Protecting Drinking Water from
Saltwater Intrusion
Carol Brzozowski
In
Miami Beach, FL, people sometimes must navigate streets wearing boots because
during weather events, the streets get flooded twice a day for a couple of
hours, notes Jayantha Obeysekera, chief modeler for the South Florida Water
Management District (SFWMD).
Scientific
studies published earlier this year in Proceedings of the National Academy of
Sciences show that sea levels on Earth are rising faster than they have in the
past 28 centuries at an accelerated rate — they are expected to rise between
11–52 inches by 2100 — because of man-made global warming.
Miami
Beach stands as a sign of the times in which coastal regions are being impacted
by sea level rise.
Some
seven million people in four south Florida counties rely on the Biscayne
Aquifer for their drinking water.
As
a coastal aquifer connected to the floor of Biscayne Bay and the Atlantic
Ocean, it is vulnerable to potential contamination.
The
question dogging water managers not only in south Florida but in other US areas
relying on groundwater for drinking water is: what is the potential for
saltwater intrusion into the drinking water supply?
According
to EPA, more than one-third of the US population relies on groundwater from
public water systems or private wells.
Not
only is sea level rise affecting groundwater, but so is over-pumping.
According
to the California Water Foundation, over-pumping of coastal aquifers allows the
ocean to push inland, with saltwater detected in wells 5 miles from the coast.
Leonard
Konikow, a USGS emeritus scientist, notes that there are many serious concerns
in addition to the effects of sea level rise on water sources and water
supplies.
“The key issue is the salinization of coastal land, water
resources, and water supplies,” he says.
“This can have a major impact on drinking water
supplies — both surface water and groundwater — supplied from wells.”
“As sea level rises, storm events and storm surges will
bring saltwater further and further inland,” he adds.
“This saline water will infiltrate through the soil
into underlying aquifers, thereby contaminating a freshwater resource with
saltwater, and affecting the usability of that water for drinking water or for
industrial and agricultural purposes,” says Konikow.
“A single event with saline infiltration can introduce
salts that might take years or decades to flush out.”
Deeper
and confined aquifers would be better protected and insulated from the effects
of storm surges and saltwater infiltration because the groundwater at the
greater depths generally is derived from flows from recharge areas much further
inland, he adds.
“In a sense, the fresh quality of the deeper aquifers
will probably outlast our ability to live in coastal areas inundated by rising
sea levels,”
says Konikow.
The
2014 State of the Climate report addressing sea level published in the July
2015 issue of the Bulletin of the American Meteorological Society notes that
global average sea level was 2.6 inches above the 1993 average, which is the
highest yearly average in the satellite record (1993 to present).
Sea
level measurements highlighted recent shifts in both the El Niño-Southern
Oscillation and the North Atlantic Oscillation. The report echoes that of
others: the pace of sea level rise has increased in recent decades.
The
State of the Climate report points out that in the US, nearly 40% of the
population lives in relatively population-dense coastal areas where sea level
plays a role in flooding, shoreline erosion, and hazards from storms.
Globally,
eight of the world’s 10 largest cities are near a coast, according to the UN
Atlas of the Oceans.
The
report makes the point that in urban settings along coastlines around the
world, rising seas threaten infrastructure necessary for local jobs and
regional industries.
Transportation,
energy production, and waste disposal systems that support these populations
are also at risk from rising waters.
In
the natural world, rising sea level is a stressor on coastal ecosystems that
provide recreation, protection from storms, and habitat for fish and wildlife,
including commercially valuable fisheries.
As
seas rise, saltwater is intruding into freshwater aquifers, many of which
sustain municipal and agricultural water supplies and natural ecosystems.
Another
study released earlier this year from scientists at NASA’s Jet Propulsion
Laboratory (JPL) in Pasadena, CA, and the University of California, Irvine, shows
that while ice sheets and glaciers continue to melt, changes in weather and
climate over the past decade have caused Earth’s continents to soak up and
store an extra 3.2 trillion tons of water in soils, lakes, and underground
aquifers, temporarily slowing the rate of sea level rise by about 20%.
