USGS Groundwater Study

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New research by the U.S. Geological Survey conducted on the Delmarva Peninsula, which forms the Eastern Shore of the Chesapeake Bay, indicates it may take several decades for many water-quality management practices aimed at reducing nitrogen input to the Bay to achieve their full benefit due to the influence of groundwater.

The USGS 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 U.S. 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.

The responses of watershed systems and ecosystems to environmental management actions at any location can vary from rapid changes (such as the swift beneficial effect of a wastewater treatment plant upgrade) to longer improvement intervals of several decades. In the Chesapeake Bay, “lag response time” refers to the time between implementing management actions and the resulting improvements in water quality. Lag times will vary for nitrogen, phosphorus, and sediment.

This USGS study focused on nitrogen. 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.

For this study USGS scientist Ward Sanford developed a complex model for water, geology, and chemical interactions that he applied to seven separate watersheds on the Delmarva Peninsula. 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.

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.

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.

The study highlights the complexities of environmental restoration of the Bay. The findings help refine the expectations of resource managers and citizens alike of how long it may take to see substantial water-quality improvements in the Bay, and they may provide additional insight into the effectiveness of different types of land management practices given the time lag created by local groundwater response times.

The study was done as part of increased federal efforts under the President’s 2009 Chesapeake Executive Order, which directs Federal agencies, including the EPA and the Department of the Interior, to “begin a new era” in protection and restoration of the Chesapeake Bay. With a watershed that spreads across six states and Washington, DC, the Chesapeake Bay is the largest estuary in the United States and one of the largest and most biologically productive estuaries in the world.

source: U.S. Geological Survey

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