Nearshore Morphology, Benthic Structure, 

Hydrodynamics, and Coastal Groundwater Discharge 

near Kahekili Beach Park, Maui, Hawaii

By Peter W. Swarzenski, Curt D. Storlazzi, M. Katherine Presto, Ann E. Gibbs, Christopher G. Smith, 

Natasha T. Dimova, Meghan L. Dailer, and Joshua B. Logan

Open-File Report 2012–1166

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director 

U.S. Geological Survey, Reston, Virginia: 2012 

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Suggested citation:

Swarzenski, P.W., Storlazzi, C.D., Presto, M.K., Gibbs, A.E., Smith, C.G., Dimova, N.T., Dailer, M.L., 

and Logan, J.B., 2012, Nearshore morphology, benthic structure, hydrodynamics, and coastal 

groundwater discharge near Kahekili Beach Park, Maui, Hawaii: U.S. Geological Survey Open-File 

Report 2012–1166, 34 p. (Available at http://pubs.usgs.gov/of/2012/1166/.)

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright 

owners to reproduce any copyrighted material contained within this report.

Cover: Left, example radon survey map offshore from Kahekili Beach Park, Maui, Hawaii; middle, photo 

of the radon time-series mooring at Kahekili; and right, example radon activities at the primary vent site

at Kahekili. 
iii

Contents

Abstract........................................................................................................................................................... 1

Introduction ..................................................................................................................................................... 1

Methods .......................................................................................................................................................... 2

Nearshore Mapping..................................................................................................................................... 2

Oceanographic Measurements................................................................................................................... 3

Submarine Groundwater-Discharge Measurements................................................................................... 3

Multichannel Electrical Resistivity ............................................................................................................... 4

Land-Based Resistivity............................................................................................................................ 4

Continuous Resistivity Profiling ............................................................................................................... 5

Results and Discussion .................................................................................................................................. 5

Nearshore Mapping..................................................................................................................................... 5

Oceanography............................................................................................................................................. 5

Submarine Groundwater Discharge and Nutrient Loads ............................................................................ 6

Resistivity.................................................................................................................................................... 8

Conclusions .................................................................................................................................................... 9

Acknowledgments........................................................................................................................................... 9

References Cited ............................................................................................................................................ 9

Additional Digital Information ........................................................................................................................ 12

Direct Contact Information ............................................................................................................................ 12

Appendix. Acoustic Doppler Current Profiler (ADCP) Information................................................................ 12

Tables

1. Experiment personnel........................................................................................................................... 13

2. Experiment location information............................................................................................................ 13

3. Select trace-element concentrations in surface water at the primary vent site at Kahekili Beach 

Park....................................................................................................................................................... 14

4. Select trace-element concentrations in discharging vent water at the primary vent site at Kahekili 

Beach Park. .......................................................................................................................................... 14

5. Nutrient concentrations in surface water at the primary vent site at Kahekili Beach Park. ................... 14

6. Nutrient concentrations in discharging vent water at the primary vent site at Kahekili Beach Park...... 15

7. Wave statistics at the Acoustic Doppler current profiler (ADCP) site.................................................... 15

8. Current and temperature statistics at the Acoustic Doppler current profiler (ADCP) site. .................... 15

9. Mean radon-222 (222Rn) derived submarine groundwater discharge (SGD) rates at the four timeseries moorings at Kahekili Beach Park. .............................................................................................. 15

10. Mean discharging groundwater nutrient concentrations in select coastal groundwater systems in 

Hawaii. .................................................................................................................................................. 16

11. Loading estimates per meter of shoreline of nutrient concentrations from select nearshore sites in 

Hawaii. .................................................................................................................................................. 16

Figures

1. Map depicting Maui, Hawaii, and surrounding islands. Study site is located on the northwest coast of 

Maui. ..................................................................................................................................................... 17
iv

2. Aerial photograph and SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) lidar 

bathymetric map of the west Maui study area showing the video and side-scan sonar tracklines....... 18

3. Aerial photograph and SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) lidar 

bathymetric map of the west Maui study area showing the location of instruments, resistivity lines, 

vent sites, and major landmarks. .......................................................................................................... 19

4. Aerial photograph and SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) lidar 

bathymetric map of the west Maui study area showing the location of the continuous resistivity 

profiling lines......................................................................................................................................... 20

5. Aerial photograph and SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) lidar 

bathymetric map of the west Maui study area overlaid with the side-scan sonar data collected in 

the study area. ...................................................................................................................................... 21

6. Side-scan sonar map and video images of the seafloor in the northern part of the the west Maui 

study area. ............................................................................................................................................ 22

7. Aerial photograph of the west Maui study area showing percent (%) coral coverage interpreted 

from the seafloor video imagery. m, meters.......................................................................................... 23

8. Time-series plots of tide, current, and water temperature data from the Acoustic Doppler current 

profiler (ADCP) at the primary submarine groundwater vent site. ........................................................ 24

9. Map showing the mean (vectors) and variability (ellipses) of waves and currents at the primary 

submarine groundwater vent site.......................................................................................................... 25

10. Map showing modeled cumulative flow distance for 3 hours following low tide at the near surface 

(gray vectors) and near bed (black vectors) at the primary submarine groundwater vent site.. ........... 26

11. Time-series plots of salinity of the discharging submarine groundwater and tide level (in meters, 

m) at the primary vent site. ................................................................................................................... 27

12. Time-series plots of surface water and bottom water salinity and tide level (in meters, m) at the 

primary vent site.................................................................................................................................... 28

13. Time-series plots of radon-222 (222Rn) activities, in disintegrations per minute per cubic meter of 

water (dpm/m3), salinity, and tide level, in meters (m), at the primary submarine groundwater vent 

site ........................................................................................................................................................ 29

14. Time-series plots of surface water radon-222 (222Rn), in disintegrations per minute per cubic 

meter of water (dpm/m3), and water level, in meters (m), at the primary submarine groundwater 

vent site. ............................................................................................................................................... 30

15. Time-series plots of calculated radon-222 (222Rn)-derived advection rates, in centimeters per day 

(cm/d), and salinity for the primary submarine groundwater vent site. ................................................. 31

16. Aerial photograph and SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) lidar 

bathymetric map of the west Maui study area overlaid with radon-222 (222Rn) activities, in 

disintegrations per minute per liter of water (dpm/L), in coastal surface waters................................... 32

17. Fence diagrams showing continuous resistivity profile results from the shore-parallel transects in 

the west Maui study area...................................................................................................................... 33

18. Fence diagram showing a land-based, multichannel electrical resistivity survey conducted along 

the beach face in the west Maui study area adjacent to the primary submarine groundwater vent 

site (horizontal axis is meters). ............................................................................................................. 34
Nearshore Morphology, Benthic Structure, 

Hydrodynamics, and Coastal Groundwater Discharge 

near Kahekili Beach Park, Maui, Hawaii

By Peter W. Swarzenski1, Curt D. Storlazzi1, M. Katherine Presto1, Ann E. Gibbs1, 

Christopher G. Smith1, Natasha T. Dimova2, Meghan L. Dailer3, and Joshua B. Logan1

Abstract

This report presents a brief summary of recent fieldwork conducted off Kahekili Beach 

