Question

Explain how replacing asphalt with gravel or other permeable material can improve water quality.

Answer 1

Replacing asphalt with gravel or other permeable surfaces (permeable pavers, porous concrete, porous asphalt, gravel, crushed stone, or vegetated surfaces) improves water quality mainly by changing how stormwater is handled and treated. Key ways this helps:

How permeable surfaces improve water quality
- Increase infiltration and reduce runoff volume: water soaks into the ground instead of running off the surface. Less runoff means fewer pollutants are washed into storm sewers, streams, and lakes.
- Slow down flow and increase residence time: slower-moving water has more time to settle suspended solids and to be exposed to biological and chemical processes that break down or transform pollutants.
- Filter and trap pollutants: the surface layer (gravel/aggregate, filter media, or soil) physically removes sediments and associated pollutants. Soil and organic matter adsorb and retain metals, phosphorus, hydrocarbons and other chemicals.
- Promote biological treatment: soil microbes degrade hydrocarbons, nutrients (through microbial uptake and denitrification under certain conditions), and pathogens as water percolates through the soil zone.
- Reduce peak flows and downstream erosion: smaller, delayed peaks limit streambank erosion and the re-suspension of contaminants in receiving waters.
- Reduce thermal pollution: infiltrated water is cooled by ground temperatures; less hot runoff from asphalt lowers temperature stress to aquatic life in receiving waters.

Factors that control effectiveness
- Soil permeability and depth to groundwater: coarse, deep soils maximize infiltration and treatment. Shallow soils or a high water table limit storage/treatment capacity.
- Pollutant type and load: permeable systems are very good at removing sediments, suspended solids, and many metals and hydrocarbons; they are less effective or require design controls for soluble contaminants (salts, some pesticides) or large concentrated spills.
- Pretreatment and design: inlet sediment traps, vegetated swales, and grit chambers prevent clogging and extend treatment performance. Layered filter media and underdrains improve reliability.
- Climate and storm size: permeable pavements can often infiltrate most small-to-moderate storms; very large storms may exceed storage capacity and produce runoff.
- Maintenance: regular sweeping, vacuuming, and replacement of clogged surface media are essential to preserve infiltration and pollutant removal.

Potential drawbacks / cautions
- Groundwater risk: in areas with very permeable soils and contaminant sources (fuel stations, industrial sites), infiltration can mobilize contaminants into groundwater. In those cases, use treatment trains, liners, or direct runoff to centralized treatment.
- Clogging: if not properly maintained or if sediment loads are high, pores can clog and performance declines.
- Not always suitable for heavy traffic or steep slopes without appropriate engineered design.

Practical design/maintenance recommendations
- Use pretreatment (sediment forebays, vegetated strips) to remove coarse sediments before water reaches the permeable surface.
- Design sub-base storage and underdrains sized for local rainfall and soil conditions.
- Implement a maintenance plan: frequent sweeping (especially first year), vacuuming every 1–3 years, replenishing gravel or joint material as needed.
- Restrict use in high-risk contaminant areas unless additional treatment is provided.
- Combine permeable surfaces with other green infrastructure (bioretention, swales, rain gardens) for a treatment train that maximizes pollutant removal.

Bottom line
Replacing impermeable asphalt with permeable materials reduces the volume and velocity of runoff, filters and biologically treats pollutants as water infiltrates, and lowers peak flows to receiving waters — all of which lead to improved water quality when the permeable system is properly designed and maintained.