Hydrological study of Laura area in Majuro atoll, Republic of Marshall Islands

Andreas Antoniou

Supervisors: Vincent Post, Peter Sinclair

PHYCOS (Pacific Hydrological Cycle Observing System) – key messages

Strengthen the capacity of small island countries to conduct water resources assessment and monitoring as a key component of sustainable water resources management.

There is a need for capacity development to enhance the application of climate information to cope with climate variability and change.

PHYCOS (Pacific Hydrological Cycle Observing System)• Jointly funded European Union and SOPAC regional

initiative

• 14 participating countries in the Pacific

• Project aims to assist countries in developing their data sets and knowledge on their available water resources by employing equipment and technical support that will lead to appropriate decision making in the future

Majuro atoll – Marshall Islands

• Northern hemisphere of the western Pacific Ocean• Total land area of the entire atoll is 11.14 km2

• Population in Majuro was estimated to 25400 in 2004

Laura provides 60% of the total reticulated water requirements for Majuro

Laura provides 60% of the total reticulated water requirements for Majuro

Laura provides 60% of the total reticulated water requirements for Majuro

Majuro atoll - Climate

• Average annual temperature: 27.2 °C• Average annual rainfall: 356 cm/year • Period from June to December typically receiving about

75% of the annual rainfall (www.noaa.gov)

Majuro atoll - Vegetation

• 70% of Laura is covered by forest (mainly coconut trees)• 27% consists of grassland and low vegetation types• 3% is used for agricultural purposes

Majuro atoll - Geology

Hamlin & Anthony, 1987

Majuro atoll - Geology

• Holocene deposits• Upper sediment (6 - 12 m thickness)

• unconsolidated, calcareous, well-sorted beach sand• Lower sediment (10-12 m thickness)

• more cohesive and heterogeneous mixture of calcareous silts, sands and coarse-coraline materials

• Pleistocene sediments• Lower limestone (reached between 17 and 25 m)

• dense, well-consolidated limestone with greater overall porosities and permeabilities than the relatively unaltered Holocene deposits.

Laura – Existing infrastructure

• 10 monitoring sites (3 – 4 boreholes each site)

• 7 public pumping wells (only 4 operating today)

• Private hand-dug wells

• Meteo station – Daily temperature and rainfall measurements

Mission purpose

• Quantifying lens size• Monitoring of water quality• Raise public awareness

Minor tasks• Rehabilitation of monitoring

network• Survey to determine site

elevations

Mission purpose

• Geophysics• 6 EM-34 transects• 10 VES

• 10 Monitoring borehole sites

GeophysicsEM-34 drawbacks:

• When using it for simple profiling with a single separation length, changes in conductivity cannot be connected to specific depths

• In high-conductivity environments such as seawater-saturated sediments, conductivity measurements obtained with the coils in the horizontal coplanar position are not stable

• VES measurements as well as borehole data were used for verificationFrom Tony Falkland

Electrical Conductivity measurements in monitoring boreholes

Private wells

Groundwater assessment interpretations

Storage – Lens size• Based on the 6 transects where

the EM-34 measurements took place (extrapolating lens area of each profile)

• Uniform unsaturated zone of 1.42 meters was assumed to exist in the whole area occupied by the lens as well as a porosity of 20%

• Area occupied by freshwater lens: 1.51 km2

• Potable water: 2,678,000 m3

IDEXX Colilert® reagent

Water quality

• 33 monitoring boreholes – weekly basis – 160 samples

• Bacteria analysis (E.Coli test)

IDEXX Colilert® reagent

Water quality

5 col/100mL

10 col/100mL

20 col/100mL

Monitoring boreholes

Pumping wells

Tidal & rainfall impact on water table

11/19/07 0:00 11/24/07 0:00 11/29/07 0:00 12/4/07 0:00 12/9/07 0:00 12/14/07 0:00 12/19/07 0:00 12/24/07 0:000

0.5

1

1.5

2

2.5

3

Fluctuation of water level in pumping well 5

Wat

er le

vel b

elow

gro

und

surfa

ce

(ft)

Significant rainfall event causing increase in water level (November 25)

3-hours delay

Tidal & rainfall impact on water table

0

0.5

1

1.5

2

2.5

3

24-hour water table fluctuation

Time

Wat

er le

vel (

ft)

Sea tides

Tidal impact on groundwater level

Pumping well 5

Rainfall influence on water table and EC of well 3-38A

Calculating tidal lags and damping

Calculating tidal lags and damping

1/30/2008 14:24 1/30/2008 19:12 1/31/2008 0:00 1/31/2008 4:48 1/31/2008 9:36 1/31/2008 14:24 1/31/2008 19:120

0.5

1

1.5

2

2.5

3 2.2

2.3

2.4

2.5

5-28

WLEC

time (hours)

wat

er le

vel (

m)

cond

uctiv

ity (m

S/cm

)

2/3/2008 12:00 2/4/2008 0:00 2/4/2008 12:00 2/5/2008 0:001.3

1.351.4

1.451.5

1.551.6

1.651.7

1.751.8 0.7

0.8

0.9

WLEC

time (hours)

wat

er le

vel (

m)

Con

duct

ivity

(mS/

cm)

delay 1:17 h delay 1:13 h delay 1:58 h

Tidal delays – efficiencies

0 10 20 30 40 50 60 70 80 90 1000.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

site 1site 2site 3site 4site 5site 6

Tidal efficiency (%)W

ell s

cree

n de

pths

(m)

0:00 0:28 0:57 1:26 1:55 2:24 2:52 3:210.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

site 1site 2site 3site 4site 5site 6

Tidal delay (hours)

Wel

l scr

een

dept

hs (m

)

Tidal delay decreases with depth as we approach the transition zone

Tidal efficiency increases with depth as we approach the transition zone

Calculating heads – horizontal fluxes• Density differences

• Convert all the measured hydraulic heads into theoretical fresh water heads for each depth referenced to the same elevation (10 m depth)

• Correct for delay of each borehole

Horizontal flows analysis – 10 m depth

head values of some wells intersect the head values of other wells in time, meaning that there is not a single flow direction between these wells but it can invert during the same tidal cycle

Horizontal flows analysis

16 meters depth

Horizontal flows analysis

16 meters depth

• Possible explanation: combined effect of water abstraction from the nearby pumping wells and the intensive irrigation in the farm

• Existence of low permeability layer at relatively shallow depth

Laura lens simulation using SEAWAT

• Simulating the creation of a freshwater lens in a salt water aquifer (Salt concentration = 35000 mg/L) without tidal boundary conditions

Laura lens simulation using SEAWAT

Transect 6

Laura lens simulation using SEAWAT

• Simulating the tidal effect on the freshwater lens for a period of 6 months using a variable head boundary condition

Laura lens simulation using SEAWAT

Transect 6

Public awareness• 2 meetings with Laura community at Laura high school

• 2 meetings with Laura Lens Protection Committee and with RMI ambassador in Fiji

Summary - Conclusions

• Lens extension – depth• Lens size estimated with

combined use of geophysics, borehole information and private wells

• Water quality survey• Identified contours of pollution.• Differences in concentration

with depth• Monitoring is continued on

monthly basis by EPA and MWSC staff

Tidal impact Water table can fluctuate

up to 1 m on daily basis Salinity of groundwater is

also affected by sea tides – EC can vary with a range of up to 3000 μS/cm within one tide cycle

When sampling a well, always account for tidal impact

Propagation of tidal signal is much more important in the vertical sense