Park, Maui, Hawaii, the site of the newly established U.S. Coral Reef Task Force priority study 

area at Kaanapali and the Hawaii Department of Land and Natural Resources, Division of 

Aquatic Resources, Kahekili Herbivore Fisheries Management Area (HFMA). The goals of this 

fieldwork are to provide new baseline information to help guide future studies and to provide 

first insights into rates and drivers of coastal groundwater discharge and associated constituent 

loadings into the priority study area’s coastal waters. This study presents the first swath acoustic 

mapping information, in situ oceanographic instrument measurements, and coastal groundwater 

discharge estimates at this site based on the submarine groundwater discharge tracer radon-222 

(

222Rn). Coastal groundwater discharge rates ranged from about 22 to 50 centimeters per day, 

depending on proximity of the sampling mooring to the primary discharge vent. The water 

chemistry of the discharging groundwater was at times dramatically different than ambient 

seawater. For example, at the primary vent site at Kahekili, the concentrations of total dissolved 

nitrogen (TDN), dissolved silicate (DSi), and total dissolved phosphorus (TDP) in the 

discharging groundwater were 43.75 micromolar (μM), 583.49 μM, and 12.04 μM, respectively. 

These data extend our basic understanding of the morphology, benthic structure, and 

oceanographic setting of this vent site and provide a first estimate of the magnitude and physical 

forcings of submarine groundwater discharge and associated trace metals and nutrient loads here. 

Introduction 

The Kaanapali region north of Lahaina in west Maui, Hawaii, is a very popular tourist 

destination, and the shoreline is consequently largely developed with hotels, resorts, and 

associated infrastructure. This report presents a brief summary of recent fieldwork done off 

Kahekili Beach Park in Kaanapali (fig. 1; see table 1 for list of experiment personnel) to better 

characterize the seafloor, nearshore oceanography, and hydrogeologic exchange processes. This 

site has just been established as a U.S. Coral Reef Task Force (USCRTF) priority study area and 

the Hawaii Department of Land and Natural Resources, Division of Aquatic Resources 

 

1

U.S. Geological Survey. 2

University of Alabama, Department of Geological Sciences, Tuscaloosa, Alabama. 3

University of Hawaii, Department of Botany, Honolulu, Hawaii.
2

(DLNR/DAR), Kahekili Herbivore Fisheries Management Area (FMA). Over the past decade, 

there has been a notable change in bottom type at this location; areas once covered by abundant 

corals are now covered mostly by turf algae or macroalgae, suggesting a likely local nutrient 

imbalance that warrants further investigation. Because of this, the site has previously been 

studied to examine how municipal wastewater plumes may be conveyed to the coastal waters by 

discharging groundwater (Dailer and others, 2008, 2010; Hunt and Rosa, 2009). At this site there 

are multiple spring vents close to shore (water depths are usually less than 2 meters), where 

much lower salinity water can readily be observed discharging into the nearshore water column. 

Previous efforts have used dissolved nitrate (δ15N) records (Dailer and others, 2010), as well as a 

suite of telltale organic pollutants (for example, Hunt and Rosa, 2009), to infer focused 

municipal wastewater discharges at this location. In 2010, the USCRTF designated the Kaanapali 

linked watershed-reef system (or “ahupua‘a” in Hawaiian) as a USCRTF Hawaiian Local Action 

Strategy to Address Land-Based Pollution (HI-LBP/LAS) priority study site so that research and 

restoration could be directed to the area. At the same time, DLNR/DAR established the Kahekili 

Herbivore Fisheries Management Area (FMA) in an effort to increase the number of herbivores 

to help reduce the turf algae or macroalgae that have overgrown the corals at the site. 

Our study presents the first swath acoustic map, in situ oceanographic instrument 

measurements, and first estimates of coastal groundwater discharge to this site based on the 

submarine groundwater discharge tracer radon-222 (222Rn). These data extend our basic 

understanding of the morphology, benthic structure, and oceanographic setting of this location, 

and provide a first estimate of the magnitude and physical forcings of submarine groundwater 

discharge (SGD) and associated metals and nutrient loads here. The goals of this effort are to 

provide baseline information for future studies and to assess the viability of proven SGD tracers 

to assess SGD magnitude and constituent loadings into the area’s coastal waters. Based on four 

surface-water 222Rn time-series measurements that were conducted at the primary vent site and 

surrounding environs, the groundwater flux was quantified and used to derive constituent 

loadings for suite of trace elements and nutrients. Loading estimates were also compared to 

similarly derived load estimates for select other Hawaiian coastal waters. 

Methods

Nearshore Mapping

Geo-referenced underwater video and side-scan sonar/acoustic backscatter data were 

collected between Kekaa Point (Black Rock) and the north end of Honokowai (fig. 2) between 

March 21 and 22, 2011, as part of U.S. Geological Survey (USGS) Field Activity A-2-11-MU 

(http://walrus.wr.usgs.gov/infobank/a/a0211mu/html/a-02-11-mu.meta.html, last accessed 

May15, 2012). This fieldwork was reconnaissance in design and not intended to acquire the 

extent or quality of imagery necessary to perform a comprehensive or quantitative analysis on 

coral diversity, benthic habitat, or health of the Kahekili-Honokowai reef tract. 

Nearly 2 hours of underwater video were collected at 84 stations using a Splashcam 

Sidewinder 360 video system (http://www.splashcam.com/sidewinder/sw.htm, last accessed 

May15, 2012). The camera was hand deployed over the side of the boat, at stations that generally 

corresponded to the 5, 10, 15, 20, 25 and 30 meter (m) isobaths, along multiple shore-normal 

transects. Standard definition National Television System Committee (NTSC) video was 

recorded to mini-DV videotape in the field. Back in the lab, the video imagery was converted to 

digital MPEG2 files and adjusted for white balance. Still images were grabbed every 10 seconds 
3

along individual video tracklines and used to interpret benthic structure following the 

classification scheme of Battista and others (2007). 

Navigation and position information was acquired using a Novatel ProPak-G2plus. 

National Marine Electronics Association (NMEA) Global Positioning System (GPS) information 

was recorded on the right audio channel of the videotapes using Red Hen Systems VMS 

hardware (RHS; http://www.redhensystems.com/, last accessed May15, 2012). This audio 

encoding allows geolocation of the video imagery and random access to the video in a 

geographic information system (GIS), using software developed by RHS. Navigation was also 

recorded to a computer on the fly using RHS Media Mapper software. 

Side-scan sonar was collected along 12 lines over an area of 0.92 square kilometer (km2

; 

227 acres) using a Humminbird 898C SI fishfinder system 

(http://store.humminbird.com/products/341721/898c_SI_Combo, last accessed May15, 2012). 

Data were processed and an acoustic backscatter image was produced using SonarWiz software 

(http://chesapeaketech.com/prod-wizmap.html, last accessed May15, 2012). 

