Nutrient time lag may slow Chesapeake Bay cleansing - Delawareonline.com
Ground-water "lag" may delay Bay cleanup - B'More Green
Groundwater pushes nitrogen into Bay, delays effects of restoration - Chesapeake Bay Program
New USGS Study on Chesapeake Bay: Groundwater Delaying the Effects of Some Water Quality Actions - USGS Newsroom
I'll steal from the USGS, if only because I, like most of you, paid for these words too. Besides, their spin will be slightly less flamboyant than the EPA Bay Program's.
...Based on the concept of nitrogen mass-balance regression, the model was able to reproduce the time history of nitrate concentrations in area streams and wells, including a recent slowdown in the rate of concentration increase in streams. The model was then also used to forecast future nitrogen delivery from the Delmarva Peninsula to the Bay under different nitrogen management scenarios.Which makes it sound like things are going to get worse before they get better, if they get better. And more severe reductions will be required to make even minor improvements. The farmers are gonna love this...
The new study shows that ages of groundwater and associated nitrogen from the Delmarva Peninsula into the Chesapeake Bay range from less than a year to centuries, with median ages ranging from 20 to 40 years. These groundwater age distributions are markedly older than previously estimated for areas west and north of the Bay, which has a median age of 10 years. The older ages occur because the porous, sandy aquifers on the Delmarva yield longer groundwater return times than the fractured-rock areas of the Bay watershed.
The USGS research found that in some areas of the Delmarva the groundwater currently discharging to streams is gradually transitioning to waters containing higher amounts of nitrate due to fertilizer used during the 1970s through the 1990s. Similarly, the total amount of nitrogen in the groundwater is continuing to rise as a result of the slow groundwater response times.
Without additional management practices being implemented, the study forecasts about a 12% increase in nitrogen loads from the Delmarva to the Bay by 2050. The study provides several scenarios for reducing nitrogen to the water table and the amount of time needed to see the reductions in groundwater discharging to streams. For example, the model predicts that a 25% reduction in the nitrogen load to the water table will be required to have a 13% reduction in load to the bay...