In Proceedings of the Coastal Water Resource Conference, John R. Lesnik (editor),
American Water Resources Association, Middleburg, Virginia,
TPS-02-1, 2002, 31-36.
THE VULNERABILITY OF PUBLIC WATER SYSTEMS TO SEA LEVEL RISE
John Furlow, Joel D. Scheraga, Randall Freed, Ken Rock1
ABSTRACT: Global average sea level is rising more rapidly as a result of climate change, posing risks to estuaries,
aquifers, wetlands, lowlands, beaches, and infrastructure. The U.S. EPA has initiated a study to assess the potential impacts
of sea level rise on drinking water systems along the Gulf and Atlantic Coasts in the U.S. This paper reports on the first
phase of the study, which is intended to identify systems whose intakes are vulnerable to sea level rise. A multi-stage
screening process and vulnerability assessment was developed and applied to a sample of about 500 systems. Results
suggest that several million people are served by coastal surface water systems that are unprotected (by a dam or other
structure) from sea-level rise. Five surface water systems serving over 100,000 people are ranked highly vulnerable to salt
water intrusion, meaning they are unprotected and within a tidal fresh-water reach with estuarine wetlands nearby
(indicating slightly saline water a short distance downstream).
KEY TERMS: sea level rise, salt-water intrusion, climate change, wetlands, drinking water, infrastructure.
INTRODUCTION
Global average sea level is rising more rapidly as a result of climate change, posing risks to estuaries, aquifers,
wetlands, lowlands, beaches, and infrastructure (National Assessment Synthesis Team, 2000). Observations indicate that
sea levels have risen from four to eight inches in the 20 th century, and the models used in the U.S. National Assessment
indicate the possibility of an additional rise of eight to 15 inches over the 21 st century (National Assessment Synthesis
Team, 2001). Changes will vary locally, and impacts will vary as well.
The U.S. EPA is conducting assessments of the possible consequences of climate change and climate variability on
water quality. Our initial focus in these assessments has been on the ability of drinking water systems to continue to provide
clean, safe water as climate changes. One area of concern is the possible impact of sea level rise and salt-water intrusion on
community water systems. In an effort to assess the potential magnitude of the problem, we developed a screening
methodology to identify systems that are potentially vulnerable.
Factors other than sea level rise will impact the vulnerability of freshwater intake points to salt intrusion. For surface
water systems, river flow, the distance of the intake point from the salt front, and withdrawal rates are among the factors
that can influence vulnerability and complicate the evaluation of that vulnerability. For ground water systems, withdrawal
rates, the depth and gradient of the aquifer, and recharge rates affect vulnerability. Even within a small geographic area,
these factors vary, making it difficult to provide reliable estimates of impacts for the large population of coastal drinking
water systems. Multiple factors affect the vulnerability of freshwater intake points at any one location and local changes in
sea level deviate from the global average. Therefore, in order to begin identifying those systems that are most vulnerable,
we developed a multi-tiered screening process to rank vulnerability (see Table 1).
Approach
We realized early that we would have to limit the scale of our assessment. Data and resource limitations dictated a
scoping exercise that would evaluate the potential magnitude of the problem. We decided to focus initially on surface water
systems both because they typically serve larger populations than ground water systems (and thus have higher resource
value) and because information needed to assess their vulnerability is more readily available than for ground water systems.
We limited the geographic range of the study to the Gulf and Atlantic Coasts, where most of the country’s low-lying areas
1
Furlow is with the Global Change Research Program within the Office of Research and Development in the U.S.
Environmental Protection Agency, 1200 Pennsylvania Av. NW, MC 8601-D, Washington, DC, 20074, Phone: (202) 5643388, Fax: (202) 565-0075, E-Mail: furlow.john@epa.gov. Scheraga is National Program Director, EPA Global Change
Research Program. Freed and Rock are with ICF Consulting. The views expressed are the authors’ own and do not reflect
official EPA policy.
vulnerable to sea level rise are found. The screening analysis was designed to identify systems withdrawing water from
tidal, freshwater reaches of rivers. Though these situations are not common, they represent systems vulnerable to sea level
rise (as well as more frequent droughts, another possible effect of climate change) due to the potential for the intake water
to become brackish.
