In a normal year, Star Valley wells pump out 15 percent less water than rainfall puts in, according to a consultant’s draft of a sustainable yield study.
But during long-term or short-term droughts, well levels can drop sharply as demand far outstrips natural recharge, according to a just-completed study.
During a severe drought, when the amount of water that can be extracted falls dramatically, Payson’s Tower Well, running at full capacity, could suck the aquifer dry, the study says. Payson’s purchase of the Tower Well was a key factor in the grassroots campaign that ended in Star Valley’s incorporation, in large measure, to protect its water supply.
Because the shallow, layers of crushed granite beneath the town cannot store much water, drought could easily have a pronounced effect, concluded the report.
“The only water stored in the aquifer is in cracks and wedges,” said Senior Associate Hydrogeologist Vit Kuhnel with LFR. “And there is not that much of it. When there is nothing going in, that would have severe results.”
The report appears to challenge results of some previous studies funded by Payson that concluded the Tower Well had little or no effect on the water table or on most other groundwater wells. Even at the height of the long-term drought that was interrupted by two wet winters, Star Valley’s water table remained high enough that Brooke Utilities pumped water from its Star Valley wells and trucked it to drought-stricken Pine and Strawberry.
However, Star Valley has long worried about the potential effect of the Tower Well and the long-term capacity of its water system. Payson has operated the Tower Well at only a fraction of its full pumping capacity in recent years. Moreover, Payson recently locked up rights to some 3,000 acre-feet of water from the Blue Ridge Reservoir, which will secure a long-term water supply, even without running the Tower Well at full capacity.
Nonetheless, the most recent study concluded that the Tower Well alone could pump out more water than rainfall puts into the system in a drought year.
In June, Star Valley asked Phoenix-based LFR Inc. to conduct a study of its underground water resources to determine how much water can be safely pumped from the fractured granite aquifer and to determine the long-term and drought-based limits of the water supply.
The study also provided more information about previous studies from LFR Inc. and Clear Creek Associates, which gave the town conflicting numbers on the impact of the Payson’s Tower Well on Star Valley’s groundwater supply.
The study could have long-term impacts on Star Valley’s general plan. In July, town members voted to halt general plan discussions in hopes the water study could determine how big the town could grow, given the amount of water available.
The study provided background about two, highly contentious studies completed several years ago.
An October 2005 study by LFR concluded that many of Star Valley’s wells tap into the same water table as the Tower Well, which feeds Payson’s municipal water system.
A developer drilled the well and sold it to Payson several years ago, just before Star Valley’s incorporation. Payson told residents the well would not affect Star Valley’s wells, but LFR concluded that overpumping from one or more wells would eventually lower water levels in all wells.
In Arizona, the amount of water extracted from a well is unregulated. In Star Valley, water use is regulated by a 1980 doctrine that assumes groundwater is a common resource that can be used by all, as long as it is used for beneficial purposes in the area.
“Groundwater depletion is often a consequence where the common resource is exploited in the absence of regulation or sustainable practices,” the LFR report states.
A follow-up study by Clear Creek Associates, which Payson paid for, concluded that natural water recharge significantly exceeds water demand. The study did not address the negative impacts of the Tower Well.
The recent LFR study concluded that all current wells owned by residents, Payson and Brooke Utilities, use 807 acre-feet per year, which is below the sub-basin’s safe yield of 954 acre-feet per year.
Safe yield is not the total amount of water that flows into the aquifer, but the amount that can be used, without affecting the ability of the underground water table to store water.
The balance between rainfall and use changes dramatically in drought years, the report concluded.
Sustainable yield for six-year drought
During a long-term, six-year drought, the sustainable yield would drop to just 196 acre-feet per year, which is less than half of the current groundwater demand.
During a short-term drought, generally lasting eight months, the amount of water available drops even more sharply, due to the reservoir response time.
Kuhnel explained that during a long-term drought, generally a few storms will replenish the aquifer, but during a short-term drought, there is no replenishment.
“As long as the aquifer gets one or two storms a year, you will replenish it,” he said. “In a short-term drought, there is nothing going in.”
In a short-term drought, the amount of water that can be safely extracted is 126 acre-feet.
The Tower Well alone extracted 136 acre-feet over an eight-month period in 2007, the report concluded.
The report does not explain why the town didn’t run out of water in previous droughts, even when the Tower Well was operating.
Kuhnel said several wells were impacted by the recent drought, which peaked in 2005, and had to be deepened.
LFR explains that the sharp drops during droughts occur because Star Valley’s groundwater is located in a fractured granite aquifer, which has limited storage.
“Its yield and outflow are therefore strongly dependent on precipitation as recharge,” the study states.
Kuhnel said the last two wet winters have recharged the aquifer enough that it should sustain itself for the next three years at the current rate of consumption.
During the wet winter of 2007, water levels fluctuated more than 28 feet daily in some wells. On the other hand, levels fell very little in others.
“Therefore, this study focused on the source water in a specific area within the basin where the businesses and most individual production wells and pumping clusters are located.”
LFR focused its analysis on a 15-square-mile stretch of town, where most businesses are located on Highway 260 and where approximately 1,700 people live. The area is where the majority of groundwater pumping occurs. However, the entire Star Valley basin covers close to 33 square miles, most of which is remote. LFR assumed that an average person would use 100 gallons per day — around 190 acre-feet per year.
To get a clear view of the water situation, researchers studied the amount of water removed, groundwater level changes over two years and precipitation records.
According to the study, researchers did not differentiate between natural outflows, such as how much plants suck up, and man-made uses, such as wells.
Fifteen wells were monitored on an hourly basis, and eight rain gauges located inside and outside town limits. Star Valley also placed three rain gauges near wells in 2006, which would have provided the best basis to compare water level fluctuations. However, these gauges were not used because their record is not long enough.
Besides monitoring rain gauges, well levels were tracked because, “water level in a well is a direct indication of storage of that system,” the report states. The higher the water level in a well, the more water can be removed, regardless of the well’s depth.
For example, the Lamplighter Well is 180 feet deep, but has a reservoir storage coefficient of 400 days, while the Bradley Well is 160 feet deep but only has a 190-day storage coefficient.
The wells used in the study vary in depth from just under 100 feet to 560 feet.
Star Valley’s Water and Sewer Commission Chairman Vern Leis said since the study only looks at a 15-mile section of town, results could be different in a wider area with growth factored in.
The Water and Sewer Commission is still discussing the water study, and plans to make several changes before submitting it to council.
A draft of the general plan calls for expanded growth along a business corridor east of town. The impact of additional homes and businesses on water supply is unknown, but it would likely increase usage.
“Groundwater is currently the only water source available to the Town of Star Valley, and over-exploitation through high-yield wells and export of water outside the basin threaten the resource and should be avoided,” the LFR study concluded.