Tag: groundwater

Water Scarcity in Yemen

Groundwater tends to be a common property resource. In places like Yemen, where ownership rights are not clearly defined it tends to be overexploited. So much so, that they’re looking at running out within the next 10 years. Peter Salisbury has an article in Foreign Policy.

Most potable water in Yemen is produced from a series of deep underground aquifers using electric and diesel-powered pumps. Some of these pumps are run by the government, but many more are run by private companies, most of them unlicensed and unregulated. Because of this, it is nigh on impossible to control the volume of water produced. By some (conservative) estimates, about 250 million cubic meters of water are produced from the Sanaa basin every year, 80 percent of which is non-renewable. In recent years, the businessmen who produce the water have had to drill ever-deeper wells and use increasingly powerful pumps to get the region’s dwindling water reserves out of the ground.

–Salisbury (2012): Yemen’s water woes in Foreign Policy.

The Saltwater Lens on Small Islands

The freshwater lens beneath small islands is fed by rainwater and maintained because the freshwater is less dense than the surrounding saltwater.

One of the questions that came up when we were talking about dealing with the highly contaminated leachate that drains out of landfills, is what would happen to it if it was just put into a lake or the ocean. Would the liquid just mix into the water, or would it stay separate.

Rainwater seeping through a landfill picks up a lot of nasty chemicals, and by the time it gets to the bottom it is a highly contaminated leachate.

I’m afraid I did not go with an easy answer. It depends after all on two things: how different the density is of the leachate from seawater; and how turbulent is the water.

Turbulent water will make the leachate more likely to mix, while a greater density difference would cause them to “want” to remain separate. An extremely dense leachate might just settle to the bottom of a lake and stay there.

Small Islands

One example of two fluids that are in contact but stay separate is in the groundwater beneath small islands. Rain water falls on the island and seeps into the ground. It’s fresh, but the water in the surrounding ocean and the water that’s already underground are both salty. Salty water is more dense than the fresh so the freshwater will float on top of the salty water creating a thin lens.

Dimensions of a freshwater lens. Image via the USGS.

How thick is the lens? For every meter that the fresh groundwater is above sea level, there are 40 meters of fresh water below sea level (1:40). This is because saltwater has density of about 1.025 g/cm3, while freshwater has a density of about 1.000 g/cm3 (note that I use four significant figures in each of these values).

The freshwater lens can be a great source of drinking water on these isolated small islands, but like the islands themselves, they are threatened by rising sea levels due to global warming.