THE INFLUENCE OF GROUNDWATER ON NITROGEN DELIVERY TO THE CHESAPEAKE BAY AND IMPLICATIONS FOR WATER-QUALITY RESPONSE TO MANAGEMENT PRACTICES

Resource managers need to understand the “lag time” between implementation of management practices and improvement in water quality in the Bay to help develop future nutrient- and sediment-reduction strategies. Lag times are the delays between implementing management practices and seeing the full benefits in water quality. Lag times can vary based on the kind of restoration work completed, the geology of the restoration site, its distance from a nearby river or stream, and other factors. One of the major factors is the slow movement of groundwater and the nutrients it often carries. Travel times in ground water for nitrogen can vary from months to centuries depending on the length of the flow path and aquifer characteristics.

Initial studies by USGS revealed the age of ground water in shallow aquifers in the Chesapeake Bay watershed ranges from modern (less than 1 year) to more than 50 years. Samples from springs showed median age of all samples was 10 years, with 25 percent of the samples having an age of 7 years or less and 75 percent of the samples having an age of up to 13 years.

A more recent study of the Delmarva Peninsula, which utilized a modeling approach, estimated that the range of groundwater ages was from months to centuries, with median ages spanning from 20 to 40 years. The dramatic differences in age occur as a result of variation in the landscape; for instance, Delmarva’s porous, sandy aquifers yield much longer groundwater return times than the fractured-rock areas in the Bay’s watershed. The study found that groundwater currently being discharged to streams in certain areas of Delmarva are slowly transitioning to waters with larger amounts of nitrate, due to the heavy increase in the use of fertilizers between the 1970s and 1990s.

The effect of ground-water residence times is one factors contributing to the “lag time” between implementation of management actions and improvements in water quality. The amount of nutrients being applied to the land surface and the type of management practice will also effect water quality.