The principal source of information used to determine the locations and characteristics of water supplies was BASINS
(Better Assessment Science Integrating Nonpoint and Point Sources) (http://www.epa.gov/waterscience/basins/). It
includes latitude and longitude coordinates for public water system (PWS) intake points. The Safe Drinking Water
Information System (SDWIS) (http://www.epa.gov/safewater/sdwisfed/sdwis.htm) includes information on more PWSs,
but its location identifiers are expressed in terms of the community served, rather than the intake point. (This poses a
problem in cases where the source water and the community are not near one another. For example, New York City, a
coastal community, is served by inland lakes.) BASINS does not include all PWSs, further limiting the breadth of our
study. A PWS is an official designation given to systems that have at least fifteen service connections or regularly serves at
least twenty-five individuals (http://www.epa.gov/safewater/pws/pwsdef2.htm).
Once we established the domain of PWS intakes for consideration (surface water systems on the Gulf and Atlantic Coasts
and covered by BASINS), we began a screening process to rank their vulnerability. The first cut was to eliminate all
systems above the “fall line.” The fall line is an imaginary line that forms a boundary where upland rivers drop to the
coastal plain, forming waterfalls and related navigational barriers. We chose this line as a first cut in the screening process,
assuming that a waterfall would pose a physical barrier to protect upstream intakes. To identify intakes below the fall line,
we used Geographic Information System (GIS) techniques to digitize the fall line, based on the 1946 USGS map, “Physical
Divisions of the United States.” We then overlaid the intake points as identified in BASINS. There were 933 PWS locations
below the fall line, from Texas to Massachusetts. Of these, 416 were identified in BASINS as surface water sources.
Another 50 without a source designation were assumed to have surface water sources because of their names (e.g., a system
with “lake” or “reservoir” in the title) (Figure 1).
Table 1: Number of Public Water Systems after Each Screening Stage.
Number of PWSs
933
466
416
156
57
39
25
Description of Systems Remaining
PWSs below fall line and in BASINS
Surface water and suspected surface water systems
Known surface water systems
Low lying systems
Unprotected systems
Unprotected systems not on Mississippi River
Somewhat vulnerable systems with data available. See Table 2
Figure 1: Surface Water Intakes Below Fall Line.
The next step was to sort the sites by elevation class, using 90 meter USGS Digital Elevation Models for the entire
coast. Each intake point was labeled according to the following code: 0 = below 0.5 meters; 1= 0.5-1.5 meters; 2 = 1.5 to
2.5 meters; 3 = 2.5 to 3.5 meters; 4 = above 3.5 meters. Although the elevation data apply to the average elevation of the
land in a 90 meter-square area around the intake and not to the intake itself (which is below the level of the land), for
screening purposes we considered intakes below 3.5 meters elevation to be in low-lying areas and therefore vulnerable to
sea level rise. This screening step yielded 156 low-lying, coastal surface water intakes.
Working with these surface water intakes in low-lying areas, we then screened for intakes protected by a dam. BASINS
contains data on dam locations, based on a 1980 census by the U.S. Army Corps of Engineers. This screen indicated 57
surface water intakes that appear to be unprotected. Next, we decided to eliminate systems with intakes on the Mississippi
River and its tributaries, based on the assumption that the flow of the Mississippi is such that salt is not likely to intrude
upstream, leaving systems along the river at low vulnerability. This left 39 public water systems with unprotected surface
water intakes at low elevation.
The next challenge was to find an indicator of vulnerability to sea level rise. We decided that sites closest to brackish
water would be the first to be affected by a migrating salt front, and needed an indicator of the proximity of brackish water
to the remaining intakes. Wetland plants are often grouped by biologists into freshwater, brackish, and saltwater species.
