Muddy Waters
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Muddy Waters
Is it safe to swim in your local lake? Are the fish
from that lake good to eat? NASA satellites will soon help find
out....
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Sept.
5, 2002: Dozens of shallow-draft bass fishing boats
creep along the cypress-lined shore, each guided by a seasoned
fisherman. It's the annual bass tournament everyone's been
waiting for ... yet the fish aren't biting.
Tugging at the rim of his threadbare cotton fishing hat, one
veteran angler eyes the clear blue sky above, the ripples on
the lake surface, and the pattern of tree trunks and reeds along
the water's edge. Sensing a clue, he toes the control of the
trolling motor and glides slowly toward a shallow cove where
a trophy-winning fish is certainly lurking--just out of view.
Indeed, there's something there. But it's not a bass.
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Buried down in the sand and silt of the lake's bottom lies a rainbow of different noxious chemicals--relics of 100 years of industry in the region. The boat's trolling motors stir them up and so do wind-driven waves. The fisherman doesn't notice what's happening, but a satellite passing 400 km overhead does. It snaps a picture of the lake and beams the data to Earth, where scientists note areas of water that are less reflective than usual--a result of the stirred-up sediments.
City officials and environmental regulators can't wait to
see the data. They hope it will help answer some important
questions: Are the lake's legendary bass fit for the dining room
table? How much sediment is dumped into the lake by the adjoining
river? Do pollutants buried in a patch of lakebed near an abandoned
paper mill pose any threat to swimmers at a beach on the far
side of the lake? And why is this year's tournament a bust?
In
real-life, they'll have to keep waiting. Satellite views of stirred-up
murky water (scientists call it "resuspended sediments")
aren't yet available to answer their questions. Currently, monitoring
suspended sediments is done by hand, a challenge for bodies of
water that cover hundreds or even thousands of acres. Scant data
gathered at a few monitoring stations provide only a glimmer
of what's going on.
Above: One way to measure water clarity by hand: a "Secchi
disk." The photographs show increasing murkiness from
left to right. Credit: Minnesota Pollution Control Agency.
Around the country, there are dozens of reasons to monitor stirred-up
sediments. Shellfish harvests in Northeastern bays, for example,
are affected by sediment levels. So is the rich biodiversity
of Atlantic, Pacific and Gulf coastal estuaries. Further inland,
nutrients released by stirred-up sediments can nourish microscopic
phytoplankton in freshwater lakes and trigger algal blooms that
choke-off the lakes' plant and animal life.
This need for wide-area monitoring is what has motivated scientists at NASA's Stennis Space Center in Mississippi to explore how satellites might help. And after 6 months studying Lake Pontchartrain, just north of New Orleans, Louisiana, they think they have a system that works.
Above: This high-resolution satellite image (Terra/MODIS) shows sediments from the Mississippi River spilling into the Gulf of Mexico. Lake Pontchartrain lies near the top center of the photo. [more]
"We've talked to city planners, [environmental regulators, and other] decision makers--and they've said they would like this," says Richard Miller, chief scientist for NASA's Earth Science Applications Directorate and the manager of the project.
Miller's team monitored Lake Pontchartrain using two instruments in space: NASA's SeaWiFS satellite and NOAA's Advanced Very High Resolution Radiometer (AVHRR). Both measure the reflectance of the water--an indicator of turbidity and stirring.
A certain amount of stirring will occur just because of the action of wind-driven waves. This is called "natural resuspension." To account for it, Miller's group uses a computer model to calculate the expected amount of stirring based on wind speed, wind direction, and the depth and shape of the body of water.
The
computer runs its simulation and "spits out" a number
the scientists call the "index of resuspension intensity."
Plotted over the area of the body of water (in the form of false
colors or contours) this number maps out the expected resuspension
due to wind and waves.
Left: In this false-color map of Lake Pontchartrain red squares denote cloudy water; blue squares denote clear water. [more]
"At least for the environments in Lake Pontchartrain, our index of resuspension intensity correlates really well with our satellite imagery," Miller says. Sometimes, though, they spot suspended particles in a place not predicted by the computer model. Such anomalies might be evidence of human activity--such as fishing in shallow waters--or perhaps a movement of turbid water from another area, set in motion by a passing storm front.
The results so far are "very encouraging," says
Miller, but there's more to do. For example, each pixel in the
images from these satellites represents one square kilometer
on the ground, so the application of this remote sensing technology
is currently limited to large bodies of water.
The research team is now starting a new phase of field trials
that incorporates a different satellite sensor that has better
resolution. Called the MODerate-resolution Imaging Spectro-radiometer
(MODIS), the pixels in images from this sensor are only 1/16th
of a square kilometer on the ground. MODIS rides aboard two NASA
satellites--Terra and the recently launched Aqua -- which together
will provide two snapshots per day, one in the morning and one
in the afternoon.
The
field trials have also expanded to a new site at Pamlico Sound
in North Carolina. In collaboration with Reide Corbett of East
Carolina University, this phase of the trials will focus on the
effects of fishing trawls or bottom nets.
Right: A thin line of land separates North Carolina's
Pamlico Sound from the Atlantic Ocean. The wetland communities
of this area are vital to productive fisheries and water quality.
Image credit: Terra/MISR. [more]
Ultimately, the researchers want to construct a system for
delivering an executive-summary version of the satellites' observations
to the regulators and decision makers who need it. Miller says
that his team's goal is to collaborate with decision makers in
the region to design a system to suit their needs. He expects
that the project could be producing these executive reports in
six months' time.
Putting this knowledge into the hands of decision makers will
help keep our waterways clean, so that fishers in the future
can safely make a meal of the day's catch ... not just a trophy.
Earth Science Applications Directorate -- (NASA/Stennis)
Testing the waters: using satellites to monitor lake water quality -- (EarthObservatory.com)
Satellite links: Terra; MODIS; SeaWIFS; MISR
Lake Pontchartrain Basin -- information about bottom sediments and pollutants in Lake Pontchartrain, from the USGS
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