[No. 1] Artificial turf fields should be included in the definition of the term “impervious surface.” SynTurf.org, Newton, Mass. 31 December 2021. In this article we explore the reasons why artificial turf fields should be classified as “impervious” as a matter of definition of the term “impervious surface/structure” as well as the functional equivalent of such structures even though to the naked eye the field seems permeable. The geographical context of this article is Massachusetts, but discussion has a global message.
Why get worked up about impervious surfaces to begin with? Well, when rainwater washes over impervious surfaces such as rooftops, parking lots, and roads, it collects and carries pollutants that ultimately flow into waterways. According to the United States Environmental Protection Agency, “[s]tormwater runoff is a major contributor to water pollution …. Smart growth strategies can help to reduce the impact of new development on stormwater runoff by preserving open spaces and focusing new construction in developed areas, which reduces the amount of new impervious surface per unit of development.” [Fn 1].
Because of the characteristics of the materials used in impervious surfaces – and other factors such as compaction of otherwise pervious materials – impervious surfaces tend to heat up and contribute to the heat island effect. According to the EPA, unlike trees, vegetation, and water bodies that tend to cool the air, “[h]ard, dry surfaces in urban areas – such as roofs, sidewalks, roads, buildings, and parking lots … contribute to higher temperatures.” For example, according to the EPA, “[o]n a warm day with a temperature of 91°F, conventional roofing materials may reach as high as 60°F warmer than air temperatures. Surface heat islands tend to be most intense during the day when the sun is shining.” [Fn 2]. These structures and surfaces where they are highly concentrated and greenery is limited they become “islands” of higher temperatures, hence the term “heat island.” [Fn 3]. Heat islands increase energy consumption, increase emissions of air pollutants and greenhouse gases, compromise human health and comfort, and impair water quality. [Fn 4]. Heat islands contribute to climate change. [Fn 5].
Statutory framework for mitigating damage to the environment. The Massachusetts Environmental Policy Act (MEPA) requires that all authorities of the state review, evaluate, and determine the impact on the natural environment of all works, projects or activities conducted by them and shall use all practicable means and measures to minimize damage to the environment. The statute defines “damage to the environment” as “any destruction, damage or impairment, actual or probable,” to any natural resource. In considering and issuing permits, licenses and other administrative approvals and decisions, the respective authority is required to consider reasonably foreseeable climate change impacts, including additional greenhouse gas emissions, and effects, such as predicted sea level rise. The statute applies also to an agency, department, board, commission or authority of any of the state’s political subdivision – that means local and municipal entities as well. G.L. c. 30, §§61-62. Under the Code of Massachusetts Regulations, one of the thresholds that triggers mandatory review of a project is when the alteration to land creates 10 or more acres of impervious area. 301 CMR 11.03 (1) (a). Municipal zoning and stormwater management bylaws may require impervious surfaces associated with a project be even less than 10% of the total development area.
Definition of the term “impervious surface.” In Massachusetts, apparently, the only general definition for the term “impervious surface” appears in the state’s model groundwater protection regulations. It defines the term as “material, or structure on, above, or below the ground that does not allow precipitation or surface water runoff to penetrate into the soil.” [Fn 6]. In the next-door state of New Hampshire, the definition of “impervious surface” is baked into its Revised Statutes with examples of such surfaces: “[A]ny modified surface that cannot effectively absorb or infiltrate water. Examples of impervious surfaces include, but are not limited to, roofs, and unless designed to effectively absorb or infiltrate water, decks, patios, and paved, gravel, or crushed stone driveways, parking areas, and walkways.” [Fn 7].
For the sake of comparison, Belle Meade (Tennessee) defines the term as “those areas which prevent or impede the infiltration of stormwater into the soil as it entered in natural conditions prior to development. Common impervious areas include, but are not limited to, rooftops, sidewalks, walkways, patio areas, driveways, parking lots, storage areas, compacted gravel and soil surfaces, awnings and other fabric or plastic coverings. [Fn 8].
