Saltwater and Sewage
Daily Environment Repost

Saltwater and sewers: A nasty combination

Life in a warm, low-lying coastal city is not all about another day in paradise. Fort Lauderdale, Florida, faces some harsh realities as the city soaks in sewer waste. Six sewage spills recently leaked over 126 million gallons of raw sewage into the city waterways. The volume of discharge is large, but half of it is actually groundwater. Of course, the purpose of sewage lines is to separate nasty waste materials from the rest of the environment. What has gone wrong?

The word summarizing this problem is corrosion. In the case of Fort Lauderdale, corrosion exacerbated by salty seawater creates the problem. The salt in ocean water makes it more corrosive than freshwater. Saltwater and sewers don’t mix well, so why is there saltwater in the ground below the city?

Sea level rise and flooding

Mean sea level around Southern Florida rose about eight inches between 1950 and today. The rate is picking up, and one inch of sea-level rise is now expected every three years. Take a ruler and stand on an average beach. Place the ruler at the water’s edge and observe the eight-inch mark. It certainly doesn’t look like much. But there are some knock-on effects of tidal flooding to consider. 

Living at sea level increases a person’s exposure to tidal changes. Anyone who has lived on the Florida coast is familiar with king tides that usually come during the autumn. When the gravitational pull of the sun and the moon align, and the Gulf Stream slows, the combined effect is to increase water levels during high tides by about one foot. This effect leads to nuisance flooding in coastal areas, and the increasing frequency of excessive tidal flooding is, in part, a result of rising sea levels. The difference between a 12-inch rise in high tide levels and a 20-inch rise is significant. The 8-inch sea level rise makes noticeable difference during king tides.

Saltwater from flooding seeps into the ground and contributes to the corrosion of the city sewage pipes. More flooding translates into more corrosion. However, there is a second factor contributing to the sewage problem. Seawater does not always soak into the ground from above; it can flow in laterally through a process called saltwater intrusion.

Porous limestone

The coast of Florida is particularly susceptible to saltwater intrusion due to its geology. Very porous limestones underlie much of the state. Geology doesn’t stop at the coastline, and these same porous limestones extend into the ocean. The high porosity of these limestones provides a direct flow connection between the sea and the coastal groundwater aquifers. 

Generally, in a coastal aquifer, water from rain will form a freshwater lens over the saltwater intruding from the ocean. If too much groundwater is pumped from the freshwater lens, then saltwater will rise to fill the void. Rising sea levels and extra high tides also take a toll on the freshwater supply. The added water pressure forces more saltwater into the aquifer, displacing existing freshwater. 

All of this is bad news for Florida’s coastal cities. Saltwater and sewers don’t mix. Corrosion, enhanced by saltwater, turns sewage pipeline networks into leaky sieves. Groundwater leaks into the pipes and raw sewage leaks out. Fixing the problem in Fort Lauderdale will take $1.4 billion they don’t currently have. Surely, a lot of political finger-pointing will happen before any real change occurs.


Archeanweb:

Saltwater invades Florida’s water supplies (Source: ArcheanWeb) – https://archeanweb.com/2019/12/09/saltwater-intrusion-florida-water-supplies/ Also:

Climate change at the margins (Source: ArcheanWeb) – https://archeanweb.com/2019/12/16/climate-change-at-the-margins/  Also:

King tides, bringing reality home (Source ArcheanWeb) – https://archeanweb.com/2020/03/02/king-tides-bringing-reality-home/ Also:


Sources:

Half the stuff in Fort Lauderdale’s sewage system isn’t waste. Sea rise makes leaks worse (by BY ADRIANA BRASILEIRO AND  ALEX HARRIS – Miami Herald) – https://www.miamiherald.com/news/local/environment/article239005633.html Also:

Why Are Sea Levels Rising? – https://sealevelrise.org/states/florida/ Also:

Feature Image: Pipeline Repair (USEPA Environmental-Protection-Agency) (Modified)

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.