This document discusses a study on submarine groundwater discharge (SGD) at Kiholo Bay, Hawaii, focusing on the volume, composition, and seasonal changes of groundwater contributions to coastal ecosystems. It highlights the use of radon as a tracer to quantify groundwater flow and its nutrient content, which is crucial for supporting coastal ocean productivity. The ongoing research aims to enhance understanding of SGD's role in nutrient delivery and its impact on marine life amidst environmental changes.

1. u p dat e
to
Hui Aloha O Kiholo
By: Henrieta Dulaiova and Christine A. Waters (University of Hawaii at Manoa and Hawaii EPSCoR)
(808) 956-0720 | hdulaiov@hawaii.edu
Research Questions
Hawaii’s coastal aquifers and environments
play a fundamental role in the state’s history,
economy, and culture. The need to maintain
the appeal and general health of Hawaii’s
coasts has been recognized and was
addressed with the creation of watershed
partnerships and the continuation of
traditional land divisions (ahupua’a). This
study will provide a necessary baseline
for how groundwater aquifers on these
traditional lands contribute to Hawaii’s
coastal ecosystems.
1. What volume of groundwater discharges to Kiholo
Bay, what is its composition, and how does it change
over time?
2. What amount of nutrients does submarine
groundwater discharge (SGD) deliver to the coast?
3. How long does it take for the currents to flush the
groundwater-derived nutrients from the nearshore
region?
4. How important are groundwater-derived nutrients
for coastal ocean productivity, i.e. growth of algae,
plankton, et cetera?
the many seasons of sgd
The Kona coast on leeward Hawaii is one of the
driest places in the Hawaiian Island chain, and it
is a place where streams only rarely reach to the
sea. Yet, groundwater from rain falling high on
the mountainsides of the Hualalai, Mauna Kea,
and Mauna Loa provides tremendous amounts
of water to the coastal ocean, fed through the
subterranean aquifers (Figure 1). Because groundwater in coastal aquifers is slightly brackish, we have to rely on tracers
other than salinity to identify and quantify SGD. We use radon, a natural radioactive isotope that is present as the daughter
product of uranium in the groundwater aquifers. Elevated coastal ocean radon levels reveal locations of groundwater
discharge and allow us to quantify rates of SGD flow.
Our team used radon as a geochemical tracer of groundwater
at Kiholo Bay in order to calculate the amount of groundwater
discharged into the sea during different seasons. You may
have noticed our dinghy anchored for 2-3 days at a time,
housing measuring apparatuses, during our several trips to
this location. Radon is produced naturally in the groundwater
aquifer and accumulates in it. It is also radioactive, so we may
rely on its decay (~3.8 days) to assure us that the groundwater
we measure is newly discharged to the coast.
Kiholo Bay
Figure 1. Cross-section showing a simplified island aquifer
(modified from Johnson, 2008).
Using this tool, in addition to salinity and temperature, we
calculated total groundwater discharges at three identified groundwater outlets in Kiholo Bay (Figure 2) . The purpose for
sampling twice a year was to see if there was a distinction between discharge dynamics and chemistry between wet and
dry seasons. As you can see in Figure 3, total groundwater discharge was the greatest at the head of the inlet, amounting on
average to about 4,000 m3/d (as measured in June 2010). The error bars on the figure illustrate the large range of measured
discharge over a 3-day period.
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2. This range is expected, as groundwater discharge is modulated by tides.
During high tide, the ocean level is higher and the hydraulic gradient,
the difference between groundwater level and ocean level, is smaller than
at low tide. The larger the hydraulic gradient, the more groundwater
can flow out of the coastal aquifer. During dry summer months,
groundwater discharge at the mouth of the inlet and on the southwest
corner of the bay (~2,000 - 2,200 m3/d) was less than in measurement
periods during/following winter months (~2,400 - 3,000 m3/d).
