Discussion of Issues and their Impact on Water Quality

Impervious Surface. As urbanization proceeds, it is accompanied by an increase in impervious surfaces. The primary hydrologic result is an increase in runoff and a decrease infiltration and groundwater recharge. This has been termed the “urban stream syndrome". Numerous studies have demonstrated that this also changes the pattern of stream hydrographs such that the high flows are made higher, and last for a shorter duration. In addition to the changes in streamflow characteristics, the increase in impervious surfaces also has important consequences for water quality.

Any chemical constituent deposited onto the impervious surface will likely be incorporated directly into storm runoff rather than being retained within soils and vegetation within the local landscape. The increased stormflow runoff caused by impervious surface can also increase suspended matter concentrations. Hence urban pollutants introduced to the environment from nonpoint sources, such as fossil fuel burning, combustion, and engine or tire wear can be rapidly introduced into the aquatic environment, bypassing introduction/absorption/retention by the soil.

This has been documented in Columbus, Ohio, US, where total Hg yields to the Scioto River were ~1.7x higher in the urban portion of the watershed than in the upstream agricultural portion. In addition, the particulate Hg yield was ~2.75x higher in the urban portion of the watershed. High inputs of Zn, V, and Ni into urban rivers from highly traveled highways have also been well documented in this same region.

Much work over the last 15 years has documented the increases in Cl^- concentrations in surface waters due to the addition of road de-icing salt in urban areas in the eastern and Midwestern portion of the US. A recent investigation in a number of river systems in central Ohio has indicated increased concentrations and fluxes of Cl^- since the 1960s. Overall salinity shows a statistically significant increase in 37% of the drainage area of contiguous US, with the biggest increases in the most densely populated Northeast and Midwest regions.

These inputs can greatly enhance the background concentrations of these elements in urban river systems. Increased urbanization can lead to the burial or covering-over of headwater streams, which in can turn impact nutrient retention, usually reducing N uptake. Increases of alkalinity in a number of urbanized streams has also been attributed to increasing impervious surface and the use of concrete as landscape cover. A portion of this bicarbonate increase may also be due to increased organic carbon input and its subsequent mineralization as well.

These factors appear to be especially important where urban infrastructure is older. Kaushal et al. recently introduced the term “freshwater salinization syndrome," which describes the concurrent and linked processes of increasing salinization and alkanization in a large fraction of North American river systems attributed to both the addition of salts to surface waters through road deicers and sewage, and the weathering of carbonate-containing minerals such as concrete.

The term “urban karst" has been given to the stormwater that is able to infiltrate within urbanized watersheds and mixes with water leaking from subsurface urban infrastructure. Its effect on base flow contributions and groundwater geochemistry is not well understood, and the fate of these waters is generally unknown. Recent work has clearly demonstrated that urban groundwater can be an important source or sink of dissolved constituents to urban stream waters under different conditions. For instance, urban aquifers can act as biogeochemical reactors and process both carbon and nutrients, while groundwater discharge into gaining streams can add both Cl^- and NO₃^-.

In addition, flood-induced groundwater discharge can introduce such soluble components as B from the urban unsaturated zone into stream waters. Potter et al. have suggested that sewage effluent from either leaking or illegally connected pipes has a detrimental impact on surface water quality in San Juan, Puerto Rico. Baseflow of the Rio Piedras demonstrated increases in Cl^-, NH₄⁺, DOC, DON, PO₄^3-, and increases in NO₃^- and O₂ with increasing urban infrastructure.

There is a need to better quantify surface water- groundwater interactions in urban settings, especially in situations where the urban subsurface has been contaminated by previous activities. These impacts can change through time depending on the management policies and practices of the city. In addition, the discharge of municipal treated water into urban streams can also affect their geochemistry. For example, increased F^- concentrations in low order streams in Columbus, OH have been attributed to the use of municipal water supply for golf course irrigation. Hence, both surface and groundwater in urban regions probably represent mixtures of water of different sources, histories, and transport paths.

As urban regions are expected to expand both outward and upward, the impact of infrastructure on both the quantity and quality of urban water will continue to evolve. Aging infrastructure and the associated weathering of urban concrete and building materials will also influence the geochemistry of urban waters. Given the continuous changes of urban landscapes coupled with the historic land use legacies, predicting the future of the biogeochemistry of urban water resources will be difficult, but critical to the development of sustainable cities. However, urbanization tends to drive the convergence of waters in the US cities, such that surface water distribution and abundance of individual cities are more similar to each other than their own natural surroundings. This may suggest that comparisons between urban water systems could hold predictive power among cities.