Oceanographic Measurements

In situ oceanographic measurements of tides, waves, currents, and water temperature 

were made July 14–17, 2010, just north of the Kahekili Beach Park (fig. 3; table 2). These 

measurements were made using a 2-MHz Nortek Aquadopp acoustic Doppler current profiler 

(ADCP; see appendix for ADCP information) that collected data in 0.25-m bins from 0.5 m 

above the seafloor up to the surface for 250 seconds (s) at 2 hertz (Hz) every 5 minutes (min) to 

allow calculation of tides (m), mean current speeds (m/s), and mean current directions (in 

degrees relative to true north). Directional wave data were recorded for 1,024 s at 2 Hz every 

hour; these data included wave height (m), wave period (s), and mean wave direction (in degrees 

relative to true north). Water temperature (in degrees Celsius, °C) was measured every 5 min. 

Submarine Groundwater-Discharge Measurements

Radon-222 is an effective tracer to assess water transport and exchange processes, such 

as submarine groundwater discharge, because this isotope has a short half-life (t1/2) of 3.8 days 

(d), is a noble gas and so chemically inert, and is highly enriched in groundwater relative to 

surface water (Swarzenski, 2007). Recent advances in the analyses of 222Rn have been greatly 

simplified by a commercially available Rn-in-air monitor and a simple water/air exchanger that 

enables one to calculate the aqueous Rn concentration from the measured air concentration 

(Burnett and Dulaiova, 2003; Dulaiova and others, 2006; Swarzenski and others, 2006). At 

Kahekili Beach Park, a suite (fig. 3) of geochemical field samples were collected and 

geophysical surveys conducted over a 1-week period in mid-July 2010. Geochemical analyses of 

time-series sampling included 222Rn, ammonium (NH4

+

), dissolved silicate (DSi), soluble 

reactive phosphorus (SRP), nitrate-nitrite (NO3

−

+NO2

−

), dissolved inorganic nitrogen, (DIN), 

dissolved organic nitrogen (DON), total dissolve nitrogen (TDN), total dissolved phosphorus 

(TDP), molybdenum (Mo), barium (Ba), rhenium (Re), vanadium (V), iron (Fe), cesium (Cs), 

uranium (U), chromium (Cr), and manganese (Mn). Geophysical surveys included both 

continuous-resistivity-profiling mapping and land-based multichannel electrical resistivity. 

Select trace-element concentrations (in nanomolars, nM) are shown in table 3 (surface water) and 

table 4 (discharging vent water), whereas select nutrient concentrations (in micromolar, μM) s 

are shown in table 5 (surface water) and table 6 (discharging vent water). 
4

Time-series measurements of surface water 222Rn (t1/2=3.8 d) were obtained using a 

single Durridge Company, Inc., RAD7 radon monitor set to 30-min counting intervals and were 

modeled to derived total (fresh+brackish) SGD rates (Burnett and Dulaiova, 2003; Burnett and 

others, 2001, 2006) at the primary vent site and also at three additional sites surrounding the 

main vent. The purpose of these deployments was to observe how SGD rates may vary spatially 

and within different benthic structures. Using methods described in Swarzenski and others 

(2007b), nutrients were determined on a Lachat Instruments QuickChem 8000, and the suite of 

trace elements was analyzed on a HR-ICPMS at University of Southern Mississippi. 

To adequately capture discharging vent water, a piezometer was carefully inserted into a 

prominent spring vent by a diver using scuba to minimize contamination of the discharging 

groundwater by ambient seawater. Time-series measurements of 222Rn, as well as of nutrients 

and select trace elements, were collected from this piezometer and complimented with 

concurrent surface water samples. Water quality parameters were monitored using a calibrated 

YSI multiprobe. In all instances, the dissolved oxygen concentrations in groundwater samples 

were close to 1 milligram per liter (mg/L) (strongly sub-oxic), indicating that the groundwater 

samples were not compromised by seawater intrusion. 

Multichannel Electrical Resistivity

Electrical resistivity techniques are often used to measure the conductivity (resistivity = 

electrical conductivity−1

) of coastal sediment and pore fluids and have recently proven useful for 

assessments of the detailed dynamics of the freshwater/saltwater interface with the development 

of multichannel resistivity systems (Manheim and others, 2004; Swarzenski and others, 2004, 

2006; Dimova and others, 2012). Advantages of these newer systems include tailored stream 

arrays, rapid data acquisition, and user-friendly post-processing software. The resistivity 

measurements were post-processed using an updated inverse modeling routine within Advanced 

Geosciences, Inc. (AGI), EarthImager software that accommodates the water-column salinity 

and depth. Processing resolution was optimized by using a starting model with the apparent 

resistivity pseudosection coupled to real bathymetry and topography data. A best-fitting layered 

model was then developed using an iterative least-squares smooth-model inversion method 

(Swarzenski and others, 2007b). 

Land-Based Resistivity

At Kahekili Beach Park, the resistivity of the underlying beach sediment was surveyed 

using an AGI, eight-channel Marine SuperSting R8 system connected to an external switching 

box that controlled the flow of current along the 56-electrode cable. This system was 

programmed to measure current potentials in a distributed array so that resolution and signal-tonoise ratio is maximized. For one resistivity measurement, the receiver injects an optimized 

current, reverses the polarity and then re-injects the current again to cancel potential spontaneous 

voltages. This process was repeated until the error is below a prescribed threshold value. Landbased electrical-resistivity surveys were conducted along the beach face adjacent to the primary 

vent site as well as at two adjacent mooring sites. For this work, the electrical resistivity streamer 

was placed just landward of the high tide line (Swarzenski and others, 2006, 2007a,b). Current 

flow was routed down-cable from a user-defined command file using a dipole-dipole 

configuration. These settings enabled the collection of more than 760 discrete resistivity readings 

in less than 25 min. Recent applications of this geophysical tool have ranged from creating 

hydrogeologic perspectives (Bratton, 2011) to qualitative identification of groundwater seepage 
5

sites (Day-Lewis, 2006, Swarzenski, 2007), and actual SGD assessments (Swarzenski and others, 

2006; Dimova and others, 2012). 

Continuous Resistivity Profiling

Continuous resistivity profiling (CRP) is the unique adaption of a common geophysical 

technique to measure the electrical resistivity of sediment beneath a water body such as river, 

lake, or the coastal ocean. An advantage of this method is that relatively long transects (tens of 

kilometers) can be surveyed from a small boat with access to very shallow water. Most of the 

interest in these towed-array resistivity measurements comes from coastal scientists and 

hydrologists that are interested in the dynamics of the freshwater/saltwater interface and general 

hydrogeological controls of coastal aquifers. 

A CRP survey was made of the nearshore region off Kahekili Beach Park to help confirm 

the regional extent of discharging groundwater (fig. 4). Surveys were done using the AGI 

SuperSting R8/IP and a 120-m long towed cable specifically designed for marine resistivity 

acquisition. The cable consists of two current electrodes and nine potential electrodes with 10-m 

spacing. Total depth penetration below the sea bottom using this electrode spacing and dipoledipole configuration is approximately 25 m but is water depth dependent. The real-time position 

of the boat was streamed directly to the SuperSting from a Lowrance LMS-480 GPS. Basic water 

quality parameters and 222Rn were also measured during these surveys. Temperature and specific 

conductance were measured using a YSI-556 probe mounted 0.5-m below the water surface. 

Radon-222 was measured using three RAD7 radon-in-air monitoring devices that were 

connected in parallel to one air/gas exchanger, as described by Dulaiova and others (2006). 

