Heterogeneity and Water Quality. Urban Areas as "Global" Water Polluters

The hydrologic connectivity of urban watersheds is complex. Kaushal and Belt described the “urban watershed continuum" that recognized both engineered and natural flowpaths that impact both the flow of water and the fluxes and processing of chemicals in urban systems. They point out that there should be changes in biogeochemical cycling in urban aquatic environments as both land use and infrastructure change. As mentioned earlier, the heterogeneity of urban landscapes is complex and “patchy", but, this heterogeneity is also changing, sometimes very rapidly over time.

Stucker and Lyons investigated three low-order urban streams in Columbus, Ohio, the 15th largest city in the USA. These streams had subtle but quantifiable different land use patterns. Although dissolved trace metals (Cr, Cu, Mo, Ni, Pb, V, and Zn) had concentrations higher than the mean global river values, rivers draining different land covers had no statistically different metal concentrations. They concluded that urbanization may help to homogenize the sources and fluxes of these metals, even in the urban “headwater" streams. Understanding the role that heterogeneity plays controlling urban water quality is an important aspect that clearly needs more study.

Urban Areas as "Global" Water Polluters. It is not surprising that recent work has clearly demonstrated a strong relationship between urban land-use change, water quality, and water treatment costs. The analysis of 309 large cities indicated that although both the yields of N and P had increased from 1900 to 2005, the N increase was larger than P. A more specific study in seven subwatersheds in St. Paul, Minnesota, USA provided a different perspective, showing that these urban watersheds retained more N than P. The increased P transport was thought to be due to the high street densities and enhanced import through stormwater flushing. Denitrification could account for a portion of the N retention in these systems, while the rest is probably lost to the urban groundwater system. Again, these studies demonstrate the complexity of urban aquatic systems and that an understanding of water movement is paramount in order to predict water quality.

The US Geological Survey reported pesticide concentration trends in US streams and rivers between 1992 and 2001 and found the percentage of stream with concentrations exceeding aquatic-life benchmarks increased from 53% to 90% in urban streams during that period. Conversely, the percentage of agricultural streams that exceeded the benchmark essentially stayed the same.

In addition, the relatively recent identification pharmaceuticals, personal care products, endocrine disrupting compounds, nanoparticles, and microplastics have become significant contaminants in many urban water bodies. One example is the observed increases in Li concentrations in urban streams, brought about by the use of lithium carbonate as a medication as a treatment for bipolar disorder. Because Li is very soluble, it is extremely difficult to remove it via wastewater treatment.

Trace metals have been demonstrated to drastically increase in rivers/streams as they transition from rural to urban. This increase occurs in both in the particulate and dissolved (<0.45 µm) forms. A percentage of the metal introduced in urban streams is associated with particulate matter and ends up in soil. Both urban storm runoff and sewage input have been demonstrated to be potential sources of metal to urban water bodies. High rainfall events can re-suspend and/or erode this material and introduce it into urban waterways where it can be transported outside the urban area.

This is particularly true for more particle reactive metals like Pb and Hg. Sanudo-Wilhelmy and Gill demonstrated that although management practices developed through the Clean Water Act in the USA helped lead to a reduction of Cu, Cd, Ni, and Zn in the Hudson River estuary over a 23-year period, Pb and Hg did not decrease as significantly. This is attributed to the fact that the former metals were removed from point sources, while Pb and Hg were enhanced through the erosion of soils. A case study evaluating the historical flux of particulate metal discharge from the metro-Atlanta, Georgia, US region, suggests that although the total fluxes have increased over time, the per capita increase had decreased.

These data suggest although changes in policy have had an effect in decreasing pollution in urban settings, increases in population also play a significant role. Clearly, population and population density changes have very important consequences on urban water quality and may counteract enhanced policies and management practices attempting to mitigate urban water quality degradation.

Wastewater treatment plant effluent in urban areas can contribute significantly to trace elements, as well as nutrients, in surface waters. For example in Switzerland, wastewater effluent accounts for 24% of the Zn and 83% of the Gd observed in river waters. These authors even suggest that urban sewage sludge could potentially be profitable exploited for many elements. It has been further speculated that human excrement may be an important global source of phosphorous in the future as economically minable phosphorous deposits are depleted.

Finally, it has been suggested that urbanization can increase local chemical weathering rates, thus increasing the dissolved concentrations of rock-forming elements such as major cations Na, Ca, K, as well as dissolved Si. It has also been clearly shown that urban sewage can be a significant source of H₄SiO₄ to discharge waters as well. Clearly, anthropogenic activities in urban areas can accelerate the cycling of most of the periodic table, thus increasing the elemental loads in urban water bodies.

As urban populations continue to grow, it is expected that water quality problems could become more severe. The total population living in urban centers is expected to reach 6.3 billion by 2050, which translates to ~67% of the world’s population living in cities. Megacities with populations of 10 million or more are projected to number 41 by 2030. With these predictions in mind, the ecological footprint of cities, and their need for clean water will both continue to increase. Even without accounting for climate change, which could potentially exacerbate water scarcity issues, it has been anticipated that many urban areas will face potentially serious water resource availability in the coming decades. Clearly, the need to better understand urban aquatic systems will continue to be of immense societal importance.

Solving these problems will require multidisciplinary and transdisciplinary approaches. Important new work is currently being conducted to better enhance urban water quality and urban water security. This includes new innovative approaches in dealing with stormwater drainage and runoff, reducing overall water usage, and better ways to manage water pollution at its source, to name a few. The development of green infrastructure can also result in cleaner water being transported from urban landscape surfaces into the aquatic environment. In addition, the development of new technologies and materials could greatly aid in solving these issues in the future. We anticipate that research in these applied aspects of urban hydrology will continue to develop.

References:
1. Wong, C.I., Sharp, J.M., Hauwert, N. et al. (2012). Elements 8: 429-434.
2. Chambers, L.G., Chin, Y.-P., Filippelli, G.M. et al. (2016). Appl. Geochem. 67: doi: 10.1016/j.apgeochem.2016.01.005.

3. Groffman, P.M., Cadenasso, M.L., Cavender-Bares, J. et al. (2017). Ecosystems 20: 38-43.
4. Wigginton, N.S., Fahrenkamp-Uppenbrink, J., Wible, B., and Malakoff, D. (2016). Science 352: 904-905.
5. Cadenasso, M.L., Pickett, S.T.A., and Schwarz, K. (2007). Front. Ecol. Environ. 5: 80-88.

6. Grimm, N.B., Faeth, S.H., Golubiewski, N.E. et al. (2008). Science 319: 756-760.
7. Kaushal, S.S., McDowell, W.H., and Wollheim, W.M. (2014). Biogeochemistry 121: 1-21.
8. Albanese, S. and Cicchella, D. (2012). Elements 8: 423-428.
9. Gessner, M.O., Hinkelmann, R., Nutzmann, G. et al. (2014). J. Hydrol. 514: 226-232.
10. Lyons, W.B. and Harmon, R.S. (2012). Elements 8: 417-422.