THE SUBSURFACE STRUCTURE OF
THE COASTLINE
We used a physical technique called electrical resistivity sensing of the
subsurface aquifer to study the groundwater conduits. We imaged the
below ground composition of the coastal aquifer along the southwest
side of Kiholo Bay and found that, here, brackish groundwater of salinity
about 5, discharged through isolated conduits in rocks such as lava tubes.
We also showed (Figure 3) that, at low tide, the coastal aquifer is more
dominated by brackish groundwater (red areas representing high resistivity
regions, because freshwater is less conductive than seawater) than at
high tide when seawater inundates the coastal aquifer and the freshwater
recedes landward. This confirms our findings using the radon tracer that
showed higher groundwater discharge at low tide as described above. Figure 2. Total groundwater discharge at the
Figure 3. Electrical resistivity sensing images of the
subsurface aquifer at the black pebble shoreline of Kiholo
Bay. At low tide, the subsurface coastline is dominated by
red areas of higher resistivity freshwater than during high
tide.
Higher High Tide
Lower Low Tide
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inlet head and mouth and at a coastal site on
the southwest side of Kiholo Bay.

3. Is there spatial Variability to sgd at kiholo
bay?
We measured radon concentration in the coastal ocean along the coastline in order to identify groundwater discharge
sites. Figure 4 shows surveys for radon and salinity along the coastline. Higher radon (red, yellow, and green circles)
with salinities below ambient marine salinity (< 36.0, shown as light blue in the blue-gradient swathes) indicate locations
of SGD. These occurred during all excursions, regardless of season, at the head of the inlet and from basalts along the
middle, eastward side of the inlet. There is groundwater discharging along the pebble beach and from the southwest
corner of the bay, which is discernible by the slightly elevated radon levels and lowered salinity in the middle bay area.
These are consistent with findings from previous studies using thermal infrared imaging (Johnson et al., 2008). Possible
groundwater outlets unnoticed in previous studies were discovered in September’s lengthier shoreline survey (box
C) along the western coast, possibly flowing through natural conduits in basalts just outside Kiholo Bay. There were
significantly elevated levels of radon measured in surface waters during the month of March 2011 (box D), corresponding
These measurements were done
to increased groundwater fluxes during this period.
A
B
C
one week after the Japan tsunami,
which inundated the land and
forced seawater to infiltrate
the into the coastal aquifer.
We believe the elevated coastal
groundwater discharge was due
to this inundation. Because our
measurements only lasted 2 days,
D
E
we were unable to determine the length of time required for
the aquifer and the lagoon to get back to their natural state.
Figure 4. From upper left to lower right: Groundwater surveys
conducted on A) June 23 and B) June 25, 2010, C) September
29, 2010, D) March 24, 2011, and E) March 29, 2012. Hot
colors (reds, yellows, and oranges) show areas of higher
radon concentrations (associated with greater groundwater
discharge). Light blue swathes indicate fresher salinities.
Nutrient Fluxes Originating
from Groundwater Discharge
Coastal groundwater is enriched in nutrients in comparison to ocean nutrient
levels. It is therefore important to understand what role SGD has in delivering
nutrients to the coastline. An optimal range of nitrogen and phosphorus in
the coastal ocean is preferred, as these nutrients support algal and plankton
growth, supporting the bottom of the food web. Too many nutrients, on the
other hand, may cause overpopulation of algae, phytoplankton blooms of
undesired levels that potentially affect the coral reefs. The nutrient composition
of the groundwater has to be optimal, just the right ratio of nitrogen to
phosphorus for coastal plants. We therefore measured groundwater nutrient
concentrations and combined these with corresponding groundwater discharge
estimates (Figure 2). The calculated nutrient fluxes via groundwater discharge
are shown in Figure 5 (DIP: dissolved inorganic phosphorus, DIN: dissolved
Figure 5. Nutrient fluxes derived from groundwater fluxes at time
series (yellow stars in Figure 2) locations at Kiholo Bay. DIP: dissolved
inorganic phosphorus, DIN: dissolved inorganic nitrogen, DON:
dissolved organic nitrogen, and DOP: dissolved organic phosphorus.
