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….