GGroundwater Salinity Simulation of roundwater Salinity Simulation of A Subsurface Reservoir in Penghu IsA Subsurface Reservoir in Penghu Is

land (Pescadores), Taiwanland (Pescadores), Taiwan

GGroundwater Salinity Simulation of roundwater Salinity Simulation of A Subsurface Reservoir in Penghu IsA Subsurface Reservoir in Penghu Is

land (Pescadores), Taiwanland (Pescadores), Taiwan

•Professor Yih-Chi TanProfessor Yih-Chi Tan •Department of Bioenvironmental Systems EnDepartment of Bioenvironmental Systems Engineering, National Taiwan Universitygineering, National Taiwan University•Director, Hydrotech Research InstituteDirector, Hydrotech Research Institute

INTRODUCTION INTRODUCTION

Chi-Ken subsurface reservoir Chi-Ken subsurface reservoir The subsurface reservoir is located in ChThe subsurface reservoir is located in Ch

i-Ken Basin,i-Ken Basin, Pescadores (a group islands Pescadores (a group islands

located at western part of Taiwan, betweelocated at western part of Taiwan, betwee

n Kinmen and Taiwan) n Kinmen and Taiwan) The reservoir has had the salinity problem since The reservoir has had the salinity problem since

19911991

Chi-Ken subsurface reservoir Chi-Ken subsurface reservoir The subsurface reservoir is located in ChThe subsurface reservoir is located in Ch

i-Ken Basin,i-Ken Basin, Pescadores (a group islands Pescadores (a group islands

located at western part of Taiwan, betweelocated at western part of Taiwan, betwee

n Kinmen and Taiwan) n Kinmen and Taiwan) The reservoir has had the salinity problem since The reservoir has had the salinity problem since

19911991

INTRODUCTION

annual average temperature is 23.3annual average temperature is 23.3ooCC average moisture is 80~85%average moisture is 80~85% annual average rainfall is 913 mmannual average rainfall is 913 mm annual average ET rate is 1575mmannual average ET rate is 1575mm no single river in the basin; the major recharge of no single river in the basin; the major recharge of

groundwater is by infiltration of rainfallgroundwater is by infiltration of rainfall

annual average temperature is 23.3annual average temperature is 23.3ooCC average moisture is 80~85%average moisture is 80~85% annual average rainfall is 913 mmannual average rainfall is 913 mm annual average ET rate is 1575mmannual average ET rate is 1575mm no single river in the basin; the major recharge of no single river in the basin; the major recharge of

groundwater is by infiltration of rainfallgroundwater is by infiltration of rainfall

INTRODUCTIONINTRODUCTION

basin area of the subsurface reservoir is basin area of the subsurface reservoir is 2.14 km2.14 km22

discharge of groundwater is 1.27discharge of groundwater is 1.27××101066mm33 annual planning water supplies is annual planning water supplies is

7.97.9××101055mm33

Chi-Ken subsurface reservoirChi-Ken subsurface reservoir

North-west toward south-eastNorth-west toward south-eastNorth-west toward south-eastNorth-west toward south-east

Chi-Ken subsurface reservoirChi-Ken subsurface reservoir

south toward north south toward north south toward north south toward north

What is subsurface reservoir?

subsurface reservoirsubsurface reservoir

Possible salinity reasons of subsurface reservoirPossible salinity reasons of subsurface reservoir

shortage of rainfall shortage of rainfall leakage of the subsurface barrier causes the leakage of the subsurface barrier causes the

intrusion of seawaterintrusion of seawater Percolation from weathering basaltPercolation from weathering basalt

NUMERICAL MODELNUMERICAL MODEL NUMERICAL MODELNUMERICAL MODEL

GMS (Groundwater Modeling System)GMS (Groundwater Modeling System)define soils layer datadefine soils layer data hydro-geological parametershydro-geological parameters initial conditionsinitial conditions boundary conditionsboundary conditions generation of three dimension meshesgeneration of three dimension meshes

NUMERICAL MODELNUMERICAL MODELNUMERICAL MODELNUMERICAL MODEL

A three dimension FEMWATER(Yeh , 199A three dimension FEMWATER(Yeh , 1996) numerical model is adapted to find the po6) numerical model is adapted to find the possible leakage position of the subsurface barssible leakage position of the subsurface barrier and location of seawater intrusion by corier and location of seawater intrusion by comparing the different possible leakage simumparing the different possible leakage simulated result with the observations lated result with the observations

Boundary of the modelBoundary of the model

Location of subsurface barrierLocation of subsurface barrier

Subsurface barrierSubsurface barrier

HYDRO-GEOLOGICAL LAYERING HYDRO-GEOLOGICAL LAYERING

1.The upper layer is white middle to grossly 1.The upper layer is white middle to grossly coralline sand with screenings coral, coralline sand with screenings coral,

2. The middle layer is gray loam sand, 2. The middle layer is gray loam sand, 3. The substratum layer is brownness clay,3. The substratum layer is brownness clay, 4.the bottom layer is weathering basalt 4.the bottom layer is weathering basalt The major aquifer is in the upper and middle The major aquifer is in the upper and middle

layerslayers

HYDRO-GEOLOGICAL LAYERINGHYDRO-GEOLOGICAL LAYERING

HYDRO-GEOLOGICAL LAYERINGHYDRO-GEOLOGICAL LAYERING

determine the leakage position by numerical modeldetermine the leakage position by numerical model

