Making 'Chemical Cocktails' - Evolution of Urban Geochemical Processes across the Periodic Table of Elements

Abstract

Urbanization contributes to the formation of novel elemental combinations and signatures in terrestrial and aquatic watersheds, also known as 'chemical cocktails.' The composition of chemical cocktails evolves across space and time due to: (1) elevated concentrations from anthropogenic sources, (2) accelerated weathering and corrosion of the built environment, (3) increased drainage density and intensification of urban water conveyance systems, and (4) enhanced rates of geochemical transformations due to changes in temperature, ionic strength, pH, and redox potentials. Characterizing chemical cocktails and underlying geochemical processes is necessary for: (1) tracking pollution sources using complex chemical mixtures instead of individual elements or compounds; (2) developing new strategies for co-managing groups of contaminants; (3) identifying proxies for predicting transport of chemical mixtures using continuous sensor data; and (4) determining whether interactive effects of chemical cocktails produce ecosystem-scale impacts greater than the sum of individual chemical stressors. First, we discuss some unique urban geochemical processes which form chemical cocktails, such as urban soil formation, human-accelerated weathering, urban acidification-alkalinization, and freshwater salinization syndrome. Second, we review and synthesize global patterns in concentrations of major ions, carbon and nutrients, and trace elements in urban streams across different world regions and make comparisons with reference conditions. In addition to our global analysis, we highlight examples from some watersheds in the Baltimore-Washington DC region, which show increased transport of major ions, trace metals, and nutrients across streams draining a well-defined land-use gradient. Urbanization increased the concentrations of multiple major and trace elements in streams draining human-dominated watersheds compared to reference conditions. Chemical cocktails of major and trace elements were formed over diurnal cycles coinciding with changes in streamflow, dissolved oxygen, pH, and other variables measured by high-frequency sensors. Some chemical cocktails of major and trace elements were also significantly related to specific conductance (p<0.05), which can be measured by sensors. Concentrations of major and trace elements increased, peaked, or decreased longitudinally along streams as watershed urbanization increased, which is consistent with distinct shifts in chemical mixtures upstream and downstream of other major cities in the world. Our global analysis of urban streams shows that concentrations of multiple elements along the Periodic Table significantly increase when compared with reference conditions. Furthermore, similar biogeochemical patterns and processes can be grouped among distinct mixtures of elements of major ions, dissolved organic matter, nutrients, and trace elements as chemical cocktails. Chemical cocktails form in urban waters over diurnal cycles, decades, and throughout drainage basins. We conclude our global review and synthesis by proposing strategies for monitoring and managing chemical cocktails using source control, ecosystem restoration, and green infrastructure. We discuss future research directions applying the watershed chemical cocktail approach to diagnose and manage environmental problems. Ultimately, a chemical cocktail approach targeting sources, transport, and transformations of different and distinct elemental combinations is necessary to more holistically monitor and manage the emerging impacts of chemical mixtures in the world's fresh waters.

Keywords: freshwater salinization syndrome; human-accelerated weathering; urban evolution; urban karst; urban watershed continuum.

Figure 1.

Conceptual model illustrating how groups of elements can be grouped as ‘chemical cocktails’ and transported in urban waters. Elemental inputs to the watershed originate from different urban geochemical sources and processes. Chemical cocktails are mixed hydrologically across time and space in watersheds through surface flowpaths (impervious surfaces, construction materials, altered geomorphology, etc.) and subsurface flowpaths (stream burial, urban karst, pipes, soil compaction, etc.). Urban geochemical processes form distinct elemental combinations of nutrients, metals, salt ions, and organics that are transported and transformed along hydrologic flowpaths based on factors such as chemical mobility, particle size, and reactivity. These different chemical cocktails can become mixed together along hydrologic flowpaths and their sources may be traced using diverse analytical approaches.

Figure 2.

Long-term changes in geologic materials used to make concrete in the United States. This widespread proliferation of geologic materials used to make concrete can degrade over time through accelerated weathering of impervious surfaces contributing to transport of chemical cocktails of major ions to urban waters. Data are from U.S. Geological Survey Minerals Yearbook.

