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PowerPoint for University of Guam course, EV 511-01 Spring 2014. Professor John Jenson Northern Guam Lens Aquifer
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The Northern Guam
Lens AquiferCarbonate Island Karst
Model (CIKM)
Sustainable Yield
EV511
Groundwater Module
Lecture #8
0
5
10
15
20
25
30
35
40
Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec
Me
an
Mo
nth
ly R
ain
fall
(cm
)
.
Mean Monthly Rainfall (1982-1995)
Dry Season
Wet Season
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)
Pro
port
ion
of to
tal r
ecor
ded
rain
fall
.
Annual
Wet season
Dry season
Distribution of Rainfall Intensity
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
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
Optimizing Well Locations
Best practices for production volume and water quality
Topographic Map of Basement Volcanic Rock
Vann, 2000
Habana, 2010
Agafa GumasSubbasin
MangilaoSubbasin
FinegayanSubbasin
YigoSubbasin
AganaSubbasin
AndersenSubbasin
N
2 0 2 4 Miles Chloride Profile Wells
BasementTopography
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
5500050000 50000
55000 55000
']
']
#S
#S
Tamuning-Yigo
FaultTamuning-Yigo
Fault
Tumon Bay
Agana BayTamuning
Jonestown
Tumon
Harmon Industrial Park
Guam International Airport
Oka Point
Marine Drive
3.63.6
3.73.7
3.83.8
3.93.94.04.0
Fujita Hotel#
Harmon Sink Injection Pit
Airport Injection WellAirport Sampling Well
Alupang Beach Stream130 m/d
Dungca's & BBQ Beach Springs350 m/d
Dungca's Stream & Spring650 m/d
Ypao & Pacific Island Club Springs70-94 m/d
Pacific Islands Club Spring175 m/d
Airport Sampling Well38 m/d
Harmon Sink Sampling Well38 m/d
• Defining features1. Young limestone &
eogenetic karst2. Freshwater-saltwater mixing
at base and margin of lens3. 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. Simple2. Carbonate covered3. Composite4. Complex
Carbonate Island Karst
CIKM Zones& History
Closed Depressions
• Sinkholes and dolines
• Structural control• Lithologic control• Quarries and
ponding basins
Recharge Features
• Argillaceous vs. pure limestone aquifers
• Distributed vs. concentrated recharge
• Sinkholes, ponding basins, injection wells
Agana Sub-Basin
• Argillaceous Mariana LS
• Alifan LS• “Miniature”
classic karst• Classic karst
plumbing….
limestone aquifer
water tablesea level
volcanic basement
Chloride Benchmarks
Safe Drinking Water guideline 250 mg/l
Saltwater19,000
parabasal range < 30 mg/l
< 30
parabasalwater
saltwater intrusion > 150 mg/l
> 150mixing zone
saltwater toe range > 30 to 70 mg/l
< 70
saltwatertoe
basal range > 70 to < 150 mg/l
basal water
< 150
Surface Contaminants Potentially Carried by
Recharging Water
industrial spills
agricultural runoff
storm water
coastal contamination
septic tanks & sewage spills
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
Having said all that…
The Myth of “Sustainable
Yield”
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
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 resourcesProduction
Sal
inity regulatory limit
Cos
t
affordable limit
“green” technology
“bro
wn” technology
high-cost tunnel wells
low-cost vertical w
ells
*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
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….