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Researchers say stormwater from solar fields needs careful management

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By Whitney Pipkin
Bay Journal News Service

Solar panels are going up across the Chesapeake Bay watershed to help states reach their renewable energy targets. But, while working to achieve climate-related goals, solar fields have the potential to generate water pollution — through increased stormwater runoff.

And, until recently, little work was being done to understand the impact of solar fields on the way stormwater runs off the landscape and into local waterways.

As the science begins to come in, policymakers in Virginia are grappling with a dilemma: How much should solar fields be subject to stormwater controls? It’s a pressing question because solar development in the state is charging ahead. The state ranked fourth in the nation in 2021 for its pace of new solar installations, and hundreds of thousands of acres there could be converted to solar projects in the coming years.

The crux of the problem lies with whether solar fields should be considered pervious or impervious land cover. Pervious areas allow water to soak into the ground. Impervious areas, like roads, rooftops and parking lots, do not. Polluted runoff from those hard surfaces causes problems for waterways across the Bay region — making them subject to regulation.

Solar fields have both pervious and impervious elements: often enormous acreage covered by the panels and a range of soil conditions and groundcover below them.

Many states consider solar fields pervious, which cuts regulatory red tape. Also, the volume and velocity of runoff from the panels falls somewhere between that caused by farmland and parking lots, depending on the type of groundcover under the panels. That makes solar facilities difficult to regulate under existing models.

“There is a whole lot of science around stormwater regulation, but not for the kind of land use that is a ‘solar farm,’” said Brian Ross, vice president of renewable energy for the Great Plains Institute for Sustainable Development, a Minnesota-based firm researching ways to improve renewable energy.

In the Chesapeake watershed, Maryland and Pennsylvania have policies that either consider the panels pervious under most conditions or exempt them from being considered impervious for the purpose of stormwater management.

Until March, Virginia did, too. That’s when the state’s Department of Environmental Quality Director Mike Rolband announced that solar projects there would be subject to stronger post-development stormwater regulations, effective immediately.

In a memo announcing the change, Rolband said that treating solar installations as pervious cover could “underestimate the post-development runoff volume or runoff rate from solar panel arrays, which in turn has the potential to negatively impact downstream waterways or properties.” He noted that the Chesapeake Bay Program considers solar fields “unconnected impervious” when calculating the impact of land use on water quality in the Bay and its rivers.

Industry concerns rose quickly. Two weeks later, the agency said it would allow more time for projects to comply and indicated that stakeholder feedback would be considered in shaping how the policy will be applied.

The agency guidance document is awaiting approval.

As it stands, David Murray, director of solar policy for American Clean Power, said the changes Virginia regulators proposed for dealing with stormwater could require solar facilities to acquire 20% more land for projects to offset the impervious areas. That would have “a significant impact,” he said.

Research that could help inform such decisions is just beginning to come out.

Seeking science

Decisionmakers are looking to the scientific community for more research that could help balance the need for cleaner energy with commitments to improve water quality.

So far, studies indicate that one of the biggest factors in reducing the impact of solar panels on runoff could be the types of soil and groundcover underneath them. But places that may be ideal for solar development from a big-picture perspective — using former industrial sites, for example — are often not the most economically attractive. Not to mention, the regulatory landscape leaves solar placement decisions to individual landowners, zoning boards and county officials, all of whom stand to benefit financially from leasing to solar suppliers.

Seeing the smattering of different regulations facing solar development, the U.S. Department of Energy contracted the Great Plains Institute to study how stormwater runs off solar panels on a variety of landscapes. Their study measured how water runs off solar installations in five states, each with soil types ranging from rocky to sandy to clay based.

In their nearly complete, three-year effort, researchers found that one of the best ways to reduce problems with stormwater runoff from solar sites is to avoid compacting the soil during construction. Driving heavy equipment across a site or grading it has an outsized impact on the volume of runoff both during and after construction.

The soil type also matters a great deal. Sandy soils like those of Minnesota, where one research site was located, can quickly absorb rainfall coming off solar panels. Clay soils like those studied in New York struggle to absorb runoff if they are compacted or lack vegetation.

