City Backs off Storage of Drinking Water in Underground Aquifer

The Missouri Department of Natural Resources is requiring additional geological and chemical testing of the aquifer as a prerequisite for renewing a permit that allows for storing water underground

The Missouri Department of Natural Resources is requiring additional geological and chemical testing of the aquifer as a prerequisite for renewing a permit that allows for storing water underground.

The city spent $858,100 on the two wells to accommodate water storage in the aquifer, but they have seen little use.

Water and Light Director Tad Johnsen, in an October letter to the Water and Light Advisory Board, recommended the city let the permits for the underground water storage expire in June. The Water and Light Advisory Board agrees, Assistant Director Ryan Williams said.

In 2003 and 2008, Columbia converted two of the city’s deep wells, which were abandoned in the early 1970s when the city began piping water from the Missouri River bottoms near McBaine, for aquifer storage and recovery. 

The treated drinking water from McBaine — 40 million to 100 million gallons a year — was injected into the old wells that tap the Ozarks Plateau aquifer beneath the city. The new water sits “in a bubble” over the groundwater in the aquifer, Williams said.

“Back in 2000, someone thought it would be a really good (idea) to pour treated water down the hole," Williams said. "At the time we were doing it, it was a relatively new science. The way we were originally sold it, it was like free or almost free storage, if you will.”

According to aquifer storage and recovery principles, the treated water and the existing groundwater should not mix, he said. That means the city can pump up softened, treated water for consumption as needed.

The advantages of injecting and withdrawing water became less attractive with the state requiring an analysis to show that the treated water did not react with the existing water or the aquifer rock, Williams said.

“Essentially, they’re wanting us to develop a pretty precise geological model of the aquifer and then go into the mathematical hydrology model and prove where that plume that we inject is,” Williams said. “Prove to us that it’s staying where you said it is and it’s not migrating."

To retain the system, the city must report:

• The chemistry of the injected water, native water and when they're mixed.
• How the treated water reacts with underground rock.
• Whether the process disturbs other aquifers around the well site.
• The direction and extent the treated water travels underground.
• The tests would cost an estimated $300,000, according to Johnson’s memorandum.

In November 2014, Water and Light's engineering department approached Martin Appold, an associate professor in geological sciences at MU, to submit a proposal for conducting tests. Appold studies how underground water transports dissolved minerals as it travels through aquifers, including the Ozarks Plateau aquifer.

Groundwater trickles through aquifers at rates of a few millimeters to a few meters per year, he said. “It typically takes a long time for a plume to move from its source.”

A standard test measuring rock permeability at different depths would allow him to model more precisely how the drinking water and the groundwater move through the aquifer, Appold said.

Testing water chemistry was as straightforward as collecting groundwater samples across the city from wells that tap into the aquifer, samples of city drinking water piped in from the McBaine water treatment plant, and mixed samples of the two waters at different ratios, then analyzing the samples through an instrument that can detect dissolved elements in concentrations as low as 10 parts per billion.

Based on the composition of the mixed waters, Appold said, he can predict the solubility of different rocks and minerals found in the aquifer when exposed to the mixed waters.

The Ozarks Plateau aquifer is composed of limestone, dolomite and sandstone.

While there is a slight chance that the aquifer contains traces of minerals that might release toxic heavy metals such as arsenic or lead when dissolved in the treated water, Appold said he suspected that dissolved calcium and magnesium, which contribute to the "hardness" of the water, were far more likely.

Source: Missourian

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Taxonomy

• Aquifer
• Drinking Water
• Water Storage