H2O - Healthy Hawaiian Oceans

“Malama o kekai, kekai o ke malama”

Take care of the sea, and the sea will take care of you

Post Office Box 895

Honaunau, Hawai`i 96726


Sunday, June 2, 2019

Summary of Kahalu‘u Bay Water Quality Research

R.H. Bennett Ph.D.
Applied Life Sciences LLC

The Kahalu‘u Bay Education Center actively strives to inform visitors and the community about the protection and conservation of the bay’s marine ecosystem.  A group of committed volunteers and staff deliver educational programs daily.  Similarly, they collected scientific data about the bay for over a decade.   This trove of data is priceless.  It tells us much about the interaction of the ocean with the adjacent urban lands. With the possible exception of the marine waters of Kaloko Honokōhau National Historic Park, no other marine site has been studied so thoroughly and for so long.

The data from Kahalu‘u Bay gave rise to three technical research reports.  They are:
I.    Tidal Influence on Temperature and Salinity
II.   Elevated Kahalu’u Bay Enterococci Associated with Large Tidal Flux
III.  Kahalu‘u Bay Nutrient Trends 2009 to 2019

It is the purpose of this summary to provide an easy to understand synopsis of the three technical reports, to enable the community to “see below sea level” so that we may be better informed for the best stewardship of our nearshore waters.

I.  Tidal Influence on Bay Waters

The mountains above Kailua Kona are blessed with abundant rainfall.  Some sites get over 70 inches of rain per year.  Yet there are no rivers or streams. On the Hilo side of the island, there are over 200 streams.  So, in Kona where does all that water go?  It percolates quickly through the fractured lava rock and forms a fresh groundwater lens that floats upon the seawater that permeates the island.   From there, fresh groundwater flows toward the shore.  It forms a nearshore subterranean estuary (SE). In the estuary, groundwater and seawater mix under the forces of tidal action.  Four times a day the tides rise and fall.  Each month during the Spring Tides massive tidal fluxes of over two feet push and pull on the estuary with great force.

Groundwater eventually discharges along the coast as a cold mixture of freshwater and recirculated seawater. For those that swim in the ocean, encountering cold spots is inevitable.  For snorkelers, these surface cold spots can have hazy visibility.  This is refraction caused by the incomplete mixing of seawater and groundwater.  The fresher water is less dense and floats on the ocean surface in many locations.

In Kahalu‘u Bay shore area, there are pronounced discrete locations where flows of water can be seen, and the coolness perceived.  In other places, the brackish water is more diffuse.  And yet in some offshore sites, the cold brackish water emerges from the floor of the ocean.   Most of all, water that does not evaporate flows into the sea one way or another.   Water is almost a perfect solvent.  That is a high number of things dissolve in it even though it may remain crystal clear.  In the kitchen, we can dissolve a lot of salt or sugar in a glass of water with no change in its appearance.

This site map of the bay shows where water samples are collected.  Site one is where water flow at low tide can be seen. At low tide, the salinities at site 1 were different from the others.  At high tide, the salinity of site 1 - 4 was different from 5.   This shows that water inflow can be very localized.  We know that lava tubes and fractured rock can provide such a discrete conveyance.

Similarly, it should not be too hard to imagine how the flow of the tides can influence the temperature of the nearshore water.   The graph is an example of some real data for site 1.  Ocean water is warmer and more saline.  Thus, a good high tide will render the nearshore water warmer and saltier. At a big low tide, the converse is also true.

At low tide, groundwater flows increase, and salinity and temperature decrease quite dramatically.   This suggests the volume of water flowing into the bay is massive.

UH researchers using some advanced physics estimate for each mile of the Kona Coastline between one to three million gallons of groundwater flow into the sea per mile of coastline.  On the Hilo side, this can be seen in rivers and streams.  On the Kona side, it is unseen but still vast.

One way we can see this flow is to use cameras that see the temperature, called infra-red (IR).  This IR photo was taken of a submarine groundwater discharge, just north of the Kona Airport.  The cold, fresh water (shown by the cooler colors in the image) extends over 200 yards out to sea.

