Vegetated buffers adjacent to wetlands and stream channels up to 50 feet in width provide substantial benefits for protecting and enhancing water quality, based on a review of the published literature on wetlands and other regulated waters. State, regional, and local regulations vary greatly in their requirements for buffer widths. In four counties of Southeastern Pennsylvania, regulatory requirements for stream and wetland buffers vary from zero to 300 feet wide. Many of these regulations do not have published or stated goals and objectives supporting the choice of buffer widths or provide a scientific rationale for the width chosen.
ENTRIX reviewed 137 published scientific papers, written over the preceding 40 years, on riparian and wetland buffer widths, and on the subject of providing “ecosystem services”: the processes by which the environment produces resources, such as clean water. We attempted to establish a scientific rationale for selecting buffer widths for streams and wetlands. We also hoped to find a uniform buffer width, which could satisfy resource protection requirements for most buffer applications. The published studies were grouped by types of ecosystem services provided, buffer widths examined, and capacities for protection of water quality. For streambank stability, temperature control, minimization of direct impacts, and pollutant removal capacities, substantial benefits are achieved within the first 50 feet of vegetated buffer width. Marginal increases in benefits may accrue when buffer widths are increased beyond 50 feet. The application of some stormwater best management practices (BMPs), when used in conjunction with riparian and wetland buffer strips, can result in a significant increase in water-quality benefits from vegetated buffers less than 50 feet in width. Variable-width buffers and the use of “buffer averaging” can result in significant benefits to water quality from vegetated buffers that are less than 50 feet in places.
Pennsylvania does not have a statewide standard set of buffer requirements for riparian habitats or other wetlands, although a proposed rule would require a 50-foot standard buffer (PA DEP 2005). Wetland and riparian buffer widths are instead decided at a county or township level. In four counties of southeastern Pennsylvania (Chester, Bucks, Montgomery, and Delaware) there are 278 municipalities creating a multitude of state, regional, and local jurisdictions, each with various requirements for set-aside, protection, or creation of riparian and wetland buffers. For example, East Pikeland Township requires 300 feet (S&LDO 2004), while Bensalem Township requires 20 to 100 feet (NRP 2005). Developers of small- to mid-sized residential communities and office/commercial campuses must therefore negotiate many different levels of political jurisdictions to remain in compliance with these regulations. Based on a review of regulations alone, it is unclear whether differences in buffer widths among municipalities were developed to represent real geographic variation in the contribution of buffers to ecological function. Jurisdictions do not necessarily appear to have previously established or defined their goals for maintaining water quality or other ecosystem services prior to setting requirements for buffer width.
Because the range of widths adopted by the many jurisdictions in southeastern Pennsylvania does not appear to have a scientific basis, ENTRIX questioned the scientific validity of these buffer widths as they relate to riparian and wetland resource protection. In this study, we attempted to (1) review existing buffer regulations in southeastern Pennsylvania and their basis in ecosystem value protection, (2) review and summarize the scientific and technical literature addressing buffer widths and compositions and how changes in these variables affect ecosystem function, and (3) review and summarize literature addressing the use of BMPs in conjunction with vegetated buffers to create a “treatment train” that would improve water quality and increase protection of ecosystem values. This article expresses the major finding and conclusions of this review.
Material and Methods
ENTRIX reviewed existing environmental regulations in southeastern Pennsylvania and published scientific literature. We began the analysis by reviewing local environmental ordinances for municipalities in Chester, Bucks, Montgomery, and Delaware Counties that specified wetland or riparian buffer requirements.
ENTRIX collected and reviewed 137 published studies and reports relating to riparian and wetland buffer widths and vegetative composition. These studies included primary literature as well as technical reports and review papers on the subject. Where possible, we verified conclusions in review papers and technical reports by evaluating references cited in those papers. Literature reviewed was ranked for relevance to riparian and wetland areas in southeastern Pennsylvania. Ecological services provided by riparian buffers were identified. The published studies were divided into categories for the types of ecosystem services provided, the buffer widths examined, and the capacities for protection of water quality and other ecosystem services. Ranges of buffer widths from relevant papers were recorded for each of the ecological services.
