Water From Beside the River: Riddlesworth's Collector Wells

For seventy years, the fictional town of Riddlesworth pumped its drinking water straight out of the Oxmarket River. An intake pipe reached into the channel, and a screen kept out fish and sticks, so everything else the river carried went to the treatment plant.

That arrangement worked well enough on ordinary days, yet it went badly during storms. Rain washed soil off the fields upstream, and the river turned brown within hours. Cloudy water is hard to treat. Operators had to add far more coagulant, the chemical that makes fine particles clump together so they can settle out. Sometimes the plant could not keep up, and the operators shut the intake and drew down the town's storage tanks until the river cleared.

There was a second problem. In late summer, algae grew in the slow water upstream. The algae gave the finished water an earthy taste. The taste was harmless, but residents called the town office about it every year.

In 2009 the town tried something different. Instead of building a bigger plant, Riddlesworth drilled three wells on the riverbank, about two hundred feet back from the water. Each well reaches down about sixty feet into a thick layer of sand and gravel. That layer was left behind by a glacier, and it runs along the whole valley floor beneath the town.

The wells do not tap a separate underground lake. Most of the water they pump is river water. When the pumps run, they lower the water level in the sand around each well. River water then seeps down through the riverbed and moves sideways through the sand toward the wells. The town still uses river water, but that water now reaches the wells by moving through the ground rather than through a pipe.

The route through the ground is what changes the water. Sand and gravel hold water in the narrow spaces between grains. The water creeps slowly along a winding path through those spaces. As it moves, the silt and clay that make a river look brown settle out or stick to the grains and are left behind. Bacteria and larger organisms are caught the same way. Riddlesworth measured the travel time by adding a harmless tracer to the river and watching for it at the wells. The trip took about three weeks.

Three weeks underground changes water in more than one way. Microbes living on the sand grains feed on organic matter in the seeping water, and much of the material that caused the earthy taste is consumed along the route. The temperature evens out too, because the ground stays near the same temperature all year while the river swings from cold to warm. Steady temperature makes the treatment plant easier to run, since coagulants behave differently in cold water than in warm water.

The change showed up quickly in the plant's records. The water arriving at the plant was far clearer than river water, and its clarity barely moved during storms. Operators cut their coagulant use by more than half. Complaints about taste nearly stopped. The plant that had once struggled to keep up now had room to spare.

Riverbank filtration has real limits, and the town's engineers were careful to say so. The sand removes particles well. It does much less to substances that are dissolved, because dissolved material travels with the water rather than being strained out of it. Road salt, nitrate from fertilizer, and some manufactured chemicals all pass through. If a tanker spilled fuel upstream, the wells would give the town about three weeks of warning, but they would not clean the water on their own.

For that reason, Riddlesworth still treats and disinfects everything it pumps. State regulators classify these wells as being under the influence of surface water. That classification requires the same monitoring and the same disinfection a river intake would need. The collector wells are a first stage of treatment, not a replacement for the plant.

The method also depends entirely on what lies underground. Riddlesworth could do this because a glacier left a coarse, permeable layer beside the river. Sand and gravel let water move through at a useful rate. A town sitting on clay could drill the same wells and get almost nothing, because water moves through clay far too slowly to supply a public system. A town on solid rock would face the same problem. Whether the idea was possible at all depended on the valley's geology rather than on the size of the town's budget.

There is one more limit. Fine material collects in the riverbed above the wells over the years, the streambed slowly clogs, and yields drop. Riddlesworth watches its pumping levels closely, and the town expects that a flood large enough to scour the bed will eventually reset the system. Until then, the wells produce about three million gallons a day, and the intake pipe in the channel sits idle, kept in place only as a backup.

1. What two problems did Riddlesworth have when it pumped water directly from the Oxmarket River?
2. How far back from the river are the collector wells, and how deep do they reach?
3. Explain what happens to the water in the sand and gravel that makes it clearer than river water.
4. The passage says road salt and nitrate pass through to the wells while silt does not. Explain the difference in a sentence or two.
5. Why does the state still require Riddlesworth to disinfect water from these wells?
6. Using evidence from the passage, explain why this method worked for Riddlesworth but would not work for a town built on clay.
7. The passage says the wells give the town about three weeks of warning if fuel spilled upstream. Explain what makes that warning possible, and why the warning alone does not solve the problem.