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Urban 'evolution' turns fresh-water streams salty

Eroding stream banks and aging sewer lines contribute to evolving water pollution problems in cities. A sewage pipe that was originally placed in the stream bed of Gwynns Run in West Baltimore developed leaks, and is now surrounded by a concrete casing.
Eroding stream banks and aging sewer lines contribute to evolving water pollution problems in cities. A sewage pipe that was originally placed in the stream bed of Gwynns Run in West Baltimore developed leaks, and is now surrounded by a concrete casing. (Tamara Newcomer Johnson)

If current trends continue, Marylanders may not have to go "down-y oshun" in future to dip their toes in salt water. Thanks to liberal de-icing of our roads in winter and chronic sewage leaks, salinity levels are rising in Baltimore area streams  - with at least one already a third as tangy as Ocean City surf.

That's just one of the insights researchers have gained by studying how cities have altered their environment as they "evolve" over time.

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In a collection of studies published Wednesday in a special issue of the journal Biogeochemistry, scientists find common threads in how urban development in disparate regions affects water quality, geology and the landscape in general. The changes - both good and bad - can be sudden or gradual, depending on how people manage them, the researchers conclude.

"The city is constantly breaking down, but how you build it (and) the materials you build it out of will affect the pathways by which it breaks down - and can be repaired," said Sujay Kaushal, a University of Maryland geologist.

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Kaushal, who recently documented how stream alkalinity is increasing, is one of the lead authors of an overview paper that argues for tracking the evolution of cities. Co-authors were a pair of ecologists from University of New Hampshire, William McDowell and Wilfred Wollheim.

Fourteen studies were carried out in variety of places with varied climates, from Boston and Baltimore to San Juan, Puerto Rico, Tucson, Ariz. and Southern California. Some were sites in the National Science Foundation's Long Term Ecological Research (LTER) network.

Among the findings:

Urbanization increases the saltiness of streams flowing through it. In addition to runoff of road salt, salinity is raised by cracked sewage pipes leaking chloride-laden waste - from humans' salt-heavy diet.

Urban streams also have elevated calcium levels, a byproduct of rainfall and weather dissolving concrete frequently made with limestone.  Researchers dubbed the proliferation of such concrete a new geologic formation - "urban karst."

How cities develop and how they're maintained can have far-reaching and lasting impacts, researchers say.

"Basically these cities evolve over time like an organism," Kaushal said. "Cities need food, they need water, they need to eliminate their wastes." And, he added, later, "some cities die or decay."

But keeping in mind the cycles through which all cities go can help urban managers and residents shape their future, he said.

"We hope scientists, managers and citizens will work together to make decisions that allow for what we call 'urban evolution,' – that is, changes in the ecology of cities over time, " Kaushal said in a statement accompanying the publication.

"If we do that, we can find effective ways to understand and manage the trajectory of urban ecosystems, from decline towards sustainability."

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