Storing Winter Water Underground: Havenridge's Recharge Wells

Havenridge sits on a plain where the Wrenmere River runs high from December through April and thin from June through September. The city of about 40,000 people has never had a shortage of water in the year as a whole. What it has is a timing problem. The river carries far more than the city can use in winter, and by August the flow drops to the point where the state limits how much the utility may withdraw.

For most of the twentieth century the answer to a timing problem was a dam. Havenridge studied one twice and set the idea aside both times, partly because the only usable canyon held a working ranch and partly because a shallow reservoir on the plain would lose a great deal of water to evaporation each summer. In its place the utility built something less visible. Six wells now reach a layer of sand and gravel that begins about 240 feet down and continues to roughly 380 feet, sealed above by a thick bed of clay. In winter the plant treats river water to drinking standards and then pumps it down those same wells. In late summer the pumps run the other direction and bring it back.

The approach is called aquifer storage and recovery. Its appeal is that the storage space already exists in the pore openings between sand grains, so the city does not have to buy land, move a road, or build a wall to hold water back. Water kept underground does not evaporate, and it does not warm in the sun or grow algae. The limits are real, though. A well can only accept water as fast as the aquifer will take it, which is far slower than a river fills a reservoir behind a dam. Havenridge can inject only during the months when the plant has spare capacity and the river has surplus flow, and the total it can store in a season is set by the number of wells rather than by the size of the aquifer.

Water that goes down does not sit in place waiting. It spreads outward from each well, forming a mound in the aquifer's pressure level that slowly relaxes, and part of it drifts away along the aquifer's natural slope toward the river valley. Summer pumping does the reverse, drawing the level down around the well field, and once the pumps stop the level climbs back toward its natural position as water moves in from the surrounding aquifer. It also mixes at its edges with native groundwater, which has spent centuries in contact with the same sediments and carries more dissolved minerals. Because of that mixing, the last water an operator pulls back is saltier and harder than the water that went down, so the utility stops recovering before the well runs dry. What comes back is therefore always less than what went in, and the fraction that returns is called the recovery efficiency.

Two operating problems occupy most of the staff's attention. The first is clogging. Fine particles, trapped air bubbles, and bacterial growth all collect on the well screen during injection, and a clogged screen accepts less water each week. Havenridge treats the injected water to full drinking-water standards partly for regulatory reasons and partly because clean water clogs a screen more slowly, and crews backflush each well weekly by reversing the pump for several minutes.

The second problem is chemistry. Treated river water is rich in dissolved oxygen and carries a chlorine residual, while the sediments deep in the aquifer have sat without oxygen for a very long time. When oxygen-rich water meets those sediments, it can dissolve minerals that had been stable, releasing iron, manganese, or arsenic into the stored water. Havenridge ran a two-year pilot before expanding the well field, and it samples a ring of monitoring wells around the site every month. Those samples decide whether the recovered water goes to customers directly or through an additional treatment step.

The record from the most recent storage season is shown below. Injection ran from December through April, recovery from May through September, and the depth to water in the storage zone was measured in a monitoring well at the center of the well field.

The utility's manager describes the well field as a savings account rather than a spring. Havenridge deposits water in the months when the river gives more than the city needs, accepts that a share of each deposit will not come back, and withdraws the rest when the river runs low.

1. Using the table, calculate the total volume Havenridge injected during the storage season, in million gallons.
2. According to the table, in which month was the water in the storage zone closest to the ground surface, and what was the depth to water that month?
3. Recovery efficiency is the volume recovered divided by the volume injected, expressed as a percent. Using the table, calculate Havenridge's recovery efficiency for this season, rounded to the nearest tenth of a percent.
4. The passage says Havenridge studied a dam twice and set the idea aside. Give two reasons from the passage that underground storage suited the city better, and one limit of underground storage the passage names.
5. Explain why the water Havenridge recovers late in the season is saltier and harder than the water it injected, using the passage's account of what happens at the edges of the stored volume.
6. The table shows depth to water shrinking from 69 feet in September to 66 feet in November, so the water stands closer to the surface, even though nothing was injected in those months. Explain what the passage says is happening.
7. The passage states that Havenridge treats the injected water to full drinking-water standards for two reasons. Name both, and explain which one is about keeping the wells working rather than about water quality.
8. A neighboring town proposes injecting river water that has been screened but not filtered or disinfected. Using the passage, predict two problems that town should expect, and name the monitoring the passage describes that would detect one of them.