This review focused on studies that were applicable to residential development, suburban office and commercial development, and the conversion of rural land use. This study did not consider large-scale commercial development or associated BMPs. ENTRIX considered the following ecosystem services preformed by buffers:
- Temperature control
- Streambank stability and sediment control
- Minimization of direct human impact
- Removal of total suspended solids
- Maintaining surface water supply and quality
- Nitrogen removal
- Phosphorus removal
- Removal of pesticides
- Removal of bacteria
- Removal of metals
Buffers provide additional ecosystem services, including providing habitat for water- or wetland-dependent wildlife, but an evaluation of buffer width on these ecological services is beyond the scope of this study; this study on focused the relationship between buffer widths and water quality.
Results of our literature search were compiled into a matrix listing the ranges of effective buffer widths for each of the ecosystem services. The matrix also reflected criteria relevance ranking and interpretation of effective buffer width ranges for each ecosystem service. Ranking criteria were developed based on (1) applicability to small-scale residential and commercial development, (2) similarity to southeastern Pennsylvania, (3) quality of research, (4) publication level, and (5) test parameters.
A second literature search was performed to evaluate the effectiveness of common stormwater BMPs combined with the construction and maintenance of wetland and riparian buffers.
Results
Jurisdictions and Regulation of Environmental Factors in Southeastern Pennsylvania. Pennsylvania does not have a state minimum buffer width outside the Dam Safety and Encroachment Act of 1979, which requires that dams be set back 300 feet from “important” wetlands and water courses. The Riparian Buffer Ordinance Act Senate Bill #453 (Rafferty 2007), if approved, would require a 35- to 100-foot-wide buffer adjacent to each stream or nontidal wetland. This rule would apply to any activity where a building, grading, or encroachment permit is needed or when zoning board, subdivision/land development, or conditional-use approvals are needed.
In the absence of state-level rules, buffer widths are determined on the county and township jurisdictional level. Local buffer requirements range greatly in southeastern Pennsylvania from no buffer requirements to 300 feet (e.g., S&LDO, 2004). Local environmental approvals that may require riparian and wetland buffers may include Sediment Erosion and Sediment Control Plans, County Grading Permits, and Nontidal/Tidal Wetland Permits.
Published Literature: Buffer Widths and Ecosystem Values. Tables 1 and 2 show recommended buffer widths for physical and chemical ecosystem services.
- Temperature Control. Vegetated buffers bordering rivers, streams, and wetlands help maintain lower water temperatures in summer and reduce temperature decreases in the winter by providing shade and cover. The relationship between buffer width and water temperature control is determined by vegetation type, height, density, and cover. Broderson found that forested buffers 50 feet in width provided adequate shade for small to moderate streams and that buffer widths along slopes could decrease with increasing tree height with no significant loss of shade (Broderson, 1973 in Castelle et al. 1994). Buffers as narrow as 10 feet provide adequate temperature control if vegetation is tall enough to shade the wetland or water body (Brazier and Brown 1973). Vegetated buffers 75- to 100-feet-wide stabilized temperatures when vegetation was not tall enough to create direct shade or cover (Lynch et al. 1985).
- Streambank Stability, Erosion Control. Vegetated buffers stabilize streambanks with plant root structures providing tensile strength and forming a physical barrier. Roots maintain soil structure and physically restrain otherwise erodible soils (Castelle et al. 1994). Vegetation reduces the channelization of water and slows surface water flow, reducing erosion and soil transport. The relationship between buffer width, streambank stability, and erosion control is based on soil type and erodibility; vegetation type, density and cover, and bank slope.
Ghaffarzadeh et al. (1992) examined sediment removal by grass vegetated filter strips ranging from zero to 60 feet in width on 7% and 12% slopes. They found no difference in performance after 30 feet on either slope, at which point 85% of the sediment was removed; furthermore, they found no difference in sediment removal after the first 10 feet (Castelle et al. 1994). Young et al. (1980) found that an 80-foot vegetated buffer reduced the suspended sediment in feedlot runoff by 92% (Castelle et al. 1994). In a study by Lynch et al. (1985), a 98-foot buffer between actively logged areas and wetlands and streams removed approximately 75% to 80% of the suspended sediment in stormwater.
- Minimize Degradation From Direct Human and Livestock Impact. Buffers protect wetlands and waterbodies from direct human and livestock impact by limiting access and reducing noise, light, odors, and debris. Adjacent land use type accounted for much of the variation found in the level of human (Shisler et al. 1987, Castelle et al. 1994) and livestock (Wenger 1999).
