Watershed Management for Potable Water Supply Assessing the New York City Strategy (2000) / Chapter Skim
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10 Setbacks and Buffer Zones
10 Setbacks and Buffer Zones
Pages 427-467
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From page 427... ...
Although the use of setbacks is quite common in watershed regulations across the country, little research has been done regarding the effectiveness of setbacks per se in preventing contamination of waterbodies from nonpoint source pollution. Rather, research has focused on the use of buffer zones for nonpoint source pollutant removal.
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From page 428... ...
STRUCTURE AND FUNCTION OF RIPARIAN BUFFER ZONES Riparian buffer zones refer to lands directly adjacent to waterbodies such as lakes, reservoirs, rivers, streams, and wetlands. These land areas have a significant impact on controlling nonpoint source pollution and on the associated water quality in nearby waterbodies.
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From page 429... ...
Subsurface flow 4. Groundwater flow \ 1 ~ / FIGURE 10-1 Hydrologic pathways surrounding riparian buffer zones.
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From page 430... ...
Shallow subsurface flowis much more likely to interact with riparian buffer zones than groundwater flow because it passes laterally through the root zone. In some instances, shallow subsurface flow in upstream areas can become saturation-excess overland flow by the time a buffer zone is reached (exfiltration or seepage)
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From page 431... ...
By design, when channelized flow reaches Zone 3, it is usually converted to sheet flow and subsurface flow by the hydraulically rough surface and the enhanced infiltration of the buffer zone. Coarse sediment removal from stormwater is also predominantly accomplished in Zone 3, although it can also occur in Zones 2 and 1.
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From page 432... ...
A literature review found later in this chapter discusses the extent of pollutant removal that can be achieved when rainfall runoff travels through riparian buffers. This review focuses on the pollutants of greatest concern in the New York City watersheds, including phosphorus, microbial pathogens, and sediment.
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From page 433... ...
In addition, small-scale variations in slope, woody debris, herbaceous plants, and leaf litter on the forest floor present additional barriers to overland flow. ACTIVE MANAGEMENT OF BUFFER ZONES The most important management practice for influencing functioning of buffer zones is to stabilize the hydraulic properties of stormwater so that channelized flow does not reach nearby streams.
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From page 434... ...
Converting channelized flow to sheet flow may require the installation of a structural BMP in Zone 3. For example, at sites with significant overland flow parallel to the buffer, water bars should be constructed perpendicular to the buffer at 45- to 90-ft intervals to intercept runoff and force it to flow through the buffer before it can concentrate further.
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From page 435... ...
Additional suggestions for the active management of buffer zones are given below in relation to specific activities that produce nonpoint source pollution. Agriculture In general, agricultural land uses tend to increase surface runoff and decrease infiltration and groundwater recharge in comparison to perennial vegetation such as forest or grassland.
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From page 436... ...
By contrast, a large proportion of agricultural fields or urban areas can generate overland flow and associated nonpoint source pollution because of changes in soil surface conditions. Riparian forest buffers are subject to special operating restrictions, often specified by state forest practice acts, to minimize undesirable changes in site conditions.
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From page 437... ...
In many cases, no amount of management will allow riparian buffers (in the absence of other BMPs) to convert this channelized flow to sheet flow.
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From page 438... ...
The activities for which setback distances are proscribed are not all-inclusive. For example, agriculture, a contributor of nonpoint source pollution, is specifically excluded from setbacks.
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From page 439... ...
a New aboveground and underground petroleum storage facilities with NYC DEP registrations New home heating oil tanks installed underground New aboveground and underground petroleum storage tanks >185 gallons without NYC DEP registrations Subsurface discharge from a wastewater treatment plant (WWTP) Absorption field from new septic tanks Raised septic systems Impervious surfaces (basic guidelines only)
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From page 440... ...
EFFECTIVENESS OF THE MOA SETBACKS Several approaches can be used to evaluate the effectiveness of the setback distances prescribed in the MOA, some of which were used in the 1993 environmental impact statement (EIS) prepared for the Watershed Rules and Regulations.
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From page 441... ...
Courtesy of the NYC DEP. yet been accomplished anywhere in the New York City watersheds, primarily because of the newness of the Watershed Rules and Regulations and because it is not a stated goal of the NYC DEP's monitoring program.
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From page 442... ...
If combined with models of reservoir water quality, setback models can potentially be used to determine the net effect of setbacks on reservoir health. However, no such data on pollutant concentrations in runoff for the Catskill/Delaware watershed are available.
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From page 445... ...
36) was adopted, establishing a "primary buffer zone" extending 400 It from either reservoir and 200 It from any
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From page 446... ...
(10-1) Other watershed programs in North Carolina consider vegetation as well as slope when calculating setback distances.
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From page 447... ...
