Impervious surfaces are human-made structures, such as roads, buildings, sidewalks, and parking lots, that do not allow water to penetrate to the soil, which prevents infiltration, increases surface runoff, and leads to flooding in urban areas (AP Enviro EK EIN-2.M.3).
Impervious surfaces are exactly what they sound like. "Impervious" means water can't get through. Roads, rooftops, sidewalks, and parking lots are all hard, sealed surfaces, so when rain hits them it can't soak into the ground the way it would on grass, forest floor, or farmland. Instead, that water becomes surface runoff that rushes across the pavement, picks up speed and pollutants, and overwhelms storm drains and streams.
The CED definition (EK EIN-2.M.3) is short and specific. Impervious surfaces are human-made structures that do not allow water to reach the soil, leading to flooding. But the ripple effects go further than flooding. When water can't infiltrate, it also can't recharge groundwater aquifers. So a paved-over city gets a double hit. More water on the surface during storms, and less water stored underground for later. Think of a city as a watershed wearing a raincoat. Everything that falls on it slides off instead of soaking in.
This term lives in Topic 5.10, Impacts of Urbanization, in Unit 5 (Land and Water Use), supporting learning objective AP Enviro 5.10.A (describe the effects of urbanization on the environment). It's one of the most testable urbanization impacts because it has a clean cause-and-effect chain you can trace on any question. Pavement replaces soil, infiltration drops, runoff and flooding increase, groundwater recharge decreases. That chain also sets up the rest of Unit 5, since the mitigation strategies you learn later (permeable pavement, planting trees, building up rather than out) only make sense once you understand what impervious surfaces broke in the first place. It's a perfect example of the APES theme that human systems alter natural cycles, in this case the hydrologic cycle.
Keep studying AP® Environmental Science Unit 5
Urban heat islands (Unit 5)
Same pavement, different problem. Impervious surfaces block water infiltration, while urban heat islands form because those same dark surfaces absorb and re-radiate heat. One asphalt parking lot causes both effects at once, which is why these two terms show up together on urbanization questions.
The hydrologic cycle (Unit 1)
Impervious surfaces are basically a human edit to the water cycle. They cut off the infiltration step, so water that should become groundwater becomes surface runoff instead. If you can sketch the hydrologic cycle from Unit 1, you can explain exactly which arrow urbanization shrinks and which one it inflates.
Saltwater intrusion (Unit 5)
EK EIN-2.M.1 pairs these for a reason. Paved cities recharge their aquifers slowly while pumping groundwater quickly, and in coastal areas that drawdown lets salt water creep into freshwater aquifers. Impervious surfaces are part of the setup that makes saltwater intrusion worse.
Urban runoff and water pollution (Unit 8)
Runoff isn't just extra water. As it sheets across roads and parking lots, it picks up motor oil, road salt, fertilizer, and sediment, then dumps that mix straight into streams. Impervious surfaces are why urban runoff is a classic nonpoint source pollution example in Unit 8.
On multiple choice, impervious surfaces usually show up as the answer to a cause-and-effect stem. A typical question describes a city flooding after rapid development, or asks which hydrological change follows when farmland gets converted to suburbs, and you have to identify increased impervious surface cover (and decreased infiltration) as the mechanism. You may also get the reverse direction, where a question asks which stormwater management technique best fixes flooding from impervious surfaces, and the answer involves permeable pavement, rain gardens, or other infiltration-boosting designs. No released FRQ has used the term verbatim, but it fits perfectly into FRQ prompts about urbanization or solving an environmental problem, where you'd propose a solution like permeable pavement and justify it by explaining how it restores infiltration and reduces runoff. The skill being tested is the chain of reasoning, not the vocab word alone.
Both are urbanization impacts caused largely by the same pavement and buildings, so it's easy to blur them. The difference is what the surfaces do. Impervious surfaces are about water (blocking infiltration, increasing runoff and flooding). Urban heat islands are about heat (dark surfaces absorbing solar energy and making cities warmer than surrounding rural areas). If the question mentions flooding or runoff, it's impervious surfaces. If it mentions temperature, it's the heat island.
Impervious surfaces are human-made structures like roads, buildings, sidewalks, and parking lots that prevent water from penetrating to the soil (EK EIN-2.M.3).
Because water can't infiltrate, it becomes surface runoff, which increases urban flooding during storms.
Blocked infiltration also means less groundwater recharge, which can contribute to aquifer depletion and saltwater intrusion in coastal cities.
Runoff from impervious surfaces carries pollutants like oil, road salt, and sediment into waterways, making it a major nonpoint source of water pollution.
Mitigation strategies include permeable pavement, rain gardens, planting trees, and building up instead of out, all of which restore infiltration.
On the exam, the winning move is the full chain of reasoning: development adds impervious surfaces, infiltration drops, runoff rises, flooding follows.
They're human-made structures such as roads, buildings, sidewalks, and parking lots that don't allow water to reach the soil. The CED (EK EIN-2.M.3) ties them directly to increased flooding, and they're tested in Topic 5.10, Impacts of Urbanization.
No. Permeable (or porous) pavement is specifically designed with gaps or porous material that lets water filter through to the soil below. That's why permeable pavement is one of the go-to mitigation answers for urban runoff and flooding questions.
Impervious surfaces are a water problem, and urban heat islands are a heat problem. The same asphalt causes both, but impervious surfaces block infiltration and cause flooding, while heat islands form because dark surfaces absorb solar energy and raise city temperatures above nearby rural areas.
Rain that would normally soak into soil hits pavement instead and runs off immediately. All that water reaches storm drains and streams at once, faster and in greater volume than the system can handle, so it backs up and floods.
No, flooding is just the most visible effect. They also reduce groundwater recharge (which can worsen aquifer depletion and saltwater intrusion) and generate polluted runoff that carries oil, road salt, and sediment into waterways.
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