The National Wetlands Inventory (NWI), conducted by the U.S. Fish and Wildlife Service, has produced a series of maps
(http://mac.usgs.gov/mac/isb/pubs/factsheets/fs08099.html) derived from aerial photography and field checked by
biologists. The NWI maps show estuarine, palustrine (non-tidal wetlands and tidal wetlands with an ocean-derived salinity
below 0.5 parts per thousand (ppt)), lacustrine (lakes), and riverine (rivers) wetlands. The boundary between estuarine and
freshwater wetlands is defined by salinity, during average annual flow, of 0.5 (ppt). A PWS drawing water at 0.5 parts per
thousand or higher would have difficulty meeting the EPA’s secondary Maximum Contaminant Level (MCL) for chloride
and total dissolved solids (2.5 parts per million (ppm) or 0.0025 ppt and 5 ppm or 0.005ppt, respectively
(http://www.epa.gov/safewater/mcl.html)) unless it used desalination techniques. Therefore, an intake in an estuarine area
might face salinity problems now, and one in close proximity to an estuary might be vulnerable to sea level rise. The NWI
contains digitized maps for 25 of the 39 potentially vulnerable PWS intakes. We considered proximity to an estuarine
wetland an indicator of vulnerability for a public water system intake. Using the intake coordinates from BASINS, we
generated maps of the area around the intake. These maps were overlaid on the NWI wetland maps to measure the distance
from the intake to the nearest estuarine area. The intakes ranged in distance from 0 km (i.e., the intake is in an estuary) to
27 km from the nearest estuary. Six intakes appeared to be within an estuary. Nine intakes seemed to be within one km of
an estuary. Of these, five were on fresh water creeks or rivers that are separated from the estuary by land, and were
designated as being at medium vulnerability. Four more were in low-lying lakes or ponds, and were classified as having
low vulnerability to contamination from sea level rise. The remaining 10 intakes were classified as having low vulnerability
because they are more than one km from the nearest estuarine wetland (Figure 2).
Vulnerabilities of Selected Public Water Supplies with
Surface Water Intakes
Figure 2: Vulnerabilities of Selected Surface Water Intakes
Because we had examined the SDWIS database for violations of the total dissolved solids Maximum Contaminant
Level Secondary Standard and did not find violations at these PWSs (or at more than a few others), we were concerned that
this approach might have yielded a number of errors or uncertainties. The coordinates given in BASINS for the intake could
be wrong, or the wetlands maps could be inaccurate. The boundary between fresh and estuarine waters is not precise, or
may have changed since the NWI maps were prepared. An area that is home to estuarine plants might be only slightly
brackish, and therefore suitable for a drinking water supply. The water systems may use desalinization processes, enabling
them to meet the EPA’s secondary standards for salt. Table 2 shows those PWS intakes that are 25 km or less from
estuarine wetlands. (Three points on the map are not included in the table: two were more than 25 km from estuarine areas,
and one appears to be a duplicate entry.)
To verify our findings, we contacted six of the PWS operators by phone (indicated with an asterisk (*) in Table 2). One
facility in Biscoe, North Carolina, turned out to be inland. It appears that the longitude coordinate in BASINS is incorrect.
The remaining five seem to be where we thought they were, and all of the remaining five operators indicated that they
already have, or recognize that they could have, problems with increasing salinity. For example, the PWS in Wilmington,
NC, relocated its intake 23 miles upstream in 1943, but the original intake (identified in our screening analysis) continued
to be used to provide a backup source, though it was useable only at low tide. Finally, in 1995 it was taken out of use after a
series of hurricanes essentially destroyed it. Although none of the other system operators use desalination technology,
several have experienced salinity problems associated with storm surges. One facility relies on stored treated water to
alleviate periodic quality and quantity problems, while another (in Panama City, FL) maintains a small reservoir of surface
water that can be released during a storm, as was done after Hurricane Opal in 1995. All four active plants reported that
they have had salinity problems following storms or during droughts.
Table 2: Vulnerabilities of Selected PWSs with Surface Water Intakes.
City
Prichard
Panama City *
Biscoe *
Beaufort
Corpus Christie
Corpus Christie
Distance
to
Estuarine
Wetland
[km]
State
AL
0.0
FL
0.0
NC
N/A
SC
0.0
TX
0.0
TX
0.0
Comments
Estuarine
Estuarine
Longitudinal data error in BASINS
Creek next to emergent est. wetland
Estuarine section of river
At estuarine & palustrine boundary
Vulner- Elevation
ability
Class
3
H
1
H
N/A
N/A
0
H
3
H
3
H
High Vulnerability Pop Served:
Port Charlotte *
Wilmington *
Anahuac *
Freeport *
Port Neches
FL
NC
TX
TX
TX
0.1
0.2
0.4
0.2
1.0
Creek not on maps; distance is est.
Creek not on maps; distance is est.
