Two years ago, the Ashumet Plume Citizens
Committee began exploring options to improve water quality in Great, Green, and
Bournes ponds in East Falmouth. At the time, one simple and inexpensive option seemed
to hold lots of promise: enlarge the pond inlets and let the tides wash away
the pollution. Unfortunately, the
committee has recently concluded that enlarging the inlets will do little to
improve pond water quality.
Water quality in the three ponds has
deteriorated over the past twenty years as nitrogen concentrations in the water
have increased. Intensive development within the watershed, particularly around
the perimeter of the ponds, has multiplied the amount of nitrogen being
discharged into the ponds from septic tanks and use of fertilizers.
The committee was formed in 1998 to identify and quantify the nitrogen overloading problems in the ponds, and to survey the options the town has to improve water quality. The committee will use this information to guide their recommendations to the town selectman next spring for how to spend the $8.5 million pledged by the US Air Force to offset the impact of the Ashumet Nitrogen Plume generated at Otis AFB.
The committee had hoped that enlarging the
inlets to the ponds would reduce nitrogen concentrations in the water by
facilitating exchange of water between Vineyard Sound and the ponds. Nitrogen levels in the ponds are already
kept down by a natural flushing mechanism that washes some nitrogen out to sea
with each tidal cycle. During a flood tide, ocean water, which is low in
nitrogen, rushes into the ponds and mixes with the pond water. During the
following ebb tide, the mixed water empties into Vineyard Sound, carrying with
it a considerable amount of the excess nitrogen.
Water entering and leaving the ponds must
pass through relatively small inlets. The committee initially had hoped that
enlarging the inlets so that they were a little bit deeper and/or a little bit
wider would allow ocean water to exchange more freely with pond water. More
exchange would mean more nitrogen flushing out into Vineyard Sound. Water
quality problems would literally be flushed out to sea.
Enlarging the inlets by dredging or other
steps would have been an attractive solution because it would be relatively
inexpensive, and would not require any changes in the management of waste-water
or fertilizers. Indeed, the first
report completed by the consultants hired by the town to advise on water
quality matters was focused exclusively on determining the flushing
characteristics of the three ponds.
In general, flushing characteristics are
determined by three factors: (1) the tidal range above and below mean water
depth; (2) the configuration of the inlets from the ocean; and (3) the physical
shape of the ponds e.g. length and width plus shape and depth of the
bottom. The tidal range is a given, and
is only 1 1/2 feet in the area of the three ponds. That means the natural “pump” driving low-nitrogen water into the
ponds is not very strong, which naturally imposes a limit on the potential
benefit from enlarging the inlets.
Nevertheless, the existing inlets to Great
and Green Ponds do allow water to flow freely between the ocean and the ponds.
The consultants made that determination by setting up a series of tidal gauges,
in the ponds and the Sound, to continuously measure water depth between January
12 and February 10, 1999. If the
inlets significantly impede water flow into the ponds, then the tidal cycle in
the ponds would lag behind the cycle in the Sound. For example, when the tide rose in the Sound, water would slowly
drain into the ponds, held back by the resistance of the inlets. Water levels
in the ponds would rise, but only after a certain amount of time delay; the
greater the resistance, the greater the delay. The consultants found essentially
no time lag between tidal cycles in Vineyard Sound and those in Great and Green
Ponds, demonstrating that water flows freely in those inlets.
In Bournes pond, the story is a little bit
different. Here the inlet does restrict water flow, although less than might be
expected from simply observing the narrowness of the channel because the water
flow speed is relatively high [4.2 knots compared with 2.7 and 1.7 knots for
Green and Great Ponds]. High tide in Bournes
Pond occurs about an hour later than high tide in the Sound. That finding seemed to indicate that
enlarging the inlet would improve the flushing of Bournes Pond, at least until
looking at the third factor.
The three ponds have similar surface shape;
each is long and relatively narrow.
Bournes Pond, however, also is relatively
shallow. Compared with Green Pond, for
example, Bournes Pond has about 17% more surface area but contains about 12%
less water. Moreover, the southern
portion of Bournes Pond, where tidal mixing has more immediate effect, is much
shallower than the corresponding portion of Green Pond.
As a result, each incoming tide mixes clean
ocean water and removes relatively more polluted pond water from Bournes Pond
than from Green Pond. Indeed, each tidal cycle already exchanges more than 70%
of the water in Bournes Pond for clean Sound water. So, enlarging the inlet would have little impact on nitrogen
concentrations in Bournes Pond, particularly in the northern portions that are
experiencing the worst pollution.
Naturally High Flushing Rates
Bournes Pond has an exceptionally high
flushing rate, but Great and Green Ponds have high rates as well. All three are characterized as “rapidly
flushing” by the consultants. The average amount of time that a parcel of water
spends in the pond before it is flushed out to sea is 0.64, 0.99, and 1.38 days
for Bournes, Great, and Green Pond, respectively.
It is important to maintain the inlets in
order to maintain these naturally high flushing rates, which provide a buffer
against the effects of nitrogen pollution. Without this buffer, nitrogen would
accumulate rapidly, and concentrations would skyrocket to the point that the
ponds would hardly be recognizable. They would be overgrown with algae, smell
awful, and be completely devoid of fish due to low oxygen levels.
Yet despite this beneficial flushing by
nature, the water quality in the ponds has steadily declined. This means that
the source of nitrogen pollution has been so great that it has overwhelmed the
natural capacity of the ponds to correct for pollution.
Since enlarging the inlets will not
significantly increase the amount of nitrogen flushing for any of the ponds,
the town is left with the only other alternative for improving water quality: sharply
reduce nitrogen inputs to the ponds.
Reducing inputs is, of course, much harder to
accomplish than it is to simply dredge the inlets to the ponds more deeply. It
will require a combination of sewering, on-site denitrifying systems, and
reductions in fertilizer use. While these changes will be difficult, the health
of the ponds depends upon them.
This is the second in a series of articles explaining the findings, conclusions and remediation options being examined by the Ashumet Plume Citizens Committee. Everyone who is interested in those subjects is invited to attend the third public forum being sponsored by the committee to present its report and seek public input. Please come to the Gus Canty Center in Falmouth on Tuesday, December 12 at 7 PM. The committee’s consultants will be there to respond to questions, and copies of the committee report will be available to take home and review for future reference.