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4. inorganic nitrogen, DON: dissolved organic nitrogen, and DOP: dissolved organic phosphorus). DIP ranged from
2.0-7.0 mol/d and DIN ranged from 38.4-170.3 mol/d. These graphs show that the largest fluxes of bioavailable
phosphorus and nitrogen are delivered by groundwater fluxes through the head and mouth of the lagoon during
the winter months (March and September). For comparison, nutrient fluxes in the less pristine Honokohau Harbor
are 182 mol/d and about 4,000 mol/d of bioavailable phosphorus and nitrogen, respectively (Holleman, 2011).
Where do we see possible areas of increased
primary productivity (i.e. plankton growth)?
Because phytoplankton are either the direct
or indirect food source for all marine animals,
knowledge of its population mass and growth can
inform us of the general state of an ecosystem.
We use chlorophyll-a (chl-a), the green pigment
essential for photosynthesis in algae and
phytoplankton, which was measured in the surface
waters, as our proxy for primary productivity. We
measured chl-a during three sampling excursions
which are shown in Figure 6 as grey circles (June 2010),
squares (March 2011), and triangles (October 2011),
with chl-a concentrations ranging from 0.1-0.7 µg/L.
Figure 6. Chl-a concentration measured in surface
water samples, shown with radon measured during
March 2011 survey (from Figure 2, box D) to
indicate groundwater discharge locations (in red).
The March 2011 samples (squares) had the highest overall average chl-a concentrations. The highest concentration
(0.7 µg/L in October 2011) was measured outside the lagoon in the open bay. Surprisingly, chl-a concentrations
at the head of the lagoon (where groundwater discharge occurs at large volume), were the smallest (0.1 µg/L). This
led us to investigate possible reasons for minimal uptake of groundwater-delivered nutrients directly at the source.
The graphs below, in Figure 7, show chl-a concentration (green fill) with time of day (x-axis) and tide (blue
fill). Sampling was conducted during low, low rising tide during every excursion. It is important to note
that chl-a concentrations vary with the availability of sunlight as the sun’s energy is needed for the plants to
perform photosynthesis. For comparison, mean chl-a concentration for the sampling excursion is shown
as the green dotted line and mean chl-a concentration of all samples is given by the straight black line.
Figure 7. Chlorophyll-a (chl-a) and mean chl-a concentration with tide at different times of the day. Chl-a concentration varies with
the availability of sunlight, temperature, and salinity of water.
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5. Figure 8 shows chl-a concentration with salinity, radon, and the time since groundwater entered the ocean, i.e.
the age of the nutrients assuming 0 age at the point of discharge. Our data suggest that cold temperatures and
low salinity of SGD plumes near their source are not favored by marine organisms, despite the abundance of
bioavailable nutrients. At the same time, older aged waters are warmer and have higher salinity because they mix
with ocean water. Yet, they still contain elevated nutrient levels in comparison to the ambient ocean concentration.
Thanks also to the longer presence of nutrients, these water masses exhibit higher primary productivity.
Figure 8. Chlorophyll-a (chl-a) concentration measured in surface water samples with salinity, radon, and plotted against the apparent
age of waters. Chl-a concentration increases with increasing salinity and age of waters. Also, March 2011 “tsunami” samples were
brackish samples that had atypically high chl-a concentration, possibly due to inundation of seawater into the groundwater aquifer.
