Urbanization

There is an impact on the hydrologic cycle as well. Soil, vegetation, and wetlands (which are drained and filled in) are converted to impervious surfaces like cement. This increases the incidence of runoff and decreases infiltration and groundwater recharge.

In addition, saltwater intrusion is a risk in coastal cities. This can occur due to excessive groundwater withdrawal lowering the water table, which is made worse by the fact that the impervious surfaces prevent groundwater recharge. In addition, it can be caused by sea level rise due to thermal expansion and ice cap melting. 

A combination of expanded highways, increased car use, and personal preference for low-density all lead to more and more people moving to the suburbs.

As more and more people move to the suburbs, this continues to occur. Increased car use and longer commutes lead to more revenue from fuel taxes, which goes towards building more highways, which leads to more car use and tax revenue, etc. At the same time, as residents leave cities for the suburbs, so do businesses, which causes more businesses to leave.

This is a problem because it leads to increased land use (and therefore habitat destruction), increased resource use, and more driving which leads to the burning of more fossil fuels. It can be prevented from walkable cities and increased public transport which encourage people to stay in the city, zoning laws that prevent development beyond a certain area, and mixed land use where residential, business, and entertainment buildings are all intermixed in the same area of the city. Mixed land use cities are more efficient uses of land and are more practical for residents.

Common pollutants that are impacted by urban runoff are:

• Salt, particularly from when roads are salted in the winter. This can lead to plant and insect death due to dehydration. It can also lead to soil salinization.

• Sediment from nearby soil or that has broken off from manmade surfaces can increase the turbidity of water.

• Fertilizer that has been added to gardens or other plants can cause eutrophication when brought to bodies of water, greatly damaging aquatic ecosystems.

• Pesticides can cause non-target death in other species. They are

• Oil and gas from cars can suffocate fish and kill aquatic insects.

There are a variety of methods that can be used to decrease the effects of urban runoff.

Permeable pavement can be designed that allows for infiltration of stormwater into the soils beneath the pavement. This is more expensive than traditional pavement, but it decreases runoff and flooding, increases infiltration and groundwater recharge, and promotes treatment of pollutants (as the soil and its microbes can help to filter the polluted water).

Rain gardens are gardens that surround storm drains. The soil and plants will take in water, reducing runoff and flooding. In addition, they provide a habitat for pollinators and other species, while also sequestering CO2.

Increased public transit results in fewer cars being used. This results in fewer pollutants on the road (such as motor oil, gas, antifreeze, and tires pieces), fewer car lanes and parking lots needed (which reduces runoff and can reduce urban heat island effects), less traffic, fewer CO2 emissions (reducing impacts on climate change), and fewer air pollutants from car emissions.

Green roofs are when plants are grown on the sides and tops of buildings. This absorbs water, reducing runoff. It also sequesters CO2 and filters air pollutants, leading to better air quality and helping to combat climate change. It can also be combined with taller rather than wider buildings to increase the positive effect.

Building up, not out is the name for the strategy mentioned above of building taller, rather than wider buildings. This reduces the amount of impervious surfaces (and the amount of habitat destruction due to needing less land), which decreases the amount of runoff.