This collective and massive flow of groundwater into the sea is part of our island water cycle, and it has been flowing this way long before the Hawaiians came to inhabit the island.   Now that the shore area is urbanized or otherwise altered in the last century; what is flowing in this water now is of great interest and concern.

The take-home story in this section is that groundwater flows dramatically alter the nearshore ocean and it is easily detected and measured with simple instruments like a thermometer or salinity meter.  This will take on greater significance in Part III.

The Tidal Forces that Drive the Subterranean Estuary (SE) on the Kona Shore

II.    Influence of Tides on the Recreation Safety Bacterial Water Quality Indicator

When the local news recently (Jan 8, 2019) reported several beaches on Oahu were closed due to elevated Enterococci (ENT) indicator bacteria, and Kahalu‘u Bay was “Posted” at about the same time, it raised curiosity.   There had been no significant rain events on the islands, and no sewer spills to cause health department warnings. Instead, the warnings arose for beaches widely separated geographically and simply because the levels of ENT were elevated above the regulatory threshold.

The State Department of Health monitors the indicator bacteria Enterococci.  The official belief was that ENT was a useful marker for fecal contamination.  A lot of science has shown that it is not a valid indicator, nonetheless, it is still the official test.  This ENT monitoring data is part of the Kahalu‘u Bay collection.  When the Bay was posted as contaminated in January, the curiosity it generated posed a question.  We were having colossal Spring Tides at the time.  Could the tides have something to do with the bacteria levels?  To cut to the chase, indeed they do.  But how?

The ENT bacteria are common to land and water sources even where there is no fecal matter at all.   They grow in wet places like culverts, compost piles, drain pipes, and wetlands.   From there they flow into the sea, and they make a home there too.   They can be found floating in the sea water, and that is where the state measures them.  But more critically, they live and grow in wet beach sand above and below water level.  Many times, more ENT can be found in the beach sand than in the water above.
(Lee 2017)

Along comes a super high tide followed by a minus low tide and allows for a huge water outflow from the SE.  The water flows above and through the sand dispersing ENT up into the water column where the ENT can be detected and counted.  When the ENT count is high, beaches get warning signs and may be closed.
This graph shows the ENT count and the moon cycle.  Fourteen days after a full moon are the astronomical Spring Tides and the associated rise in the ENT (Boehm 2005). This tidal force literally stirs things up.

There remains a distinct possibility that elevated ENT counts are often an artifact of the tide cycle at the time of sampling.   This bias likely triggers official warnings where there are no apparent health risk events like sewage spills or flooding.

  However, given that the region near Kahalu‘u Bay is not served by sewer and most homes utilize cesspits for decades, sewage components may be conveyed to the ocean by groundwater. Microbiologists have documented the presence of a human virus in seawater when the ENT is low or absent.   The need for a better risk measure is great, and there are signs that a human-specific fecal marker test is in development by the EPA.  Even then its application may be years off, so we will continue to guess.

The real good news about the safety of recreation water in Hawaii is the sun.   The intense midday UV sunlight penetrates clear water and kills microbes in a matter of hours. We shall see, however, in the next section, clear ocean water is less so these days.

III. The Nutrients Nitrogen and Phosphorus in Nearshore Groundwater

It is well established that brackish groundwater flowing in discrete and diffuse SE is abundant in the nutrients nitrogen and phosphorus.  According to University of Hawaii researchers, where ever there is cold SE water flows, elevated nutrients are present.   This flow results in thousands of pounds of the nutrients being conveyed daily to the nearshore waters.   These waters of the tropics are naturally low in nutrients, but things have been changing for decades of human activities.

This graph looks complicated, but it’s not.   Let’s interpret.   The line says, when seawater is saltier there is less N in it.  Conversely, the fresher the water the higher N is there.   Taken over 10 years in many locations in the bay and from other Kona Coast sites, this pattern is very consistent. Simply, it means the elevated N source is from the land and groundwater and not the sea.

