Environment

"Downwelling" could be a temporary fix for aquatic dead zones

"Downwelling" could be a tempo...
A downwelling testing rig, at the Searsville Reservoir in California
A downwelling testing rig, at the Searsville Reservoir in California
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A wider view of the Searsville Reservoir
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A wider view of the Searsville Reservoir
A downwelling testing rig, at the Searsville Reservoir in California
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A downwelling testing rig, at the Searsville Reservoir in California

Aquatic hypoxia can be a serious problem, producing oxygen-depleted "dead zones" in lakes or seas. New research suggests that a process known as downwelling may help keep those zones from forming – although it wouldn't be a cheap solution.

Hypoxia typically occurs when excessive amounts of nutrients from land-based agriculture or other human activities flow into large water bodies, creating algae blooms. The overly-abundant algae proceed to use up much of the dissolved oxygen in the water, causing fish and other organisms to die off. Because there's already less oxygen the deeper one goes in a lake or the ocean, the problem is particularly severe at lower depths.

Ideally, the formation of dead zones would be minimized by altering farming practises, creating better wastewater treatment facilities, and otherwise getting people to do things differently. This could be a slow and complex process, however, so researchers at Washington, DC's Carnegie Institution for Science began investigating new ways of keeping the existing pollution from causing hypoxia.

Current approaches include utilizing bottom-laid pipes to bubble air up through the water column, or using fountains to increase oxygenation at the surface. Because neither of these are highly efficient, the Carnegie scientists instead looked to downwelling – this involves pumping more-oxygenated water down from the surface, so that it can disperse oxygen into the hypoxic depths.

In tests performed at the Searsville Reservoir in Woodside, California, it was found that downwelling could increase oxygen saturation by 10 to 30 percent – reportedly enough to keep many marine organisms from dying of oxygen deprivation. The effect was only noted within a few meters of the water pipes, however. This means that an extensive network of piping, and a large amount of energy for operating the pumps, would be required to make an appreciable difference over a typical dead zone.

A wider view of the Searsville Reservoir
A wider view of the Searsville Reservoir

According to the team's calculations, using downwelling to treat the dead zones that occur annually in Chesapeake Bay would cost somewhere between US$4 million and $47 million – that figure climbs to a range of $26 million to $263 million for use on the dead zones in the Gulf of Mexico.

Nonetheless, it is believed that in the short term, it would still be considerably cheaper (and thus more doable) than making sweeping changes to agriculture, wastewater treatment, and other well-established human activities. Additionally, based on computer models, large-scale downwelling should be three to 100 times more efficient at oxygenation than the use of air-bubbling pipes, and 10,000 to a million times more efficient than fountains.

"Reducing nutrient pollution is the only way to eliminate hypoxia permanently," says Carnegie atmospheric scientist Ken Caldeira, who is leading the study along with marine scientist David Koweek. "However, our work shows that downwelling is a technological solution that could mitigate the risk of low-oxygen dead zones while nutrient management strategies are put in place."

A paper on the research was recently published in the journal Science of the Total Environment.

Source: Carnegie Institution for Science

11 comments
PhilDor
Downwelling is too expensive, upwelling is the way to go, but not by laying airpipes at the bottom of the lake. Every aquarium hobbyist knows what is called an air-lift, a vertical pipe sucking the water from the bottom by having air pumped in 1/4 from the surface with the use of a ventury system. The air bubbles being confined inside the pipe will push the water upwards in a constant and very efficient way as well as a water pump can do. In the case of this lake, the 10" PVC vertical pipes with ballasts & floats could be sucking the anoxic water from any depth to have it being aerated once it reaches the surface of the lake and if this process in continued for long enough all the anoxic water could be replaced by oxygenated water in weeks or days. This is very basic hardware, easy to use & install, economic and super reliable and efficient; why would you want to do this any other way?
Jay Gatto
@PhilDor - that rings true for me. Why was it not even mentioned as a possibility, even if were found not to be (I'd doubt) far more effective?
CarolynFarstrider
The relative costs here make no sense. If you can reduce the pollution at source, that's a one off cost of whatever tech or operating system change is required, which can be defrayed over an appropriate period of say, ten years. Indeed, it might even be a cheaper way of operating than wasting nutrients, as happens at present. But installing any type of pumping system represents a continual cost, in perpetuity, with associated fuel costs and carbon emissions. Bite the bullet, stop the pollution at source rather than trying to cover it up. In any case, what happens when the entire water column is toxic?
Aross
I had the same thought as PhilDor. In addition to pushing anoxic water up this system will aerate it along the way increasing the benefit. The system could be driven by solar or wind energy there by almost eliminating any additional pollution caused by conventional energy production. My only concern with this is what effect moving this oxygen-less water to the surface will have on the upper water table and visa-versa.
Brian M
A thought - If you increase the oxygenation - any risk of increasing the algae blooms, the ones depleting the oxygen? Logical solution is stop the additional nutrients entering the system. Another thought what is the natural consumer of algae?
Signguy
Wow, I used to swim in this water! Philidor has it right, BUT, why don't we go back to more organic ways of planting; see the video "Back to Eden". https://www.backtoedenfilm.com/ The more we wait, the more expensive it will be...
Laura Farley
Phildor...hope there are people with inte llegence who read your post and act on it....bravo
Gregg Eshelman
Build a holding tank upstream to capture part of the water flowing into a lake. Run a pipe from that tank down into the lake, through a manifold that branches to pipes that let out directly into the deepest parts of the lakebed. Gravity does all the work and water with a higher oxygen level gets fed to the deepest levels.
HighlanderJuan
There are some excellent suggestions here, including one from Carolyn Farstrider that suggests going to the source and getting the pollution stopped from flowing into the water. I note that adults usually pick up after themselves, and so perhaps getting the farmers to clean up their act might be a good target. Use some of that federal money, or maybe creating some tax incentives, on improving the existing system,
Kpar
One question- isn't algae a plant? Do plants not consume CO2 and release oxygen? Someone will have to explain to me how an algae bloom causes anoxia.