Star Valley: How Much Is Down There?

Consultants will estimate underground water storage and use in the face of drought's effect on well levels


Star Valley this week approved a $52,500 study of its underground water, in the wake of a drought that sent well levels plunging and raised questions about how much water lies in the pockets of fractured granite underlying the town.

Star Valley has long benefited from some of the most shallow and productive wells in Rim Country, but it's also vulnerable because at least half of the residents have their own wells and the town has little water storage or rights to surface water.

The town council on Tuesday voted unanimously to hire LFR Inc. to do a "Water Sustainability Study," to determine the long-term and drought-based limits of the town's water supply. The study should be complete late this year.

"We need to keep what we have," said Mayor Chuck Heron. He said previous studies have looked at long-term average rainfall and groundwater recharge, but haven't considered the impact of drought and sustainable levels of withdrawal.

"This will give us the best case and worst case scenario," Heron added.

The study will help the town figure out how much water it needs on an annual basis and whether the complex layering of underground crushed and fractured granite, bedrock and loose alluvial fill that has washed down from higher ground will sustain the town's long-term future.

Previous studies have attempted to estimate the amount of water that makes it into that water table in an average year, but Star Valley officials have questioned the statistics and assumptions in those studies -- many of which were done by neighboring Payson.

Moreover, in the past two summers, well levels have dropped 20 feet in many cases, before climbing back upward as a result of three record-wet months this past winter. Although the long-term studies suggest that the area gets more than 23 inches of rain annually, the bottom of the drought year brought less than three inches annually.

The town's wells reported "alarmingly low water levels," said LFR Hydrogeologist Vit Kuhnel. During the drought, "people were right at the edge" of pumping dry many of the wells in the heavily pumped core commercial area.

Kuhnel said the study will help the town anticipate when the town's 250 registered wells are likely to start sucking air, which would interject air into the system and damage both the pumps and the network of pipes, since the air pockets would promote corrosion of the pipes.

The study could also provide more information about the impact of the Payson-owned Tower Well, a high-capacity pump in Star Valley that feeds water into Payson's municipal water system. The well was drilled by a developer, when Payson limited new development that didn't bring along its own fresh water supply. Drilling the Tower Well effectively triggered the incorporation of Star Valley, on a crusade to protect the town's groundwater from its much larger neighbor.

Payson insists that the Tower Well pulls water out of a deep water table that should not affect Star Valley's much shallower wells.

Kuhnel, who has been monitoring about 30 Star Valley wells on a contract with the town, said that the preliminary data suggests that the Tower Well affects at least five nearby wells, one of them substantially. Payson rejects claims that its well affects any others and maintains that the deep water table is separated from the shallow water table by layers of impermeable rock. Kuhnel disagrees.

"In Star Valley we are dealing strictly with fractured granite," and the deep granite pockets from which the Tower Well is drawing water is connected in complicated and still unclear ways with the formations saturated with water closer to the surface, he said.

"The situation is pretty complex," he said. Generally, the fractured, crushed, water-bearing pockets lie along fault zones and remain interconnected -- although some of the pockets may lie on different faults or in isolated areas that keep them separated.

He said the study will shift many of the roughly 50 monitoring devices installed now in wells scattered throughout town, to gather more data on well levels. In effect, measuring the water levels in many different wells can map the boundaries of the water-bearing layers.

Moreover, most of the wells in Star Valley aren't lined in concrete, which means geologists can lower cameras into the well shaft and photograph the rock layers, which will give additional information on the structure and fracturing in those layers.

The study will also revisit some of the earlier estimates on how much rainfall actually makes it into the underground water table, said Kuhnel.

An earlier study by Payson used 1984 maps to estimate how much vegetation was growing in the forested areas of the watershed.

Plants normally use about 85 percent of the rainfall before it can get past their root zones. Therefore, estimates of the total number of plants would have a critical impact on figuring out how much water makes it into the water table.

Kuhnel said that plant densities in the forest have increased dramatically in the past 20 years, so that previous estimates of the annual recharge into the water table may be too optimistic.

Finally, the study will attempt to estimate the total amount of water that can be stored in the layers of rock and sand beneath Star Valley.

The amount of storage will obviously have a major impact on how easily the town can weather droughts, when the amount of water soaking into the ground is far less than the amount residents pump out.

"If your water gets shut off and you can draw buckets of water out of the water stored in a swimming pool, you can last a long time," said Kuhnel. "But if all you have stored is a kitchen sink full, then you're in trouble."

Commenting has been disabled for this item.