Solving Environmental Problems through Engineering

Background

Unit Content

This unit is divided into several sections.  The first section provides basic information about stormwater runoff, watersheds, and their path through Tulsa.  The second section discusses the impacts of stormwater, nonpoint source pollutants, and the Clean Water Act.  The third section discusses the process, rationale, and current methods of stormwater runoff containment and mitigation.  After grasping the basics, students will have the opportunity to evaluate current mitigation techniques, then will either improve upon an existing technique or create a mitigation of their own design, which will eventually be implemented. 

Stormwater Runoff Basics

Stormwater Runoff Management

As a concept, stormwater is straightforward: when it rains, or when snow melts, it runs off permeable, semipermeable, and impermeable surfaces and eventually (often directly via storm drains) makes its way to streams and rivers.⁴ The methods for managing stormwater runoff vary from city to city.  Older cities (which includes over 700 cities in the United States) use a combined sewer system, in which stormwater and sewage flow through pipes together and travel to a sewage treatment plant.  The very significant disadvantage with combined systems is that hard rains overwhelm the treatment systems and require that the excess untreated stormwater/sewage mix be discharged directly into local rivers and estuaries, into the local watershed system.⁵

Tulsa’s system is new enough that it has storm drains and collection systems separate from the sanitary sewer system, which nearly eliminates untreated sewage from entering the watershed.  However, there are homes and neighborhoods in Tulsa that chose not to tie into the city sewer system and will occasionally have system flooding problems with their septic tanks during heavy rains as a result.  Those areas, fortunately, are few and far between.  Tulsa storm drains flow directly or via a system of smaller watersheds to either Bird Creek or the Arkansas River.⁶ Overall, Tulsa is fortunate to not have the issues that older cities with combined sewer systems experience.  Unfortunately though, this means that untreated stormwater enters our natural watershed.

Watersheds

When imagining water pollution, watersheds are frequently the physical area being discussed. “Watershed” is a broad term that refers to an area of land on which all the rain and snow, etc. that falls on it flows.  The size of these areas varies greatly, depending on their location.  There are varying standards of quality of water and watersheds; the EPA’s goal is to have them be minimally “swimmable and fishable.” “Watershed management,” then, is a similarly broad term; it refers to all the “plans, policies, and activities used to control water and related resources and processes in a given watershed.”⁷

The U.S. Environmental Protection Agency’s (EPA’s) Watershed Protection Approach is carefully integrated and holistic in that it focuses on all water quality aspects, “including chemical water quality (toxics and conventional pollutants, e.g., fecal coliform and total phosphorous), physical water quality... habitat quality... and biodiversity.”⁸ This unit will focus on the chemical water quality aspect, since this aspect of water quality is one that students can easily observe and more directly impact. 

Tulsa’s Watershed System

Oklahoma has seven major watershed basins and nearly 7343,000 acres of freshwater wetlands.  These include rivers, streams, lakes, reservoirs, and ponds.⁹ In Tulsa, watersheds can be as small as a person’s yard or as large as the Arkansas River, which is the 9th largest watershed in the United States as shown in Figure 1.¹⁰  The stormwater that washes into Tulsa’s storm sewers generally flows into one of many storm drains or creeks before being discharged into either Bird Creek or the River.  Monroe Demonstration Academy’s watershed area is Flat Rock Creek, which breaks away from Bird Creek.  Hale Junior High’s watershed is Mill Creek, which flows into Mingo Creek and through a large area of sunken land used as drainage ditches on its path (which is otherwise used as a soccer field complex in drier seasons.)¹¹ 

Figure 1: A map of the major streams comprising Tulsa’s watershed system.¹²

Impact of Stormwater

The Clean Water Act

The major federal regulations for water pollution were signed into law in the early 1970’s. The Clean Water Act is what we tend to think of when considering regulations concerning water quality.  The Clean Water Act of 1972 (CWA) expanded the 1948 Federal Water Pollution Control Act and specifically addressed industry wastewater standards, the need for permits to discharge pollutants into navigable waters, and water quality criteria.  An initial goal was to have “fishable and swimmable” waters, which meant that fish, shellfish, and wildlife needed to be protected, and that humans could enjoy recreation in and on the water.  The CWA was further amended in 1987 to include more sources of impairments, including stormwater,¹³ and Section 208 specifically requires the control of nonpoint source pollution in stormwater.¹⁴  While the goal of 100% fishable and swimmable waters has not yet been met, there has been roughly a 20% improvement in the quality of our waterways since the CWA was passed.¹⁵ 

