Kendall Webster – Assignment 5
Goal of Analysis
The Alewife Brook has been listed by the EPA as an “impaired water body” for the following pollutants
which probably originate from storm water runoff: metals, nutrients, oil and grease. The State of
Massachusetts is required by the EPA to create a total maximum daily load (TMDL) plan for this water
body. According to the EPA TMDL web site, a TMDL “is a calculation of the maximum amount of a
pollutant that a water body can receive and still safely meet water quality standards”.
The discipline of low impact development (LID) offers several solutions for sequestration and treatment
of storm water runoff. Many of these solutions involve retrofits of impervious surfaces in order to allow
storm water to infiltrate into the aquifer rather than running off contaminated surfaces such as roads,
lawns and roofs directly into a receiving water body.
The analysis will use basic queries in GIS to assess the potential for urban retrofits for reducing storm
water runoff in environs surrounding the Alewife Brook. These queries will apply to the towns of
Belmont, Alewife and Somerville.
Layers used: Named Streams and Rivers, LANDUSE2005_POLY, TOWNS_POLY
Steps
Step 1
For the area selected for this project there were 29 different land use types in the LANDUSE_POLY layer.
In order to simplify this layer, I ranked these land use types into levels of permeability. First I created an
additional field in the LANDUSE_POLY layer attribute table (Figure 1). Then I used the select by attribute
tool to group the land use types in LANDUSE_POLY into the categories listed in Table 1.
Figure 1. Additional attribute field with cover category rankings
Table 1. Land use cover categories
Rank
Category
1
Impervious_Residential
2
Impervious_Other
3
Semi-pervious_Residential
4
Semi-pervious_Other
5
Pervious
Cover Type
high density residential;
medium density residential;
multi-family residential
commercial; industrial;
junkyard; transportation;
urban public/institutional;
waste disposal
low density residential; very
low density residential
cemetery; cropland; golf
course; marina; nursery;
participation recreation;
spectator recreation;
transitional
brushland/successional; forest;
forested wetland; nonforested wetland; open land;
orchard; pasture; saltwater
sandy beach; water; waterbased recreation
Color
Dark yellow
Red
Light yellow
Rose
Green
Step 2
Then I used the field calculator to group the selected attributes of the LANDUSE_POLY layer in to
spatially represented categories (some of these categories are visible in Figure 1). Figure 2 depicts the
result of Step 2, the colors in the map are explained in Table 1.
Figure 2. Map for Step 2
Step 3
Then I identified the impervious residential cover surrounding Alewife Brook that would be able to
absorb storm water if retrofitted using LID principles. Using the select by location tool, I identified the
residential impervious cover within 100 feet of the Alewife Brook (Figure 3). (This distance was
arbitrarily chosen.) According to the Massachusetts Draft Regulated Impervious Area General
Stormwater Permit, any retrofits to impervious cover should decrease impervious surfaces by 40%. So
the area selected around the Alewife Brook, should be required to reduce impervious cover by 40%. This
selection is depicted in Figure 4.
Figure 3. Selection procedure for identifying residential impervious retrofit buffer
Figure 4. Residential impervious retrofit buffer
It is clear from Figure 4 that the residential impervious cover layer won’t allow selection of exactly a
100-foot retrofit buffer zone. I’m not sure why this is, it may have something to do with the definition of
the polygons. For instance, to make the buffer exact, the select by location tool would have to slice
through polygons which it is unable to do.
Step 4
I used the statistics tool to calculate the sum of the total area within the supposed 100-foot buffer
around Alewife Brook. This sum, displayed in Figure 4, includes all cover types in the buffer zone. This
represents all the land use cover types in the buffer zone for which LID retrofits would be beneficial.
Figure 5. Calculation of sum of land use in the buffer zone
Step 5
I used the summarize tool to calculate the area of the 5 types of land use cover within the buffer zone
(Figure 6). Because there were no semi-pervious residential polygons within the buffer zone, the
summarize tool skipped that category. However, it looks as though the area is highly impervious.
Caveats
This analysis is limited by the nature of the LANDUSE2005_POLY layer. The polygons for this layer were
created from aerial photographs which ArcGIS is unable to divide (for instance, ArcGIS cannot split the
polygons in order to create a 100-foot radius around Alewife Brook).
The LANDUSE2005_POLY layer was sectioned into categories at the beginning of this project. The
actually imperviousness of each category and/or parcel may be highly variable. For example, I had no
idea whether the category “spectator recreation” was impervious or not. The LANDUSE2005_POLY layer
may not be ideal for assessing impervious cover.
Also, the pollutants metals and oil and grease originate from road runoff. These pollutants may not be
significantly reduced by residential retrofits. Some other LID techniques would be more appropriate to
mitigate road runoff.
Also, cities and towns cannot use zoning reform to require property owners to install LID retrofits to
reduce storm water. Requiring changes to existing structures through zoning is sticky business, so
requiring retrofits to all impervious residential land uses within this buffer may not be the best way to
reduce storm water discharge to Alewife Brook.