Briefly, surface water was pumped to an air/water exchanger where the radon in water was 

aspirated into the gas phase and drawn into the three RAD7s. Radon-in-air concentration was 

converted to radon-in-water concentration using the exchanger temperature and a computed 

partitioning coefficient (Burnett and others 2001). 

Results and Discussion

Nearshore Mapping

Preliminary evaluation of the underwater video and acoustic backscatter imagery suggest 

the Kahekili/Honokowai reef tract supports medium to high coral cover from just south of 

Kahekili Beach Park to the northern extent of our survey in north Honokowai, with the densest 

coral cover observed offshore of Honokowai Point and Kahekili Beach Park (figs. 5, 6). The 

dominant coral species observed included Porites compressa, Porites lobata, Montipora 

capitata, and Pocillopora meandrina. Live coral is restricted to water depths between 

approximately 3 and 20 m, with highest coral coverage between 10 and 15 m (fig. 7). In water 

depths greater than about 20 m, the seafloor is dominated by sand colonized with the calcified 

green algae Halimeda sp. South of Kahekili Beach Park, the seafloor transitioned to an extensive 

sheet of uncolonized, rippled sand and undulating, uncolonized carbonate pavement. 

Oceanography

Oceanographic data were acquired for 61 hours over more than 2 days during the period 

from July 14 to 17, 2010 (2010 Year Days 195.75–198.25). More than 10,000 data points were 

recorded by the ADCP. The raw data were archived and copies of all the data were postprocessed for analysis. The ADCP data generally appeared to be of high quality. 
6

The tides of west Maui are mixed, semidiurnal (Storlazzi and Jaffe, 2008); the maximum 

tidal range during the deployment was 0.74 m (fig. 8), transitioning from spring to neap tide 

conditions. The wave heights decreased slightly during the deployment, ranging from 0.31 to 

0.78 m, with a mean±1 standard deviation of 0.50±0.12 m (table 7). The wave periods and 

directions varied a bit around the mean but showed no long-term trends during the deployment, 

ranging from 4.2 s to 11.0 s and from 225.9° to 263.6°, with a mean±1 standard deviation of 

8.2±1.6 s and 233.9±34.0°, respectively. The currents appear to be primarily driven by the tides, 

and the current speeds decreased slightly during the deployment, likely due to the decrease in 

tidal range (fig. 8). The near-surface and near-seabed current speeds ranged from 0.01 m/s to 

0.11 m/s and from 0.00 m/s to 0.09 m/s, with a mean±1 standard deviation of 0.05±0.02 m/s and 

0.04±0.02 m/s, respectively (table 8.) Whereas the primary orientation of flow was alongshore, 

likely due to tidal forcing, the mean current direction throughout most of the water column at the 

deployment site was directed offshore to the west-southwest, likely balancing the net wavedriven onshore flow to the east-northeast (figs. 9, 10). The water temperatures appeared 

primarily driven by the daily insolation cycle, peaking in the afternoon and reaching a minimum 

in the early morning, with no apparent long-term trend over the short deployment. Over the 

course of the deployment, the water temperature ranged from 20.62 to 22.24°C, with a mean±1 

standard deviation of 21.28±0.38°C (table 8). For more information on tides, waves, currents, 

and water-column properties along west Maui, see Storlazzi and others (2003, 2006), Storlazzi 

and Field (2008), and Storlazzi and Jaffe (2003, 2008). 

Submarine Groundwater Discharge and Nutrient Loads

Salinity of the discharging vent water varied directly with the tide and was surprisingly 

fresh, although the salinity fluctuated from under 2 to more than 12 (fig. 11). Ambient seawater 

salinities above the vents were consistently between 34 and 36, indicating only slight freshening 

relative to open-ocean salinities (fig. 12). The surface water is noticeably lower in salinity than 

bottom water during much of the time series. The mean 222Rn activity in the vent water showed 

inverse systematic variations with the tide (fig. 13). A representative value of the discharging 

vent water (that is, groundwater) 222Rn activity (27,045 disintegrations per minute per cubic 

meters, dpm/m3

) was used in the calculation of submarine groundwater discharge rates at all four 

surface water sites at Kahekili. Figure 14 shows the 222Rn time-series results for the surface 

water at the primary vent site. Although there are systematic fluctuations in 222Rn that appear to 

be modulated by the tidal stage, the surface water 222Rn activities are quite low (fig. 14). 

Acknowledging the various assumptions that are inherent in the application of a Rn box model 

(Burnett and Dulaiova, 2003; Burnett and others, 2001, 2003, 2006, 2007; Dulaiova and others, 

2006; Swarzenski and others, 2006, 2007a,b, 2009) to derive a rate of SGD, the calculated SGD 

rate was highest (mean=55±56 cm/d; max=277 cm/d; fig. 15) at the primary vent site (fig. 15) 

and consistently lower at the three surrounding sites (table 9). The three more distal Rn mooring 

sites were chosen based on bottom sediment/bedrock type as well as distance from the main vent 

site; site A targeted sand deposits, site B was situated seaward of the primary vent site to discern 

the potential offshore extent of the discharge field, and site C targeted volcanic outcrops. Based 

on these radon data as well as simple visual observations, it appears that groundwater discharge 

was most pronounced at the shallow active vent site but is still enhanced at least out to the most 

distal Rn-mooring site (site B). This suggests that discharge may not be contained only within 

the shallow vent sites but may be more regional in scale. 

Based on the peaks in 222Rn activity (fig. 13) at and in the 3 hours following low tides, a 

rough estimate of the distance and direction of submarine groundwater discharged into the 
7

nearshore at the vent site can be made using a series of 3-hour progressive vector diagrams of 

projected cumulative flow starting from the vent site at low tide. These progressive vector 

diagrams were constructed using the method described by Siegel and others (2003) during the 

study period (fig. 10). These calculations make the assumption that the flow measured at 

Kahekili is uniform alongshore and cross shore from the vent site, which is not always the case 

(Storlazzi and others 2006; Storlazzi and Field, 2008). The projections do, however, provide 

information regarding the relative direction and magnitude of flow and the flux of nutrient-laden 

submarine groundwater discharged from the vent during the projected time period if the forcing 

mechanisms (wind and tides) were to remain uniform over the projected travel path. Our 

measurements off Kahekili showed that there was net alongshore flow, and therefore net 

transport of nutrient-laden water, to the south from the vent site towards Keka’a (Black Rock) 

Point during the 3 hours following low tides when the SGD rate was the greatest. 

The majority of the radon data collected during the CRP surveys was near the limits of 

detection for the multidetector instrumentation. The highest surface water radon activity 

measured was 0.4 dpm/ L (fig. 16). This is likely attributable to the deep water column that 

would considerably dilute any focused high Rn groundwater source. Another prominent factor in 

these low Rn numbers is probably a result of the sea state at the time of sampling; unfortunately, 

the prevailing winds were consistently strong and created relatively large waves that disrupted 

the air/sea interface. 

In addition to the surface-water and groundwater 222Rn time-series data, discrete surfacewater and groundwater samples were also collected on July 14, 2011, at the primary vent site. 

