1. Building Analytical Source Protection Areas (SPAs): Comparisons of Methods for the DWMA Version 2 & the DWMA Version 3
December, 2010

2. SPA construction approaches for the DWMA Version 2 & the current DWMA Version 3
Earlier versions of the DWMA used convex hulls as robust Source Protection Areas (SPAs) for NHD flowline networks within 15 miles upstream of drinking water intakes. With the enhanced NHD (the NHDPlus), better analytical one day upstream time of travel SPAs can be developed based on the NHDPlus value-added attributes. 2

3. DWMA Version 2 Convex Hulls Applied an Oracle Convex Hull Procedure
Start with surface drinking water facility (the blue triangle) georeferenced to the NHD
Navigate upstream and collect NHD flowlines (in red) within 15 miles (robust estimate of a day’s time of travel) of the drinking water facility point
Apply Oracle convext hul procedure to create a polygon that surrounds the end points of all the selected NHDPlus flowlines

4. Enhanced National Hydrography Dataset (NHDPlus) provides more flexible ways to develop SPAs
Includes Value Added Attributes providing estimates of flows, velocities, and time of travel through the NHD network
Provides a set of catchment polygons for each NHD flowpath
NHDPlus only became available in 2006 4

5. SPA construction for current DWMA Version 3
(1) Navigate 24 hours time of travel (TOT) upstream from georeferenced drinking water facility points along NHDPlus flowlines
(2) Identify NHDPlus catchments related to the 24 hour TOT flowlines
(3) Dissolve catchment boundaries to create 24 hour TOT SPA 5

6. The 24 Hour TOT criterion could be adjusted as EPA is able to study operational SPAs used by the States
Analytical SPAs follow watershed-based ridgelines
NHDPlus-based SPAs in many cases similar to older pre-NHDPlus convex hull SPAs
Differences more apparent in areas with high gradient typography
Some examples comparing DWMA Version 2 and DWMA Version 3 SPAs … 6

7. MD0120016 Harford County D.P.W. – moderate gradient example

8. ME0090990 Millinocket Division – high gradient example

9. MT0000215 City of Forsyth – another high gradient example

10. Building Analytical Source Protection Areas (SPAs): Comparisons of Methods for the DWMA Version 2 & the DWMA Version 3
December, 2010 10