As
glaciers melt due to climate change, an increasingly hot and parched Earth is
absorbing some of that water inland, slowing sea level rise, according to NASA
experts.
Measurements
from a NASA satellite have enabled researchers to identify and quantify for the
first time how climate-driven increases of liquid water storage on land have
affected the sea level rise rate.
The
global hydrologic cycle occurs each year as a large amount of water evaporates
from the ocean, falls over land as rain or snow, and returns to the ocean
through runoff and river flows.
Small
changes in the cycle by persistent regional changes in soil moisture or lake
levels, for example, can change the rate of sea level rise from what is expected
based on ice sheet and glacier melt rates.
Until
the current availability of instruments to accurately measure global changes in
liquid water on land, scientists did not know how large the land storage effect
would be.
“We always assumed that people’s increased reliance on
groundwater for irrigation and consumption was resulting in a net transfer of
water from the land to the ocean,” says the study’s lead author, JPL’s J.T.
Reager.
“We didn’t realize until now that over the past decade,
changes in the global water cycle more than offset the losses that occurred
from groundwater pumping, causing the land to act like a sponge — at least
temporarily.
“These new data points are vital for understanding
decadal variations in sea level change. The information will be a critical
complement to future long-term projections of sea level rise, which depend on
melting ice and warming oceans.”
Michael
Campana, professor of hydrogeology and water resources management at Oregon
State University and technical director of the American Water Resources
Association, has been conducting research and teaching hydrogeology and related
subjects for 41 years.
He
agrees that seawater has the potential to impact both surface water and fresh
water as a result of sea level rise.
“Surface freshwater supplies could be compromised as
they become contaminated by seawater. This would impact ecosystems, fisheries
and M & I [municipal and industrial] supplies and even irrigation water.
Many crops don’t grow in brackish water,” says Campana.
“Fresh water does not have to be 100% displaced by
seawater,”
he adds.
“For example, as salty water moves up a river, the
salinity may change enough to cause problems long before the native river water
becomes 100% seawater, if it ever does.”
There
also is the potential for increased coastal erosion, bigger storm surges and
flooding during storms, as well as flooding irrespective of storm surges, says
Campana.
Campana agrees sea
level rise can cause water level rises in coastal aquifers and that “even if the shallow groundwater remains
fresh, the rising water would compromise subsurface infrastructure, such as
pipes, cables, tunnels, foundations, and basements. If the rising water is
brackish/salty, then corrosion problems could also be significant.”
Sea
level could also seep into the groundwater from the land surface and
contaminate water supplies and even damage the crop-producing capabilities of
soils because of salt buildup in the soil, he adds.
“In coastal aquifers, rising sea level and/or declining
water levels will cause the seawater to move inward and displace fresh water,” he says.
“This could mean that supply wells would start pumping
a mixture of fresh water and seawater, which would likely render the water from
the well unusable.”
This
has always been a problem for coastal regions that rely on groundwater, such as
south Florida, Georgia, South Carolina, Louisiana, and Texas, notes Campana.
Obeysekera
notes that sea level rise affects water resources in several ways. Flood
protection is one aspect.
“South Florida is very flat and when the area was
developed in the 1950s and 1960s, the federal government, together with the
state, developed a coastal flood control infrastructure of salinity barriers
along each of the canals that are discharged into the ocean,” he says.
The
control structure prevents saltwater from coming into the canals, but they also
have to be open during storms in places like Miami and Fort Lauderdale.
“These structures have been there for 50 years and were
not defined for the higher sea levels that we are going to be experiencing,” says Obeysekera.
“Even today, in places like Miami-Dade County, we can’t
open those gates during higher tides because the ocean side is higher than the
inland. That’s a concern for future flood protection which we are addressing.”
