Protecting Drinking Water from Saltwater Intrusion

Editors Note: This article first appeared in the July-August 2016 issue of Water Efficiency.

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 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’salready 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.”