Geochemistry of Urban Water Systems

Introduction. Applying the principals of environmental sciences such as geochemistry, ecology, and hydrology to urban systems is greatly complicated by the complex infrastructure found in cities and the large numbers of humans located in small geographic areas. The impact of urbanization is to greatly change an existing “natural" system by relocating and repurposing surficial geologic material, creating impervious surfaces, channelizing streams, and modifying the ecological community, which in turn impacts ecosystem function. All of these activities greatly affect the hydrology and hydrogeology of the urban environment. Superimposed on these processes is the increase in biogeochemical cycling of elements brought about by the high concentration of people in a limited area.

This “cycling" is due to the manufacturing of products and their disposal, the burning of fossil fuels, increased transportation of people and products, the production of human waste, and so on. These anthropogenic activities greatly alter the environment, making urban systems a specialized subset of other types of landscapes because their ecological, hydrological, and biogeochemical footprints are much greater than the area of the city itself.

Although all of these issues have been recognized in the past, only recently have urban systems been the focus of their own, more specific, and unified approach to investigation. Yet, urban landscapes can be highly heterogeneous, and the role of these usually fine-scale differences on the role of elemental transport and fate is not well quantified, let alone well understood. This heterogeneity is also made more complicated by legacy effects brought about by historical changes in land-use. Thus, processes and fluxes that control water and elemental distributions are less understood in urban aquatic environments than more natural ones, and worthy of fundamental research.

Many specific human activities affect the geochemistry of water, especially in urban environments. These include both the input of industrial and human waste, construction activities, spills, and long-term leakage. Many of these are point source inputs, but significant nonpoint sources exist in the form of storm runoff. Thus, scale, both spatial and temporal, is a very important factor in delineating the impact of these sources in urban water quality.

The introduction of excess nutrients, toxic elements, xenobiotic compounds, sediments, and labile organic matter that depletes O₂, all impact urban water quality and therefore threaten both human and ecosystem health. In developed countries, best management practices and regulation have helped minimize the degradation of water quality, but even in these settings, the overall impact of urbanization on water geochemistry is measurable. The combination of enhanced chemical fluxes and the “re-plumbing" of natural hydrologic systems help make urban aquatic geochemistry/biogeochemistry such an interesting and complicated topic.

The geochemical influences of urban areas on water resources have evolved significantly from the early industrialized cities of the nineteenth century through the more environmentally regulated cities in the developed world and sprawling megacities in developing countries. These changes are driven by the stage of a city’s economic development, population growth, changes in technology, and environmental regulations. As such, any individual city’s environmental issues are related to where they are along these stages of development, leading to a large diversity of cities around the world and a diversity of environmental problems. At the beginning of a city’s development, vegetation loss and the loss of headwater streams to impermeable surfaces creates channelized flow and alters biogeochemical cycles.

As populations increase, more energy and materials move into and out of urban areas, which also grow spatially to include suburban and exurban areas. Because culture, climate, geology, environmental regulation, and technological advances of cities vary significantly, the specific geochemical impacts on water resources are different among cities, though most cities share some similarities. Both early and modern cities have problems with water pollution related to human waste removal and stormwater runoff as well as enhanced particulate emissions and other degradations of air quality, including the ramifications of increased fossil fuel usage during industrial development. Newly identified “emerging" contaminants such as endocrine-disrupting compounds and pharmaceuticals that pass through wastewater treatment plants are a common problem among developed urban areas. All of these issues are additionally affected by evolving cultural attitudes and environmental regulation.

Wolman introduced the concept of “urban metabolism," which argues that urban areas can be equated to an organism and ecological principles can be used to investigate the exchange of energy, water, and materials within and between cities and their outside environment. This concept has been modified and refined over the past ~50 years. For the long-term sustainability of urban areas, resource utilization and waste emission must be reduced, as both of these processes have important implications for water quantity and quality.

The role of the “hinterland" on supplying water and other important resources to urban regions has also stimulated much discussion, and depending on the per capita historic water usage of any given city, the water withdrawal from its hinterland has either grown or decreased. Others have approached urban water consumption in a similar manner, proposing the urban funnel model, which integrates the resource consumption and waste generation of urban areas at multiple spatial scales. Although the role of water withdrawal vs the input of wastewater and treated water obviously plays an important role in water quality (especially downstream from urban centers), it will not be explicitly discussed here.

Rather, in this article, we will introduce a few of the issues that confound the understanding of the geochemistry of urban aquatic systems. It is not meant to be a comprehensive view, but rather a summary of a number of noteworthy considerations that greatly differentiate urban regions from more “natural," less anthropogenically modified, landscapes.