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Mon, Dec. 09

Pilot program for rainwater harvesting to feed acquifer

This rainwater harvesting system shows how rainwater can collect and be processed.

This rainwater harvesting system shows how rainwater can collect and be processed.

An idea for recharging aquifers could increase the efficiency of traditional rainwater harvesting systems and direct the water back to the aquifer, according to a Well Owner’s Rainwater Concept Paper by John Munderloh, water resources manager for Prescott Valley.

Rainwater Harvesting fast facts:

A 2,500-square-foot roof can capture 1,558 gallons of one inch of rainfall in an hour with a traditional harvesting system using gutters and storage units. An average annual rainfall of 8.5 inches exceeds the capacity of most tanks resulting in the loss of about 78 percent of available rainwater.

A French Drain directs all rainwater throughout the year back into the ground, recharging the aquifer.

Typical systems capture rainwater from rooftops and store it in cisterns or barrels where homeowners or businesses can use it later for their landscaping needs. Munderloh would like to see a pilot project built that evaluates a different kind of harvesting, the French Drain.

Yavapai County Board of Supervisors Chair Jack Smith brought the issue to the board and the county Planning and Zoning Commission earlier this month at a joint meeting. Currently, the county has no requirements for installing a rooftop rainwater harvesting system unless the storage tank exceeds 5,000 gallons or its ratio of height to diameter does not exceed 2:1, Senior Planner Tammy DeWitt stated in her memo to the board.

Several supervisors and commissioners said they are opposed to mandating new construction to require a rainwater harvesting system, even though they encourage its use. Munderloh said Prescott Valley also urges harvesting without any requirements.

Munderloh’s concept paper points out that most systems are capable of storing only 22 percent of available annual rainwater. Pumps, filtration systems and backflow prevention devices can be costly and need maintenance. The French Drain system, at one-third of the cost, can capture all rainwater and can extend the useable life of private wells.

Smith asked if there was interest in joining Prescott Valley to test this alternative system, also known as a soakaway pit, that basically consists of a 2-foot-wide trench about 6 feet deep, filled with 2 feet of gravel and 3 feet of excavated material, with a pipe at the bottom. Rainwater directed to the French Drain would percolate down into the aquifer.

This kind of system would be most useful for properties two acres or larger whose homeowners rely on private wells, Munderloh said this week. It utilizes the captured moisture not for the landscape or yards, but to return it to the aquifer.

The idea is only on paper now, but building a pilot project would give him the opportunity to evaluate the effectiveness of this alternative system, he said.

There are some requirements to installing a French Drain system. The Arizona Department of Environmental Quality (ADEQ) does require registering the French Drain system with a $100 fee, the concept paper states. And an unknown issue relates to a required setback of 100 feet from the groundwater well that may affect the effectiveness of the system. Also, the size and depth of the ditch could depend on the makeup of the soil – Prescott Valley, for instance, has more clay-ish soil; Williamson Valley’s soil is sandier in many areas and allows for better permeability.

“The size of the roof, the drain, soils, costs – all these factors would come into the study,” Munderloh said. “We’ve been trying to find a suitable funding source for a pilot project to evaluate whether the thoughts on paper will pan out. It appears more cost effective than capturing rainwater in aboveground systems. We need to test it first.”

Board members and commissioners requested Smith provide them with a handout and more information to study the idea.

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