In Massachusetts, municipalities are left to their own devices when it comes to defining the term. In the Town of North Andover, for example, the term is defined as “Any material or structure on or above the ground that limits water infiltrating the underlying soil. Impervious surface includes, without limitation: roads, paved parking lots, sidewalks, sports courts and rooftops. Impervious surface also includes soils, gravel driveways, and similar surfaces with a runoff coefficient (Rational Method) greater than 85.”[Fn 9].
The Town of Mashpee defines the term as “[a]ny material or structure on or above the ground that prevents water infiltrating the underlying soil. Impervious surface includes without limitation roads, paved parking lots, sidewalks, and rooftops.” [Fn 10]. The town’s stormwater management program recognizes that “[t]he more impervious surface, the more stormwater runoff and therefore the greater impact to local water bodies. Clean stormwater runoff from natural areas is an important source of recharge to the groundwater; replenishing drinking water wells and supplying base flow to lakes, ponds, springs, brooks and wetlands.” [Fn 11].
Most significantly, the Town of Ashland defines the term as “Material or structure on, above, or below the ground that does not allow precipitation or surface water to penetrate directly into the soil.” [Fn 12]. On the other hand, the town’s Impervious Area Map describes impervious surfaces as “any surface that prevents or significantly impedes the infiltration of water into the underlying soil. This can include but is not limited to: roads, driveways, parking areas and other areas created using non-porous material; buildings, rooftops, structures, artificial turf and compacted gravel or soil.” [Fn 13].
Functional equivalent of an impervious surface/structure. The selling point of artificial turf fields is that the playing surface drains and dries fast, the reason being that the surface of the field (carpet) is permeable. Of course there are synthetic lawns and other synturf installations that result in runoffs because they rest on hardened surfaces like cement or are resting on compacted gravel or on soils with poor drainage. The discussion here is focused on artificial playing fields for sports, of the variety that involves an elaborate integrated system or structure, complete with a sophisticated drainage system and underdrain engineering. [Fn 4].
At first glance, it would appear as counter-intuitive to equate an artificial turf field with an impervious surface. For one thing, artificial turf is a surface that is permeable, meaning that water is supposed to passes through it. Yet, the term, as we have seen in some of the definitions above, including in Massachusetts, defines “impervious surface” as material, or structure on, above, or below the ground. In that sense, by definition, a synthetic turf field is a structure [Fn 15], with much of its cubic footage under the surface of the carpet and below grade (ground). Whether a synthetic turf field should be classified as impervious therefore depends on the answer to the question: To what extent does the field structure limit, significantly impede, or prevent water infiltrating into the underlying soil as it entered in natural conditions prior to installation of the field?
To appreciate the complexity involved in the engineering and construction of an artificial turf field structure/system, one may want to view the video of such fields being built:
- Base Construction for Synthetic Turf Sports Fields By Sunny Acres Sports Systems
The important takeaways from viewing the foregoing clips are (1) the use of gravel and other aggregates in the subgrade; the compaction of the various layers of stone and gravel; the laying down of “geotextile” or rubberized membranes; and the plumbing that ultimately transports water from the subsurface away from the field and possibly connect to the municipal drainage system.
In a recent decision of the Town of North Andover Planning Board, the board described the stormwater management system of the artificial turf field component of a project as follows: “Runoff from the synthetic turf field will collect in 8" perforated underdrain pipes and outlet to underground infiltration chambers. Overflow from some of the infiltration chambers will connect to the existing drainage systems.”[Fn 16].
In connection with the aforementioned project, the designers provided schematics that showed the structure of the proposed synthetic turf fields consisting of the following layers from bottom up: The compact sub-base is covered with non-woven geotextile fabric, followed by 8 inches of base (crushed) stone, topped by 2 inches of finishing (crushed) stone, followed by the carpet (turf) with tufting gauge of 3/8 inch fiber (filaments, plastic grass blades) to the height of 2.5 inches, topped with infill (crumb rubber pellets) primarily from truck tire grade. [Fn 17]. Another schematic showed the structure of the field underdrain system consisting of an 8-inch perforated pipe located above the geotextile covered subgrade and nestled in the 8-inch high layer of base stone. [Fn 18] and [Fn 19].