Saltwater
intrusion is a second aspect.
“We are already experiencing nuisance flooding in
places like Miami Beach and they are taking steps to fix it,” he says.
“South Florida’s underground geology is such that it
consists of very porous limestone, so even though building dams might stop
water from coming onto the land, it will go underground through the limestone,” says Obeysekera.
“As the sea level is rising, the concern is the
saltwater intrusion will reach the freshwater supply for urban communities,” he adds.
A
third aspect of the situation relates to the Everglades, home to many mangroves
and environmentally sensitive areas.
“The sea level rise sends more saltwater into those
environmentally-sensitive areas, potentially changing the ecology,” says Obeysekera.
“The peat layers in those areas will interact with
salty water and cause what is known as peat collapse. These are the concerns in
terms of water resources management.”
Projects
are underway to help address those concerns. A USGS Sea Level Rise Hazards and
Decision Support project assesses the potential impacts of sea level rise.
Tools
for coastal management decision-making draw from historical and recent
observations of coastal change, combined with model simulations of coastal
environments such as barrier islands, wetlands, and coastal aquifers.
Information
integrated from a variety of methods evaluates the probability of sea level
rise impacts for managers who face decisions to avoid, mitigate, or adapt to
future hazards.
The
USGS has echoed that changes in climate and sea level are the driving factor to
coastal groundwater system changes impacting humans and coastal ecosystems
(http://bit.ly/1rfG8L8).
Increases
in sea level will raise the freshwater table in many coastal regions, with
people seeing an increase in the potential for basement or septic system
failure as well as contamination of groundwater supplies due to landward and
upward movement of seawater incoastal aquifers.
Saltwater
intrusion into groundwater systems is expected to impact coastal ecosystems
such as marshes by changing the elevation of the fresh water-saltwater
interface.
The
potential adverse effect of the depth of that interface near public groundwater
supply wells poses a major concern for water managers.
Pumping
from public supply wells in coastal aquifers underlaid by saltwater can lower
the water table with respect to sea level, decreasing the depth to the fresh
water-saltwater interface beneath the pumping well and increasing potential for
saltwater intrusion.
This
is a potentially limiting potable water availability.
Konikow
has published several papers and reports related to coastal groundwater and sea
level rise.
In
a 1999 book Seawater Intrusion in Coastal Aquifers: Concepts, Methods, and
Practices, which Konikow co-authored with T. E. Reilly, the argument is made
that coastal aquifers serve as major sources for freshwater supply in many
countries, especially in arid and semi-arid zones, and that many coastal areas
are also heavily urbanized, which makes the need for fresh water even more
acute.
The
authors show that coastal aquifers are highly sensitive to disturbances, with
inappropriate management of a coastal aquifer potentially leading to its
destruction as a source for fresh water much earlier than other aquifers not
connected to the sea.
The
reason: the threat of seawater intrusion. In many coastal aquifers, seawater
intrusion has become one of the major constraints imposed on groundwater
utilization.
As
seawater intrusion progresses, existing pumping wells, especially those close
to the coast, become saline and have to be abandoned.
Also,
the area above the intruding seawater wedge is lost as a source of natural
replenishment to the aquifer.
William
Sweet is an oceanographer with the National Oceanic and Atmospheric
Administration (NOAA) National Ocean Service Center for Operational
Oceanographic Products and Services and an expert in the connection between sea
level rise and recurrent coastal flooding.
Globally,
sea level rise is due to climate change with oceans expanding when they’re
warmer and ice melts leaving Greenland and Antarctica mountain glaciers,
causing an upward rise of 3 millimeters a year on average, says Sweet.
Sweet’s
work has focused on impacts people see in urbanized areas near aquifers from
which localities pump water for drinking, such as the Norfolk, VA, region, the
Chesapeake Bay area, Miami, and other coastal areas.
