Urbanization’s Impact on Stream Water Quantity and Quality

 

Impacts of urbanization on stream water quantity and quality in the United States | Publications | SRS (usda.gov)

Researchers at the U.S. Department of Agriculture Southern Research Station have for some time been studying the impact of urbanization on our water resources. As we are moving ahead with the industrialization, and residential development of the Occoquan Watershed. I though it might be useful to summarize the work of two of the researchers, Ge Sun and Peter Caldwell. The below is excerpted from their article:

Sun, Ge; Caldwell, Peter 2015. Impacts of urbanization on stream water quantity and quality in the United States. Water Resources Impact, Volume 17 Number 1. 4 p.

In the United States 80% of the population lives in urban areas. The most obvious and direct impact of urbanization on watersheds is altering the hydrologic cycle that controls the flow of water in ecosystems. Manmade structures such as irrigation canals, wells, reservoirs, dams, and paved roads have shaped the natural watershed landscape.  Though there are still gaps in our understanding of the interactions among processes associated with urbanization (land conversion, increasing impervious areas, new pollutants), hydrological functions (water budget change and infiltration), and ecological functions (biota change) and the time scale of impact, we’ve learned much in recent decades. It is clear that rising populations and increased development pose major threats to our future water supply.

Converting forest lands and natural landscapes to urban uses increases the surface reflection of solar radiation, enhances storm intensity, and causes heat island effects (O’Driscoll et al., 2010). Removing forest and natural vegetation cover reduces plant transpiration, and reduces water’s ability to infiltrate into soil and the soil’s ability to accept the water. These factors result in a dramatic increase in stormwater overland flow.

To maintain water supply from an urbanized watershed requires some combination of factors such as infrastructure renovation, improved design and scale of new water and sanitation treatment systems, and expanded implementation of watershed services management will be needed. Urbanization dramatically increases population and the demand for water; and affects ecosystem processes, and as a result, water quantity and quality in streams. Alterations of watershed hydrologic cycles are the root causes of the stream ecosystem degradation observed in urban landscapes.

The Impacts of urbanization on water quality are primarily caused by two key factors – significant production of pollutants and reduction of retention capacity of the watersheds as a result of increased impervious surfaces (Sun and Lockaby, 2012). Conversion of portions of watersheds from forest to urban cover often elevates sediment and nutrient concentrations by tens to hundreds of times.

The threshold of impervious surfaces at which water quality and flow changes occur is 5%-20%. In addition to sediment and nutrients, other concerns to human health are that urban waters often contain pharmaceuticals such as antibiotics, analgesics, narcotics, and psychotherapeutics, pesticides, heavy metals, pathogenic microbial populations, and organic pollutants.

Protecting water resources across urbanizing landscapes requires management of land cover at the watershed scale by adopting urban best management practices (BMPs) and protecting source headwaters. The contemporary watershed management goal is to prevent development beyond the critical point which varies based temperature, rainfall, and geology; or otherwise, minimize impact in critical locations that are particularly sensitive to water quality and quantity. Although difficult to quantify, the opportunity cost of maintaining forest land cover in a watershed is likely less costly than allowing urbanization to occur, increasing storage and applying conventional approaches to water treatment to mitigate the water quality impairment.

In order for us to have a sustainable, adequate and quality water supply, we need to ensure that the source water of the Occoquan Reservoir is protected. Prince William County needs to convene the Occoquan Basin Policy Board and oversee a Comprehensive Study of the impacts on water quality and quantity of the planned land use changes in the county before do irreparable harm to our source water supply.