Billion Oyster Project

Pollution and Runoff Through Pipes


Steward-shed Investigation



Class Periods




Subject Areas

Science, Social Studies


Where does our local land-based pollution go?  What other parts of the city contribute pollution to our waterfront? To understand what pollution reaches their waterfront, and where their steward-shed’s pollution travels on its way into our waterways, students will map CSOs along their waterfront and WPCPs that discharge effluent along their waterfront.  They will determine what kind(s) of Storm drainage exists their steward-shed -- Combined Sewer, MS4, Direct Drainage and/or Bluebelt -- and make educated guesses about where those pipes and overland routes most likely discharge.  Finally, they brainstorm different approaches to reducing different kinds of sewage pollution of the waterways that are relevant to their steward-shed.


  • Access local, relevant environmental information from public maps such as Oasis.

  • Transfer the relevant information onto their own paper maps.

  • Use their knowledge of local infrastructure and topography to make inferences about where their local Storm water and treated effluent most likely discharge.

  • Represent their inferences on their own paper maps.

  • Brainstorm approaches to reducing different kinds of sewage pollution.

Materials and Resources


  • Thin but impactful markers -- for marking up maps

  • Highlighters in two or three colors -- one set for each map

Before you get started

Tips for Teachers

  • BOP’s lesson Watersheds Part 4 - Sewersheds and CSOs provides a good introduction to Combined Sewer Systems and CSOs.  You might like to teach it ahead of this lesson.


  • Prepare a relatively blank map of the neighborhood+waterfront -- note, this map should not be zoomed in on just the class’ Steward-shed.  Rather, this map should show the entire neighborhood, plus a good stretch of waterfront.  Ideally it includes a “downhill” section of waterfront, where your students believe their rain water drains to.

  • Remember that Storm sewers (for rain) and Sanitary sewers (for our toilets, sinks, etc.) are two different things, even though they are sometimes eventually combined.  

  • First determine where your Sanitary sewage is supposed to go (and where it actually goes in dry weather):

  • Then examine the New York City Sewer Systems map to determine which kinds of Storm sewer system you have in your watershed and steward-shed, and read up accordingly.  (See “Background” for more detail.)

  • Find the best map you can of CSOs on your waterfront.  We’ve found a number of maps that dramatically understate the number of CSOs.  In our experience, it’s unlikely that you’ll see a CSO on a map and then go to that place and discover there is no CSO.  So in general, the more CSOs are shown on the map for your area, the more accurate we think it probably is.

    • Check the Resources at Open Sewer Atlas to see if there’s one you can use for your neighborhood.  If so, you’re in luck!

    • If there isn’t, you can still use Open Sewer Atlas NYC’s All Layers Map.  

      • Be sure to click the double-arrow-heads at the upper left, and then click “Legend”, so you can figure out what you’re looking at.

      • One straightforward view is to select only “CSO Outfalls”.  One great feature is that you can click on each outfall to see if more information is available  Here’s Canarsie, with a CSO outfall clicked in East New York, to the northeast:

      • An interesting follow-up is to add “Sewer Infrastructure”, “Major Sewer Lines”, and “CSO Drainage Areas”  You still can’t tell exactly which areas drain to which CSO Outfalls, but you can make some pretty informed guesses.i

    • It’s not quite as user-friendly, but we think the Oasis map might use more recent data than Open Sewer Atlas in some cases.  Under “Environmental Impact / Cleanup” select “Combined Sewer Outfalls”.  The icons can be a little hard to make out, so it’s a good idea to click “Hide All” for all the other layers of the map, like we did here for Canarsie:
      In Canarsie, this looks like the same data.  Check in your area to decide which is the best source for CSO information for your class.

  • Look at the table in the “Evaluate” section, and choose a few scenarios that are most relevant to your steward-shed and waterfront.  

    • For instance, Canarsie has substantial Direct Drainage, so in Canarsie it would be very interesting to consider untreated Storm sewage that discharges directly to the local waterfront.  That’s probably a significant path for water pollution from Canarsie and to Canarsie’s waterfront.

    • But in an upland CSS area, for instance, it’s possible that untreated Storm sewage basically only reaches the waterways through CSOs -- in which case, it’s probably more interesting to talk about untreated Sanitary sewage!  Where does ours go?  Can we figure out whose is going to our waterfront?  And so on.

The following Preparation steps make sense if you are utterly fascinated by sewers and CSOs, as we are!

  • Sign up to get CSO discharge alerts from the NYS DEC, for an area of your choosing.  The sign-up process is a bit fussy, but you can get alerts within four hours of each discharge event.

