On Driveways and Water Quality

Driveways are a defining characteristic of many suburban neighborhoods. They have the simple utilitarian task of providing a transport route or storage place for our vehicles. Some can host basketball games or chats with neighbors. Due to their prevalence they comprise a significant portion of the developed landscape and therefore, can have an impact on water quality.

There are three ways the water from driveways can pollute streams. First, while traveling over the driveway the water picks up any oil, antifreeze, leaves or litter left there, carrying it to streams. Second, during rainstorms driveways, like all impervious surfaces, shed water that would otherwise be absorbed and filtered by soils. As a result more water enters storm drains and flows directly to streams where it causes erosion. Third, the material used to coat driveways can erode and cause pollution. Many driveways are coated with coal tar-based sealants. Coal tars are a by-product of the production of coal coke, distilled coal used as fuel and in iron ore smelting. Many contaminants are released in the erosion and breakdown of coal tars, most notably polycyclic aromatic hydrocarbons (PAHs), many of which are suspected human carcinogens and toxic to aquatic life.

Driveways that shed less water will have less of an ecological impact. Infiltration is much higher for driveways of crushed stone or pavers than it is for asphalt.  Some people eliminate their driveways and replace them with hardened plastic grids that support the weight of cars, but allow grass to grow between the grid cells.

Permeable pavements can be used to reduce the amount of water driveways shed. Permeable pavements have less fine materials in the asphalt mixture which allows water to fill some of the gaps and move through the material. Regular vacuuming of permeable pavement prevents the pours from getting clogged. In 2008 the Chicago White Sox became the first Major League sports facility to use permeable paving when they installed the largest permeable pavement parking lot in the country at their new stadium. 

Even if changing the type of driveway you have is out of the question, driveways can be managed to reduce their ecological impacts. Some actions are quite simple. Regularly sweeping your driveway prevents a build up of litter and leaves. Ensuring that downspouts from the house do not drain onto the driveway but rather onto lawns and gardens prevents the driveway from becoming a conduit for rainwater from the roof during storms and lets soil filter some of the water.

On a summer day it is not unusual to travel around a suburban neighborhood and see buckets with black ooze blocking the ends of shiny new looking driveways. Sealing driveways is generally recommended every three years or so, but the choice of sealants one uses can affect water quality. Sealants are the black liquids sprayed or painted on pavement to protect the asphalt surface. A study by the USGS in Austin, Texas compared runoff from unsealed parking lots, those with coal tar based sealants, and asphalt sealed parking areas. Export of PAHs from the lots with coal tar sealant exceeded exports from unsealed parking areas by 65 times, while asphalt sealed areas were intermediate. The study authors concluded “runoff from sealed parking lots could account for the majority of PAHs” in streams in the study area. A number of sealants that do not use coal tar are available on the market.

Car-washing on driveways is another means by which pollution can be transported to waterways, but small changes in how it is done can reduce impacts. It is important to think of ways to reduce the amount of water that runs off the property when removing grease and grime from your vehicle. Cars can be washed on the lawn or water on the driveway can be diverted into the lawn before it reaches storm drains or the street. Trigger hoses or buckets are preferable to hoses that flow continuously. Minimizing the frequency of washing can also reduce subsequent water pollution. Commercial car wash facilities offer another option. They usually use less water per wash and have specialized drainage and equipment to treat the water used. 

Driveways are a ubiquitous feature in today’s landscape; so long as we have cars they will be with us. We cannot completely eliminate the impacts driveways have on the environment, but we can minimize them.