SLIDE 1 Assessing Groundwater Availability in Hawai‘i’s Diverse Hydrogeologic Settings
Scot Izuka U.S. Geological Survey Pacific Islands Water Science Center
Water Resource Sustainability on Tropical Islands Honolulu, Hawai‘i December 1-3, 2015
SLIDE 2
Groundwater Availability — a Definition
Availability of fresh groundwater
SLIDE 3
What Limits Groundwater Availability?
Quantity Consequences
SLIDE 4
In Hawai‘i, Pumping is Less Than Recharge
(32%)
Water budget, in million gallons per day
SLIDE 5
Inflow
Consequences of Pumping
Recharge Wells Outflow Streams Submarine GW discharge Outflow
New Storage Volume
Water-table decline Reduced discharge Saltwater rise
SLIDE 6 Consequences and Availability
- Any amount of groundwater withdrawal has
consequences
- Availability of groundwater depends on what
consequences are deemed acceptable
- Type and magnitude of the consequences
depend on
- How much water is withdrawn
- Hydrogeologic setting
SLIDE 7
Hawai‘i’s Diverse Hydrogeologic Settings
High-perm. lava flows ? Perched? Caprock Low-perm. lava flows Low-perm. dikes
SLIDE 8
High-Permeability Lava Flows, no Caprock
Ocean Saltwater Freshwater lens Water table Submarine discharge
SLIDE 9
Pumping from Thin Freshwater Lens
Reduced discharge Well Water-table decline Saltwater rise & encroachment
SLIDE 10
Freshwater Lens with Caprock
Freshwater lens Discharge to streams/springs Caprock Saltwater Water table Discharge to ocean
SLIDE 11
Pumping from Freshwater Lens with Caprock
Reduced discharge to streams & ocean Well Water-table decline Saltwater rise & encroachment
SLIDE 12 Freshwater Lens in Low-Permeability Aquifer
Thick freshwater lens Discharge to streams Saltwater Discharge to
SLIDE 13 Pumping from Low-Permeability Aquifer
Reduced discharge to streams Reduced discharge to
SLIDE 14 Dike-Impounded Groundwater
Dikes Discharge to
Freshwater lens Saltwater
SLIDE 15 Dike-Impounded Groundwater
Discharge to
Saltwater Discharge to springs & streams
SLIDE 16 Withdrawing Dike-Impounded Groundwater
Reduced discharge to streams Reduced discharge to
Loss of spring discharge Well Tunnel
SLIDE 17
Approaches to Quantifying Consequences
SLIDE 18 Wailuku, Maui—Freshwater Lens
(Gingerich, 2008, USGS SIR 2008-5236)
Acceptable Consequence
- Salinity 1% that of seawater
- r better
Approach
model
SLIDE 19
Pumping rate (Mgal/d) Number of well fields Percentage of yield meeting acceptable criterion 20.1 4 23% 27.1 14 89%
Wailuku, Maui—Freshwater Lens
(Gingerich, 2008, USGS SIR 2008-5236)
SLIDE 20 Līhu‘e, Kaua‘i—Thick Lens, Low Permeability
(Izuka and Oki, 2002, USGS WRI 01-4200)
Acceptable Consequence
Approach
model
streamflow depletion
Reduced discharge to streams Thick freshwater lens
SLIDE 21
Effects of Pumping Additional 1.2 Mgal/d from Līhu‘e
Decrease in GW discharge to streams = 1.1 Mgal/d
SLIDE 22
USGS Groundwater-Availability Assessments
SLIDE 23
USGS Groundwater Availability Study in Hawai‘i Numerical Models
Kaua‘i O‘ahu Maui
SLIDE 24 Summary
- Groundwater availability limited by consequences
community is willing to accept
- Hawai‘i—diverse hydrogeologic settings, each with its
- wn set of pumping-related consequences
- Assessing groundwater availability in Hawai‘i requires:
Identifying hydrogeologic setting and associated consequences of groundwater withdrawal Setting acceptable limits for those consequences Using approaches that quantify consequences for the desired pumping