  • If you’re determined to find out all you can about where those CSOs come from, it’s also worth investigating this DEP map.  If you click the “Contents” icon in the upper left, you can select “All CSO Tributary Areas” and “Combined Sewer Area Watersheds”.  The data are similar to Open Sewer Atlas data, but with a few bonus nuggets for the sewershed adventurer. Here’s a screenshot of those layers of Canarsie and environs on that map:

  • Check to see if your waterbody has a Long-Term Control Plan for reducing CSOs.  If so, you can access a several-hundred-page report that will give you incredible details about CSO-related problems in your waterbody, and steps NYC DEP has recently taken to mitigate those problems.


So far in the Steward-shed Investigation, your students’ research has focused on runoff as a major route through which land-based pollution gets transported into our waterways.  It’s important to understand how topography shapes that process.  Now they will consider the topography-of-pipes that captures Storm water and directs it -- and its pollutants -- underground and ultimately into our waterways.

Some parts of NYC have Combined Sewer Systems (CSS), meaning that pipes direct Storm water from the ground into the same pipes that carry Sanitary sewage.  Ideally that would mean that Storm water would pass through a water pollution control plant (WPCP) before reaching the waterways, but in reality the converse happens: when it rains, our local WPCPs don’t have the capacity to treat the increased water volume.  So, because almost anything is better than letting this combination of Storm sewage and Sanitary sewage back up into toilets, sinks, baths, and showers throughout the city, special pipes called Combined Sewer Outfalls (CSOs) divert the excess directly into our waterways.  Unfortunately, it takes very little rain to make that happen.  

Other parts of NYC have Municipal Separate Storm Sewer Systems (MS4s), which divert Storm water directly to the waterways.  The disadvantage is that this Storm water can never be treated at a WWTP, and Storm water is responsible for a significant proportion of our water pollution.  The advantage is that this Storm water does not contribute to CSO outflows -- another significant contributor to our pollution.

Compare and contrast CSS and MS4 in the CSO vs. MS4 Diagram.

Some parts of NYC have no Storm sewers at all.  Storm water just travels downhill over land and into our waterways -- carrying its pollutants.  When this is allowed to happen without much management, it is generally called Direct Drainage.  

In contrast, a carefully managed way to discharge runoff without sewers is through a bluebelt.  A large part of Staten Island drains into the Staten Island Bluebelt, a managed landscape of wetlands, streams, and ponds that conveys, stores, and filters storm water the old-fashioned way.

NYC’s WPCPs process most of our Sanitary sewage, but at the end of that process, they discharge treated effluent directly into our waterways -- and that is a lot of effluent!  (Here you can read more about what goes on at our WPCPs).  Although treated, the effluent still:

  • Contains a lot of nitrogen (from most NYC WWTPs) and phosphorus, which are considered nutrient pollution.

  • Is nearly always warmer than the receiving waterbody.  If you’ve ever suddenly changed the temperature of a fish tank, you’ll understand how that could be very impactful.

  • Contains a variety of other substances and materials, notably pharmaceuticals and microplastics.

To understand what pollution reaches your waterfront, and where your steward-shed’s pollution travels on its way into our waterways, your students will look at:

  1. CSOs along your waterfront, if any

  2. WPCPs that discharge effluent along your waterfront, if any

  3. What kind of sewer system is in your neighborhood (Combined Sewer, MS4, Direct Drainage and/or Bluebelt) -- and make some educated guesses about where those pipes most likely discharge.

Instruction Plan


  1. Students or pairs have a relatively blank map of the neighborhood+waterfront.  They draw in the boundaries of their steward-shed, and highlight the area of waterfront they think the steward-shed drains to.

  2. Pairs share their maps, and share with the full class any interesting questions or points of disagreement.

  3. Tell your students, “So far we have been talking about Storm water that falls on our steward-shed -- rain, snow, etc. -- but there is a lot more water coming into NYC than just our local rain!”  Show NYC’s Water Supply Map.

    • Ask your students, “This is where we get our tap water.  What inferences can you make about the higher and lower elevation areas on this map?”

    • “Do you think our tap water is mostly flowing downhill, uphill, or on the level as it moves from these upstate watersheds into our city pipes?”

    • “Do you think that over the course of, say, a year, more rain falls on these two watersheds, or on NYC?”

    • “After we use that tap water and flush it or let it go down our drains, where do you think it goes?”


  1. Show New York City's Wastewater Treatment System - Map of Plant Locations and Capacities

  2. Students make observations and inferences, and raise questions -- while you record their ideas.

  3. If it doesn’t come up, ask your students things like:

    • “How many Water Pollution Control Plants” do we have in NYC?”

