Installing solar at a large scale requires increasingly bigger plots of land to host acres and even miles of PV arrays on a single tract. Accompanying that growing scale of project scope is the responsibility to implement proper stormwater runoff measures to protect the environments and communities the projects are built in.

In any construction project requiring civil work, excavated land that isn’t seeded or is missing topsoil has higher risk for erosion. Stormwater runoff, like rainfall or melted snow, can take substances like pollutants and debris that are detrimental to local waterways and carry them into neighboring land and municipal sewer systems.

The National Pollutant Discharge Elimination System (NPDES) is a federally mandated permit program with measures for stormwater runoff remediation at construction sites. This permit is issued either directly by the Environmental Protection Agency or through an authorized state, which most are qualified as. From there, states can have their own requirements for stormwater runoff remediation at worksites.

Any construction project with land disturbance exceeding one acre is generally required to have an NPDES permit. It describes design parameters for building proper stormwater runoff management at a construction site and how to close the permit once a project is complete.

“The biggest challenge is managing stormwater to prevent any impacts off site that could impact sensitive habitat,” said Christina Hebb, stormwater pollution prevention plan and vegetation senior manager at McCarthy Building Companies. “The permit language states that we have to do everything in our power during planning designs to really minimize the release of sediment off site.”

There are many preventative measures that can be taken to curb stormwater runoff on solar projects. Hebb oversees solar project site preparations for McCarthy, a national construction company with a utility-scale solar installation arm. She said the most important part of preparing solar sites is establishing vegetation on the land before construction begins.

“Until you really get vegetation established, you’re not going to have a fully stabilized site,” Hebb said. “Any type of seeding we can do ahead of pile installation is going to significantly reduce your costs, it reduces your risk and it will increase production during construction because you’re not going to work on muddy soil conditions.”

Rooted plants stabilize topsoil — the first several of inches of dirt starting from the ground surface. As precipitation and runoff percolate and pass over that soil, the roots of plants prevent it from washing away. It’s nutrient-rich and the ideal place to sow native seed crop.

While ground-mount racking and solar tracker systems have become more adaptable to undulating topographies, there’s still the possibility that some grading at a solar project site will be necessary to install the array. If the land must be graded, Hebb implores contractors to save the topsoil they remove, because it can be replaced later.

Failing to establish initial seeded topsoil can require additional ground amendments, soil compaction and possibly reseeding multiple times after construction to grow vegetation.

Once a site has the proper vegetation in place, other methods can ensure stormwater runoff is properly managed, such as:

• Sediment basins are dug out on construction sites to gather and pool water so the sediments within it can remain onsite and settle into the ground.
• Check dams are temporary structures made of stone placed along construction site water channels to stop sediment passing through. They’re used in combination with other runoff retention devices.
• Swales are dug channels with grass-cover that act like topsoil and guide water flow. Water passing through a swale is exposed to grass, which catches sediment and encourages the water to infiltrate the soil along its route.
• Silt fences are installed on worksites during construction and until vegetation is grown. They are made of fabric that acts as a barrier to keep soil and sediment on a worksite.

“The idea is that we’re flowing water down in the arrays and then catching any sediment that might be moving, but really the perimeter and the basins are there to be that line of defense so that when water is leaving the site, it’s clear,” Hebb said.

Any tamped or graded land must be decompacted and loosened to promote future plant growth in that soil. Contractors should follow the natural drainage patterns of the land they’re working on and maintain those water routes throughout and after construction. It might seem like a lot of work up front, but in totality it pays off.

“We’re really only working on a project from start to finish for two years, including the planning and pre-construction design phase through operation,” Hebb said. “Solar projects are designed to last 30 years or so. In the scale of the project, the thing that we’re doing in that window, you have such a significant impact on the success of the project.”

Solar stormwater conflict

Stormwater runoff controls on construction projects are required everywhere in the United States. But that doesn’t mean every state will issue the same regulations for each type of structure being built. That’s the case with stormwater controls on solar projects built in Virginia.

In Spring 2022, the Virginia Department of Environmental Quality (DEQ) issued an update to the Virginia Stormwater Management Program, amending that solar arrays are considered impervious surfaces. An impervious surface is one that doesn’t let water soak into the ground. DEQ considers sidewalks, streets, industrial buildings, lawns and now solar panels as impervious surfaces.

“There are no additional required stormwater controls specific to solar development over other land-disturbing activities,” said Mike Rolband, director of Virginia DEQ, in an email. “However, DEQ does have guidance which requires that any solar project that does not obtain an interconnection approval by a regional transmission organization or electric utility by December 31, 2024, to consider all ground-mounted solar panels as unconnected impervious cover.”

DEQ purports that impervious cover is typically attached to stormwater controls. Solar projects are being considered “unconnected impervious cover” because precipitation falling from the modules must travel across the ground before reaching a stormwater channel. This change to post-construction stormwater management could require developers to use more land for stormwater controls on their solar arrays.

Opponents to this change for stormwater controls on solar projects believe it’s unfair and a detriment to the industry in the state. Yet, Virginia has a history of issues with stormwater runoff, and the state has upped efforts to meet minimum erosion controls on construction projects.

“Without proper stormwater management and erosion control devices in place, erosion and sedimentation of natural receiving channels and waterways may occur,” Rolband said. “In addition, increased runoff due to the change in land cover conditions may result in increased flooding of downstream properties and resources.”

Not every state will regulate stormwater controls on solar projects like Virginia. Each region has individual environmental and regulatory characteristics that solar installers must abide by to keep the land they’re building on as green as the PV modules they’re installing. Starting with a baseline of proper topsoil and vegetation management and adding the appropriate stormwater channels can help developers keep the surrounding waterways clean during construction and the project’s entire lifespan.