Tables 3 and 4 report associated select trace-element analyses (in nM) obtained during this 

sampling. The discharging vent water was strongly depleted in dissolved oxygen (as measured 

with a YSI multiprobe; data not shown), which was also manifested in some of the observed 

redox-sensitive trace-element concentrations (that is, Mn, U, V, and Mo). In contrast, Fe does not 

appear enriched in the reduced groundwater. Some trace element concentrations (for example, 

Fe, Cr, and Mn) increased after the 10:00 a.m. Hawaii Standard Time (HST) low-tide event, 

verifying that the tide modulates groundwater discharge rates. 

Nutrient (NH4, DSi, SRP, DIN, DON, TDN, and TDP) concentrations (μM) in surface 

water and discharging vent water, respectively, are summarized in tables 5 and 6. Very little 

ammonia was present either in the surface water (mean concentration=0.07 μM) or discharging 

groundwater (mean concentration=0.3 μM). Mean silica concentrations were roughly 36-fold 

greater in discharging groundwater (mean=583.5 μM) than ambient surface seawater 

(mean=15.97 μM). Dissolved inorganic nitrogen (DIN) concentrations were also much higher in 

discharging groundwater (mean=34.8 μM) than in ambient surface seawater (mean=0.2 μM); 

this trend was also observed for unacidified PO4 (Total Dissolved Phosphorus, TDP) where the 

mean groundwater concentration was 12.04 μM and the mean surface seawater concentration 

was 0.11 μM. Mean unacidified PO4 (as soluble reactive phosphorus, SRP) concentrations in the 

discharging groundwater (11.41 μM) were comparable to mean TDP values. The N:P (DIN/SRP) 

molar ratio for discharging groundwater was between 1.8 and 3.5 (mean=2.98) and the mean 

surface seawater value was 5.5. 

One can use mean coastal groundwater nutrient concentrations (table 10) and calculated 

SGD rates to derive submarine groundwater discharge-associated nutrient loading estimates into 

nearshore waters (Garrison and others, 2003; Swarzenski and Izbicki, 2009; Johnson and others, 

2008; Peterson and others, 2009; Knee and others, 2010). The calculated mean SGD rate for the 
8

primary vent site at Kahekili Beach Park (55 cm d-1 or 55 m3/d/m of shoreline assuming that the 

discharge extends out to 100 m) is on the same order of magnitude as the Rn-derived SGD rates 

obtained recently for two sites in Maunalua Bay, Oahu (Black Point=33.1 m3

/d/m and Niu=29.8 

m3

/d/m) (Swarzenski and others, 2009), as well as two additional SGD rate estimates reported in 

Johnson and others (2008) and Peterson and others (2009) for the Kona coast of Hawaii, 133.9 

m3/d/m and 86.5 m3/d/m, respectively. 

From these SGD rates, the following SGD-derived nutrient loading estimates are 

calculated for the primary vent site at Kahekili per meter of shoreline: NH4

+

, 0.02 mol/d/m; DSi, 

32 mol/d/m; SRP, 0.70 mol/d/m; [NO2+NO3

-

], 2.18 mol/d/m; DIN, 2.2 mol/d/m; DON, 0.54 

mol/d/m; TDN, 2.73 mol/d/m; and TDP, 0.67 mol/d/m. Table 11 summarizes these results from 

the vent site at Kahekili and compares them to similarly obtained SGD-derived nutrient fluxes at 

Niu and Black Point in Maunalua Bay, Oahu. 

Resistivity 

In the shallow waters off Kahekili, focused submarine groundwater discharge provides a 

direct conduit for the coastal aquifer to readily exchange with the nearshore water column. 

Changes in the pore-water resistivity cause a large increase in the resistivity according to wellknown empirical laws governing the resistivity in porous rocks. For example, Archie’s Law 

relates bulk resistivity (ohm meter, Ω•m) of a porous rock to a formation factor, as follows: 

ρT = ρf F; F=ɸ−m

where 

ρT = the bulk resistivity (Ω•m) of the saturated material, 

ρf = the resistivity (Ω•m) of the pore fluid, 

F = Formation Factor (unitless), 

ɸ = fractional porosity of matrix material, and 

m = a empirical exponent based on lithology. 

On the basis of Archie’s Law, one can estimate that the bulk resistivity of a 

unconsolidated sand layer (m=~2) may be approximately four times the resistivity of the 

seawater (typically about 0.3 Ω•m). Deviations from this may be interpreted in terms of a 

hydrologic control (that is, the presence of a freshened water parcel). 

Depth penetration below the sediment surface varied considerably during the survey 

because of the steep offshore seafloor gradient at Kaanapalii Beach. Water depths varied 

between 5 and 15 m, restricting the surveys to the upper 20 m of the sub-seafloor or less. 

Approximately six north-south lines, 2 kilometers (km) each, were surveyed parallel to the 

Kaanaplii Beach area. Data are shown in figure 17 for four of the six lines collected. Lines 

beyond approximately 100 m from the shore had reasonably constant resistivity values (average 

= 0.61 Ω•m) due in part to deeper water and homogenous subsurface. The most nearshore line 

provides some indication of more resistive units in the subsurface (ρT>2.5 Ω•m). One resistive 

body occurred at the southern end of the embayment where a channelized drainage lagoon at 

Keka’a (Black Rock Point) provides a hydrogeologic anomaly. The second resistive unit occurs 

approximately half-way along the shoreline, in a region known for groundwater seepage. The 

unit is less resistant (~ 2.0 Ω•m) than the unit to the south (> 2.5 Ω•m) but is considerably 

different than elsewhere in the bay (0.61 Ω•m). 

 One processed land-based electrical resistivity image is shown in figure 18. Based on 

this model, a sharp contact is observed along the entire line roughly 2–5 m below the beach face. 

It is likely that this contact represents a transition between surficial beach sands and deeper 
9 

geologic deposits. These deeper layers are likely hydraulically transmissive limestone, caprock, 

and gravel alluvium that is situated variably on top of older volcanic-rock aquifers. Groundwater 

could be conveyed in these deposits towards the coast where they would rise to the surface due 

their density contrast with seawater (Manheim and others, 2004; Swarzenski and others, 2006, 

2007a,b). 

Conclusions The seabed in the study area was characterized by dense coral cover offshore of 

Honokowai Point and Kahekili Beach Park between depths of 3 and 20 m, with the highest coral 

coverage between 10 and 15 m. In water depths greater than about 20 m, the seafloor was 

dominated by sand colonized with Halimeda sp. South of Kahekili Beach Park, the seafloor was 

primarily an extensive sheet of uncolonized, rippled sand and undulating, uncolonized carbonate 

pavement. Overall, the primary orientation of currents at the deployment site was alongshore, 

likely due to tidal forcing. The mean current direction throughout most of the water column was 

directed offshore to the west-southwest, likely balancing the net wave-driven onshore flow to the 

east-northeast.

The discharging groundwater at the primary vent site north of Kahekili Beach Park was 

successfully captured using a piezometer that was installed into an active spring vent. This 

piezometer was used to collect continuous 222Rn measurements as well as discrete water samples. 