    • “Which one(s) treat the Sanitary sewage from our neighborhood and steward-shed?”

    • “Why do you think that areas 1, 2, 4, 5, and 9 span more than one landmass?  Why do you think the city is piping Sanitary sewage underwater from one place to another?”

    • “Why do you think there are some orange and yellow areas with no WPCP?  Does that look like the mapmaker made a mistake, or could there be other explanations?”

    • “Look at the small black dots that show the locations of the WPCPs.  What do you notice about where our sewage treatment plants are located?”

    • “Where do you think the treated effluent from the WPCPs gets discharged?”  

      • If it doesn’t come up, tell your students, “all the treated effluent from NYC’s WPCPs goes directly into our waterways.”

      • Optional: “One question is, exactly where?” Then students can look at the SPDES addresses for each WPCP, map those locations, and make educated guesses about where each WPCP discharges its treated effluent.

    • “Which WPCPs do you think discharge effluent that makes its way the fastest to our waterfront?  Remember how much water moves, and remember that it moves differently in different times and places!”

  4. Students draw arrows on their neighborhood maps, showing how they suspect Sanitary sewage travels underground from their stewardshed, to one or more WPCPs, and then into the waterways.


  1. Reiterate that there are two distinct sources of water in NYC pipes:  one is the rain that falls directly on the city, and the other is piped in from upstate.

  2. Show or hand out NYC Sewer Systems map.  Mention that this is actually a map of Storm sewers, not Sanitary sewers.  That’s why it looks so different from the Map of WPCP Locations.  

    • Check to see if students understand the distinction -- it’s about where the water comes from (local precipitation vs. plumbing) that ultimately drains into these two different sets of pipes.

  3. Students locate their neighborhood and steward-shed on NYC Sewer Systems map.

  4. Using highlighters on their blank paper maps, students indicate the type of Storm sewer system in their area.

    • You might remind them to start a Legend or Key to show what the highlighting means on their maps.

  5. Based on the type(s) of Storm sewers in the area, students make observations, draw inferences, and raise questions in response to, “Where does our Storm sewage go?”

    • Note: in most places, it’s impossible to give a complete, evidence-based answer to this question, because no public map shows all the sewage pipes.  Based on the more limited information available, students should make inferences and support them with evidence and reasoning, and it’s wonderful if they disagree!

  6. Using a different color than they used for the Sanitary sewage arrows, students draw arrows on their neighborhood maps to show how they suspect Storm water travels from their steward-shed, overland and underground, and ultimately into the waterways, and:

    • Whether any of it ever goes to a local WPCP

    • Whether any of it goes into the waterways without being treated

    • Optional:  ask students to make the thickness of an arros correspond to the proportion of the steward-shed’s storm water that they think travels that route


If you have Combined Sewer Systems in your area:

  1. Show CSO vs. MS4 Diagram

  2. Ask your students, “what do you think are the pros and cons of CSS?  And MS4?”

  3. Show the most complete map of CSOs in your area that you can find (see “Preparation” for more detail).

  4. Students locate their neighborhood and steward-shed on the map of CSOs

  5. Students add the CSOs to their paper maps of their neighborhood and steward-shed.

  6. Ask your students, “Do you think a lot of untreated Sanitary sewage is discharged along our waterfront?”

    • “Do you think that untreated Sanitary sewage comes from our steward-shed?  If not, do you think untreated Sanitary sewage from our steward-shed gets discharged somewhere else?  If so, where?”

      • Note: in most places, it’s impossible to give a complete, evidence-based answer to these questions, because no public map shows all the sewage pipes.  Based on the more limited information available, students should make inferences and support them with evidence and reasoning, and it’s wonderful if they disagree!

  7. Using yet another color, students draw arrows on their maps to show the where they suspect that untreated Sanitary sewage travels from their steward-shed into the waterways.


Choose a few of the following scenarios that to your class’ steward-shed and waterfront (see “Preparation” section), and then ask your students to brainstorm:  “what might be some different approaches to decreasing this kind of pollution of our waterways?”