The idea of an underdrain is for the water that seeps through the carpet to collect in the perforated pipe and be transported away from the field into an open-air area detention area such as a bioretention pond or swale where the water is expected to infiltrate into the ground (and some of it evaporate). Or, the water is carried by the pipes away from the field to an underground retention or infiltration chamber where the water is expected to gradually infiltrate into the ground. Naturally, the infiltration of the water and the rate of it any milieu depends on the character of the soil and the level of the water table. The lack of an adequate drainage system is likely to destabilize the structure of the field, resulting in damage to the drainage system and the playing surface itself.
According to the Synthetic Turf Council, the artificial turf industry’s premier trade organization, the stormwater measurements for synthetic turf fields “are meant to control the transport of sediments, pollutants and other undesirable elements into stream, lakes or the potable water system. They are also intended to limit the strain on existing storm water systems caused by any addition of impervious surfaces in the environment. A synthetic turf surface acts just like an impervious surface, since the rain water it collects is most often redirected directly into the rain sewer system instead of being left to percolate into the soil.” [Fn 20]. For all intents and purposes it is this feature of synthetic turf fields that makes the artificial turf field functionally an impervious structure.
What heat has got to do with it? Due to the materials used in impervious surfaces/structures they tend to heat up to temperatures higher than the ambient air. It is no secret that artificial turf fields heat up. [Fn 21]. To illustrate, the measurements taken in late summer in 2007 at fields in eastern Massachusetts revealed that that synthetic turf field surfaces got considerably hotter than asphalt [Fn 22], among them the following:
1. Date & Time: August 3, 2007, 2PM
Location: Lincoln-Sudbury Regional High School
Weather: Hazy sun
Old synthetic turf field: 143
New synthetic turf field: 156
2. Date & Time: August 16, 2007, 11AM
Location: Waltham Veteran's Field
Weather: Hazy sun
Synthetic turf: 128
3. Date & Time: August 24, 2007, 1:30PM
Location: Greater New Bedford Vocational
Weather: High clouds, 20MPH wind
Synthetic turf: 136
4. Date & Time: August 28, 2007, 11:45 AM
Location: Sudbury Cutting Field
Weather: Partly cloudy
Synthetic turf: 140
5. Date & Time: September 20, 2007, 12 Noon,
Location: Wayland High School
Weather: Clear, calm
Synthetic turf: 142
The scientific basis for the heating up of synthetic turf fields is basic physics. Solar energy is the fundamental input causing temperature elevation in surfaces. Light energy is absorbed by the surface of the field which is made of plastic carpet blades (filaments made from polyethylene or polypropylene) usually in dark green, and infill consisting of crumb rubber or other material such as sand and thermoplastics. Solar reflectance, or albedo, is the percentage of solar energy reflected by a surface. The “cool” materials have a high solar reflectance – meaning they absorb and transfer less of the energy that reaches them. Albedo alone can significantly influence surface temperature, with the white stripe on the brick wall about 5 to 10°F (3-5°C) cooler than the surrounding, darker areas. Although solar reflectance is the most important property in determining a material’s contribution to urban heat islands, thermal emittance is also a part of the equation. Any surface exposed to radiant energy will get hotter until it reaches thermal equilibrium (i.e., it gives off as much heat as it receives). A material’s thermal emittance determines how much heat it will radiate per unit area at a given temperature, that is, how readily a surface gives up heat. When exposed to sunlight, a surface with high emittance will reach thermal equilibrium at a lower temperature than a surface with low emittance, because the high-emittance surface gives off its heat more readily.[Fn 23].
It has long been understood that surfaces made of concrete, brick, stone, tar, and asphalt used in buildings, sidewalks and roads, parking lots, other pavements and rooftops are among materials that absorb heat, retain it and then release it during radiation cooling (night) time thus contributing to the heat island effect. [Fn 24]. Synthetic turf fields (with plastic carpets and crumb rubber/sand infill) and rubberized playground surfaces (poured-in rubber) are in the same category of heat island material as concrete, asphalt and other surfaces associated with heat island effect. [Fn 25].
