Groundwater delaying effects of Chesapeake Bay water quality actions
Water-quality management practices aimed to reduce nitrogen in the Chesapeake Bay may take several decades due to groundwater influences.
Nov. 14, 2013 -- Many water-quality management practices aimed at reducing nitrogen input to the Chesapeake Bay may take several decades to take effect due to a signficant influence of groundwater, according to new research by the U.S. Geological Survey (USGS) conducted on the Delmarva Peninsula, which forms the Eastern Shore of the Chesapeake Bay.
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.
The findings provide critical information on how long it may take to see the water quality in the Bay improve as more stringent practices are implemented to reduce nutrients and sediment to tidal waters. Having established a calculation for the total nitrogen, phosphorus and sediment pollutants that are allowable for the Chesapeake watershed, known as the total maximum daily load (TMDL), the Environmental Protection Agency (EPA) is working with Maryland, Pennsylvania, Virginia, and the four other Bay watershed jurisdictions to ensure that all water-quality practices needed to reduce the flow of nutrients and sediment to the Bay are in place by 2025.
Focusing on nitrogen, the USGS study indicated that some of the nitrogen will run off directly into a stream, but a large portion on the Delmarva (more than two thirds) is affected by the slow travel times of nutrients moving from their land source through underground aquifers to a receiving stream or estuary. Sources of nitrogen include fertilizer and manure applications to agricultural land, wastewater and industrial discharges, runoff from urban areas, domestic septic drain fields, and air emissions. Excess nitrogen contributes to algal blooms that cause low dissolved oxygen in the Bay and related fish kills each summer and impact recreational activities.
Without additional management practices being implemented, the study forecasts about a 12 percent 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 percent reduction in the nitrogen load to the water table will be required to have a 13 percent reduction in load to the Bay.
However, the results also indicate that nutrient management practices implemented over the past decade or so have begun to work and confirm that the amount of the nitrogen loading to streams in the future will depend on the rigor of water-quality practices implemented to reduce nutrients at present.