Tying it all Together
Highest Chl-a
“Oldest” Water
8-12 Days Old
Lowest Chl-a
“Youngest” Water
0-2 Days Old
up to 2,400 m3/d
up to 8,000 m3/d
up to 6,500 m3/d
Figure 9. We found that as much as 11,000 m3/d of groundwater discharged to Kiholo Bay every day. Groundwater
is enriched in nutrients and among others, contributes nitrogen, phosphorus and silica to the coastal ocean. Our data
suggest that cold temperatures and low salinity of SGD plumes near their source are not favored by marine organisms,
despite the abundance of bioavailable nutrients. At the same time, older aged waters are warmer and have higher salinity
because they mix with ocean water. Thanks also to the longer presence of nutrients, these water masses exhibit higher
primary productivity.
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6. Continuing Work & the role of the sgd sniffer
Ongoing work includes analyzing data for salinity, nutrients, radon and temperature across the whole depth
of the water column in the bay and lagoon regions in order to get an idea of how the SGD plume changes/
does not change with seasons throughout the water column. These data will also provide us ideas about
benthic groundwater fluxes that do not have a prominent surface signature. Lastly, net primary productivity
will be better constrained by performing carbon uptake measurements along the SGD plumes in the bay.
Figure 10. Groundwater is naturally enriched in radon,
and elevated coastal radon levels (shown in units of
decay per minute per liter of seawater) reveal locations of
groundwater discharge. Thanks to the newly developed
SGD Sniffer (shown in the inset), we can now investigate
this long-term record of submarine groundwater
discharge to the coastal ocean to better understand landocean-climate interaction. The SGD Sniffer would be
best positioned at the location proposed on this image,
as that is one of the major SGD outcrops in the protected
lagoon. More details on the proposed deployment
can be found in the attached plan of operation.
The SGD Sniffer MISSION
Over several years, we have performed 2-3 day long measurements of coastal radon to calculate groundwater
discharge and coastal nutrient fluxes in different seasons. We created maps of the coastline indicating the
locations of focused groundwater outflow. We found that discharge is not uniform along the coastline but
occurs through well-defined conduits. We also found that groundwater fluxes across the land-ocean interface
vary significantly throughout the tidal cycle and also within and between wet and dry seasons.
An excellent example of the temperamental nature of the magnitude of these fluxes occurs near the mouth of
the inlet at Kiholo Bay (Figure 10). From June 2010 to March 2012 (five, 1-week periods), fluxes of groundwater
from the coast at this one location varied in magnitude from about 5,000 gallons per day, to more than one
million gallons per day. More-brackish discharges occurred just one week after the March 2011 tsunami brought
up to 11-foot high waves to Kona. Discharges from March 2011 also had a noticeable effect on nearshore
phytoplankton growth compared to previous sampling periods.
It is evident that one cannot take “snap-shot” measurements of SGD to adequately understand how groundwater
and its dissolved constituent fluxes respond to changes in climate and/or sea level change. To address this
problem, we designed a unique coastal radon detector, the SGD Sniffer, which is capable of self-sustained longterm high-frequency monitoring of SGD. This is a first of its kind continuous radon monitor that will allow us to
measure groundwater fluxes to the ocean over durations of months and years. The newly developed SGD Sniffer
will provide continuous data on submarine groundwater discharge and will make it possible to investigate the
mechanisms of long-term trends in SGD and its impact on coastal ecosystems. Thanks to this methodological
advance, we can, for the first time, investigate long-term trends in SGD within the larger context of our rapidly
expanding awareness and knowledge about climate change, sea-level rise and watershed processes. At the same
time, we will be able to couple how land-ocean water fluxes change in magnitude and chemistry in response to
local environmental and anthropogenic stressors. All data from the SGD Sniffer will be available to members of
the Kiholo Community through a public data portal: PacIOOS.org.
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7. For More Information, Please Contact:
Henrieta Dulaiova and Craig Glenn: (808) 956-0720 | hdulaiov@hawaii.edu
University of Hawaii at Manoa
Marine Geology
1680 East-West Road POST 701
Honolulu, HI 96822
™
And
View Our Team At:
www.epscor.hawaii.edu/ender
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