Stormwater Guidelines for Solar Farms

Utility-scale solar farm developers often have questions about National Pollutant Discharge Elimination System (NPDES) permits and requirements. We’ve put together these stormwater guidelines for solar farms to answer some of the most common inquiries. The key takeaways of this guide are:

• Solar farms that disturb more than one acre of earth will need NPDES permits.
• Land disturbance caused by creating solar farms can affect water pollutants.
• Pervious-cover construction sites can help manage stormwater runoff.
• Infiltration trenches and berms can support solar farms with less vegetation. 

How NPDES Affects Solar Farms

NPDES influences the development process for solar farms. These FAQs offer more clarity on how it works.

What Is the Purpose of the NPDES?

The NPDES was created by the Clean Water Act (CWA) in 1972. It aims to regulate sources of water pollution in the U.S. by setting technology-based limits and water-quality-based limits. 

NPDES affects solar panel farms since these farms can cause earth disturbance, which can, in turn, affect water pollutants. 

What Constitutes “Earth Disturbance”?

25 Pa. Code §102.1 defines earth disturbance as a human activity, such as construction, that disturbs the land surface. Activities that are often associated with earth disturbance include any kind of grading, clearing or grubbing. For a solar farm, this disturbance often applies to the installation and mounting of the solar devices, and the creation of any access roads or support buildings.

What Solar Farms Need NPDES Permits?

25 Pa. Code §102.5(a) states that NPDES permits are required for any land disturbances of one acre or more. Since utility-scale solar farms cause land disturbance in an area of one acre or larger, these projects will require an NPDES permit.

Does My Solar Farm Need a PCSM Plan?

If your solar farm requires an NPDES permit, you’ll need to create a post-construction stormwater management (PCSM) plan. This plan details how developers will mitigate the effects of their construction on stormwater runoff, both during and after the construction process. 

25 Pa. Code §102.8 outlines several PCSM requirements. It states that the rate of runoff volume after construction must be equal to or less than the runoff occurring prior to development.

How Does Solar Farm Earth Disturbance Affect Water Pollution?

The Erosion and Sediment Pollution Control Program Manual states that construction sites of any size can cause these hazards:

1. Adverse changes to surface runoff patterns
2. Soil being exposed to erosive drivers
3. Greater amounts of stormwater runoff, faster erosion rates and more sediment yield
4. Exposed subsurface materials that are detrimental to existing surface vegetation
5. Changes to groundwater regimes that can affect slope stability, vegetation and drainage systems

Locations that are flat, cleared and accessible will minimize earth disturbance and its associated risks. Where earth disturbance is unavoidable, developers should protect the existing vegetation and drainage systems as much as possible. They should also minimize stormwater runoff and soil compaction.

To reduce the risk further, developers should use best management practices (BMPs).

What BMPs Can I Utilize for My Construction Site?

If your solar farm meets the threshold for NPDES requirements, you’ll have to get a permit. However, if your site is considered to be pervious cover, you’ll be permitted to model runoffs like a vegetative meadow rather than a paved area. This means you can use additional methods and tools to manage stormwater runoff. 

For your construction site to be considered pervious cover, you’ll need to use several BMPs:

• Grading and disturbance specifications: Developers should disturb and grade the earth as minimally as possible through construction that causes minimal adverse effects to preserve natural vegetative cover. This cover should be dense enough to combat increased rates of sedimentation and erosion, and have at least 90% permanent vegetation.
• Slope requirements: Each photovoltaic panel should be situated on a slope of less than 10% and allow runoff passage between each unit. It should also permit vegetation to grow between the solar arrays and underneath the panel. In certain circumstances where the slope exceeds 10%, there are BMPs to mitigate the energy of increased stormwater flow at the drip edge of each solar panel.
• Optimal distance: Developers also need to find the ideal distance between the array and the ground. The arrays should be high enough to encourage vegetation growth beneath each array, but not so high as to accelerate erosion between them.

Not every solar farm will meet all these criteria perfectly, but the closer a developer can get to meeting them, the better the site can increase pervious coverage.

Are There Different Requirements for Tracked-Panel Units Compared to Fixed Units?

Yes, developers should be aware of several differences. Without sufficient area between each one, tracked-panel units may overlap and cast shade over neighboring panels. Compared to fixed units, tracked-panel units will need additional space. The design of tracked-panel units is more complicated, too, requiring additional mechanical features to successfully track the sun.  

All of this results in an increased earth disturbance. However, if BMPs are used effectively, this disturbance can still be minimized.

Can I Grow Agricultural Crops Underneath My Panels?

It’s a great idea to grow crops under your panels, as this creates more efficient land use, as long as:

• The crops can thrive in the shade.
• The crops don’t need moldboard plowing.
• The panels are set higher from the ground when the crops need the space to grow.
• Pesticides, herbicides and chemical fertilizers are only used for the crops’ agronomical needs.
• An erosion and sediment control plan is written to control agricultural plowing and tilling.

How Can I Better Manage Wider Solar Panel Modules?

Wider solar panels make it more difficult for vegetative cover to grow and survive, but BMPs can be used to overcome this challenge.

Installed between each row and facing downhill, infiltration trenches are filled with stones and perforated throughout. They also have a level bottom and can be installed without heavy machinery. The purpose of these trenches is to evenly distribute stormwater into the surrounding soil. This reduces runoff volume and improves groundwater quality.

Infiltration berms can also be used to redirect stormwater, infiltrate it into the soil more evenly and reduce runoff. These compacted earth mounds are usually constructed on gently sloping ground. 

For more information on how to construct infiltration trenches and berms, see the PA Stormwater BMP Manual, BMP 6.4.4 and BMP 6.4.10.

Are There Additional Requirements if My Proposed Construction Site Needs to Be Regraded?

If regrading the area is necessary, soil amendments and restorations should be used afterward. To do this, developers can restore soil porosity and add a layer of compost or other soil amendments. This promotes the soil’s infiltration and pollutant removal abilities.

Riparian buffers, made of permanent areas of trees and other vegetation, can provide ecological and water quality benefits for areas where vegetation has been removed.

For more information, see the PA Stormwater BMP Manual, BMP 6.7.1 and BMP 6.7.3.

Why Trust MPL Law to Keep Your Solar Farm NPDES-Compliant?

Our Pennsylvania environmental law services combine our legal knowledge with scientific training, allowing us to offer practical advice that helps you comply with NPDES regulations. We’ll provide honest advice, deliver forward-thinking solutions and work closely with you every step of the way. 

Whatever challenges your solar farm faces with NPDES compliance, MPL Law is here to help. 

Keep Your Solar Farm NPDES-Compliant

As a developer, you have plenty of responsibilities, with stormwater management just one of them. MPL Law can help you manage the impact your solar farm will have on stormwater runoff. 

To learn more about how your solar farm development can benefit from our legal services, contact us today.