California's stormwater environment is characterized by drought and a strategic objective to capture and reuse water. Learn more about the state's surface water challenges, regulations and solutions.

Stormwater in California

Stormwater, or surface water runoff, is the result of rain, snow and other precipitation falling on impermeable surfaces such as roads, parking lots and roofs. This surface water picks up pollutants and carries them to drainage networks and bodies of water. These pollutants are at best a nuisance, but at worst can be extremely damaging to the environment.

The stormwater situation in California is shaped by the drought that has been affecting the state for several years. Despite its drought, however, California receives on average around 200 million acre-feet of precipitation annually. Of this, roughly one third reaches rivers in California’s ten watersheds, one third reaches the Pacific Ocean, and one third infiltrates to the more than 400 aquifer basins that lie beneath some 40% of the state. At its higher elevations, snow melt contributes further seasonal surface water.

California has a robust and comprehensive stormwater management program aimed at preserving the quantity and quality of water within the state.

California surface water treatment priorities

Given the drought situation, a high priority for California is to use stormwater as a resource. This involves keeping stormwater on site and using it to replenish aquifers and other ground water, or capturing it and reusing it for other purposes. Both of these require that the stormwater is treated to remove pollutants, which adversely affect water quality, impede both infiltration and reuse, and cause harm to both the fresh water and marine environments.

The California Department of Transportation, Caltrans, identified the following six types of pollutants as causing the most harm to California’s water quality:

• Gross solids, or trash
• Total suspended solids (TSS), sometimes also referred to as silt or sediment
• Nutrients, sometimes also referred to as biochemical oxygen demand or BOD
• Heavy metals, such as copper, zinc and lead
• Bacteria
• Pesticides

A significant treatment priority for California is the principle of “full trash capture”. This entails installing treatment systems certified for the full capture of all solids 5 mm in diameter and larger, at design flow and without bypass, in order to prevent gross pollutants from entering California’s waterways. To achieve this, Trash Amendments to the state’s stormwater regulations were developed and went into effect in 2015; they must be implemented in full by 2030. 

Stormwater regulations in California

Stormwater regulations in the US stem from the 1972 Clean Water Act, and the associated National Pollutant Discharge Elimination System (NPDES). These are ultimately managed by the EPA, but the agency delegates authority to the states for local management and enforcement. In California, the authority is delegated to the State Water Resources Control Board and the nine Regional Water Quality Control Boards.

These boards implement multiple stormwater permitting programs, which regulate the discharge of surface water in the state. These include:

• Municipal Separate Storm Sewer System (MS4) Permits - Phase I (medium and large municipalities) and Phase II (small municipalities)
• The California DoT, Caltrans, is the largest single discharger of stormwater in the state, so it has its own Phase I MS4 permit
• Construction Stormwater General Permit
• Industrial Stormwater General Permit

In conjunction with these regulations, California prioritizes two development philosophies that have two primary objectives: to reduce the environmental impacts of stormwater pollution, and to use stormwater as a resource. These philosophies are known as Low-Impact Development, or LID, and Green Infrastructure.

LID is a design approach in which a development is structured to mimic as closely as possible the drainage patterns of the pre-development environment. For example, if an undeveloped area allows 90% of rainfall to infiltrate into the ground to replenish the water table, then the developed area should also aim to have 90% of rainfall infiltrate. This is often referred to as runoff reduction.

Green Infrastructure takes this one step further, applying similar principles on a larger scale. Where LID might apply to a parking lot or a commercial center, Green Infrastructure applies to the surrounding area and community, and applies runoff reduction and stormwater treatment techniques and technologies wherever possible.