1. The Northern Guam
Lens Aquifer
Carbonate Island Karst
Model (CIKM)
Sustainable Yield
EV511
Groundwater Module
Lecture #8

4. 0
5
10
15
20
25
30
35
40
Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec
MeanMonthlyRainfall(cm). Mean Monthly Rainfall
(1982-1995)
Dry Season
Wet Season

5. 0.17
0.28
0.33
0.22
0.01
0.11
0.27
0.36
0.25
0.00
0.26
0.30
0.27
0.16
0.01
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
less than 0.6 0.6 to 2 2 to 5 5 to 20 more than 20
Daily total rainfall (cm)
Proportionoftotalrecordedrainfall.
Annual
Wet season
Dryseason
Distribution of Rainfall Intensity

6. discharging
fresh water
Northern Guam
Lens Aquifer
percolating water
infiltrating rain water
sea
water
fresh water lens
volcanic basement rock
limestone bedrock
Annual Rainfall 2.4 m (~100 in)
Annual Water loss 0.8 m (~33 in)
Annual Recharge 1.6 m (~67 in) = ~310 mgd
Extraction ~43 mgd = 14% est. recharge

7. discharging
fresh water
Northern Guam
Lens Aquifer
percolating water
infiltrating rain water
sea
water
fresh water lens
volcanic basement rock
limestone bedrock
Annual Rainfall 2.4 m (~100 in)
Annual Water loss 0.8 m (~33 in)
Annual Recharge 1.6 m (~67 in) = ~310 mgd
Extraction ~43 mgd = 14% est. recharge

8. #8
Optimizing Well Locations
Best practices for production volume and water quality
Topographic Map
of Basement
Volcanic Rock
Vann, 2000
Habana, 2010

9. Agafa Gumas
Subbasin
Mangilao
Subbasin
Finegayan
Subbasin
Yigo
Subbasin
Agana
Subbasin
Andersen
Subbasin
N
2 0 2 4 Miles
Chloride Profile Wells
Basement
Topography

10. Dye trace study of Harmon Sink to
Tumon & Agana Bays
Joint project with Guam EPA: 2000-2002
1 0 1 2 Kilometers
N
EW
S
55000
55000
50000 50000
55000 55000
']
']
#S
#S
Tamuning-Yigo
FaultTamuning-Yigo
Fault
Tumon Bay
Agana Bay
Tamuning
Jonestown
Tumon
Harmon Industrial Park
Guam International Airport
Oka Point
Marine Drive
3.63.6
3.73.7
3.83.8
3.93.9
4.04.0
Fujita Hotel#
Harmon Sink Injection Pit
Airport Injection Well
Airport Sampling Well
Alupang Beac h Stream
130 m/d
Dungca's & BBQ Beach Springs
350 m /d
D ung ca's Stre am & S prin g
6 50 m /d
Ypao & Pacific Island Club Springs
70-94 m /d
Pacific Islands Club Spring
175 m /d
Airport Sampling Well
38 m/d
Harmon Sink Sampling Well
38 m /d

11. • Defining features
1. Young limestone &
eogenetic karst
2. Freshwater-saltwater mixing
at base and margin of lens
3. Glacioeustasy has moved
the lens up and down
- Tectonic uplift and
subsidence may have also
occurred
- Still-stands affect aquifer
development
4. Basement-sea level-surface
relationships affect aquifer
development
1. Simple
2. Carbonate covered
3. Composite
4. Complex
Carbonate
Island Karst

12. CIKM Zones
& History

13. Closed
Depressions
• Sinkholes and
dolines
• Structural control
• Lithologic control
• Quarries and
ponding basins

14. Recharge
Features
• Argillaceous vs.
pure limestone
aquifers
• Distributed vs.
concentrated
recharge
• Sinkholes,
ponding basins,
injection wells

15. Agana
Sub-Basin
• Argillaceous
Mariana LS
• Alifan LS
• “Miniature”
classic karst
• Classic karst
plumbing….

16. limestone aquifer
water table
sea level
volcanic basement
Chloride Benchmarks
Safe Drinking Water guideline 250 mg/l
Saltwater
19,000
parabasal range < 30 mg/l
< 30
parabasal
water
saltwater intrusion > 150 mg/l
> 150mixing zone
saltwater toe range > 30 to 70 mg/l
< 70
saltwater
toe
basal range > 70 to < 150 mg/l
basal water
< 150

17. Surface Contaminants
Potentially Carried by
Recharging Water
industrial
spills
agricultural
runoff
storm
water
coastal contamination
septic tanks &
sewage spills

18. Aquifer Studies: Previous SY Estimates
Aquifer Sub-Basins
and Estimated
Sustainable Yields
Reserve
Production
(1982 Study)80 mgd (1991 Study)
1982 SY Estimates
1998 Production
• 1982 Northern Guam Lens
Study: 57 MGD
– First comprehensive field, lab,
and geophysical study
– $1.2M, 3-year effort
– GEPA, WERI, CDM (Mink)
– Still the departure point….
• 1991 Update (Mink): 80 MGD
• Current fresh groundwater
production*: 43 mgd
– 54% of 1991 SY est.
- GWA: 37 (2010)
- AF: ~3 (2008)
- Navy: ~2 (2008)
- All others (private) ~1 (2008)
~20% recharge
~25% recharge
*GEPA database
~14% recharge

19. Having said
all that…
The Myth of
“Sustainable Yield”

20. The Myth of
“Sustainable
Yield”
• “Sustainable yield” as a percent of recharge is an old but persistent
misconception
– Still provides a useful “rule of thumb”…but it’s not the real determinant
• Thiess first pointed out the error in 1940
– Even today, even many hydrologists still don’t get it
– Recent papers by prominent hydrologists notwithstanding….
• One use of models is to help evaluate how pumping changes the system
– In terms of both quantity and quality
• But models cannot replace the need for observed data
– In fact, they require data—lots of it!
• Proper management
– Comes from observing the trends and responding according
– Requires effective inter-agency cooperation and decision-making
- Which requires an inter-agency framework for ongoing consultation
• Finally, it addresses only the supply question; overlooks the demand
aspect of water management

21. Economics of
Sustainability (101)
• Higher salinity is the natural
consequence of production
• Green* technologies seem at
first to be the obvious choice
– But they may be (and usually
are) more expensive…
- Unless the incremental cost
is lower in the long run…
• The affordable limit may change
– Upward if the economy is
growing…
• Regulatory limits should be
routinely re-examined
– And adjusted based on
observable or predictable
consequences…
• “Sustainable” really means
“economical”
– i.e., meeting the objective with
least expenditure of resources Production
Salinity regulatory limitCost
affordable limit
*Green = satisfies a particular environmental
objective that is widely acknowledged as desirable
vertical wells
tunnel wells
High start-up, high ops cost
desalination
Low and high
switch places

22. Sustainable
Use
• The old question….
– “What is the sustainable yield…?”
• Is the wrong question!
• The right question…?
– “How can we economically meet the potable water needs of the
entire community?”
- Demand—just as important as supply
– Pricing structure
– Conservation incentives
- Supply—how much of a given quality at a given cost?
– Alternative technologies…and their costs
– Modeling can help—if the model is accurate
– Is only as good as the data
– The right kinds of data, and enough of them
– Sufficiently accurate and precise to do the job….