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Hui Aloha O Kiholou p dat e

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By: Henrieta Dulaiova and Christine A. Waters (University of Hawaii at Manoa and Hawaii EPSCoR)

Research Questions1. 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 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.

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.

the many seasons of sgd

Figure 1. Cross-section showing a simplified island aquifer (modified from Johnson, 2008).

(808) 956-0720 | hdulaiov@hawaii.edu

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.

Kiholo Bay

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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).

Figure 2. Total groundwater discharge at the inlet head and mouth and at a coastal site on the southwest side of Kiholo Bay.

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.

THE SUBSURFACE STRUCTURE OF THE COASTLINE

Lower Low Tide

Higher High Tide

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.

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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 to increased groundwater fluxes during this period. These measurements were done

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,

we were unable to determine the length of time required for the aquifer and the lagoon to get back to their natural state.

Is there spatial Variability to sgd at kiholo bay?

CA B

D E

Nutrient Fluxes Originating from Groundwater DischargeCoastal 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 dischargeestimates (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.

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.

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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.

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).

Where do we see possible areas of increased primary productivity (i.e. plankton growth)?

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).

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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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.

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.

Highest Chl-a “Oldest” Water8-12 Days Old

Lowest Chl-a “Youngest” Water

0-2 Days Old

up to 8,000 m3/d

up to 2,400 m3/d

up to 6,500 m3/d

Tying it all Together

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Continuing Work & the role of the sgd snifferOngoing 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.

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 long-term 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.

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 land-ocean-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

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Henrieta Dulaiova and Craig Glenn: (808) 956-0720 | hdulaiov@hawaii.edu

University of Hawaii at ManoaMarine Geology1680 East-West Road POST 701Honolulu, HI 96822

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