- Nutrient Removal. While many studies confirm the value of forested and nonforested buffers in removing nutrients from runoff (e.g., Doyle et al. 1977, Lynch et al. 1985, Shisler et al. 1987, Hubbard and Lowrance 1992, Palone and Todd 1997, and Jackson 2006), the literature varies widely in its description of the relationship between buffer width and nutrient removal function. Dillaha et al. (1989) reported that 15-foot-wide vegetated areas removed an average of 70% of suspended solids, 61% of phosphorous, and 54% of nitrogen, while 30-foot-wide areas removed an average of 84% suspended solids, 79% of phosphorous, and 73% of nitrogen. Madison et al. (1992) found similar results using simulated one- and 10-year storm events, and did not record increased nutrient removal as buffer widths increased beyond 30 feet. Riparian buffers between 5 and 15 feet wide removed significant proportions of total nitrogen, although wider buffers more consistently removed nitrogen entering the riparian zone (Mayer et al. 2005). Nitrogen reductions of 25% to over 90% of total loadings occur in the first 35 to 90 feet of forested buffers (Palone and Todd 1998).
Nieswand et al. (1990) determined that slope and width were the main factors influencing the effectiveness of buffers in trapping sediment and associated pollutants. They developed a simple formula for determining width based on a modified Manning’s equation. The equation uses a constant “50 feet” based on common buffer recommendations, with the assumption that a 50-foot buffer at 1% slope provides adequate protection to streams (Nieswand et al. 1990).
- Pesticide Removal. Hatfield et al. (1995) found that 40- to 60-foot-wide grassy buffers removed 10% to 40% of the atrazine, cyanazine, and metolachlor from runoff. Arora et al. (1996 in Wenger 1999) found that 66-foot-wide riparian buffers at 3% slope retained from 8% to 100% of the herbicides (atrazine, metolachlor, and cyanazine) that entered during storm events. Payne et al. (1988) found that a 65-foot-wide forested buffer reduced permethrin spray cloud dispersal reaching surface waters to concentrations protective of fish.
- Bacteria Removal. Riparian buffers can trap waste transported in surface runoff in the same way that they trap sediments and associated nutrients (Wenger 1999), where pathogenic microorganisms can be exposed to oxygen and sunlight and either destroyed, or their concentrations reduced. Buffers as narrow as 12.5 feet can reduce fecal coliform levels in runoff (Doyle et al. 1977, Coyne et al. 1995).
- Total Suspended Solids (TSS) Removal. Vegetated buffers remove suspended solids from runoff, with the rate of removal affected by buffer width, vegetative composition, adjacent land use, and slope. Grassed areas 15 and 30 feet wide removed 70% and 84% of TSS (Dillaha et al. 1989). However, Ghaffarzadeh et al. (1992) found that vegetated filter strips removed 85% of sediments within the first 30 feet of width, on slopes between 7% and 12%, with no additional removal up to a total of 60 feet on either slope. Similar results were observed by Peterjohn and Correll (1984) in a riparian buffer in the Middle-Atlantic coastal plain. Lynch et al. (1985) found that a 98-foot-wide buffer strip removed 75% to 80% of suspended sediment from runoff from an upslope logging operation.
- Metals Removal. Grassy swales and other vegetated areas can be effective in removing lead and other pollutant metals from runoff. Herson-Jones et al. (1995), citing data from a 1992 study by the Metropolitan Seattle Water Pollution Control District that found removal rates exceeding 40% for lead (Pb) and 60% for copper (Cu), zinc (Zn), and iron (Fe), concluded that urban buffers have a moderate to high ability to remove or retain hydrocarbons and metals from surface runoff. Studies in Rhode Island (Groffman et al. 1991 in Wenger 1999) also found high metal retention rates.
Published Literature: BMPs. In addition to or in concert with buffers, the operation of wet and dry retention areas and other BMPs to collect and treat stormwater can be protective of riparian and wetland ecosystem values. Stormwater management BMPs trap and remove nutrients and other contaminants in runoff. Horner and Mar (1982), reported that a 200-foot-wide grassy swale removed 80% of suspended solids and total recoverable lead (Pb), and a study by Young et al. (1980) found that a 200-foot-wide grassy area reduced fecal coliform by 87%, total coliform by 84%, and biological oxygen demand (BOD) by 62%.