Most crumb rubber infill (made from used tires) is black, which we know from high school physics means they are excellent thermal radiation absorbers. Indeed, ground up tires, commonly called “crumb rubber,” contain large quantities of carbon black, which is the blackest substance known and absorbs radiation throughout the solar spectrum with high efficiency. We know from elementary physics that black surfaces will be efficient radiators of infrared energy just as they are efficient absorbers, and that the magnitude of energy radiated will be proportional to the temperature. In addition, crumb rubber infill contain great quantities of air space and rubber itself is a poor thermal conductor, making the layer of rubber infill an excellent thermal conduction insulator. [Fn 26].
Thermal energy is absorbed on the surface of the field but will not be dissipated into the mass of the field material. This allows temperatures to rise far higher than on other nearby black surfaces, such as asphalt. The asphalt mass absorbs the thermal energy and thus integrates the temperature rise over time, reducing it during the day but allowing it to remain high after the radiation input is removed at night. Temperatures on the rubber field surface, on the other hand, will drop almost immediately as solar input drops. [Fn 27].
In keeping with the requirements of the law of conservation of energy heat is obviously dissipated from the fields or the temperature would increase without bound. And just as obviously, all three thermal transport mechanisms – radiation, conduction, and convection – are involved in the cooling of synthetic turf fields. [Fn 28]. The conduction of thermal energy (heat) from the field occurs to the material beneath the surface, to the air immediately above it, and to whatever is in direct contact with the surface (in the case of playing fields, to the footwear and shoes of players). A major factor in allowing the surfaces of synthetic turf fields to reach temperature extremes is the much lower significance of conduction of energy to the material beneath the surface. Because conduction of heat from the fields to the air above it is a major cooling mechanism for the fields, conduction and convection are therefore also a major factor in warming of the air above and vicinity of the field. [Fn 29], [Fn 30].
The two major reasons for stormwater management is to prevent pollution of water resources and to protect the quantity and quality of groundwater. While mechanical aspects of artificial turf fields may keep effluents of various sizes out of the runoff, there is no separation of contaminants that are dissolved in the runoff. The leaching of substances of concern in the synthetic turf fields and micro-particulates of the same do adversely affect the quality of the water. At the same time, the underdrain system of the field carries the water away from the field where it naturally would have entered the soil, thereby reducing the quantity of the water available otherwise for groundwater recharge.
Conclusin. In view of synthetic turf fields sharing the same characteristics of impervious surfaces – that is, washing away pollutants, transporting runoff to municipal drainage systems, and registering higher-than-ambient temperatures – we ask: Shouldn’t synthetic turf fields be considered as impervious structures?
14. At this stage, it may be useful for the reader to consult Synthetic Turf Council’s “Guidelines for Synthetic Turf Base Systems” (February 2017) at page 2 (terminology) (pdf) to become acquainted with the jargon of the trade.
15. The term “structure” is defined as something made up of a number of parts that are held or put together in a particular way.” The American Heritage College Dictionary (3rd ed. 1993) at 1347.
16.Site Plan Review Special Permit (pdf) at Item No. 9 (emphasis added).
17.Schematic from Permitting Plans at page D-9 (pdf).
23. Expert Affidavit of Thomas Sciacca, dated 27 January 2020 (without Exhibits) (pdf), paras. 7-10.
24. Ibid., para. 11.
25. Ibid., para. 14.
26. Ibid., para 20.
27. Ibid., para 21.
28. Ibid., para 22.
29. Ibid., para 23.
30. For an excellent depiction of the relationship between artificial turf fields and heat island effect, see Affidavit of Camilo Perez Arrau, dated 22 April 2020 (with partial graphics), regarding “Surface temperature of synthetic and natural grass fields in North Andover Massachusetts (USA) as seen by the satellite Landsat 8 (August n19th, 2018 at 10:25am), dated 1 March 2020 (pdf).