Water Quality of East Falmouth's Coastal Ponds: Past,
Present, and Future
Imagine throwing on your bathing suit on a
hot, summer day and walking down to Great Pond, towel in hand, to find this:
dense algal mats a half a foot thick floating on top of the water, dead fish
washed ashore, and a stench so strong it stops you in your tracks.
Of course, water quality in East Falmouth's
coastal ponds is not this bad, but without major changes to reduce nitrogen
loading to the ponds, those conditions are in store for the future.
Over the past two years, the Ashumet Plume
Citizens Committee, in conjunction with the environmental consulting firm
Horsley & Witten, has identified and quantified the nitrogen over-loading
problems in Great, Green, and Bournes ponds. The committee will use this
information to guide their recommendations to the town selectman for how the
town should spend the $8.5 million that it will receive from the Air Force to
offset the effect of the Ashumet Nitrogen Plume.
The first step in assessing
the health of East Falmouth's ponds was to develop a water quality
classification system. The system has five levels, ranging from 'Excellent' to
'Severe Degradation', and is based on the amount of nitrogen in the water.
At very low levels of nitrogen
(<0.30 parts per million), coastal ponds are healthy. They have dense eel
grass communities, plentiful scallops and other shellfish, and high oxygen
levels for fish. As nitrogen levels rise, the health of the ponds steadily
deteriorate.
Nitrogen is a nutrient, and
too much nitrogen can stimulate growth of phytoplankton and algae that shade
out the eelgrass beds. Eelgrass and scallops are among the first victims of
declining water quality. As nitrogen levels continue to increase, other
shellfish begin to decline as well. Oxygen levels in the water begin to dip
periodically, threatening the health of animals that live the water column.
At high levels of nitrogen
(>0.70 ppm), the water column occasionally loses all of its oxygen, causing
large numbers of fish to die and float to the surface. Thick, smelly algal mats
develop on the surface of the water, and dense phytoplankton blooms make the
water cloudy. Most people would not like to even be near a pond like this, let
alone swim in it.
According to the water quality
classifications recommended by the
consultants, the water in Great, Green, and Bournes ponds range from
'significantly impaired' in the centers of the ponds, to 'severely degraded' in the upper reaches. This means that the
native eelgrass communities are completely gone. They are replaced by murky
water filled with phytoplankton and macroalgae. Occasionally oxygen levels in
the water dip down below 4 mg/L, low enough to suffocate fish.
The
lower or southern sections of the three ponds are a bit more healthy. Their
water is at least moderate quality, and the lower basin of Bournes Pond still
supports shellfish beds. The main reason that the lower sections are healthier
than the upper is that ocean water entering through inlets at the base of the
ponds flushes out some of the nitrogen with each tidal cycle.
The low water quality in
Great, Green, and Bournes ponds has resulted from rapid development within the
watershed that has dramatically increased the amount of nitrogen loaded into
the ponds through groundwater. Septic systems and fertilizers are the main
culprits, together accounting for more than 2/3 of the total nitrogen loading.
Changes in water quality have been rapid. Fifteen years ago, most of
the water in Green Pond was of moderate quality, or better. Only about 20% of
the pond's surface area had become significantly impaired. The line of
significant impairment marched steadily southward in succeeding years,
encompassing 60% of the total area by mid-1990's. Today, the line has reached
to just south of the bridge, enclosing the vast majority of the pond. Only a
small section of water that exchanges heavily with the ocean is still
moderately healthy.
If the land within the
watersheds for these three ponds is developed to the maximum extent allowed by
current zoning restrictions, nitrogen levels will increase by 16 percent in
Great Pond, 13 percent greater for Green Pond, and 21 percent greater for
Bournes Pond. This will cause even more water to become significantly impaired
or severely degraded. The changes will be most dramatic in the lower basins,
where the water is still relatively clean. In the upper reaches, increased
nitrogen loading will not have as obvious an impact since nitrogen levels are
already so high.
The future of the ponds is not
as dim as it may seem. Models developed by Horsley & Witten show that if
all of the nitrogen inputs from septic systems and fertilizer application are
removed, the water in most of the ponds will be restored to reasonably-acceptable
quality. This means that shellfish could make a recovery, and eelgrass
communities could re-establish.
Of
course, it will not be possible to remove all the nitrogen loading from septic
systems and fertilizer application. However, the town does have a unique
opportunity to dramatically reduce those inputs. The $8.5 million from the US
Air Force can be put to one of several nitrogen-reduction plans, ranging from
sewering a limited number of homes, to establishing a comprehensive Management
District. If approved by Town Meeting and voters, the District would generate
its own revenue and use a combination of seering and on-site wastewater
treatment to systematically reduce nitrogen inputs. How te town decides to
spend the money will set in motion the
rocess for determining the fate of these fragile coastal ponds.
The first step in that process occurs at Town Meeting, where Members
are being asked to vote on Article #70 which asks the Selectmen and thus the
Committee to pursue meaningful improvements to the water quality of the ponds
versus limited improvements. The
meaningful improvements would significantly reduce nitrogen pollution of the
three ponds but would require considerably more expensive measures than the
U.S. Air Force commitment.