When
it rains, stormwater outfalls discharge high levels of enterococcus
into the ocean that causes most beach advisories.
Especially when the beach is near a stormwater outfall (map
on page 20).
How
do you guess your risks when it rains but no water
samples are being taken? There's a lot to consider.
Is
the wind blowing that light, less saline stormwater offshore, or is
it holding it in the swimming zone?
Then
there are all the other variables: tidal and lunar stages, water
temperature, wave height and intensity, and how sunny it is, since
the
ultraviolet light in sunlight inactivates bacteria.
Complicated.
That's why some states along the Great
Lakes, and California,
are using models now to predict bacterial water quality at beaches on
days when there is no sampling.
Rain
is complicated. You need to look beyond the amount, intensity, and
timing of just that one, most recent rainfall. You also need to ask:
how wet or dry was it before it rained?
The
effect of “antecedent rainfall” on stormwater quality is better
understood for metals and suspended solids than for bacteria. This
study in California found that, for up to about a month, the
longer it didn't rain, the more pollutants accumulated in parking
lots.
When
it rained, stormwater runoff had more pollutants when it was preceded
by a dry period than when it was preceded by a wet period, because
more pollutants had built up.
The
opposite happens with more frequent rainfall. That keeps the pavement
flushed. It produces stormwater plumes with less pollutants – less
impacts - per storm.
The
City
of Stamford, Connecticut has found that it takes less rainfall to
increase bacteria counts at their beaches on Long Island Sound when
that rainfall is preceded by unusually dry weather.
Their
data shows that when “the occurrence of drought or near-drought
conditions occurs, then less
rain is needed to influence water quality.”
Their
beach Closure Guidelines state that: “the current policy of the
city of Stamford is to close beaches for 24 hours following a
rainfall event of 1 inch or more under normal conditions. During
periods of low rainfall or drought conditions, advisories are issued
following a rainfall event greater than or equal to 0.5 inch” (page
I.3-7, Section 3.3.1.3).
They
define dry
weather conditions as “less than two inches of rain in 30 days
and less than one inch of rain in 10 days”.
This
suggests that stormwater, not sewage, is the primary cause of
elevated bacteria levels when it finally rains - perhaps due to more
animal droppings, and more bacterial regrowth in biofilms
in stormwater systems during dry those periods (page
3).
If
Combined
Sewer Outfalls, Sanitary
Sewer Overflows, or illicit
cross connections in stormwater system were the primary cause of
beaches closures, you would think that bacteria levels would rise
during periods with more frequent rainfall - not less.
On
the other hand, ongoing research in California didn't find antecedent
dry periods to be significant. But perhaps that is because, unlike
the East Coast, Southern California has “rarely measurable rainfall
in the summer season” anyway (page
113). “The summer dry weather in California also contributes
to the weaker dependence of [bacteria] concentrations on rainfall;
there is rarely measurable rainfall in the summer season” (page
113). And: “Rainfall in the summer is usually due to trace
rainfall events due to the passing of the monsoonal storms” (page
429).
We
have so much more rainfall in NJ: 40-51
inches a year, compared with a little more than 15
inches in Los Angeles. Dry periods lasting longer than a week
during the summer are not as unusual in southern California as they
are in NJ.
But
rainfall is still just one variable. Wind and currents can quickly
disperse bacteria from the swimming zone - or hold them there,
causing more beach closures (slide
1).
Weekly
sampling is expensive, and Public
Health has not done well since 2009. More sampling means higher
state and local taxes.
That's
why the EPA is nudging states to use
forecasting models to supplement their water sampling.