This chart looks even more complicated, but it is just three sets of years P plotted against salinity.  It shows a very similar relationship as the N chart.  Yet here we are looking at a ten-year period in three phases. 

It answers the question, are things changing over time?   For both N and P, there is no evidence of a time trend.  The nutrient concentrations in groundwater are staying about the same.

Just as for salinity and temperature in the first report, tidal action dilutes the groundwater and hence the N concentration.  Thus, if one was trying to say there is no problem or a lesser problem, the sample would be collected at the highest tide to get the lowest N level. 

 In all future work, we must account for the tidal effect on nutrient concentration in nearshore waters.

We are talking about vast masses of nutrients flowing into the ocean for almost the entire Kona Coastline.   Some have conveniently wanted to say the N and the P are natural, meaning this system was here before humans.  While it is true, the mass of nutrients were far less as there was little importation of nutrients other than through fish consumption.

Given that, what is the reason for the large mass of nutrients?   We are!   A lot of research on this island and others shows very clearly that the N and P are from human activity.  We say the elevated nutrients are anthropogenic, or human-associated.   That includes the fertilizers imported and dispersed in landscape and agriculture.  It includes livestock manures from animal agriculture. Yet the largest single source is the human diet and the nutrients we excrete daily.

For example,  protein-rich foods contain much more N than starchy foods and vegetables.  The proteins get absorbed and ultimately broken down and the nitrogen excreted in the urine as urea.   Leave urine in the bowl for a day and the bacteria break it down and form ammonia with its distinctive odor.   In the environment and underground bacteria convert ammonia to nitrate. It is the same nitrate in a bag of lawn fertilizer.

This amounts to about 21 pounds of N per household per year.   Add up all the homes in Kailua Kona and a year, and we can produce around a million pounds of N that flow into cesspits, septic systems, and treatment plants.  No matter which one is used most of the N ends up in the groundwater and the sea.

This fertilizer in the sea has the same effect as it does on your lawn.  Instead of grass growing, the microscopic green plant microbes called phytoplankton to grow and in high numbers make the water shades of cloudy green.  This growth absorbs sunlight making the natural UV disinfection of sea water less effective.

Recently, research from Kaneohe Bay demonstrates that wastewater N in the bay has an additive effect with increased temperature to cause coral bleaching.

Lastly, it is not sufficient just to know the concentration of the nutrient flowing into the bay as it tells us nothing of the mass or pounds in the flow.   Thus, we must know or estimate the volume of the flow so that we can calculate the mass of nutrients delivered to the sea from the SE

The blue arrows represent the same nutrient concentration yet at very different flow volumes.  The total mass of N, reaching the bay and impacting the ecosystem is very different. Where currents move and mix these nutrients to the open ocean, adverse impacts are less likely.  However, in protected embayments like Kahalu‘u, the transport an mixing is much less and the impact on the local ecosystem is more significant.

Think of it, this way. A little soy sauce on food gives it just the right salt taste, but a quarter cup of soy sauce on food and it is horribly salty.  The salt in the soy is at the same concentration; there was simply more salt.

In the bay, we cannot alter the volume of water flowing in, just like we cannot stop a river. So, we need to employ the tools that reduce the concentration of N and P discharged to groundwater.

The people of Long Island NY severely polluted their estuary from Septic Systems. They put their collective shoulders to the wheel and financed research.  That research demonstrated a simple technique that removes the nutrients in human wastewater by over 90%.   Kona can do this too.  The consequences of “kicking the can down the road” will be dire.


For over 35 years, Dr. Bennett has worked in the environmental science field where the land meets the water.  From Tomales Bay CA. to New South Wales Australia to the Big Island where ever there are people and oceans there are huge challenges.  As a resident of Kona since 1999, he volunteers with communities working to Malāma Ke Kai and offers guidance to the County of Hawaii as he chairs the Environmental Management Commission.