The Center for Watershed Protection (2007) analyzed data in the National Pollutant Removal Performance Database, version 3, for sediment, nutrient, and other pollutant removal efficiencies for various types of BMPs, including dry ponds, wet ponds, wetlands, filtering practices, bioretention, and open channels. BMPs removed between 29% and 86% of most metals, between 24% and 67% of nitrogen, 20% to 65% of phosphorus, and between 48% and 89% of TSS. Removal efficiencies for all bacteria ranged between 37% and 88%. The International Stormwater Best Management Practices Database (ISBMP 2008) analyzed data on metals removal by stormwater BMPs; the BMP classes included detention pond (EDD), wet pond, wetland basin, biofilter (bioretention), media (sand) filter, hydrodynamic devices, and porous pavement. BMPs ranged between 40% and 54% removal for most metals. Removal efficiency for all nitrogen ranged between 22% and 30%, while removal efficiencies for all phosphorus ranged between 28% and 39%. Removal efficiency for total suspended solids ranged between 63% and 71%.
BMPs can be combined with vegetated buffers to provide pretreatment for stormwater runoff. Stormwater treatment trains consist of several BMPs to control stormwater quantity and quality in an integrated planning and design approach whose components work together to limit the adverse impacts of urban development on downstream waters and riparian areas. When considered comprehensively, a treatment train consists of all the nonstructural and structural controls that work to attain water-quality and -quantity goals.
The Charlotte Mecklenburg (North Carolina) Office of Stormwater Services developed a pilot program in which 24 different BMP treatment trains were selected as being the most common for application in the Mecklenburg County (North Carolina Piedmont) area. Detailed water-quality modeling was performed on those 24 BMP combinations to determine the combined pollutant removal capability. Treatment trains consisted of two or three BMPs in series, and included bioretention, treatment wetlands, grassed swales, sand filters, buffer strips, extended dry detention, wet ponds, infiltration trenches, and enhanced grassed swales. Pollutant removal efficiencies for TSS and total phosphorus (TP) were determined for each of the 24 combinations. The stormwater treatment trains tested removed an average of 87% of TSS, with most combinations removing 90% and above, and 78% of TP, with many combinations in excess of 80% (CharMeck SWS 2008).
Discussion
The majority of published studies and technical reports supports the conclusion that vegetated buffers adjacent to wetlands and stream channels provide substantial benefits for protecting and enhancing water quality. For streambank stability, temperature control, minimizing degradation from direct impacts, and pollutant removal capacities, substantial benefits are achieved within the first 50 feet of vegetated buffer width. Marginal increases in benefits may accrue when buffer widths are increased beyond 50 feet.
Vegetated buffers are most protective of riparian and wetland resources when their creation and maintenance is coupled with other stormwater BMPs, such as the creation of wet and dry retention areas. Treatment trains consisting of several BMPs to control stormwater quantity and quality provide an integrated approach to limit the adverse impacts of residential and commercial development on downstream waters and riparian areas (CharMeck SWS 2008).
The use of BMP treatment trains to improve and enhance water quality prior to the site runoff reaching the riparian and wetland buffer potentially limits the need for buffers to trap or filter contaminants; therefore, buffer width can be reduced below 50 feet when paired with well-designed BMPs, when buffer goals include removal of sediment, nutrients, and other contaminants. “Buffer averaging” consists of the use of a variable-width buffer, in which some sections are greater than the required width, and some sections are less than the required width, but the average square footage for the total area is satisfied. Widths are chosen based on appropriate site conditions and site design for residential (Figure 1) and commercial (Figure 2) developments. Using this approach, buffers and BMPs can be configured to accommodate various development layouts while still protecting wetland and riparian ecosystem values.
Riparian and wetland buffer requirements found in the regional and local ordinances of southeastern Pennsylvania do not appear to be based on the conclusions of these scientific studies. However, science-based buffer requirements have been recommended in other eastern US regions. For example, 25- to 50-foot-wide averaged buffers have been recommended for Rhode Island (Roman and Good 1985, Palmstrom 1991), and buffers less than 50 feet wide used in conjunction with BMPs have been deemed effective in Mecklenburg County, NC (CharMeck SWS 2008). Well-planned developments incorporating “averaged” vegetated buffers of 50 feet or less, combined with shared BMP treatment trains, may be more protective of riparian and wetland ecosystem values than the much larger buffers required by some regional and local regulations.