Nitrate

Anyone who lives by Straits Pond would recognize this picture as typical of summer conditions on Straits Pond. Excessive growth of algae covers large portions of the Pond each summer and decays at the end of the season.

Straits Pond shares the problem of algal blooms with many fresh, brackish and saltwater bodies worldwide.  A drive around the South Shore reveals that nearly all our fresh and saltwater ponds and inlets have algal blooms to one degree or another during the summer.  It was not always this way.  In some places, there is a long history but in others, this is a relatively new development.  Why, might you ask?

Algae 101

Aquatic algae are single celled green plants that are distantly related to the multicellular vascular plants which generally grow on land.  Algae uses photosynthesis to convert sunlight and carbon dioxide to sugar.  Like all plants it also needs nutrients like nitrogen in the form of nitrate and phosphorous in the form of phosphate.  Just like your lawn, sunlight, carbon dioxide, and the nutrients nitrate and phosphate drive algae to grow and multiply. Environments naturally low in nutrients prevent extensive growth of algae despite warm temperatures and abundant sunlight.

When the Balance Shifts

Runaway algae populations are said to “bloom” which means they grow rapidly to the point where they begin to overwhelm the system.  Algal blooms are caused by an excess of nutrients, phosphate in freshwater systems and nitrate in saltwater systems.  The source of those excess nutrients is the most widely distributed animal on the planet, us.  Our sewage and manure from the animals we raise to eat or entertain us contain high levels of nitrogen.  Our agricultural systems and decorative vegetation are heavily dosed with nitrate-based fertilizer.  Though some of that nitrate is converted to plant growth on land and some of it is metabolized by bacteria releasing nitrogen gas back to the atmosphere, some of it is discharged to waterbodies.

Excess nitrate and warm water temperature are what is driving the algal blooms on Straits Pond.

How Much Nitrate are We Sending to the Pond?

Nitrate arrives in the Pond through surface and groundwater discharge neither of which we can measure accurately. We can estimate how much nitrate is present at the source meaning our residences, streets, and businesses.  Nitrate Load is the term used for this estimate.  For example, a typical residence in the Straits Pond Watershed has a house with a roof, a driveway that may be permeable (e.g. oystershell or gravel) or impermeable (e.g. asphalt or cement), a sewer connection or a septic tank, a lawn, and hopefully some remaining wild area.  Each of these areas has a nitrate load which we can estimate.  If we know the area of each of these elements in a residential lot and the number of people living there, we can estimate the total nitrate load of the property.  If we sum the nitrate load of all the residences in the watershed, we have a nitrate load model.

A model is an imperfect simulation of the real world because it is not nearly as complex as the real system.  Climate models, weather models, tide models, etc are all familiar examples of our ability to simulate the real world to try to understand it better and make forecasts of the future.  A nitrate load model may not be perfect but think of it as a tool to enable us to better understand the system and make comparisons of areas within the system.  The absolute load might not be highly accurate but the comparisons between areas are valid since each part of the model is based on the same assumptions and measurements.

Though we might have a reasonable estimate of how much nitrate is loaded into the system, we have no good way of knowing how much of that nitrate remains there and how much is ultimately discharged to the Pond.  There are natural processes that convert nitrate back to nitrogen gas which escapes to the atmosphere.  There are also organisms along the path to the Pond that consume some of the nitrate before it can be discharged to the Pond.  What we do know for sure is what is in front of our eyes each summer, algal blooms.  The evidence for excess nitrate reaching the Pond is strong.

The Straits Pond Watershed Association Nitrate Load Model

Last year, SPWA built a nitrate load model for the entire Straits Pond Watershed.  The methodology was based on the well respected load model created for the Buzzards Bay National Estuary Program.  The SPWA load model coveres a much smaller area so we were able to substitute real measurements for some parameters that were estimates or assumptions in the Buzzards Bay Model.

Specifically, we measured the areas of the roof, driveway, lawn, and wild area on each of the residences in the watershed in Hull and Cohasset (~500 residences).  Using a list of the sewer connections for each town, we could tell if a residence had a septic tank or a sewer connection.  Each residence was assigned a nitrate load based on the Buzzards Bay Model and the loads for all parcels in the Watershed were summed.

The raw model results show that the annual nitrate load within the watershed is about two tons.  Keep in mind this is not an estimate of the amount of nitrate that is entering the Pond from the residences each year.  Also consider that the further a residence is from the Pond, the smaller the percentage of the nitrate that leaves the property reaches the Pond.  With that in mind, the nitrate load in Cohasset is over ten times the nitrate load in Hull.

The results of the model and what it tells us are an incomplete picture of nitrate in the Watershed.  Some take-aways do stand out however.  For example, Hull is fully connected to the sewer system so virtually no nitrate from septic water is being loaded onto the properties in Hull.  Only about 50% of the residences in the Cohasset part of the watershed are connected to the sewer so a significant amount of nitrate enriched water is reaching the aquifer in Cohasset. The Cohasset residences that have septic tanks are the farthest from the Pond.  The aquifer ultimately discharges to the Pond though the longer the flow path, the greater the reduction in the nitrate concentration.  The size and number of lawns in Cohasset dwarfs those in Hull and fertilized lawns are a substantial source of nitrate pollution.  The near elimination of wild areas on residential lots in Hull has diminished the natural processes of nitrate removal.  The relatively short flow paths from residences in Hull to the Pond result in less natural nitrate removal before it is discharged to the Pond.

What Can We Do to Reduce Nitrate Pollution in Straits Pond?

The Watershed community has a responsibility to each other and in general to reduce pollution of all types.  There are two approaches to reducing nitrate pollution, one is as a community and the other as individuals.  Success requires both.

First, we have recognized the problem but more needs to be done to define its severity, extent, and source before we can start developing a reduction strategy.  SPWA is stepping up but we can’t do it all and we need sponsors, public and private partners, and most important, the cooperation of everyone in the Watershed.

We have been investigating some of the technologies that are available to enlist in this process.  Recognizing that lawns and toilets are the biggest source of nitrate it is obvious that any technologies that address them is our first priority.  We didn’t need to look very far to find good examples of how other coastal communities are dealing with the problem.  Foremost among them are those on Cape Cod where the problem is even more severe than that at Straits Pond.  We are looking closely at their Town bylaws for how nitrate might be addressed at the community level.  We are also looking at de-nitrifying septic tank technology that would by-pass the great expense of laying out a sewer network and expanding a centralized sewer treatment plant.  One of the great benefits of these systems is keeping water in the watershed instead of discharging it out to sea.

You can do things on your own property to contribute to the solution.  First and foremost, you can diminish the amount of fertilizer you put on your lawn each year and set up a schedule that gives your lawn more fertilizer when it needs it and actually reduce the amount of water you may be using to irrigate it.  Check out our Healthy Lawns page to see how you can help.

If you are planning to do some remodeling or landscaping, keep in mind that there are opportunities to decrease the area on your property that prevents water from infiltrating into the soil.  Storm water runoff is a quick way to deliver nitrate to the Pond before natural processes have a chance to break it down. Groundwater moves far slower giving the natural processes of converting nitrate to harmless nitrogen gas a better chance of working.

Last but not least, stay informed through our website, Facebook page, quarterly newsletter and other sources for information about the problem. You can support our ongoing advocacy, public engagement, and research by becoming a member of SPWA. Click the bird to join SPWA today!

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