Recurrent
high tide flooding is reaching local thresholds for minor flooding, with such
events occurring on an accelerated frequency and duration trajectory due to sea
level rise causing hydraulic pressures on ocean side, he says.
Sweet
and oceanographer John Marra recently completed an analysis of nuisance
flooding in 27 US cities.
Nuisance
flooding is minor, recurrent flooding that takes place at high tide. It occurs
when the ocean has reached the “brim” locally.
Because
of sea level rise, nuisance flooding in the US has become a “sunny day” event —
not necessarily linked to storms or heavy rain, notes Sweet.
NOAA
measures nuisance flooding as occurring when the water level at a NOAA tidal
gauge exceeds the local threshold for minor flooding impacts established by the
National Weather Services’ (NWS) local weather forecasting offices through
years of floodmonitoring.
NOAA
reports each location’s nuisance flood threshold as height above the long-term
average of the daily high tide. Some locations have more than one high tide
each day — for those locations, the nuisance flood level is reported relative
to the average of the higher of the location’s high tides.
During
nuisance flooding, waves may top old seawalls, water may inundate low-lying
roads, and stormwater drainage can be diminished.
Impacts
disrupt transportation, damage infrastructure, and strain city and county
maintenance budgets.
A
nuisance flood can become a more severe problem if a local rainstorm, storm
surge, or wave-topping event coincides with high tide.
Due
to sea level rise, the frequency of high-tide nuisance floods is rapidly
increasing along US coasts, the oceanographers report.
During
2014, nuisance flooding occurred most frequently along the mid-Atlantic stretch
between New Jersey and Georgia.
This
region typically experiences frequent northeasterly wind events, and the wide
continental shelf — a large area of relatively shallow water — allows high
water levels to set up during storms or from less-noticeable onshore wind
forcing.
The
West Coast and Hawaii also have experienced relatively high numbers of nuisance
floods, due in part to higher sea levels from unusual ocean warmth and onshore
prevailing wind forcing.
Along
the East Coast, El Niño-related atmospheric “teleconnections” help establish a
more zonal (west-to-east) jet stream and storm track, resulting in
higher-than-normal storm surge frequencies along much of the mid-Atlantic
coast.
Higher
sea levels set the stage for typical king tides and normal winter storms to
have a bigger impact than usual.
Nuisance
flood thresholds vary by location and depend on the surrounding landscape,
topography, and infrastructure. In general, US infrastructure is vulnerable 1–2
feet above high tide.
Along
the East and some of the Gulf coasts, acceleration in annual nuisance flooding
is already underway, implying that once impacts become problematic, they will
quickly become chronic.
Although
the trends are impacted by yearly variabilities, on average, annual nuisance
flood frequencies have been dramatically increasing.
More
common tides and storms today are causing impacts, whereas decades ago such
impacts would have been more associated with more powerful and rare storms.
The
NWS has determined on many of NOAA’s tide gauges levels to which emergency
managers need assistance, he adds.
Even though there
has been a gradual linear sea level change in the last several decades, Sweet
notes there is a “very clear exponential
increase in the number of days with tidal waters reaching elevations that are
becoming impacted. We’re talking 2 feet, 3 feet sometimes.”
With
high tide events on an accelerated trajectory in many places around the
country, it may present concern for fresh water in the ground, says Sweet.
“These are not necessarily storm-forced events, these
are tidal events, so it’s water seeping up anywhere that the tide has the reach
to come inland, such as back bays,” he adds.
“It’s water that’s defined by an elevation of that tide
that will propagate and impact areas that the tide has exposure to.”
It
will change the chemical composition of the water, with the implication that
water utilities will need to consider different approaches to water treatment.
“It’s saltwater that is now infiltrating and inundating
areas that historically hadn’t had this kind of exposure and it’s increasing
quite rapidly,”
notes Sweet.