Special appreciation goes out to Cindi Punihaole Kennedy, the founding director of the Kahalu‘u Bay Education Center and educator coordinator Kathleen Clark.  Together with many volunteers, they have collected high-quality data for over 9 years.  The data is the basis of the three reports and this summary. Also extending appreciation to the former UH researcher James. M. Bishop for his assistance understanding the hydrology of Hawai‘i Island and the review of the manuscript.

This document is covered by the Creative Commons 4.0 and is free to be distributed with attributions.


Part I.

Prouty, Nancy G., Peter W. Swarzenski, Joseph K. Fackrell, Karen Johannesson, and C. Diane Palmore. "Groundwater-derived nutrient and trace element transport to a nearshore Kona coral ecosystem: Experimental mixing model results." Journal of Hydrology: Regional Studies 11 (2017): 166-177.

Part II.

Boehm, Alexandria B., and Stephen B. Weisberg. "Tidal forcing of enterococci at marine recreational beaches at fortnightly and semidiurnal frequencies." Environmental science & technology 39, no. 15 (2005): 5575-5583.

Fleisher, Jay M., Lora E. Fleming, Helena M. Solo-Gabriele, Jonathan K. Kish, Christopher D. Sinigalliano, Lisa Plano, Samir M. Elmir et al. "The BEACHES Study: health effects and exposures from non-point source microbial contaminants in subtropical recreational marine waters." International journal of epidemiology 39, no. 5 (2010): 1291-1298.

Lee, Eunhee, Doyun Shin, Sung Pil Hyun, KyungSeok Ko, Hee Sun Moon, DongChan Koh, Kyoochul Ha, and ByungYong Kim. "Periodic change in coastal microbial community structure associated with submarine groundwater discharge and tidal fluctuation." Limnology and Oceanography 62, no. 2 (2017): 437-451.

Yamahara, Kevan M., Sarah P. Walters, and Alexandria B. Boehm. "Growth of enterococci in unaltered, unseeded beach sands subjected to tidal wetting." Applied and environmental microbiology 75, no. 6 (2009): 1517-1524

Part III.

Bishop, James M., Craig R. Glenn, Daniel W. Amato, and Henrietta Dulai. "Effect of land use and groundwater flow path on submarine groundwater discharge nutrient flux." Journal of Hydrology: Regional Studies 11 (2017): 194-218.

Bristow, Laura A., Wiebke Mohr, Soeren Ahmerkamp, and Marcel MM Kuypers. "Nutrients that limit growth in the ocean." Current Biology 27, no. 11 (2017): R474-R478.

Johnson, Adam G., Craig R. Glenn, William C. Burnett, Richard N. Peterson, and Paul G. Lucey. "Aerial infrared imaging reveals large nutrientrich groundwater inputs to the ocean." Geophysical Research Letters 35, no. 15 (2008).

Johannesson, Karen H., C. Dianne Palmore, Joseph Fackrell, Nancy G. Prouty, Peter W. Swarzenski, Darren A. Chevis, Katherine Telfeyan, Christopher D. White, and David J. Burdige. "Rare earth element behavior during groundwater–seawater mixing along the Kona Coast of Hawaii." Geochimica et Cosmochimica Acta 198 (2017): 229-258.

Klausmeier, Christopher A., Elena Litchman, Tanguy Daufresne, and Simon A. Levin. "Optimal nitrogen-to-phosphorus stoichiometry of phytoplankton." Nature 429, no. 6988 (2004): 171.

Lau, Leung-Ku Stephen, and John Francis Mink. Hydrology of the Hawaiian Islands. University of Hawaii Press, 2006.

Moore, Willard S. "The subterranean estuary: a reaction zone of ground water and sea water." Marine Chemistry 65, no. 1-2 (1999): 111-125.

Peterson, Richard N., William C. Burnett, Craig R. Glenn, and Adam G. Johnson. "Quantification of pointsource groundwater discharges to the ocean from the shoreline of the Big Island, Hawaii." Limnology and Oceanography 54, no. 3 (2009): 890-904.

Reay, William G. "Septic tank impacts on groundwater quality and nearshore sediment nutrient flux." Groundwater 42, no. 7 (2004): 1079-1089.

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