A suite of nutrients and trace elements was obtained in this discharging groundwater and also 

compared to ambient surface seawater. Surface water 222Rn was monitored for at least several 

days at the primary vent site, as well as at three surrounding sites that were chosen on the basis 

of bottom sediment/bedrock type and distance to land. A 222Rn mass balance model was used to 

develop submarine groundwater discharge rates at the four surface water moorings. Mean 

advective rates were predictably highest at the primary vent site (55 m3

/d/m) and decreased 

systematically in the surrounding sites (A=21 m3/d/m; B=30 m3/d/m, and C=22 m3/d/m). During 

the limited period of in situ measurements, these discharged waters were advected to the south 

from the vent site. Such observations provide preliminary information as to the scale, magnitude, 

and constituent loadings of submarine groundwater discharge into nearshore coastal waters off 

Kahekili Beach Park and can be used to develop subsequent sampling strategies. 

Acknowledgments 

This research was conducted under the USGS Submarine Groundwater and USGS Coral 

Reef Projects, both of which are funded by the USGS Coastal and Marine Geology Program. We 

would like to thank Russell Sparks and Darla White (HI-DLNR/Division of Aquatic Resources) 

for invaluable logistical support and Chip Hunt (USGS) for constructive discussions that helped 

define the field campaign. 

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Additional Digital Information

For additional information on the instrument deployments, please see: 

For an online PDF version of this report, please see: 

http://pubs.usgs.gov/of/2012/1166/ 

For more information on the U.S. Geological Survey Western Region’s Coastal and Marine 

Geology Team, please see: 

http://walrus.wr.usgs.gov/ 

For more information on the U.S. Geological Survey’s Coral Reef Project, 

please see: 

http://coralreefs.wr.usgs.gov/ 

Direct Contact Information

General Project Information 

Peter W. Swarzenski (USGS Coastal Aquifer Project Chief): pswarzen@usgs.gov

General Project Information: 

Curt Storlazzi (USGS Coral Reef Project Chief): cstorlazzi@usgs.gov

Appendix. Acoustic Doppler Current Profiler (ADCP) Information

Nortek Instruments 2-MHz Aquadopp upward-looking acoustic Doppler current profiler 

s/n: 1757 

Transmitting frequency: 2 MHz 

Depth of transducer: 1.7 m 

Blanking distance: 0.20 m 

Height of first bin above bed: 0.50 m 

Bin size: 0.25 m 

Number of bins: 10 

Average interval: 0:08:00.00 
13

Profile interval: 0:15:00.00 

Wave interval: 1:00:00.00 

Wave cell size: 0.5 m 

Operating mode: High-resolution 

Sound speed calculation: Set salinity, updating temperature via sensor 

Data Processing: 

The Nortek current data were processed using the Prof2NDP program and the wave data using 

the QuickWave program. 

The data were averaged over 1 hour ensembles, all of the spurious data above the water surface 

were removed, and all of the data in bins where the beam correlation dropped below 80 percent 

were removed for visualization and analysis. 

Tables

Table 1. Experiment personnel. 

[USGS, U.S. Geological Survey; UA, University of Alabama; UH, University of Hawaii]

Person Affiliation Responsibilities

Peter Swarzenski USGS Chief Scientist, oceanographer

Curt Storlazzi USGS Chief Scientist, oceanographer

Kathy Presto USGS Oceanographer

Ann Gibbs USGS Geologist

Chris Smith USGS Geologist

Natasha Dimova UA Geochemist

Megan Dailer UH Biologist

Josh Logan USGS Physical scientist

Table 2. Experiment location information. 

[ADCP, Acoustic Doppler current profiler] 

Site Name Latitude

(decimal degrees)

Longitude

(decimal degrees)

ADCP 20.938631 −156.693122

Vent site—mooring 20.938576 −156.693612

Resistivity line A (start) 20.935763 −156.692697

Resistivity line A (end) 20.934664 −156.692737

Resistivity line B (start) 20.93897 −156.69270

Resistivity line B (end) 20.93799 −156.69281

Resistivity line C (start) 20.94471 −156.69234

Resistivity line C (end) 20.94569 −156.69229

Site A—mooring 20.934477 −156.693612

Site B—mooring 20.938499 −156.694476

Site C—mooring 20.944505 −156.693707
14

Table 3. Select trace-element concentrations in surface water at the primary vent site at Kahekili Beach 

Park. 

[Molybdenum (Mo), barium (Ba), rhenium (Re), vanadium (V), iron (Fe), cesium (Cs), uranium (U), chromium 

(Cr), and manganese (Mn) in nM, nanomolar]

Sample ID Mo 

[nM]

Ba

 [nM]

Re

 [nM]

V 

[nM]

Fe 

[nM]

Cs 

[nM]

U 

[nM] 

Cr

[nM] 

Mn

[nM] 

T1-800-SW 98 29 0.044 32 97 1.80 10.34 3.6 7 

T2-900-SW 108 30 0.053 36 66 2.02 11.41 3.0 7 

T3-1000-SW (low tide) 105 30 0.039 37 226 1.92 11.28 11.1 58

T4-1100-SW 107 30 0.046 39 261 1.92 11.31 13.2 8 

T5-1200-SW 110 30 0.043 51 3,859 1.94 11.10 177.7 148

T6-1300-SW 106 30 0.036 46 2,719 1.85 10.67 121.8 320

T7-1400-SW 106 30 0.039 44 2,923 1.87 10.83 127.3 128

T8-1500-SW 108 30 0.050 42 2,066 1.93 11.17 89.0 194

T9-1600-SW 105 30 0.041 33 812 1.91 10.93 19.4 98

T10-1700-SW 105 30 0.041 35 847 1.89 11.02 23.8 72

T11-1800-SW (high tide) 108 30 0.038 36 2,543 1.91 11.20 65.6 196

Table 4. Select trace-element concentrations in discharging vent water at the primary vent site at Kahekili 

Beach Park. 

[Molybdenum (Mo), barium (Ba), rhenium (Re), vanadium (V), iron (Fe), cesium (Cs), uranium (U), chromium 

(Cr), and manganese (Mn) in nM, nanomolar]

Sample ID Mo 

[nM]

Ba

 [nM]

Re

 [nM]

V 

[nM]

Fe 

[nM]

Cs 

[nM]

U 

[nM] 

Cr

[nM] 

Mn

[nM] 

T1-800-GW 30 11 0.002 200 44 0.48 0.50 1.3 8,355

T2-900-GW 51 21 0.005 330 71 0.87 1.05 1.9 13,884

T3-1000-GW (low tide) 42 16 0.002 273 154 0.70 0.65 6.9 11,933

T4-1100-GW 54 19 0.005 348 3145 0.82 0.78 133.3 14,500

T5-1200-GW 41 15 0.002 269 2090 0.64 0.60 92.8 11,343

T6-1300-GW 43 16 0.003 276 2316 0.66 0.62 98.1 11,421

T7-1400-GW 38 13 0.002 240 2518 0.58 0.56 104.8 10,185

T8-1500-GW 51 19 0.005 337 2398 0.82 0.81 95.0 14,400

T9-1600-GW 51 21 0.004 321 n/a 0.86 1.06 n/a 14,257

T10-1700-GW 43 14 0.005 273 3244 0.65 0.74 87.4 10,589

T11-1800-GW (high tide) 53 22 0.004 309 6976 0.84 1.23 204.2 13,046

Table 5. Nutrient concentrations in surface water at the primary vent site at Kahekili Beach Park. 

