NHDPlus-based Tools for Drinking Water Source Protection
Areas with the Drinking Water Mapping Application (DWMA)
Contact: Roger Anzzolin
Office of Ground Water and Drinking Water
Drinking Water Protection Division
U.S. Environmental Protection Agency
E-Mail: anzzolin.roger@epa.gov
Phone: 202-564-4093
Background on the Drinking Water Mapping Application (DWMA)
The U.S. Environmental Protection Agency’s Office of Ground Water and Drinking
Water (OGWDW) is applying state-of-the-art, web-based mapping and database
technology to enhance Agency capabilities to identify major contaminant risks to public
drinking water supplies. OGWDW’s Drinking Water Mapping Application (DWMA) is a
Web-based geospatial application that enables queries of the Safe Drinking Water
Information System (SDWIS/FED), as well as the identification of potential contaminant
risks to surface water and groundwater used for public drinking water supplies. The
DWMA provides a secure application that EPA staff can easily use to obtain reports and
maps that help manage programs under the Safe Drinking Water Act (SDWA).
The DWMA makes extensive use of the NHDPlus to locate drinking water surface intake
locations within the NHD network and to define source water areas relative to the intake
facilities. Source water areas are the land areas that contribute water (and pollutants) to
the surface water intakes or groundwater wells that supply drinking water. Because there
are more than 8,000 surface water intakes associated with public water systems in the
United States, EPA has developed techniques to automate the process of building
analytical surface Source Protection Areas (SPAs).
Enhancements to SPAs Based on the NHDPlus
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 as an
approximation to 24 hours time of travel. The
DWMA Version 3 takes advantage of new content
and functionality in the NHDPlus to provide
significant enhancements for analytical SPAs related
to surface intakes. The NHDPlus “value-added
attributes” (VAAs) provide more precise tools for
upstream/downstream navigation on the NHD’s
network of flowlines, and the catchments defined for
each initialized flowline can be combined to produce
genuine watershed-oriented SPA polygons.
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Production Steps for Two Adjacent Analytical Source Protection Areas
A series of GIS maps illustrate how the new NHDPlus-based SPAs are created using
upstream navigation techniques to select
catchments within a day’s (24 hours) time of
travel from drinking water intake facilities.
An example shows the results for two
intakes whose SPAs reflect a pair of adjacent
watershed polygons. Using the NHDPlus,
flowline networks are built navigating
upstream one day (24 hours) time of travel
from the downstream pour point of a
drinking water intake georeferenced as a point to the NHD.
NHDPlus catchment polygons are then selected related to the one day time of travel
flowline networks.
Catchment boundaries are dissolved to create the final analytical Source Protection Area
polygons for the DWMA Version 3.
Summary
Basing the DWMA SPAs on NHDPlus catchments enhances the analytical precision of
these watershed-oriented polygons for a wide range of geospatial analyses. Cross
program comparisons using the SPAs, and the locations of the underlying drinking
water facilities, against NPDES discharge points, waste sites, and other potential
contaminant risks provides a valuable screening tool to promote the source water
protection goals of the Safe Drinking Water Act.
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