Although this means that the provisions may be relatively easy to enforce, they may also be less protective of nearby waterbodies, especially in areas with steep slopes and where vegetation is less capable of dissipating rainfall runoff and sequestering pollutants. Literature Review of Pollutant Removal in Buffer Zones A limited body of research quantifies the effectiveness of buffer zones in removing pollutants such as nitrogen, phosphorus, sediment, pesticides, and some microbes from storm and drainage water.
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From page 448... ...
However, the wide range of reported removal efficiencies makes it impossible to predict the adequacy of these setback distances. The only conclusion that can be drawn is that if managed or maintained as buffer zones, the MOA setbacks may approach the removal efficiencies found in Table 10-3.
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From page 449... ...
(1983) hardwood subsurface flow forest budgets Bacteria Most research on microbial removal by buffer zones has concentrated on fecal coliform bacteria as indicators of human and animal contamination of natural waters.
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From page 450... ...
An overloaded filter 75 It in length showed no significant filtration of either fecal coliform or fecal streptococcus bacteria in stormwater from a dairy barnyard (Schellinger and Clausen, 1992~. These studies all support the conclusion that bacteria in overland flow can move through buffer zones without significant reduction.
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From page 451... ...
Because the decay rate of viruses increases with longer residence times, greater than 300 It of buffer zone may be needed to sufficiently reduce virus levels in overland flow. The lack of data on removal of nonbacterial microbes in buffer zones makes any evaluation of the MOA setbacks regarding these pollutants highly speculative.
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From page 452... ...
Landfill Leachate There have been no published studies on removal of pollutants from landfill leachate via buffer zones of any kind. However, by examining the typical constituents of landfill leachate, it may be possible to draw conclusions about the ability of buffer zones to remove some constituents.
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From page 453... ...
Because of the wide and unpredictable variability in landfill leachate composition, evaluating the 250- and 1,000-ft setbacks in the MOA is necessarily limited. Phosphorus and nitrogen compounds, for which buffer zone studies have been conducted, can be expected to be removed along the setback distances; however, it is impossible to draw conclusions about the ability of the setbacks to greatly reduce concentrations of other chemicals.
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From page 454... ...
Expert Panel Recommendations To gain further insight into the MOA setbacks, 12 experts on buffer zone structure and functioning in the United States and Europe were polled for their opinions (see the Preface)
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From page 455... ...
has shown that buffer zone roughness (which corresponds to the amount of vegetation present) , soil hydraulic conductivity, and soil moisture can also significantly affect buffer removal efficiencies.
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From page 456... ...
Various modeling approaches, primarily Darcy's Law for subsurface flow and Manning's equation for surface flows, were used to estimate pollutant travel times or other measures given certain setback distances, slope, hydraulic conductivity, porosity, roughness, and rainfall intensity. Table 10-5 presents the travel-time results of the EIS analyses for each of the setback provisions considered.
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From page 457... ...
Source: NYC DEP (1993)
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From page 458... ...
Changing setback distances, slope, and hydraulic conductivity had minimal effect on pollutant removal. Committee Analysis Because the analysis in the EIS generates travel times rather than pollutant removals for almost all categories of setbacks considered, its use in predicting the effectiveness of the New York City setbacks is limited.
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From page 459... ...
Locations where both slope and hydraulic conductivity are high are rare since high-conductivity sand and gravel deposits typically occur in valley bottoms with limited gradients. Conductivity and gradient values less than 1 m/day and 1 degree, respectively, yield estimated travel times of hundreds or thousands of days.
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From page 460... ...
The analysis also assumes no overland flow in or through the riparian zone. Overland flow is uncommon but can occur when soil frost or compaction reduces
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From page 461... ...
, overland flow short-circuits the subsurface flow path and decreases travel time. The analysis assumes complete saturation of the soil mantle.
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From page 462... ...
NYC DEP should create incentives for managing setback areas to meet the following goals: · Setbacks function most effectively as buffer zones if runoff entering the setbacks is sheet flow rather than channelized flow. Thus, best management practices such as filter strips, bioretention areas, and level spreaders should be installed upslope from buffer zones to create sheet flow in areas prone to significant concentrated flow.
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From page 463... ...
In order to evaluate the effectiveness of particular setback distances, significantly more detailed data are needed on land use and pollutant transport through buffer areas to recipient streams and reservoirs. Performance monitoring of riparian buffer zones, in which shallow subsurface pollutant concentrations are measured above and below the buffer zone, is required.
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From page 464... ...
1989a. Vegetative filter strips for agricultural nonpoint source pollution control.
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From page 465... ...
1997. Water quality functions of riparian forest buffers in Chesapeake Bay watersheds.
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From page 466... ...
1989a. Nonpoint source pollution control effectiveness of riparian forests along a coastal plain river.
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From page 467... ...
1989. Septic tank setback distances: A way to minimize virus contamination of drinking water.
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