One of the best practices that nearly every site can apply is to grow the right kind of vegetative cover under the panels, preferably native grasses with deep roots that can reduce soil compaction. Spacing solar panels farther apart to provide more land to absorb the stormwater also helps, but less than researchers originally thought. And it’s one of the more-expensive mitigation tools.

The research did not study sites with bare earth under the panels “because we already know from stormwater research what that will give us,” Ross said, a nod to sediment easily running off such properties.

But when vegetation covers the landscape under and around solar panels, “in almost every case, you are better off [from a stormwater perspective] with well-managed solar than with agriculture,” Ross said. “Converting forest to solar is a very different circumstance. From a stormwater standpoint, the best groundcover you can have is forest.”

The researchers have produced a best practices document and will soon release equations to calculate runoff from different solar practices. An instruction manual for implementing the findings at various locations is due out this fall.

There’s little a solar developer can do about the soil type once a property is leased for solar construction, so more stormwater mitigation could be needed on some sites. National soil maps could help guide decisions about where to locate solar in the first place, Ross said.

Research like this “places more emphasis on finding sites that are suitable — not just considering where it is on the [energy] grid, but also taking water quality into account,” Ross said.

If solar developers don’t consider soils, previous land use and stormwater dynamics when selecting a site, “are [they] going to create costs for someone else who’s regulating water quality?”

The solar industry also has a taskforce researching best practices for reducing stormwater impacts. Most of the measures considered best practices by researchers, though, are not required by localities.

Bay perspective

Meanwhile, the Chesapeake Bay Program, the state-federal partnership leading the Bay restoration effort, is studying how the conversion of land to solar fields will impact the region’s ability to reach water quality goals.

Officials confirmed that, for calculations in the Bay Program computer model, solar sites are defined as “unconnected” impervious to account for spacing between panels. But the specifics of how solar acreage is incorporated into the model could change as it’s informed by additional research.

“There is the guidance on how solar should be installed and managed, but then there is the actuality of it. There may be a wide variety of compliance to those recommendations,” said Peter Claggett, a researcher with the U.S. Geological Survey who coordinates the Bay Program’s Land Use Workgroup. “And it’s not clear to us which of these solar facilities are done well and which aren’t.”

The Bay Program will offer a workshop this fall to answer some of these questions and better inform the model that demonstrates how these types of changes impact water quality.

Virginia legislators are conducting another set of meetings on the subject this fall with what one senator called “the mother of all stakeholder groups.” House bill 206 required the DEQ to assess the impact of smaller renewable projects on prime agricultural and forested lands, then propose mitigation measures.

Some organizations wonder if the effort will be too little, too late to keep pace with solar development while efforts are under way to meet Virginia’s share of the 2025 Bay cleanup goals.

The nonprofit Piedmont Environmental Council is particularly concerned about the impact of solar development on what they consider Virginia’s prime soils.

If those soils are compacted or graded, “you forever alter the runoff characteristics of that property, because you’re changing the absorption rate of that soil as well,” said Dan Holmes, a consultant on solar issues for PEC.

Holmes points to the largest solar installation recently built in Virginia on 6,000 acres in Spotsylvania County as an example of such projects bring sweeping change to land use. The site was previously used for rotational timber harvesting, so the land use change was considered significant. Virginia’s State Corporation Commission had to sign off on it, and large stormwater retention ponds were required to filter runoff from the site.

Solar development in the state, if it continues at this pace, would represent “the biggest land use change we’ve ever seen,” PEC President Chris Miller said.

Although technological innovations make energy generation more efficient, Miller said current projections (based on 1 megawatt of power being generated from seven to 10 acres of solar) indicate that 200,000–300,000 acres could be converted to solar fields in Virginia.

“That’s bigger than Shenandoah National Park,” Miller said. “So, for us, that’s a land use problem that we have to consider in aggregate, not just on a site-specific basis. Like everything else, it’s the sum of the acres.”

Whitney Pipkin is a Bay Journal staff writer based in Virginia. You can reach her at [email protected]. This article first appeared Aug. 15 on Bayjournal.com and was distributed by the Bay Journal News Service.



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