Model Methods

The way in which groundwater moves has raised questions about the pace of Chesapeake Bay cleanup and the adequacy of data to estimate progress.

First, how do we know whether we have reduced nutrient and sediment pollution to the Chesapeake Bay—and second, will we continue to do so? Two gauges are used to measure nutrient reduction in waterways: The states and the federal government look at conditions, while the Chesapeake Bay Program Office, which is managed by the Environmental Protection Agency (EPA), uses models to evaluate different practices and estimate their effectiveness.

The Chesapeake Bay Program’s model of the 64,000-square-mile watershed examines many scenarios to understand water quality. A watershed model may calculate, for example, how much nitrogen and phosphorus will be reduced under different scenarios. So, modelers can enter into their computer calculations one practice that’s been used in the past to estimate how long it might take to make a difference in nutrient levels. But until recently, that particular calculation did not factor in the lag time for groundwater—that is, the time from when a practice is put into place to when it starts to take effect.

The median time for nitrogen on the land to reach streams can be 20 to 40 years, according to a U.S. Geological Survey (USGS) study; this means that fertilizer applied during the Reagan Administration could still be working its way through the Eastern Shore’s rivers. Compare that to other practices, such as an upgrade to a treatment plant, where the impact is immediate: new pollution-capturing technology is installed, and the resulting discharge water is less nutrient laden.

The challenge with the model lies in trying to estimate how long it takes for nitrogen to go through groundwater and into the streams and then the Bay. USGS officials worked with Bay modelers to factor in the lag times for groundwater to enter Chesapeake Bay. They recognize that groundwater moves much more slowly through the system; in addition, the path that groundwater takes is not always understood, which makes response times hard to predict. But geologists and modelers have tried, and here are their estimated lag times for Maryland’s diverse geographic regions:

Header image: Aerial image of the Chesapeake Bay taken on September 13, 2011. Photo credit, NASA

Graphic by Nicole Lehming / MDSG, redrawn from USGS graphic