[All samples unacidified unless denoted. Ammonium (NH4

+

), dissolved silicate (DSi), soluble reactive phosphorus 

(SRP), nitrate-nitrite (NO3

−

+NO2

−

), dissolved inorganic nitrogen, (DIN), dissolved organic nitrogen (DON), total 

dissolve nitrogen (TDN), and total dissolved phosphorus (TDP). μM, micromolar]

Sample ID NH4

[μM]

DSi

[μM]

SRP

[μM]

[NO3

−

+NO2

−

] 

[μM]

DIN

[μM]

DON

[μM]

TDN

[μM]

TDP

[μM]

T1-800-SW <0.05 12.6 <0.05 0.1 0.1 5.7 5.8 0.1

T2-900-SW <0.05 12.0 <0.05 0.0 0.0 5.7 5.7 <0.05

T3-1000-SW (low tide) <0.05 14.9 <0.05 0.3 0.3 7.2 7.4 0.1

T4-1100-SW <0.05 15.6 <0.05 0.1 0.1 6.4 6.5 0.1

T5-1200-SW <0.05 24.2 <0.05 0.4 0.4 6.2 6.6 0.2

T6-1300-SW <0.05 25.4 0.06 0.4 0.4 6.6 7.0 0.2

T7-1400-SW <0.05 24.8 0.05 0.3 0.3 6.1 6.5 0.1

T8-1500-SW <0.05 9.5 <0.05 0.0 0.0 6.4 6.4 0.1

T9-1600-SW 0.1 10.6 <0.05 <0.05 0.1 5.7 5.7 0.1

T10-1700-SW <0.05 13.9 0.04 0.2 0.2 9.3 9.5 0.1

T11-1800-SW (high tide) <0.05 12.2 <0.05 0.1 0.1 8.5 8.7 0.1
15

Table 6. Nutrient concentrations in discharging vent water at the primary vent site at Kahekili Beach Park. 

[All samples unacidified unless denoted. Ammonium (NH4

+

), dissolved silicate (DSi), soluble reactive phosphorus 

(SRP), nitrate-nitrite (NO3

−

+NO2

−

), dissolved inorganic nitrogen, (DIN), dissolved organic nitrogen (DON), total 

dissolve nitrogen (TDN), and total dissolved phosphorus (TDP). μM, micromolar; n/a, not applicable]

Sample ID NH4

[μM]

DSi

[μM]

SRP

[μM]

[NO3

−

+NO2

−

] 

[μM]

DIN

[μM]

DON

[μM]

TDN

[μM]

TDP

[μM]

T1-800-GW 0.3 104.3 6.03 10.5 10.8 11.2 22.0 10.2

T2-900-GW 0.1 608.2 12.78 38.4 38.4 10.5 48.9 12.3

T3-1000-GW (low tide) 0.1 775.7 13.52 42.0 42.0 10.8 52.8 12.2

T4-1100-GW 0.1 790.4 13.20 42.2 42.3 9.9 52.1 12.4

T5-1200-GW 0.1 581.8 n/a n/a n/a n/a n/a n/a

T6-1300-GW 0.1 431.9 13.62 42.0 42.1 10.4 52.5 11.0

T7-1400-GW 0.1 620.0 13.52 41.8 41.8 12.8 54.7 12.5

T8-1500-GW 0.0 813.9 13.41 41.6 41.6 11.2 52.8 12.1

T9-1600-GW 0.2 587.6 13.41 41.6 41.7 9.4 51.2 13.0

T10-1700-GW 0.1 455.4 13.09 41.2 41.4 8.8 50.2 11.5

T11-1800-GW (high tide) 2.2 649.3 8.22 26.2 28.4 2.5 30.9 13.0

Table 7. Wave statistics at the Acoustic Doppler current profiler (ADCP) site. 

[m, meter; s, second; o, degrees true north]

Site Name Parameter Mean ± 1 Std Deviation Minimum Maximum

ADCP Height [m] 0.5±0.12 0.31 0.78

Period [s] 8.2±1.6 4.2 11.0

Direction [o] 53.9±34.0 45.9 83.6

Table 8. Current and temperature statistics at the Acoustic Doppler current profiler (ADCP) site. 

[m, meter; s, second; o, degrees true north; N.S., near surface; N.B., near seabed] 

Site Name Parameter Depth [m] Mean ± 1 Std Deviation Minimum Maximum

ADCP Speed [m/s] [N.S.] 0.05±0.02 0.01 0.11

Direction [o] [N.S.] 260.0±20.9 0.0 359.9

Speed [m/s] [N.B.] 0.04±0.02 0.00 0.09

Direction [o] [N.B.] 234.1±26.7 0.0 359.9

Temperature [

o

C] [N.B.] 21.28±0.38 20.62 22.24

Table 9. Mean radon-222 (222Rn) derived submarine groundwater discharge (SGD) rates at the four timeseries moorings at Kahekili Beach Park. 

[cm/d, centimeters per day]

Site ID SGD rate

[cm/d]

Vent site 55

Site A 21

Site B 30

Site C 22
16

Table 10. Mean discharging groundwater nutrient concentrations in select coastal groundwater systems in 

Hawaii. 

[Ammonium (NH4

+

), dissolved silicate (DSi), soluble reactive phosphorus (SRP), nitrate-nitrite (NO3

−

+NO2

−

), 

dissolved inorganic nitrogen, (DIN), dissolved organic nitrogen (DON), total dissolve nitrogen (TDN), and total 

dissolved phosphorus (TDP). μM, micromolar]

Sample ID NH4

[μM]

DSi

[μM]

SRP

[μM]

[NO2+NO3]

[μM]

DIN

[μM]

DON

[μM]

TDN

[μM]

TDP

[μM]

Kahekili, Maui 0.3 583.5 12.8 39.6 40.0 9.7 49.5 12.2

Black Point, Oahu 1.2 210.8 2.6 141.9 143.1 10.3 153.4 2.8

Niu, Oahu 0.3 234.4 1.2 47.1 47.4 4.2 51.6 1.7

Table 11. Loading estimates per meter of shoreline of nutrient concentrations from select nearshore sites in 

Hawaii. [Ammonium (NH4

+

), dissolved silicate (DSi), soluble reactive phosphorus (SRP), nitrate-nitrite (NO3

−

+NO2

−

), 

dissolved inorganic nitrogen, (DIN), dissolved organic nitrogen (DON), total dissolve nitrogen (TDN), and total 

dissolved phosphorus (TDP). Mol/d/m, moles per day per meter] 

ID NH4

[mol/d/m]

DSi

[mol/d/m]

SRP

[mol/d/m]

[NO3

−

+NO2

−

]

[mol/d/m]

DIN

[mol/d/m]

DON

[mol/d/

m]

TDN

[mol/d/m]

TDP

[mol/d/m]

Kahekili, Maui 0.02 32.09 0.70 2.18 2.20 0.54 2.73 0.67

Black Point, 

Oahu

0.04 6.98 0.09 4.70 4.74 0.34 5.08 0.09

Niu, Oahu 0.01 6.98 0.04 1.40 1.41 0.13 1.54 0.05
17

Figures

Figure 1. Map depicting Maui, Hawaii, and surrounding islands. Study site is located on the northwest 

coast of Maui. 

