Water Quality Standards Attainment and Monitoring
Continually improve the capacity to monitor and assess the effects of management actions being undertaken to implement the Chesapeake Bay Total Maximum Daily Load (TMDL) and improve water quality. Use the monitoring results to report annually to the public on progress made in attaining established Bay water-quality standards and trends in reducing nutrients and sediment in the watershed.
An estimated 38% of the Chesapeake Bay and its tidal tributaries met water quality standards during the 2016-2018 assessment period. This score is lower than the record high of 42% during the 2015-2017 assessment period but is still the fifth highest estimate of water quality standards attainment since 1985. A decline in open water dissolved oxygen in a large area of the Bay impacted the attainment results. Open water habitat in a large area of the Bay failed to meet its standards in this period which had a big effect on lowering the indicator score. However, dissolved oxygen conditions in deep water habitat and surface chlorophyll a measures improved from the 2015-2017 assessment period which may indicate increasing resilience in the Bay ecosystem. Nonetheless, water quality measures remain far below the 100% attainment necessary to fully support survival, growth and reproduction of its living resources, and 62% of tidal waters are estimated to be impaired during the 2016-2018 assessment period.
In 2018, research published in Science of the Total Environment described the “positive and statistically significant trend” observed in this indicator of environmental health. One factor helping drive this improvement is an increase in the acres of estuarine underwater grass beds. Total underwater bay grass acres for Chesapeake Bay tidal waters have rebounded from the impacts of Hurricane Agnes on the Bay in 1972, and most recently from a decline sustained from Hurricane Irene and Tropical Storm Lee in 2011. The long-term improving trend for this indicator is not by chance but because of our decades-long effort to reduce nutrient pollution.
Water Quality Standards Attainment (1985-2018)
Water quality is evaluated using three parameters: dissolved oxygen, water clarity or underwater grass abundance, and chlorophyll a (a measure of algae growth).
The Bay and its tidal tributaries have been divided into 92 segments. Each of these segments contains up to five “designated uses” (also known as aquatic habitats), including deep channel, deep water, open water, shallow water and migratory fish spawning-and-nursery habitat. Each of these designated uses has its own set of criteria for dissolved oxygen, water clarity/underwater grasses and chlorophyll a (a measure of algae growth), which are designed to protect those uses. If the Bay and its tidal tributaries are to function as a healthy ecosystem and be taken off of the impaired waters listings under Section 303(d) of the Clean Water Act, all water quality standards or criteria applicable in each segment must be met simultaneously.
Bay water quality is influenced by nutrient and sediment loads delivered from the watershed, which are influenced by land use, land management and river flow. Between October 2017 and September 2018, river flow to the Bay measured an above-average 70.5 billion gallons per day. Generally, when the watershed receives more rain and river flows increase, the water carries more sediment and nutrient pollution than usual. Greater flows increase pollution loads received by the Bay. This remained true from October 2017 to September 2018 as approximately 423 million pounds of nitrogen, 42.1 million pounds of phosphorus and 15,689 million pounds of sediment reached the Bay: a 66%, 181% and 262% increase from the previous year, respectively. These higher nutrient levels feed algae blooms and result in larger dead zones than drier years. The highly variable weather-dependent changes in the amounts of nitrogen, phosphorus and sediment entering the Bay from its watershed from year to year increases the importance of analyzing trends over time to understand whether the health of the Bay tidal waters is improving or declining.
The October 2017 to September 2018 water year marks the first progress update using the Phase 6 version of the Chesapeake Bay Program’s Watershed Model, also known as the Chesapeake Assessment Scenario Tool (CAST). The Chesapeake Bay Program partnership upgraded its watershed model from Phase 5.3.2 to the Phase 6 Watershed Model to accommodate insights and understanding from new science and additional years of monitoring data. For consistency purposes, nutrient loads between 1990 and 2018 have been computed using the Phase 6 Watershed Model. For detailed descriptions of differences between Phase 5 and Phase 6 versions of the Chesapeake Bay Watershed Model, please view the Analysis and Methods document.
In 2019, the USGS released its analysis of the long-term (1985 to 2018) and short-term (2009 to 2018) trends in nutrient and sediment loads at nine River Input Monitoring (RIM) stations. Together, these stations reflect the nutrient and sediment loads delivered to the Bay from nine major tributaries representing 78 percent of its watershed. Long-term and short-term nutrient and sediment load trends are summarized in the table below. An upward arrow indicates improving conditions (and downward trends in nutrient or sediment loads), while a downward arrow indicates degrading conditions (and upward trends in nutrient or sediment loads).
While the table describes the short-term and long-term trends at each monitoring station individually, in summary, over the long term, trends in nitrogen have improved at five stations including the four largest rivers and degraded at two stations. Only three stations in four different rivers show long-term improvements in phosphorus and sediment loads. The remaining stations showed degraded trends or no change. Over the short term, nitrogen loads improved at three stations and degraded at five stations. Phosphorus and sediment loads improved at two stations, but in different rivers, phosphorus and sediment degraded at four and five stations, respectively. The remaining stations showed no change in conditions.
Computer-simulated nitrogen, phosphorus and sediment loads (generated using the Phase 6 version of the Chesapeake Bay Program’s Watershed Model and wastewater discharge data reported by watershed jurisdictions and calibrated using monitoring data) are used to track progress toward the 2025 Watershed Implementation Plans (WIPs) outcome.
Enhancing Chesapeake Bay Program models using our improved understanding of water quality trends;
Informing management strategies to improve water quality; and
Adhering to the TMDL Accountability Framework.
Monitoring progress toward achieving the outcome will occur through model simulations of pollution-reducing practices and analysis of data collected from monitoring networks that track river flow; nitrogen, phosphorus and sediment in the watershed; air deposition; conditions in tidal waters relative to established water quality standards; conditions of tidal habitats; and the health of living resources.
As part of the Chesapeake Bay Program’s partnership-wide implementation of adaptive management, progress toward this outcome was reviewed and discussed by the Management Board in May of 2018.