Figure 3.

Conceptual diagram of human-accelerated weathering of impervious surfaces and corrosion of pipes illustrating how human activities contribute major ions and metals to distinct chemical cocktails in urban waters. Human-accelerated weathering and corrosion of infrastructure can be associated with acidic precipitation and anaerobic processes in sewer pipes.

Figure 4.

Mean major ion concentrations in streams for urbanized watersheds and reference watersheds for nine selected studies spanning a range of study areas. Mean dissolved concentrations of major ions are consistently higher within urban watersheds when compared to reference watersheds, despite variability in absolute concentrations among the nine studies. This result is evidence of the ‘overprint’ of urban geochemical processes influencing major ions, which are superimposed onto natural geochemical processes influencing major ion concentrations. Where applicable, the 25th percentile concentration reported by Griffith (2014) is indicated using pink lines for the corresponding US EPA Level III Ecoregion encompassing each study area. Where study areas span Ecoregions, both are indicated. Griffith (2014) only reported maximum and minimum values in some Ecoregions due to limited data availability. These are shown using blue lines for studies without a 25^th percentile concentration. Two studies are outside the United States; the world weighted average river concentrations provided by Meybeck (2003) are shown as a dashed horizontal line for reference. The nine studies reported dissolved inorganic carbon (DIC) species in different ways; these ways are listed in the bottom right plot within the figure. Supporting Information includes further details.

Figure 5.

Concentrations of major and trace elements in streams draining a land use gradient at the Baltimore Long-Term Ecological Research (LTER) site over almost 2 years of bi-weekly sampling. Center vertical lines of the box and whiskers indicate medians. Lengths of each whisker show ranges within which the central 50% of the values lie. Edges of the boxes indicate the first and third quartiles. Dark circles represent outside values.

Figure 6.

Major ions can increase in rivers downstream of cities. For example, along the Ravi River inside and outside of Lahore, Pakistan concentrations of Na⁺, Ca²⁺, Cl⁻, and SO₄² are significantly higher in the city than in the river upstream of the city over 3 decades from 1978 to 2002. Likewise, Na⁺, Ca²⁺, Cl⁻, and SO₄² are significantly higher in the Han River downstream versus upstream of Seoul, South Korea from 2010 to 2016. Data are from GEMStat.

Figure 7.

Concentrations of major ions simultaneously increase with increasing distance downstream (meters) from a fixed sampling point along the Anacostia River near College Park, Maryland. Conversely, concentrations of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) decrease with increasing distance downstream. The simultaneous changes in concentrations suggest similar hydrogeochemical processes and sources contributing to the formation of different chemical cocktails along urbanized drainage networks.

Figure 8.

Global averages of natural concentrations of trace elements in streams and rivers (shown as red dots) are in order from greatest concentration on the bottom to the least concentration on the top. Examples of elemental concentrations in some different urban streams and rivers are typically higher than the natural global averages (shown as black dots). Further information regarding numbers corresponding to specific data sets from literature references are provided in the Supporting Information.

Figure 9.

An example of synoptic patterns in elemental concentrations along the Gwynns Falls watershed of the Baltimore Long-Term Ecological Research site. Concentrations of major and trace elements increase, peak, or show pulsed patterns with increasing distance downstream based on sources, transport, and transformations. Some elemental mixtures show very similar longitudinal patterns suggesting that they can be transported and transformed as watershed chemical cocktails.

Figure 10.

Chemical cocktails of major and trace elements are formed over diurnal cycles in the Anacostia River near College Park, MD coinciding with changes in streamflow, dissolved oxygen, and pH from in situ sensor data in streams. Orange bars indicate the start of each new day. Elemental concentrations from laboratory measurements are plotted as blue circles on the left axes, while data measured by high-frequency sensors is plotted as orange lines on the right axis. Red vertical bars indicate the start of each new day.

Figure 11.

There are significant relationships between specific conductance and some major and trace elements in Paint Branch, an urban stream near Washington D.C.