NOAA
scientists are concerned with sea level rise and its impacts and are able to
start giving projections into the future of typical high tide flooding and the
events under various projections of sea levels for which regions can start
planning in order to make smart decisions about the future, says Sweet.
“We’re starting to provide seasonal outlooks,” he adds.
“This current El Niño year really exacerbates the high
tide flooding on the West and East Coasts. We’re starting to tease out climate
models like El Niño-Southern Oscillation that can exacerbate the situation
that’s already being made worse by local sea level rise.”
As
relative sea levels rise from land subsidence — the sinking and settling of
soil — and rising global ocean levels due to thermal expansion and ice melt,
there is less of a gap between the ocean and US infrastructure.
“There are two components to sea level,” says Sweet.
“Not only is the ocean rising, but oftentimes land is
sinking, more so in some places than others. Part of that land sinking can be
natural; it could be compaction of sediments built on areas that were
reclaimed.
“It could be large deltas — the Mississippi is one. The
Chesapeake Bay in general has some natural subsidence basically leftover from
the tectonic response from the last Ice Age.”
Sweet
says some anthropogenic components to vertical land motion or downward land
subsidence comes from the pumping of groundwater.
Many
coastal municipalities obtain water from groundwater and as that water layer is
removed it causes compaction and a local relative sea level rise, with some
areas documenting this more than others.
While it doesn’t
necessarily mean there’s ocean intrusion into the aquifers, it does mean that “as you pump these aquifers, it causes a
compaction that adds a local contribution to the sea level rise it might be
experiencing.”
Campana adds: “When you pump groundwater, you lower
pressure in the aquifer and remove fresh water. Once the seawater gets in, it
is hard to get it out. You have to displace it with fresh water, but if you had
this much fresh water, you probably would not have pumped so much groundwater
in the first place.”
In
Contribution of Global Groundwater Depletion Since 1900 to Sea Level Rise:
Geophysical Research Letters, v. 38, Konikow states that water removal from
terrestrial subsurface storage is a natural consequence of groundwater
withdrawals.
Cumulative
groundwater depletion represents a transfer of mass from land to the oceans
that contributes to sea level rise.
Depletion
is directly calculated using calibrated groundwater models, analytical
approaches, or volumetric budget analyses for multiple aquifer systems.
Estimated
global groundwater depletion from 1900 to 2008 totals about 4,500 km3,
equivalent to a sea level rise of 12.6 mm (more than 6% of the total).
The
rate of groundwater depletion has increased markedly since about 1950, with
maximum rates occurring during the most recent period (2000 to 2008), when it
averaged about 145 km3 per year (equivalent to 0.40 mm per year of sea level
rise, or 13% of the reportedrate of 3.1 mm per year during this recent period).
“This better understanding and quantification of the
contribution of groundwater depletion to sea level rise should facilitate an
improved understanding of 20th-century sea level rise and more confidence in
predictions of 21st-century sea level changes,” states Konikow.
“The comprehensive census of depletion in the US is based primarily on direct calculations of volumetric changes in groundwater storage.”
Konikow
adds that more assessments are needed for systems around the world with
substantial known depletion to more accurately quantify these estimates and to
complete a global census of groundwater depletion.
“Nevertheless, the data clearly indicates that
groundwater depletion, as a distinct hydrologic factor, is a small but
nontrivial and increasing contributor to sea level rise,” he says.
Konikow
points out that a growing population brings increased demands for food and
water supply.
With
groundwater use and depletion strongly linked to climate and climate change,
“probably very little” can be done to protect aquifers from sea level rise,
says Konikow.
Groundwater
depletion can affect sustainability and viability of the water supply, he says,
adding that groundwater depletion is growing.
With
a changing climate and hydrologic systems, both management and adaptation to
change will be necessary. Innovative water management actions can help mitigate
the problems, says Konikow.
A
good strategy for protecting groundwater supplies and municipal well fields may
be to keep moving the well fields further inland, when and where possible,
and/or to higher elevations — to locations less at risk from sea level rise and
storm surges, he says.
