chemicals

2H, 18O, 15N, 3H, CFCsNumerical modelNumerical model

VerificationVerification

CalibrationCalibration

Accept

Sensitivity analysisSensitivity analysis

OutputOutput

Water budget

Define Region

Water, salt, isotope BalanceNo

Yes

Model Revise

Flow Chart of the modelingFlow Chart of the modeling

Multi-tracers

Hydrogeo-chemistry

Hydrologic data

Interaction Mechanism between surface water and groundwater

Groundwater model with a finite-difference method

( ) ( ) ( )xx yy zz s

h h h hK K K W S

x x y y z z t

Partial Differential Equations

Modflow

( )( ) ( )

K KK k

ij i s s ni j i

C CD vC q C R

t x x x

1 2

KKK

n b b

CR C C

t

FlowFlow

SoluteSolute

NO3-

δt ＝ δ0 ＋ εln(Ct/C0) (Mariotti et al., 1981)

Denitrification:

nQQQQW ...321

Infinitesimalvolumeof aquifer

Q1 Q2Q3

Most interaction between ground water and surface water is lumped into the W term

Lake-aquifer and river-aquifer system

Lake inflow or outflow:

Lake stage:

Lake-aquifer

River-aquifer

Lakebed

Lake cell

Surface runoff

Interflow

Precipitation

Lake leakage

Outlet stream

Evaporation

Ground-waterdischarge

Aquifer cell with

node

Tributary stream

Lake surface

Lake bottomLakebed

Point in aquifer

Distance from base of lakebed to point in aquifer

Lakebed thickness

Conductance terms

Aquifer

aqaq

aqaq A

thick

KC

aqlkbd

lkbdlkbd A

thick

KC

Cross-sectional area

S. A. Leake

CRIV= Kv (LW)/b

Head here is river head, HRIV

Head here is aquifer head, Hi,j,k

Vertical hydraulic conductivity is, Kv

Length, L

Width, W Thickness, b

7

Groundwater concept model------as a case of Dejima region

Hydrogeology construction• Upland: １関東ローム層　（ＹＬ）；２．常総層（Ｊ）；３．木下層　（Ｋ i）；

4. 上岩橋層（ Ka）； 5. 　上泉層（ Km）； 6. 藪層　（ Yb ） . • Lowland 　（桜川低地） : 1. 沖積層（ A ）； 2. 桜段丘体積物及び相当層；３．

木下層　（Ｋ i）； 4. 上岩橋層（ Ka）； 5. 　上泉層（ Km）； 6. 藪層　（ Yb ） .

• Lowland 　（霞ヶ浦低地） : 1. 表土（ Bs ）； 2. 砂質帯水層（ As ）；３．木下層　（Ｋ i）； 4. 上岩橋層（ Ka）； 5. 　上泉層（ Km）； 6. 藪層　（ Yb ） .

• （３．木下層　（Ｋ i）； 4. 上岩橋層（ Ka）； 5. 　上泉層（ Km）） = 成田層

Boundary Conditions: Water head boundary: Lake boundary, River boundary, Flow boundary: Mountain boundary, Upper boundary and bottom boundary.

1

2

3

4

5

6

23

1

140.15 140.2 140.25 140.3 140.35 140.436.05

36.1

36.15

140.15 140.2 140.25 140.3 140.35 140.436.05

36.1

36.15

The groundwater system could be described as a conceptual hydrologic model which was a six layer, heterogeneous, horizontal isotropy, three-

dimensions, transient flow system.

Groundwater dynamics:

Water table (m)

Concentration of Nitrate (mg/l)

2007.5

2007.82007.5

2007.8

0

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