Nonpoint Source Pollution

Whenever precipitation falls or snow melts, its runoff carries pollutants of generally unknown origin through the watershed.  All these “substances of widespread origin which run off, wash off, or seep through the ground are called nonpoint source pollutants.”  Common nonpoint source (NPS) pollutants include eroded soils, nutrients (such as fertilizers and grease,) acids and salts, heavy metals, toxic chemicals, pathogens, and even heat.¹⁶ Historically, NPS pollutants have proven to be difficult to eliminate.  Several years after the CWA was passed, point source pollutants (wastewater, industrial discharges, etc.) were noticeably lower, but various NPS pollutants were still collectively responsible for most waterway contaminants.^{17, 18}

Stormwater, and urban stormwater runoff in particular, is problematic because it discharges NPS pollutants into a variety of receiving water bodies.  Both aquatic life and raw drinking water may be affected, depending on how far removed the water body is from the stormwater runoff discharge and the type and amount of pollutants discharged.¹⁹ 

Pollutants

This unit will focus on the analysis and mitigation of a select few pollutants, chosen by the ease that students can observe them.  For this unit, students will be measuring nitrates, total coliforms (which include fecal coliforms and E. coli,) and turbidity, and they will also observe petroleum sheen.  The EPA has set standards for acceptable levels of these and many other water pollutants, which can vary depending on the water’s intended use.  For our purpose, we will use the EPA’s established levels of contaminants in drinking water.  This allows students to answer the very clear question of whether or not a particular water sample is safe for drinking. 

Nitrates aren’t problematic for most of the public to consume, but when they are present in drinking water above the EPA’s drinkable standard of 10 mg/L infants under six months can become seriously ill, and potentially die.²⁰  Nitrates can be problematic in watersheds not intended to be drinking water, as well.  Since the nitrates injected into a watershed come via fertilizer , livestock waste, and septic tank leakage, it would at first glance appear that they would have a positive impact on a watershed.  After all, fertilizers accelerate plant growth.  “However, nitrate can unbalance ecosystems in large amounts, fueling rampant growth by plants and algae. Their subsequent decomposition by bacteria can pull so much oxygen out of the water that it causes hypoxia, which can stress fish and other marine creatures. In the worst cases, dead zones and red tides can result.”²¹

“Total coliforms” refers to a group of related bacteria, most of which are generally not harmful to humans, but several bacteria, parasites, and viruses can potentially be harmful if they are ingested.  The total coliform count is considered to be a useful indicator of overall water health, and it has a maximum contaminant level of 5.0%.  Of particular note are fecal coliforms and E. coli, which should not be present in drinking water at any level.²²  Discussing the possibility of finding coliforms in stormwater runoff and/or the watershed system has an “ick” factor that makes it an appealing pollutant for middle school students to track. 

Water turbidity refers to how cloudy a water sample is due to soil runoff and is a good general indicator of how well water is being filtered.  If water is conventionally filtered, its level of turbidity cannot exceed 1 Nephelometric Turbidity Unit (NTU), and 95% of samples collected monthly cannot exceed 0.3 NTUs.  If other filtration systems are used, turbidity cannot exceed 5 NTUs.  Higher levels of turbidity are correlated with higher levels of viruses, parasites, and some bacteria.  There are a couple different approaches to measuring turbidity that are fairly easy for students to make, and observations and measurements can be taken independently, should school continue to be in distance learning at the time of this unit.²³

A water’s sheen is not a specific contaminant that has an EPA-imposed limit for drinking water.  If an iridescent or color sheen is observed on water, it can indicate the presence of either a petroleum or bacteria substance, and it is another observation that students can easily make independently if needed.  If the student drops a rock into water sheen or pokes it with a stick and the sheen breaks into platelets, it is likely caused by bacteria.  If the sheen tries to reform, it could be from petroleum source.  This is useful to check a day or two after it rains so that students can observe oil and gas runoff that likely washed from parking lots and roads.  Its presence gives students an easy visual indicator of how stormwater runoff travels.²⁴

Regardless of the type or amount of these particular NPS pollutants students find in stormwater runoff around their school or community, the larger point around taking measurements of them is that students see what contaminants are being injected into otherwise natural watershed areas.  Ideally, observations lead to questions about how and why the contaminants come from, and what is and can be done to mitigate their negative effects. 

