Near the coast and in some other places, pumping groundwater out of wells for our use can cause saltwater intrusion, eventually filling the wells with water we cannot use. Freshwater has a lower density than salt water, and so floats on salt water in much the same way that an iceberg floats on water or a mountain range floats on denser rocks of the mantle. (Salty water and freshwater can mix to make less-salty water, but if the freshwater is renewed by rainfall, the mixed waters will be forced out through the beach to the ocean, and there will continue to be nearly pure freshwater sitting on salty ocean water.) If the water table is lowered by pumping fresh water for human use, the interface between salt and fresh water will rise in the same way that the bottom of an iceberg or a mountain range rises if the top is eroded. The difference in density between salt and fresh water is small; an iceberg floating in the ocean has 9/10 of its thickness below the surface, but the fresh groundwater lens of Cape Cod floating on ocean water has 39/40 below sea level. So if enough water is pumped out of the well to lower the water table by 1 m, the salt water will have risen 39 m! If the freshwater table is lowered to sea level, the salt water will rise to sea level, and there will be no fresh water left for the well to pump. Many wells on the very low land of Cape Cod were drilled below sea level into fresh water, but are starting to pump up salt water, causing large problems.
Video: Saltwater Intrusion (1:40 minutes)
Here’s a video explaining the problem, and then a single diagram if you prefer shorter explanations.
Click here for a transcript of the Saltwater Intrusion video.
Dr. Richard B. Alley: People who get their water from wells and who live near the coast, or in some other places, have to worry that if they pump too much water out of the ground they will start to get salt water into their wells and they won't be able to use that. These diagrams are from an old publication of the United States Geological Survey but the problem is still new, it's still with us. So here's a diagram that could be Cape Cod. It is of an island from this report, and there is fresh water in the ground as shown there. But it's sitting on top of salty water and there's a little zone of mixing between the two but it's not very thick. So it is possible for a well to pump up fresh water that people can use. The problem is if you pump too much and the water table drops by one foot, the level of the salt comes up by 40 feet and that can get into trouble. So here's a diagram. Start with a water well that is in fresh water at the bottom, where it's pumping from, this is before it starts pumping. When it starts pumping it will lower the water table at the top and that will raise the level of the salt water at the bottom. If it pumps too much then you raise the salt up until it hits the bottom of the well what comes out is saltwater and it's no longer useful to the people.
This diagram shows the effect of pumping too much groundwater at a place such as Cape Cod. The Cape has salt water on both sides, and this salt water extends inland beneath the sandy soils of the Cape. A lens of freshwater supplied by rain sits on top, and extends below sea level, much like an iceberg. If the upper surface of this water is lowered to sea level by pumping too much water from a well, the salt water will rise into the well.
Human impacts on the land are easy to see. We have changed the oceans greatly, but the water covers our tracks. In "The Can," Dr. Alley briefly reflects on some issues of the oceans, as he watches one of the less-beautiful pieces of the Cape Cod National Seashore.
The Can: Cape Cod National Seashore
Click here for a transcript of "The Can" video.
Dr. Richard B. Alley: A beautiful, natural morning at the beach. But if you turn to the sea, you can see that the evening revelers were there. I've been to the beaches in Greenland. I've been to the beaches of Antarctica, and everywhere I've been, the flotsam and jetsam of humanity are on the beach.
Oceanographers have traced the currents of the Pacific using shoes and rubber ducks that fell off of ships during storms. More importantly, we've probably taken 90% of the big fish out of the ocean. We don't know what an ocean ecosystem should look like because it isn't natural anymore. We've changed everything. We've put so much fertilizer into the ocean from runoff from farming and sewers and other pollution that they cause huge blooms of algae and when they die, they rot and they kill the life around them and make dead zones. Taking care of the ocean is a big deal. And it's a deal we need.
Many of the ocean’s big fish, and other denizens of the deep, rely on salt marshes as nurseries and in other ways. But, we are losing salt marshes in many places, as sea-level rise forces the “outer beach” toward the shore, but humans don’t allow the inner side of the marsh to expand into our yards or parking lots. Obvious answers are not easily available, but Dr. Alley frames the question in this next short film clip as he paddles one of the family kayaks on the Nauset Marsh of the Cape Cod National Seashore.
The Marsh: Cape Cod National Seashore
Click here for a transcript of "The Marsh" video.
Dr. Richard B. Alley: Beautiful morning for a paddle. The tide's coming in, and a really happy professor is going out to see who's running around in the salt marsh, the Nossett Marsh in Eastham on Cape Cod. This is a place for birding. This is a place for shelling. These sandpipers are out getting breakfast.
Salt marshes are remarkably productive places. They are the nurseries of the fish and the shellfish. They're the nurseries of the ocean. They, too, need care.
The outer beach is coming in as the ocean rises. But the inner side is often hardened by humans and not allowed to move. And if we're not careful, we won't have these nurseries.
Cape Cod is a gift of the glaciers. The numerous kettle ponds left by the ice contribute to the biodiversity of the Cape, but are slowly filling in with sand, peat, and other things. Many of the ponds have already filled, and a walk along the rapidly eroding outer beach often reveals where the sea has cut into one of these filled ponds. In this next clip, Dr. Alley shows one such exposed, filled kettle pond, just below the old Coast Guard station in the Nauset region of the Cape Cod National Seashore.
Meet Peat!: Cape Cod National Seashore
Click here for a transcript of the "Meet Peat! video.
Dr. Richard B. Alley: We're on the outer beach at Cape Cod at Nauset, and we're looking at where the ocean has been cutting back a little bit of the bluffs to reveal what's behind. Now, most of the Cape is sand and gravel. It's outwash from the glaciers from the Ice Age. But what we see here is the filling of a lake. A block of ice fell off the glacier, was buried in sand and gravel, and then melted out to leave a little spot, a lake, which is filled with peat that you see here. The peat is the remains of dead plants. And if you look very carefully, you will see within the peat many of the dead plants.
On top here are grasses of the modern world, but they are not down in the material. Lakes die. They fill with this stuff. When you see a lake on the landscape, you should ask yourself, why is this here? Because something recent has happened to make the lake.
