In a changing climate, cities worsen water quality
BEN LONGSTAFF/UMCES
dicting the nitrogen input into refrom a yearly dead zone caused by A study published in ES&T (DOI ceiving bodies like Chesapeake Bay extra nitrogen. 10.1021/es800264f) finds that urban and then reducing that loading,” Developers have argued that areas become a bigger source of says ecologist Gene Likens of the sprawl is actually good for the nitrogen pollution to water when Cary Institute of Ecosystem Studies. Bay’s health because strip malls rainfall patterns are more variable. Likens has studied the effects of leak less nitrogen, especially in the This concerns scientists who are human land use on ecosystems for harmful form of nitrate, into struggling to clean up water, bemore than 40 years, and he says streams than agricultural land does cause global warming is expected pulses of nitrogen can disto cause exactly the kind of rupt the biology of streams weather extremes that and of the larger bodies could make the problem they flow into. worse: dry spells followed Donald Boesch, the by intense rain. president of UMCES and In 2003, the Chesapeake an expert on the ChesaBay saw both high rainfall peake Bay, notes that “urand record-setting hypoxia ban areas are still not as (low-oxygen conditions big a source as agriculture, linked to excess nitrogen atmospheric deposition, that create lifeless dead and wastewater diszones), says biogeochemist charges, but they are the and study coauthor Sujay one source that is growKaushal of the University of ing, whereas others are Maryland Center for Enviarguably declining.” ronmental Science (UMCFarms, forests, and urban development rub shoulders on Kent Boesch adds that alES). These conditions though data for the state so spurred researchers to ask Island, Md, on the eastern shore of the Chesapeake Bay. far do not show a statistiwhether local streams were cally significant increase in precipiwith its heavy load of nitrogen ferflushing more nitrate into the Bay tation’s variability, data for the tilizers. “But this study shows [that] than before and whether booming overall mid-Atlantic region already you can’t pave the whole waterland use by humans in the region do. Recent studies reporting this shed to reduce nitrate,” Kaushal played a role. The research is part trend include a new report on says. of the Baltimore Ecosystem Study, weather and climate extremes by The researchers analyzed data a long-term project funded by the the U.S. Climate Change Science from MBSS on nitrogen levels in National Science Foundation. Program. more than 1000 streams and comFrom 1990 to 2000, the area of The Chesapeake Bay is not bined those with maps of urban, the Bay’s watershed that is covered alone, says Nancy Grimm, an agricultural, and forested land. by urban sprawl grew by 60% as ecologist studying urban and desert They compared nitrate levels in wet rising home prices drove home streams in central Arizona. There and dry years from 2001 to 2003 construction ever farther from as well, Grimm measures more niand found that the retention of niWashington, D.C. Many new develtrate rushing into streams when trate by urban lands dropped 50% opments were built near streams, storms follow droughts. Little of in wet years compared with dry and some developers even paved that region’s water ever reaches years. Forests and agricultural over streams. Of Maryland’s nearly the ocean to cause hypoxia, but lands largely kept their nitrogen9000 miles of streams, more than nitrate could concentrate in holding ability. Urban areas, with half are impaired by nitrate condrinking-water sources. Projects their storm drains and large areas centrations above a “moderate” such as the Baltimore study will of impervious surface, prevent rain level of 1 milligram per liter of nibe key to predicting and planfrom percolating through soils that trogen from nitrate, according to ning for cities’ futures, Grimm would remove nitrate. Then, when data from the Maryland Biological says. rainfall becomes heavy, stored niStream Survey (MBSS). At the same —ERIKA ENGELHAUPT trogen is flushed out fast. time, the Chesapeake Bay, the na“Data of this sort from different tion’s largest estuary and the ES8018012 [land-use types] are critical for prestreams’ final destination, suffers
5836 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / August 15, 2008
10.1021/es8018012
2008 American Chemical Society
Published on Web 07/08/2008