“Managers should assure that any facilities such as
well fields and desalination plants located in coastal or low-elevation areas
get extra levels of protection from possible flooding or inundation,” points out
Konikow.
“These are all good strategies even if sea level does
not rise significantly because salinization of coastal aquifers is probably
more threatened by over-pumping than by sea level rise itself.”
Ben Strauss, vice
president for sea level and climate impacts for Climate Central, a climate
research and communications organization, notes that while areas with porous
bedrock such as south Florida “can expect
gradual but inevitable increases of saltwater intrusion into aquifers as sea
level rises, withdrawing fresh water from aquifers will accelerate intrusion.
“One measure to slow this process would be to limit
freshwater withdrawals, and to follow other practices to keep aquifers well
charged.”
In
addressing the stormwater challenges, one-way flow preventers for stormwater
are getting installed, says Sweet.
“A lot of these systems are still gravity-fed drainage
systems where you really start losing your downhill grade, but high tides have
just reached higher and higher levels and if it rains, that rain doesn’t
clear,”
he adds.
“In some places like Miami, Charleston, Norfolk, and
others around the country, during high tides water will start coming into the
storm drains themselves and cause problems.”
Not
only are stormwater systems being impacted, but so too are wastewater systems,
often combined sewer systems, he adds.
“The rate of change of these events is what’s alarming
and it’s a telltale of what’s to come,” says Sweet.
“In many areas on the East Coast, there’s been over the
last 50 years a 300 to 900% increase in the annual frequency of these events.”
The
SFWMD is looking at individual locations for flood protection, namely the
installation of forward pumps.
“Next to the gravity coastal structure, we will add
another pump station to send water out into the ocean during high tide — basically
pump over the structure into the ocean,” says Obeysekera.
Another
action being implemented is an impoundment to temporarily store water during
the storm and when the tide subsides, send the water out, he says.
In
addition to those flood protection measures, the SFWMD is considering several
approaches to saltwater intrusion.
“Our number one strategy is what we call a ‘no-regret
strategy,'” says
Obeysekera.
“Basically, it’s to use less water through water
conservation. Miami-Dade, Broward, and Palm Beach counties have done very well
in their concerted efforts to tell people to conserve water. For future growth,
if we conserve water, we don’t have to look for additional sources.”
Other options
include moving the well fields away from the ocean, “although it’s limited in what we could do there,” he adds.
“We also would like to have various water supply
utilities interconnect, so if one area is having trouble, they can get water
from another utility nearby,” says Obeysekera.
The creation of an alternate water supply is another approach.
“In Palm Beach County, there is a limestone pit that is
a big hole in the ground and the suggestion is to create a water reservoir out
of that mining pit and use that water during dry times to send to Broward
County and others.”
As
for Miami Beach, the city is currently taking actions to mitigate flooding
through a $400 million project to install stormwater pumps to remove water from
areas flooded during king tides.
Backflow
preventers are another approach to stop water from coming through the storm
sewer system into the streets during high tides.
The
city also is considering raising sea walls, says Obeysekera.
“You can inject fresh water into aquifers to keep the
pressure high enough to keep the seawater at bay,” notes Campana,
adding that the inherent challenge is that the option is expensive and requires
a large amount of fresh water that has to come from another source, such as
treated wastewater.
California’s
Orange County Water District has used injection for years to keep the seawater
out of its aquifers.
Another option: “balance pumping against seawater
intrusion,” says Campana.
“If the sea level rises, you might have to cut pumping
to leave more fresh water in the aquifer. For seawater seeping into the
aquifers from the surface, this is harder to control because you have to keep
the seawater away from the land surface, such as through dikes, levees, and
drainage canals.”
Editors Note: This article first appeared in the July-August 2016 issue of Water Efficiency.
Carol Brzozowski specializes in
topics related to resource management and technology.
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