A new study published in ES&T finds that urban areas become a bigger source of nitrogen pollution to water when rainfall patterns are more variable. This concerns scientists who are struggling to clean up water, because global warming is expected to cause exactly the kind of weather extremes that could make the problem worse: dry spells followed by intense rain.
In 2003, the Chesapeake Bay saw both high rainfall and record-setting hypoxia (low-oxygen conditions linked to excess nitrogen that create lifeless dead zones), says biogeochemist and study coauthor Sujay Kaushal of the University of Maryland Center for Environmental Science (UMCES). These conditions spurred researchers to ask whether local streams were flushing more nitrate into the Bay than before and whether booming land use by humans in the region played a role. The research is part of the Baltimore Ecosystem Study, a long-term project funded by the National Science Foundation.
From 1990 to 2000, the area of the Bay’s watershed that is covered by urban sprawl grew by 60% as rising home prices drove home construction ever farther from Washington, DC. Many new developments were built near streams, and some developers even paved over streams. Of Maryland’s nearly 9000 miles of streams, more than half are impaired by nitrate concentrations above a “moderate” level of 1 mg per liter of nitrogen from nitrate, according to data from the Maryland Biological Stream Survey (MBSS). At the same time, the Chesapeake Bay, the nation’s largest estuary and the streams’ final destination, suffers from a yearly dead zone caused by extra nitrogen.
Developers have argued that sprawl is actually good for the Bay’s health because neighborhoods and strip malls leak less nitrogen, especially in the harmful form of nitrate, into streams than agricultural land does with its heavy load of nitrogen fertilizers. “But this study shows [that] you can’t pave the whole watershed to reduce nitrate,” Kaushal says.
The researchers analyzed data from MBSS on nitrogen levels in more than 1000 streams and combined those with maps of urban, agricultural, and forested land. They compared nitrate levels in wet and dry years from 2001 to 2003 and found that the retention of nitrate by urban lands dropped 50% in wet years compared with dry years. Forests and agricultural lands largely kept their nitrogen-holding ability. Urban areas, with their storm drains and large areas of impervious surface, prevent rain from percolating through soils that would remove nitrate. Then, when rainfall becomes heavy, stored nitrogen is flushed out fast.
“Data of this sort from different [land use types] are critical for predicting the nitrogen input into receiving bodies like Chesapeake Bay and then reducing that loading,” says ecologist Gene Likens of the Cary Institute of Ecosystem Studies. Likens has studied the effects of human land use on ecosystems for more than 40 years, and he says pulses of nitrogen can disrupt the biology of streams and of the larger bodies they flow into.
“The way we’ve been trying to manage [and] reduce nutrient runoff to Chesapeake Bay has assumed a certain constancy, which we’re coming to understand is no longer the case,” says Donald Boesch, the president of the University of Maryland Center for Environmental Science and an expert on the Chesapeake Bay. “Urban areas are still not as big a source as agriculture, atmospheric deposition, and wastewater discharges, but they are the one source that is growing, whereas others are arguably declining,” he says.
Boesch adds that although data for the state so far do not show a statistically significant increase in precipitation’s variability, data for the overall mid-Atlantic region already do. Recent studies reporting this trend include the Northeast Climate Impacts Assessment—published by the Union of Concerned Scientists, an advocacy group—and a new report on weather and climate extremes by the U.S. Climate Change Science Program. “So now we have to contend not only with urbanization increasing the risk of flooding but [also with] urban and agricultural systems that are leakier of the nutrients that we are trying so desperately to control,” Boesch notes.
The Chesapeake Bay is not alone, says Nancy Grimm, an ecologist studying urban and desert streams in central Arizona. There as well, Grimm measures more nitrate rushing into streams when storms follow droughts. Little of that region’s water ever reaches the ocean to cause hypoxia, but nitrate could concentrate in drinking-water sources. Long-term monitoring projects such as the Baltimore study will be key to predicting and planning for cities’ futures, Grimm says.
from Environmental Science and Technology News
