Sewer or septic? Hybrid or old truck? Large lot or compact development?

How people answer these questions in their daily lives can add up to a big difference for the Bay, and a new calculator helps watershed residents determine the size of their nitrogen "footprint" on the Chesapeake.

Patterned after the increasingly common carbon calculators that allow people to estimate their contribution to greenhouse gases, the Chesapeake Bay Foundation has launched an Internet-based nitrogen calculator that allows people to estimate the amount of nitrogen produced annually by their household, and to see how it stacks up against the average.

"The calculator was driven in part to increase public awareness of the impact of our day-to-day activities on water quality in this region," said Beth McGee, CBF senior scientist.

She said similar carbon calculators found on the Internet have been successful at raising public awareness among people about how their actions generate carbon dioxide emissions.

"You don't get as many blank stares as you used to when you talk about a carbon footprint," she said. "Our hope is the same thing holds for this, and that nitrogen footprints become part of our lexicon."

A household's footprint is determined by entering information about household size, electricity use, lot size, vehicle type and miles driven, as well as other factors into the calculator.

The largest contributor for most households is the waste flushed down the toilet. If that goes to a wastewater plant with conventional technology, that household contributes 7.2 pounds of nitrogen a year from sewage. But if it's piped to a plant with advanced nutrient removal technology, that drops to 3.6 pounds a year.

In comparison, the calculator shows the same household on a septic system contributes 13.6 pounds of nitrogen a year. If the owners upgrade to a septic system with advanced nitrogen removal technology, that drops to 6.8 pounds.

Likewise, the calculator shows that the lawn from a home with 10 acres that is fertilized two or three times a year contributes 16.2 pounds of nitrogen runoff. But if the owners stop fertilizing, that drops to 14.7 pounds and if they plant half the lawn in trees, it drops to 11 pounds a year.

McGee said she hopes that calculator users make "what if" comparisons to see how decisions such as cutting electricity use or changing driving habits can alter their nitrogen footprint. "If I fertilize twice a year, what happens if I don't do it at all?"

The site also offers users a variety of tips, which range from changing personal habits to supporting legislative actions, showing how people can achieve further reductions.

The average household in the Chesapeake Bay watershed generates 14 pounds of nitrogen a year, according to the CBF. Homeowners need to reduce their footprint to 8 pounds a year to meet their portion of Bay cleanup goals, according to the website.

People who want to be "nitrogen neutral," will soon have the option of buying "offsets" for their nitrogen footprint by following a link on the website to that of the Chesapeake Fund. The fund, a partnership between the CBF, the World Resources Institute and Forest Trends, will use the money to help pay for actions that reduce nitrogen runoff-much in the way that people can buy offsets from various organizations to compensate for their carbon footprint.

A link on the website also provides information on how the figures in the calculator were developed. People can also leave comments about the calculator-McGee said the calculator will be updated in the future with new ideas and new information.

The calculator is found at www.cbf.org/yourbayfootprint/.