Why Kingsloe's Tap Water Tastes Different Each October

The October Phone Calls

Every October, the water office in Kingsloe gets the same phone calls. The tap water smells like dirt. Some people say it tastes like a freshly dug garden. Nothing has broken, and no one is getting sick. The reservoir is simply doing what reservoirs do each fall.

Kingsloe Reservoir is a fictional lake, and the numbers below were written for this reading. It is about 40 feet deep at the dam. The town's water intake sits partway down the wall of the dam.

In summer, the reservoir separates into layers. The sun warms the top few feet of water. Warm water is less dense than cold water, so it floats. The cold water stays on the bottom. Between them sits a narrow zone where the temperature drops quickly. Once these layers form, they hold. Wind can stir the top layer, but it cannot reach the cold water underneath.

The layers matter because of air. Water picks up oxygen at the surface, where it touches the air and where plants release it. That oxygen reaches the deep water only if the lake mixes. In summer, the lake does not mix. Meanwhile, bacteria on the bottom break down dead leaves and dead algae, and that work uses oxygen. By late August, the deep water has very little oxygen left.

Here is what crews measured at the dam one August afternoon.

Most fish need dissolved oxygen above 5 mg/L. That is why the fish in Kingsloe Reservoir crowd into the middle depths of the lake by late summer. The surface has plenty of oxygen but is warm. The bottom is cool but has almost none.

Something else happens in that oxygen-poor bottom water. Iron and manganese dissolve out of the mud. Certain bacteria there also release compounds that people can smell at very tiny amounts. One of them is called geosmin, and the human nose is unusually good at detecting it.

Then autumn arrives. Nights get longer and colder. The surface water cools off, and as it cools it grows denser. Eventually the top water is as dense as the water below it. At that point the layers no longer hold each other apart. A few windy days finish the job, and the whole reservoir mixes from top to bottom. Scientists call this fall turnover.

Turnover is good for the lake. Oxygen returns to the deep water, and nutrients from the bottom reach the surface. But it also lifts the iron, manganese, and geosmin off the bottom and spreads them through the whole reservoir, including the water at the intake.

That is when the phones start ringing. Operators in Kingsloe know the calendar, so they prepare. They test the reservoir at several depths every week through September. They can shift the intake to a different depth, and they can add powdered activated carbon at the plant, which grabs taste compounds as the water is treated. They also send a notice to customers explaining what is happening.

The earthy taste is not a health problem. Geosmin can be noticed at levels far below anything that would harm a person. Still, people judge their water by how it tastes, so operators treat the complaints seriously. Within two or three weeks the reservoir settles, the compounds spread out and break down, and the water tastes ordinary again until the following fall.

1. According to the passage, why does warm water stay above cold water in a reservoir during the summer?
2. Using the table, at what depth does dissolved oxygen first fall below the 5 mg/L that most fish need?
3. Why does the deep water run low on oxygen by late August?
4. What causes fall turnover to happen?
5. Why do customers notice the earthy taste in October rather than in July?
6. The passage says the taste is not a health problem. What evidence in the passage supports that statement?
7. If Kingsloe operators could draw water from any depth on the August day in the table, which depth would you choose, and why? Use the table in your answer.