Professor suggests basin as solution for polluted runoff
By Edward Chiao
May 5, 2003 9:00 p.m.
Rainstorms are the Pacific Ocean’s worst enemy ““
thousands of pounds of pollutants are washed into the Pacific Ocean
when it rains in Los Angeles.
The rainwater that falls from our rooftops and cars runs onto
city streets and is eventually collected into stormdrains. The
stormwater then finds its way to the ocean through a series of
pipes, channels, creeks and rivers; there are no treatment plants
between the water that runs from our homes to the ocean.
There are, however, plenty of pollutants along the way.
Los Angeles receives an average of 15 inches of rainfall per
year with most of the rain coming between the months of November
This pattern leaves most of the year with no rain, allowing
trash and pollutants to accumulate on surface streets and inside of
The first major storm of the season, also known as the seasonal
first flush, carries tons of pollutants and trash into the Pacific
Pollution on city streets, highways and public parking lots
include oil and grease, heavy metals from brake dust and partially
burned fuels from motor vehicles that leave behind
Trash washed into stormdrains is also a major problem. Prior to
1999, Caltrans reported that 20 percent of the material removed
from freeway storm drain inlets were cigarette butts.
In less-developed areas, lawns and gardens can release nutrients
and pesticides from the soil, which can also pollute the stormwater
that enters the ocean.
In Los Angeles, most stormwater flows to the Santa Monica Bay
via two major drains: Ballona Creek and Malibu Creek.
After conducting studies of the polluted water, investigators
found that stormwaters, as opposed to treated wastewater, contained
the bulk of the pollutants that entered Santa Monica Bay.
The study, issued by the UCLA Institute of the Environment in
1999, also measured the runoff coefficient of water. The runoff
coefficient measures how impervious the land is to the absorption
For instance, undeveloped land has a low runoff coefficient,
indicating most of the water is absorbed into the land. Paved roads
and highways, however, have a high runoff coefficient, indicating
that most of the water is not absorbed, and instead is collected by
stormdrains which flow directly into the ocean.
One of the major problems with stormwater is that it travels
through many different regions, with several public agencies
responsible for its regulation and cleanup.
But UCLA Civil and Environmental Engineering Professor Michael
Stenstrom has proposed a solution which may significantly reduce
the amount of polluted water that enters storm drains in the first
Stenstrom’s idea was to capture some of the rain water on
land, giving the pollutants a chance to be removed instead of being
washed into stormdrains.
Stenstrom proposed construction of bioinfiltration basins along
freeway shoulders to help collect some of the stormwater runoff.
These basins, or trenches, would stretch two feet wide by about
four feet deep and would be filled with gravel and topsoil. The top
of the basin would be covered by a grate which would allow water to
flow freely into the trench, while keeping trash out.
The idea is to capture the dirty water that accumulates in the
first hour of rainfall. The dirty water would flow into the
bioinfiltration basin, sinking down into the gravel and topsoil,
carrying with it the hydrocarbons, heavy metals, oil and grease
that it accumulated since the last major rain storm.
The gravel would be 50 percent porous, meaning each cubic yard
of gravel could hold a cubic yard of water. The basins would
eventually fill with water, and in a large rain storm, the cleaner
stormwater would flow off into stormdrains. The soil would not need
to be replaced for at least 20 years, and possibly last up to 50
years, according to Stenstrom.
Stenstrom believes this would be a natural and cost-effective
method to trapping the pollutants.
Currently, bioinfiltration basins are being used in other parts
of the world, including the East Coast and in parts of major Asian
An alternative solution would be to treat all the stormwater
runoff, but the cost of doing so would be in the billions of
No matter what measures are taken to filter out toxins from
stormwater, minimizing stormwater pollution will require a joint
effort between individual citizens and public agencies.
The public must realize that stormdrains were not designed to
treat polluted water. Instead, they were built only to prevent
flooding and are simply “the biggest, slickest pipe to the
ocean,” according to Stenstrom.
Public agencies have put up warning signs that tell the public
not to discard trash and pollutants into stormdrains.
In beaches located near stormdrains, local advisories are issued
after every major storm; they stay effective for 72 hours after the
last rainfall, according to Eric Edwards, an environmental health
specialist for the L.A. County Department of Health Services.
The pollutant levels are tested by several agencies in over 50
locations around Los Angeles, with some agencies testing on a daily
basis, according to Edwards.
The toxin levels in the samples are then analyzed by Edwards,
who works in the Recreational Health Program.
Last weekend’s storm that ended Saturday yielded lower
pollutant levels compared to previous storms, according to Edwards.
However, the beach advisory will remain in place through today.
In beaches that are adjacent to a stormdrain, these signs are