1. Examining coastal groundwater discharge into Maunalua Bay using geochemical tracers and electrical resistivity
Peter Swarzenski1, Chris Reich2,
Sarah Rosa3,and Eric Grossman1
1US Geological Survey, Santa Cruz, CA
2US Geological Survey, St Pete, FL
3US Geological Survey, Honolulu, HI
Malama Maunalua meeting
April 2nd, 2009

2. Case study: Maunalua Bay
Talk organization
Brief history of SGD / role in coastal environments
Define SGD components, processes,
driving forces, time scales
SGD tools: 222Rn, Ra isotopes,seep
meters, dc resistivity
Case study: Maunalua Bay
Future SGD research

3. Historic observations
December, 1905
FRESH-WATER SPRINGS IN THE OCEAN
Dr. Hitchcock
L. Sonrel (1868) Le Fond de la Mer
“…there was a fortress in Maui… and under times of siege… could not have been held except for the presence of
submarine springs…”
City of Tyre
Photo shows boil on surface
Ship engulfed by spring from Project “SubGATE”
3,000 years ago, off Ruad, Syria, the Phoenicians had built a submarine GW collection system that supplied water to the City of Amrit.

4. Definitions…
Groundwater = water in the saturated zone of geologic material. So, “groundwater” = “pore water”
Submarine Groundwater Discharge (SGD) = any flow out across the seabed of the con’t shelf, regardless of composition or driving force
So, SGD not defined solely on basis of components, origin, or driving forces

5. SGD components Controlling forces:
(fresh) (brine) (marine)
Fresh-water
tunneling
So SGD includes not only meterorically-derived waters but also recirculated seawater driven by a variety of forces
Controlling forces:
Terrestrial – hydraulic gradient, geologic controls, CLIMATE
Marine – waves, tidal pumping, density-driven flow, sea-level differences (e.g., across barrier islands)

6. Assessing SGD / Impacts
-Process inherently difficult to measure —
Diffuse, small flux rates
-Cultural differences between hydrologists and oceanographers...
Same process, different perspective
SGD impacts
-vector for water and water-borne constituents
-algal blooms
-bacterial trans.
-eutrophication
The SCOR WG’s and meetings like this have helped address the 2nd point
Hydrologist or oceanographer??

7. LEE Measurement of SGD - 1 (A) Manual (B) Automatic
Direct measurements - seepage meters
LEE
(A) Manual
(B) Automatic
Electromagnetic seepage meter
CTD
CTD
inexpensive, simple
Possible artifacts:
(1) bag effects
(2) current/ topography-induced flow
(Taniguchi et al., 2002)
(Swarzenski et al., 2004)

8. Measurement of SGD - 2 2. Geochemical tracers (Rn and Ra isotopes)
1600 y
228Ra
5.7 y
224Ra
3.7 d
222Rn
3.8 d
220Rn
<1min
i – Th parent
ii – phase shift
iii – half-life

9. Stationary Resistivity
Measurement of SGD - 3
3. Indirect measurements - resistivity
Streaming Resistivity
Res = [elec. Cond.]-1
Formation Resistivity
Stationary Resistivity

10. Marine Continuous Resistivity Profiler (CRP)
CRP Streamer
2 current/9 potential electrodes
(10m spacing)
Depth of penetration ~20-30m
Dipole-Dipole configuration
<4 kts tow speeds
Measures bulk resistivity
(lithology + water)
fresh
Suwannee River
Salt water encroachment
saline

11. Time-series example—Tampa Bay, FL
Specs:
56 elect., 112m cable; 2m spacing;
8ch receiver;
switch box
Measures bulk resistivity
(lithology + water or pore space)
Scales of SGD

12. Hood Canal Site Time Series
fresher
High tide
saltier

13. Hood Canal Site Time Series

14. Hood Canal Site Time Series

15. Hood Canal Site Time Series

16. Hood Canal Site Time Series

17. Hood Canal Site Time Series

18. Hood Canal Site Time Series
Low tide

19. Hood Canal Site Time Series

20. Hood Canal Site Time Series

21. Oahu recharge, SGD

22. Examples from Maunalua Bay: CRP at Niu
NIU1CRP_OFFSHORE
West
East

23. Examples from Maunalua Bay: CRP at Niu
NIU2_TOTAL_SHORELINE
South
East

24. Offshore Continuous 222Rn Surveys
Examples from Maunalua Bay: 222Rn surveys
Offshore Continuous 222Rn Surveys
Very high activity in GW (<6000 dpm L-1)
Low activity in seawater
(2-5 dpm L-1)
Conservative, inert
Short half life
(t1/2 = 3.8 d)
Continuous 222Rn measurements

25. Black Rock site
Niu site
Black Rock site

26. Examples from Maunalua Bay: salinity surveys

27. Hawaii Kai site: not much SGD

28. Niu site

29. Niu site: coastal water time series

30. Black Rock site

31. Black Rock site: water column time series

32. Black Rock site: shallow piezometer time series

33. SGD-derived nutrient loads
NH4 Si
PO4
NO2+NO3
DIN
DON
TDN
(uM)
mean piezo conc.
0.1
104.2
6.4
0.5
SGD rate
NH4 Si
PO4
NO2+NO3
DIN
DON
TDN
(m3 d-1 m-1)
(mol d-1 m-1)
Niu site
29.8
3.3
3105.2
3.8
190.3
193.6
Black Rock site
33.1
3.6
3449.0
4.2
211.4
215.0
Kona# 23 20
0.1 2
Kahana*
0.78
0.07
1.3 

34. Summary, 1/2 1) dc resistivity: Scales and dynamics, SGD hotspots
2) Time-series 222Rn: Tidal forcing on SGD; rates of total SGD

35. loadings (mol d-1 per m of shore)
Summary, 2/2
3) Radium isotopes: water mass mixing (ages, eddy diffusivity coeff., source terms of discharging water, saline SGD
4) EM seepmeters: physical measurements of exchange, confirmation
SGD-derived nutrient
loadings (mol d-1 per m of shore)

36. Future work Next sampling effort in May, 2009
Need to discriminate between various forces; (e.g., isotopic signatures; links to physical oceanogr. transport processes)
Scaling up for regional assessments (e.g., combine isotopes with remote sensing, geophysics)
Develop SGD-derived nutrient loading estimates, and assess SGD impacts (stable isotopes)
Evaluate effects of climate change on SGD: change in sea level will cause a shift in GW fluxes from cont  ocean