HANALEI — North Shore beach-goers in mid-August may have noticed something a bit unusual: a line of four or five 32-gallon trash cans attended by one or two sunburned, bedraggled scientists.
Each water-filled trash can contained a small pump attached to a fiber-filled column. Passers-by offered various guesses about the purpose of this equipment — catching fish, desalination, even launching rockets. The truth, however, is that scientists from Stanford University are trying to estimate how much groundwater discharges directly into the coastal ocean in Ha‘ena, Hanalei and Princeville, and to investigate what impact it may have on water quality.
Submarine groundwater discharge: Why does it matter?
Fresh water can enter the coastal ocean in several ways: through rivers, streams and overland runoff, or through rainfall that percolates into the ground and emerges beneath the surface of the sea as discrete springs or as diffuse seepage. The latter process is known as submarine groundwater discharge.
Because the tides pump saltwater in and out of coastal aquifers, the discharging groundwater often consists of a mixture of freshwater and seawater, and the part of the aquifer where this mixing occurs is referred to as the subterranean estuary.
Just like a river, submarine groundwater discharge represents a connection between the land and the sea. Land uses such as residential and commercial development, agriculture, industry and waste disposal can all affect the quality of the groundwater, as well as the water quality of the coastal ocean, if the groundwater is discharging into the sea.
In Hanalei, scientists from the Hanalei Watershed Hui monitored the levels of fecal indicator bacteria, or FIB, at several sites around the bay, and noticed that FIB counts sometimes exceeded state standards, particularly after heavy rains.
Were these bacteria entering the bay exclusively through rivers and streams, or was there a groundwater connection? To answer this question, the Hui teamed up with researchers from Stanford who specialize in measuring submarine groundwater discharge, including Adina Paytan, a professor of geological and environmental sciences, and Alexandria Boehm, a professor of civil and environmental engineering, their graduate and undergraduate students, and other research assistants.
But what about the trash cans?
One way to estimate the quantity of groundwater discharging into the coastal ocean is to use the element radium. Rocks and soil containing uranium and thorium produce radium continuously, but in freshwater aquifers it is tightly bound to the surfaces of rock and sediment.
However, when freshwater mixes with saltwater in the subterranean estuary, the radium desorbs and enters the groundwater, and its concentration can be measured. Since the concentration of radium in seawater is usually very low, a large volume of water must be collected in order to get a measurable amount of radium, and a 32-gallon trash can provides a convenient receptacle for the water sample.
The water is then filtered through a special fiber coated with manganese. The radium in the sample sticks to the manganese-coated fiber, which can then be brought back to the lab for analysis.
By measuring how much the concentration of radium near shore is elevated compared to in the open ocean, scientists can estimate the contribution of groundwater, the source of the excess radium, to the coastal zone.
Groundwater: An important nutrient source
Nutrients, including nitrogen and phosphorus, are chemicals that help plants grow. They are essential for the coastal ecosystem to function, but if their concentrations become too high they can cause serious problems like algal blooms, increased water turbidity and damage to coral reefs.
When the Stanford research team visited Hanalei Bay in March 2005, they found that nutrient levels in groundwater were as high, and in some cases higher, than levels in the Hanalei River and local streams. This means that if groundwater is discharging directly into the Bay, it is likely bringing nutrients with it.
Preliminary results from last year’s trip suggest that groundwater could be contributing a nutrient load comparable to that of the Hanalei River. In order to get a more accurate number, the Stanford team and the Hanalei Watershed Hui are working with scientists from the United States Geological Survey to better understand currents and circulation patterns in the bay.
Groundwater may be bringing not only nutrients, but also FIB, into the coastal ocean. In times of high rainfall, the groundwater level or water table rises, potentially coming into contact with underground cesspools and septic systems. This could flush FIB and other pollutants into the groundwater and out to the beach.
To investigate this issue, the Stanford team is measuring FIB levels in groundwater and in the near-shore ocean, as well as rivers and streams. Because FIB levels can vary with the tides and over longer, seasonal cycles, the researchers must collect samples in summer and winter, and at high and low tides.
They will be returning this winter to sample the water again and see how the quantity and quality of groundwater discharge into the Bay changes throughout the year, so keep an eye out for scientists with trash cans!
• Karen Knee is a member of the Stanford University research team.