Type of Sewage

Treated or Untreated

Origin on land

Discharge location

Sanitary sewage

Plumbing: toilets & drains, usually indoors

Treated: WPCP

Your steward-shed

Your waterfront



Your waterfront


Untreated: CSO

Your steward-shed

Your waterfront



Your waterfront


Storm sewage

Rain and snow that fall on NYC land

Treated: only where there is a Combined Sewer System, in which case this effluent discharges from a WPCP

Your steward-shed

Your waterfront



Your waterfront


Untreated: runoff that gets past all sewers, CSO, Direct Drainage, Blubelt, or MS4

Your steward-shed

Your waterfront



Your waterfront



NGSS - Cross-Cutting Concepts

  • Cause and Effect

    • Cause and effect relationships may be used to predict phenomena in natural or designed systems.
    • Cause and effect relationships may be used to predict phenomena in natural systems.
  • Influence of Engineering, Technology, and Science on Society and the Natural World

    • All human activity draws on natural resources and has both short and long-term consequences, positive as well as negative, for the health of people and the natural environment.
    • The uses of technologies and any limitation on their use are driven by individual or societal needs, desires, and values; by the findings of scientific research; and by differences in such factors as climate, natural resources, and economic conditions. Thus technology use varies from region to region and over time.
  • Scale, Proportion, and Quantity

    • Time, space, and energy phenomena can be observed at various scales using models to study systems that are too large or too small.
  • Science Addresses Questions About the Natural and Material World

    • Scientific knowledge can describe the consequences of actions but does not necessarily prescribe the decisions that society takes
  • Structure and Function

    • Complex and microscopic structures and systems can be visualized, modeled, and used to describe how their function depends on the relationships among its parts, therefore complex natural structures/systems can be analyzed to determine how they function.
    • Structures can be designed to serve particular functions by taking into account properties of different materials, and how materials can be shaped and used.
    • Structures can be designed to serve particular functions.
  • Systems and System Models

    • Models can be used to represent systems and their interactions.
    • Models can be used to represent systems and their interactions—such as inputs, processes and outputs—and energy and matter flows within systems.
    • Systems may interact with other systems; they may have sub-systems and be a part of larger complex systems.

NGSS - Disciplinary Core Ideas

  • ESS2.C: The Roles of Water in Earth's Surface Processes

    • Water continually cycles among land, ocean, and atmosphere via transpiration, evaporation, condensation and crystallization, and precipitation, as well as downhill flows on land.
    • Water’s movements—both on the land and underground—cause weathering and erosion, which change the land’s surface features and create underground formations.
  • ESS3.C: Human Impacts on Earth Systems

    • Human activities have significantly altered the biosphere, sometimes damaging or destroying natural habitats and causing the extinction of other species. But changes to Earth’s environments can have different impacts (negative and positive) for different living things.
  • ETS1.A: Defining and Delimiting Engineering Problems

    • The more precisely a design task’s criteria and constraints can be defined, the more likely it is that the designed solution will be successful. Specification of constraints includes consideration of scientific principles and other relevant knowledge that are likely to limit possible solutions.
  • ETS1.B: Developing Possible Solutions

    • A solution needs to be tested, and then modified on the basis of the test results, in order to improve it.
  • ETS1.C: Optimizing the Design Solution

    • Although one design may not perform the best across all tests, identifying the characteristics of the design that performed the best in each test can provide useful information for the redesign process - that is, some of the characteristics may be incorporated into the new design. (secondary)
    • Although one design may not perform the best across all tests, identifying the characteristics of the design that performed the best in each test can provide useful information for the redesign process—that is, some of those characteristics may be incorporated into the new design.
  • LS2.A: Interdependent Relationships in Ecosystems

    • Organisms, and populations of organisms, are dependent on their environmental interactions both with other living things and with nonliving factors.

NGSS - Science and Engineering Practices

  • Asking Questions and Defining Problems

    • Ask questions that can be investigated within the scope of the classroom, outdoor environment, and museums and other public facilities with available resources and, when appropriate, frame a hypothesis based on observations and scientific principles.
    • Define a design problem that can be solved through the development of an object, tool, process or system and includes multiple criteria and constraints, including scientific knowledge that may limit possible solutions.
  • Constructing Explanations and Designing Solutions

    • Undertake a design project, engaging in the design cycle, to construct and/or implement a solution that meets specific design criteria and constraints.
  • Developing and Using Models

    • Develop a model to describe unobservable mechanisms
    • Develop a model to generate data to test ideas about designed systems, including those representing inputs and outputs.
    • Develop a model to predict and/or describe phenomena
  • Obtaining, Evaluating, and Communicating Information

    • Gather, read, and synthesize information from multiple appropriate sources and assess the credibility, accuracy, and possible bias of each publication and methods used, and describe how they are supported or not supported by evidence.

NYC Science Scope & Sequence - Units

  • Grade 6, Unit 2

    • Weather and Atmosphere
  • Grade 6, Unit 4

    • Interdependence
  • Grade 7, Unit 1

    • Geology
  • Grade 8, Unit 4

    • Humans and the Environment: Needs and Tradeoffs