160t W 159t W 158t W 157t W

157t W

156t W

156t W

155t W

155t W

19t N

19t N

20t N

20t N

21t N

21t N

22t N

22t N

inset

HAWAIIAN ISLANDS

Niihau

Kauai

Oahu

Molokai

Lanai

Maui

Kahoolawe

Hawaii

Study

Area

156t40' W

156t40' W

156t30' W

156t30' W

156t20' W

156t20' W

156t10' W

156t10' W

156t W

156t W

20t30' N

20t30' N

20t40' N

20t40' N

20t50' N

20t50' N

21t N

21t N

160t W 159t W 158t W

Isobath interval: 200 m

Kahoolawe

Maui

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18

Figure 2. Aerial photograph and SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) 

lidar bathymetric map of the west Maui study area showing the video and side-scan sonar tracklines. 

Side-scan sonar tracklines were collected parallel to shore from Kaanapali to Honokowai. The video 

tracklines were completed from as close as possible to shore to approximately the 25 meter (m) 

isobath along every 5-m isobath. 

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19

Figure 3. Aerial photograph and SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) 

lidar bathymetric map of the west Maui study area showing the location of instruments, resistivity lines, 

vent sites, and major landmarks. Vent and ADCP, primary submarine groundwater vent and acoustic 

Doppler current profiler (ADCP); m, meters. 

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Figure 4. Aerial photograph and SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) 

lidar bathymetric map of the west Maui study area showing the location of the continuous resistivity 

profiling lines. m, meter. 

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Figure 5. Aerial photograph and SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) 

lidar bathymetric map of the west Maui study area overlaid with the side-scan sonar data collected in 

the study area. The acoustic backscatter shows soft sand as brighter hues and harder rock as dark 

hues. Inset shows area of figure 6. m, meters. 

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22

Figure 6. Side-scan sonar map and video images of the seafloor in the northern part of the the west Maui 

study area. The acoustic backscatter shows soft sand as brighter hues and harder rock as dark hues. 

Inset photographs showing video confirmation of seafloor habitats at select sites. See figures 2 and 5 

for the location of this region in the study area. 
23

Figure 7. Aerial photograph of the west Maui study area showing percent (%) coral coverage interpreted from 

the seafloor video imagery. m, meters. 
24

Figure 8. Time-series plots of tide, current, and water temperature data from the Acoustic Doppler current 

profiler (ADCP) at the primary submarine groundwater vent site. A, Tidal height, in meters. B, Nearsurface current speeds and directions, in meters per second from true north. C, Mid-depth current 

speeds and directions, in meters per second from true north. D, Near-bed current speeds and 

directions, in meters per second from true north. E, Temperature, in degrees Celsius. The 2-day time 

series shows the currents were fairly uniform throughout the water column and were modulated by the 

tide. The temperature data showed insolation-driven diurnal heating and cooling at this site. m, meters; 

m/s, meters per second; deg, degrees Celsius. 

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Figure 9. Map showing the mean (vectors) and variability (ellipses) of waves and currents at the primary 

submarine groundwater vent site . A, wave height and direction, in meters from true north. B, Nearsurface (grey vector/ellipse) and near-seabed (black vector/ellipse) current speeds (in meters per 

second, m/s) and directions, in degrees from true north. The mean wave height is approximately 0.5 

meters (m) from the southwest and the mean current direction is offshore to the southwest, with faster 

currents measured near the surface. 

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26

Figure 10. Map showing modeled cumulative flow distance for 3 hours following low tide at the near 

surface (gray vectors) and near bed (black vectors) at the primary submarine groundwater vent site. 

Submarine groundwater discharge volumes were highest in the 3 hours following low tide, so these 

data provide insight into the dominant direction and distance of submarine groundwater transport from 

the primary vent site off Kahekili. The flow at the near surface was slightly faster and more to the 

southwest than the flow near the seabed, m, meters. 

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27

Figure 11. Time-series plots of salinity of the discharging submarine groundwater and tide level (in meters, 

m) at the primary vent site. 
28

Figure 12. Time-series plots of surface water and bottom water salinity and tide level (in meters, m) at the 

primary vent site. 
29

Figure 13. Time-series plots of radon-222 (222Rn) activities, in disintegrations per minute per cubic meter of 

water (dpm/m3), salinity, and tide level, in meters (m), at the primary submarine groundwater vent site. 

This groundwater was captured using a piezometer that was inserted into the throat of an active spring 

vent by a diver using scuba. A representative groundwater end member 222Rn value (27,045 dpm/m3) 

used in the radon mass balance was extracted from this groundwater 222Rn time series. 
30

Figure 14. Time-series plots of surface water radon-222 (222Rn), in disintegrations per minute per cubic 

meter of water (dpm/m3), and water level, in meters (m), at the primary submarine groundwater vent 

site. 
31

Figure 15. Time-series plots of calculated radon-222 (222Rn)-derived advection rates, in centimeters per 

day (cm/d), and salinity for the primary submarine groundwater vent site. 
32

Figure 16. Aerial photograph and SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) 

lidar bathymetric map of the west Maui study area overlaid with radon-222 (222Rn) activities, in 

disintegrations per minute per liter of water (dpm/L), in coastal surface waters. m, meters. 

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33

Figure 17. Fence diagrams showing continuous resistivity profile results from the shore-parallel transects 

in the west Maui study area. Transect line positions are shown in figure 4. Warmer colors indicate 

higher resistivity (inverse of electrical conductivity) values, in ohm meters (Ω•m). m, meters. 

.

200 400 600 800 1,000 1,200 1,400

2,000 2,200 2,400 2,600 2,800 3,000 3,200 3,400

200 400 600 800 1,000 1,200 1,400

400 600 800 1,000 1,200 1,400 1,600 1,800

200 400 600 800 1,000 1,200

Distance (m)

M004_Clip1 M007_Clip6 M005_Clip1 M006_Clip1 M007_Clip1

Depth (m) 0

-5

-10

-15

-20

Depth (m) 0

-5

-10

-15

-20

Depth (m) 0

-5

-10

-15

-20

Depth (m) 0

-5

-10

-15

-20

Depth (m) 0

-5

-10

-15

-20
34

Figure 18. Fence diagram showing a land-based, multichannel electrical resistivity survey conducted along 

the beach face in the west Maui study area adjacent to the primary submarine groundwater vent site 

(horizontal axis is meters). Although resistivity values are influenced both by geologic and hydraulic 

parameters (Swarzenski and others 2007a,b), it is likely that the higher resistive layers observed define 

zones of enhanced hydrologic permeability. Ω•m, ohm meters; m, meters.

0 12 24 36 48 60 72 84 96 108

Inverted Resistivity Section Iteration = 2 RMS = 3.28% L2 = 0.90 electrode spacing = 2 m

1.7

2.7

4.3

6.8

10.9

-22.8

-17.1

-11.4

-5.7

0.0

Depth (m)

.m

Distance along streamer (m)