If you think total maximum daily loads (TMDLs) make your brain ache, wait until you hear about “airsheds.”

Science has discovered over the past 20 years that airborne gases, such as methane and ammonia, can add large amounts of nitrogen to water systems, sometimes hundreds of miles away. The region over which airborne gases can travel and wind up in a body of water is called its airshed.

Airsheds can be huge. An example is the airshed for the Chesapeake Bay, which covers most of the Eastern Seaboard, from Georgia to southern Ontario, and stretches west into Indiana and Kentucky (see egg-shaped blue line in the color-coded map on page 44).

Here's the rub: Since concentrated livestock feeding operations emit significant amounts of some nitrogen-containing gases, federal regulators are now working on scientific models to determine how to regulate operations within the airsheds.

The Environmental Protection Agency (EPA) is charged with regulation of airsheds by the Clean Water and Clean Air Acts, and more specifically by the Clean Air Act Amendments of 1990.

Before EPA can regulate any airborne emissions from livestock or cropping operations, however, it must design a system to fairly assign pollutant values to differing operations.

It's important to understand that, not only is the government likely to target air quality emanating from animal production facilities, it may put similar standards on fertilization in cropping operations.

Most farmers know that roughly half of the nitrogen fertilizer applied is dispersed into the air through volatilization or into the ground through leaching. The plants take up the other 50%.

Agriculture Held Accountable

“The regulation of agriculture for air quality hasn't been done much in the past. It's a very young effort,” says Karen Flournoy, EPA Region 7 director of external programs.

Nonetheless, EPA's Web site makes it quite clear that agriculture is moving higher on the priority list. “Long-term pollution-control measures targeting large commercial, industrial and municipal facilities have done a fairly good job,” EPA says. “Today, we realize that cleaner air must come from efforts to cut pollution from smaller, less obvious, more diffuse sources.”

Further, EPA lists agricultural activities, “including fertilizer application and animal feedlots and waste lagoons,” right behind fossil-fuel-derived nitrogen compounds as important man-caused sources of nitrogen entering the atmosphere. The quote appears in an on-line publication entitled: “Frequently Asked Questions About Atmospheric Deposition, A Handbook for Watershed Managers.” (see www.epa.gov/owow/oceans/airdep/airdep_sept_final.pdf

In fact, EPA cites a rather impressive record for air-quality improvements in the U.S. over the past 20 years, despite significant increases in overall energy and automobile usage.

Thousands of monitoring stations across the country measure six principal pollutants — carbon monoxide (CO), lead (Pb), nitrogen oxide (NOx), ozone (O3), particulate matter (PM), and sulfur dioxide (SO2). Since 1970, aggregate emissions of these pollutants have been cut 48%. Interestingly, U.S. gross domestic product increased 164%, energy consumption increased 42%, and vehicle miles traveled increased 155% during that period.

Air Regulations Follow Water Template

Air-quality controls are following a pattern resembling the development of water-quality regulation. First, the obvious, point-source polluters are regulated — those dumping large amounts of something from a single, measurable point. For air quality, that would include power plants and automobiles.

Next up would be the less obvious, non-point source polluters. With air, as with water, this less apparent group includes agriculture.

One of the first recognized environmental fallouts of air pollution was “acid rain,” a phenomenon in which SO2 and NOx carry downwind of their sources and increase the acidity of rainfall above natural levels.

Our society is nearing the point where we can connect excess nitrogen in streams, lakes and oceans directly to the gaseous escape of nitrogen compounds from a livestock facility or a cornfield. EPA says some states already have developed TMDLs that include an allocation for “atmospheric deposition” as a part of the total pollutant load.

The primary barrier to agriculture regulation in airsheds is the lack of adequate scientific modeling, EPA says.

A 2002 report by the National Academies of Science (NAS), done at the behest of EPA and USDA, notes the statistical models are still incapable of fairly estimating the contribution of farms based on criteria such as size, type and climate. EPA has considered a “model farm” approach, which would seem to create categorical indexes for farm differences. NAS suggests a “mass balance approach,” which would be more of an input-begets-output accounting system.

Eventually, those issues will be solved and actual regulation of agricultural operations in airsheds is very likely.