Stormwater Contaminant Treatment

Background

Initially, the entire purpose of stormwater systems was to bring water away from developed areas quickly and without causing flooding.²⁵ This philosophy became problematic when it was observed that, “In some cases, the ability of the wetlands to naturally remove pollutants became overwhelmed by pollutant loadings from stormwater.”²⁶ This means that so much water and pollutants are discharged via stormwater runoff that nature can’t keep up anymore.  A national water quality inventory in 1994 showed that “nearly 40 percent of surveyed waters in the US remain too polluted for fishing, swimming, and other uses.”²⁷  This was overwhelmingly due to these nonpoint source pollutants, underscoring the importance of mitigating the negative impacts of stormwater runoff. 

Surface Permeability

A major contributing factor for the hindered ability of wetlands to naturally remove pollutants is the increase in urbanization.  Natural landscapes are permeable and allow rainwater and snowmelt to filter slowly into the ground, even acting as a filter for some pollutants. Urbanization has resulted in both a reduction in these permeable surfaces and an increase in impervious surfaces such as building roofs, roads, and parking lots (Figure 2).  The increase in these nonporous surfaces means that there is more water flowing directly into storm drains, and at a faster rate.  Not only does this increase in impermeable surfaces actually increase the risk of flooding, its resultant stormwater also picks up harmful pollutants like trash, road salts, chemicals, oils, bacteria and viruses, and sediment and carries them to the watershed.  Fish, wildlife, vegetation, and humans are all impacted by increased pollutants due to urban stormwater runoff.^{28, 29} Figure 3 shows Tulsa County’s land cover, and how it has been affected by urbanization.

Figure 2: The difference in stormwater runoff due to urbanization.³⁰

Figure 3: The type of land cover for Tulsa County’s 391,148 acres (Author-created using information from Davey Resource Group. ³¹)

Mitigation

In order to design, create, and implement their own method of mitigating the negative effects of stormwater runoff, students need to understand current practices.  Urban development doesn’t have to be antithetical to responsible watershed management.  A shift toward “green infrastructure” has incorporated infrastructure needs with the desire for urban green spaces at the same time there has been an increase in the amount of concern for aesthetics in urban construction.  Incorporating plants and other specific landscape design choices into urban planning to mitigate stormwater runoff has become more aesthetically pleasing, and therefore more popular. ³²

On a global scale, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services noted several interventions that are applicable to stormwater runoff and would help sustain freshwater, including slowing and reversing de-vegetation of catchments, mainstreaming practices that reduce erosion, sedimentation, and pollution run-off, decentralized rainwater collection, and integrated management of surface and groundwater.³³

Many storm drainage systems serve multiple functions and provide multiple benefits while having value aesthetically.  These systems can improve water quality while providing recreational opportunities and improving wildlife habitat. Some of these include:

• stormwater gardens and other bioretention areas, which are simply depressed areas that are planted with vegetation that helps retain water and slow its likelihood of runoff while having the benefit of being aesthetically pleasing. They also cleanse pollutants to varying degrees, depending on design, and reduce demands on potable water for landscape irrigation;
• infiltration areas, which are similar to stormwater gardens but are deliberately connected to downspouts, driveways, and other impervious areas, and provide for groundwater recharge (if downspouts aren’t eliminated entirely;)
• natural areas that are kept undeveloped and can reduce runoff volume while enhancing wildlife habitat;
• passive park areas, which can provide for infiltration and/or extended detention (Wenk)
• green roofs have been covered with a growing medium and vegetation on top of a waterproofing membrane on a roof, to absorb rainwater and reduce runoff from its source;
• rain barrels and cisterns, which are connected to gutter downspouts, which allow rainwater to be used for other nonpotable purposes, such as watering lawns and gardens;
• permeable pavements are an excellent alternative to impermeable asphalt and cement and help increase rainwater infiltration through porous areas, reducing runoff;
• riparian buffers are natural vegetation planted along stream banks, and help slow and prevent runoff into streams;
• constructed wetlands are artificial wetlands designed to mimic natural wetlands. They help treat stormwater and other types of wastewater using vegetation, filtering sediments, and microorganisms;
• Sediment control practices at construction sites to minimize the injection of eroded soils into watersheds; and
• street sweepers remove debris from gutters and lessen the amount that enters storm drains. ^{34, 35, 36, 37, 38, 39, 40}

While this is by no means an exhaustive list of interventions that can mitigate the negative effects of stormwater runoff, the diversity of these ideas can help students consider a variety of points along stormwater’s route that they can potentially intervene.