According to the observation data from TaiAccording to the observation data from Taiwan Water Supply Corporation (1998), the wan Water Supply Corporation (1998), the possible leakage positions of the subsurface possible leakage positions of the subsurface barrier are:barrier are:the interface between barrier and basltthe interface between barrier and baslt the leakage of the subsurface barrierthe leakage of the subsurface barrier

determine the leakage position by numerical modeldetermine the leakage position by numerical model

three hypothetical leakage casesthree hypothetical leakage cases Case A1Case A1：： leakage position is in the bottom leakage position is in the bottom

part of the barrierpart of the barrier Case A2Case A2：： leakage position is in the middle leakage position is in the middle

part of the barrierpart of the barrier Case A3Case A3：： leakage position is in the upper part leakage position is in the upper part

of the barrierof the barrier

Cross –section of barrierCross –section of barrier

Case A1Case A1：： leakage position is in the bottoleakage position is in the bottom part of the barrierm part of the barrier

309100.00 309200.00 309300.00 309400.00 309500.00 309600.00

2618650.00

2618700.00

2618750.00

2618800.00

2618850.00

2618900.00

2618950.00

2619000.00

2619050.00

2619100.00

The distribution of the chloride concentration for A1case The distribution of the chloride concentration for A1case

matches with the observed valuesmatches with the observed values

309100.00 309200.00 309300.00 309400.00 309500.00 309600.00

2618650.00

2618700.00

2618750.00

2618800.00

2618850.00

2618900.00

2618950.00

2619000.00

2619050.00

2619100.00

Groundwater levels distributionGroundwater levels distribution Chloride concentration distributionChloride concentration distribution

Case A1Case A1：： leakage position is in the bottoleakage position is in the bottom part of the barrierm part of the barrier

Case A2Case A2：： leakage position is in the middle leakage position is in the middle part of the barrierpart of the barrier

309100.00 309200.00 309300.00 309400.00 309500.00 309600.00

2618650.00

2618700.00

2618750.00

2618800.00

2618850.00

2618900.00

2618950.00

2619000.00

2619050.00

2619100.00

The numerical result for chloride concentration with The numerical result for chloride concentration with 1000ppm of A2case and A3case are not comparable to the 1000ppm of A2case and A3case are not comparable to the observed dataobserved data

309100.00 309200.00 309300.00 309400.00 309500.00 309600.00

2618650.00

2618700.00

2618750.00

2618800.00

2618850.00

2618900.00

2618950.00

2619000.00

2619050.00

2619100.00

Chloride concentration distributionChloride concentration distribution Groundwater levels distributionGroundwater levels distribution

Case A2Case A2：： leakage position is in the middle leakage position is in the middle part of the barrierpart of the barrier

Case A3Case A3：： leakage position is in the leakage position is in the upper part of the barrierupper part of the barrier

309100.00 309200.00 309300.00 309400.00 309500.00 309600.00

2618650.00

2618700.00

2618750.00

2618800.00

2618850.00

2618900.00

2618950.00

2619000.00

2619050.00

2619100.00

The inland chloride concentration of A2case and A3case is The inland chloride concentration of A2case and A3case is

lower than the observed datalower than the observed data

309100.00 309200.00 309300.00 309400.00 309500.00 309600.00

2618650.00

2618700.00

2618750.00

2618800.00

2618850.00

2618900.00

2618950.00

2619000.00

2619050.00

2619100.00

Chloride concentration distributionChloride concentration distribution Groundwater levels distributionGroundwater levels distribution

A3A3：： leakage position is in the upper leakage position is in the upper part of the barrierpart of the barrier

CONCLUSIONSCONCLUSIONS CONCLUSIONSCONCLUSIONS

By assuming the leakage position is in the By assuming the leakage position is in the bottom of barrier, the numerical result bottom of barrier, the numerical result predicts values closer to the observation predicts values closer to the observation than the other assumption. It showed that than the other assumption. It showed that the most possible leakage position is at the the most possible leakage position is at the bottom of the barrier.bottom of the barrier.

CONCLUSIONSCONCLUSIONSCONCLUSIONSCONCLUSIONS

The research applied three dimension The research applied three dimension FEMWATER and GMS as an interface to FEMWATER and GMS as an interface to input parameter. The simulation of water input parameter. The simulation of water level and chloride concentration already level and chloride concentration already shows the real situation, and the result can shows the real situation, and the result can be applied to the future study of the Chi-be applied to the future study of the Chi-Ken subsurface reservoir salinity problems.Ken subsurface reservoir salinity problems.

The governmentThe government decided to rebuild another decided to rebuild another cutoff to prevent sea water intrusion, and it cutoff to prevent sea water intrusion, and it has been finished in last year.has been finished in last year.

It is very important that is to continue It is very important that is to continue monitoring the water quality and preserve monitoring the water quality and preserve the limit water resources in this island.the limit water resources in this island.