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

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

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.

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

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)

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??

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)

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

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

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

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

Hood Canal Site Time Series fresher High tide saltier

Hood Canal Site Time Series

Hood Canal Site Time Series

Hood Canal Site Time Series

Hood Canal Site Time Series

Hood Canal Site Time Series

Hood Canal Site Time Series Low tide

Hood Canal Site Time Series

Hood Canal Site Time Series

Oahu recharge, SGD

Examples from Maunalua Bay: CRP at Niu NIU1CRP_OFFSHORE West East

Examples from Maunalua Bay: CRP at Niu NIU2_TOTAL_SHORELINE South East

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

Black Rock site Niu site Black Rock site

Examples from Maunalua Bay: salinity surveys

Hawaii Kai site: not much SGD

Niu site

Niu site: coastal water time series

Black Rock site

Black Rock site: water column time series

Black Rock site: shallow piezometer time series

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 

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

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)

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