Riverine
Distance to parallel estuary channel
Riverine
M
M
M
M
M
2
0
0
0
0
Medium Vulnerabilty Pop. Served:
Ft. Lauderdale
Marco Island
Port Charlotte
Kill Devil Hills
Norfolk
Washington
Texas City
Chesapeake
Port Arthur
Porterdale
Beaufort
FL
FL
FL
NC
VA
NC
TX
VA
TX
GA
SC
0.4
0.5
4.0
0.9
0.4
4.0
4.0
5.1
5.3
18.0
25.0
Apparently a man-made lake
Man-made lake
Man-made canal
Lake on barrier island
Inland lake surrounded by uplands
Palastrine, semi-permanently flooded
Inland lake surrounded by uplands
Riverine
Palustrine emergent wetland
Similar to Chatham, GA site (dupl?)
Riverine
L
L
L
L
L
L
L
L
L
L
L
3
2
2
2
3
1
2
0
0
3
0
Low Vulnerability Pop. Served:
Population
Served
10,220
35,000
N/A
13,383
46,500
385
105,488
210,000
13,383
34,000
51,000
2,000
310,383
139,590
35,000
210,000
1,218
59000
9,800
78,480
65,000
80,000
2,500
14,000
694,588
CONCLUSIONS
Through the use of the screening process described in this paper, we conclude that of 416 PWSs with surface water
intakes below the fall line, five are highly vulnerable to salt water intrusion, with around 100,000 people served by these
systems. An additional 15 systems serving one million people are at medium to low risk. This is relatively good news,
especially considering the numbers of people that live in coastal communities. Several questions remain, however.
First, it is important to keep in mind that we only looked at surface contamination of surface water systems. Many
people are served by groundwater sources, including most people in Florida. And, groundwater salinization may have
impacts on surface water sources that our approach did not find. A groundwater study is being considered.
Second, the screening approach used in this study only looked at the systems included in the BASINS database. The
SDWIS database is comprehensive of all public water systems. We have found that with proper security clearances, we
could use SDWIS in the same way we used BASINS, and we would have a more complete picture. It is also important to
note that neither database includes systems that serve fewer than 25 people, or individual wells. Because the capital costs
associated with a small system or well are smaller, such systems may not have taken the same precautions that larger
systems have. Then again, the cost of relocating such a system might also be smaller. An individual relying on well water
might have the option of going on a public system if it were not practical to move the well upstream.
This screen, by design, looked for systems that are vulnerable because of their proximity to currently brackish waters.
Therefore, it offers little information on systems that are vulnerable to surprises. For example, during a drought in 1999, the
water system in Poughkeepsie, NY, watched with anticipation as the salt front moved up the Hudson River to within a few
kilometers of the intakes. Poughkeepsie is over 50 kilometers upstream. The vulnerability of that system depends on the
occurrence of drought as well as sea level rise, yet a coping strategy should still be developed. Better screening tools that
would capture vulnerabilities such as this should be developed.
It would be good to model the migration of the salt front up each coastal river basin in order to get a more specific
understanding of PWS vulnerability. Unfortunately, the time and funding requirements for such a project would be
prohibitive for EPA. However, we hope that these results may spark interest among PWS managers, Departments of
Natural Resources, or academic groups that might model individual at-risk rivers.
Given that some systems are vulnerable, and others may be vulnerable, it is important to consider adaptation strategies.
Examples include those already employed by the six system operators we spoke with: operate the plant at low tide; prepare
for storm surges by storing fresh water or construct barriers to block the storm surge; and, move to higher ground when it is
no longer practical to rely on the existing intake. Each of these options comes with costs that will have to be borne by the
communities being served. Other adaptation options exist, some of which are foresighted in their consideration of future sea
level rise. It would be a shame for a permit to be issued for a new system and a community to invest in a capital project
whose useable life will be cut short by salt water intrusion.
REFERENCES
National Assessment Synthesis Team, 2000. Climate Change Impacts on the United States: The Potential Consequences of
Climate Variability and Change. US Global Change Research Program, Washington, D.C. p.39.
National Assessment Synthesis Team, 2001. Climate Change Impacts on the United States: The Potential Consequences of
Climate Variability and Change, Report for the US Global Change Research Program, Cambridge, University
Press, U.K. pp. 52-53.
http://www.epa.gov/waterscience/basins/
http://www.epa.gov/safewater/sdwisfed/sdwis.htm
http://water.usgs.gov/GIS/metadata/usgswrd/physio.html
http://mac.usgs.gov/mac/isb/pubs/factsheets/fs08099.html
Note: The U.S. Fish and Wildlife Service webpage was ordered off-line by a federal judge in December, 2001.