Chesapeake Bay Water-Quality Loads and Trends

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Explore resources here describing water-quality load and trend results for nontidal rivers of the Chesapeake Bay watershed.

The health of the Chesapeake Bay is largely driven by changes in streamflow and the amount of pollution it contains. Runoff in the Chesapeake Bay watershed carries pollutants, such as nutrients and sediments, to rivers and streams that drain to the Bay. Scientists use estimated streamflow entering the Chesapeake Bay to assess the health of the Bay and make ecological forecasts.

Loads and Trends

The Chesapeake Bay Nontidal Monitoring Program quantifies nutrient and sediment loads in the bay's nontidal rivers and estimates changes over time (trends) in sediment and nutrient loads.

What are the Objectives of the Chesapeake Bay Nontidal Monitoring Program?

Quantifying nutrient and sediment loads in the nontidal rivers of the Chesapeake Bay watershed, and estimating changes over time (trends) in these loads, are the two primary objectives of the Chesapeake Bay Nontidal Monitoring Program.

Loads are defined as the mass of nutrient or sediment passing a monitored location per unit time.

Estimates of change over time (trends) in sediment and nutrient loads are made in a manner that compensates for any concurrent trend(s) in stream discharge. Trends estimated in this manner can indicate changes in the watershed, such as the effects of best management practices, that cannot be attributed primarily to climatic fluctuation.

How the Program Works

  • Monitoring data are collected by numerous agencies through the nontidal monitoring partnership.
  • Results are updated on even-numbered water years for the network of water-quality monitoring stations distributed throughout the Chesapeake Bay watershed.

What Data and Related Information Are Available?

Methods, data, results, and result interpretations are available for:

  • Nutrient and sediment loads and yields (per-acre loads).
  • Trends in nutrient and sediment loads.

Many nontidal load and trend resources are available, including:

The nine river input monitoring stations:

View and download data and nutrient and suspended-sediment load and trend results for the following nine major rivers, known collectively as the Nine River Input Monitoring Stations.

 

Estimated Streamflow    Water-Quality Loads and Trends    Maps, Tables, and Figures    Partners

Water-Quality Loads and Trends

Estimated Streamflow    Water-Quality Loads and Trends    Methods of Data Compilation and Analysis    Water-Quality Model Used for Load and Trend Determination    Maps, tables, and figures    N, P, and SS Loads and Trends: 2016 Update    Glossary     Bibliography

 

The health of the Chesapeake Bay is largely driven by changes in streamflow and the amount of pollution it contains. Runoff in the Chesapeake Bay watershed carries pollutants, such as nutrients and sediments, to rivers and streams that drain to the Bay. Scientists can use estimated streamflow entering the Chesapeake Bay and measurements of pollutant loads and trends to assess the health of the Bay and make ecological forecasts.

Chesapeake Bay has been adversely affected by nutrient and sediment enrichment. Excess nutrients stimulate algal blooms that decay and consume dissolved oxygen, creating areas of low dissolved-oxygen concentration in the bay. Algal blooms and sediment reduce sunlight needed by underwater grasses. Because of slow improvements in water-quality conditions, the bay was listed as an impaired water body under the regulatory laws related to the Clean Water Act.

The Chesapeake Bay Program has developed water-quality criteria and is requiring all jurisdictions in the bay watershed to develop and implement watershed implementation plans (WIPs) that would reduce nutrient and sediment loads entering the Bay to levels prescribed by the TMDL by 2025.

The current nontidal monitoring network consists of 115 water-quality monitoring stations that are sampled in a coordinated manner by the Chesapeake Bay Program partnership. Water-quality monitoring is performed by the following partners:

The U.S. Geological Survey (USGS) reports constituent loads and trends. In partnership with the Chesapeake Bay Program watershed water-quality monitoring partnership, the USGS routinely reports monthly and annual constituent loads, as well as trends in load, for water-quality monitoring stations across the Chesapeake Bay watershed. Reporting constituent loads and trends is USGS' primary role. These reported loads and trends are developed based on:

  • continuous streamflow monitoring,
  • extensive water-quality sampling, and
  • advanced statistical analysis.

Information and data from the network are used to help scientists and managers assess water-quality conditions and long-term trends as management practices are implemented to reduce the amount of nutrients (primarily nitrogen and phosphorus) and sediment reaching the streams in the watershed and the bay. Data are also used to help measure progress toward meeting the Chesapeake Bay Total Maximum Daily Load (TMDL). The TMDL is a "pollutant diet" designed to reduce nutrients and sediment to improve water-quality conditions for fish and underwater grasses in the bay.

Methods of Data Compilation and Analysis

USGS methods of data compilation and analysis used to analyze water-quality data collected in the Chesapeake Bay watershed are described in Hirsch and others (2010), Moyer and others (2012), Hirsch and De Cicco (2014), Hirsch and others (2015), and Chanat and others (2015). The following summary describes how scientists construct the dataset and use the water-quality model to determine nutrient and suspended-sediment loads and trends.

Dataset construction: Updated streamflow and water-quality data are compiled each spring to prepare for the annual computation of loads and trends. Daily mean streamflow data are retrieved for all sites to be analyzed directly from the USGS National Water Information System (NWIS). For water-quality analysis, the USGS has compiled a database of historical (1972 through 2016) observed water-quality data collected at each of the nontidal network stations. Since 2011, all observed water-quality results collected by the multiple monitoring agencies that operate the nontidal network are reported to U.S. Environmental Protection Agency (EPA), Chesapeake Bay Program and stored in the Chesapeake Environmental Data Repository (CEDR).

Acquiring and incorporating data from other agencies: Annually, the USGS receives water-quality records from EPA and the CEDR database for the previous water year, defined as the 12-month period from October 1 through September 30. These new water-quality observations are combined with the historical observations to create a complete record of water-quality observations for each nontidal network station.

The primary water-quality constituents considered are total nitrogen, dissolved inorganic nitrogen, total phosphorus, orthophosphate, and suspended sediment. All records of water-quality observations are reviewed by the collecting agency to ensure data completeness and accuracy.

Accounting for changes in Laboratory Procedures: Various evolutions in the long-term history of the program have resulted in slight changes to laboratory analyses and methodologies that must be accounted for prior to the data analysis. These evolutions include:

  • Historically, most programs analyzed samples for total suspended solids (TSS), but now, many programs analyze samples for suspended sediment concentration (SSC) at all stations. Glysson and others (2001) provides details about the differences between these analyses. Because of this shift in procedure, TSS and SSC were sometimes combined in water-quality input files. If TSS and SSC were collected at the same time, priority was given to the SSC sample. 
  • In addition, where supported by available information, missing observations of total nutrient concentrations were calculated as the sum of constituent species; for example, total nitrogen (TN) may be calculated as the sum of total dissolved and total particulate nitrogen.

Water-Quality Model Used for Load and Trend Determination

Concentration data retrieved from the nontidal database and daily streamflow data from NWIS are used for load and trend analyses. Recent advances in the statistical tools available to compute loads and trends have led to the use of revised data-analysis methodologies. For water year 2016, all load and trend estimates were made using a multiple linear regression model known as Weighted Regressions on Time, Discharge, and Season (WRTDS; Hirsch and others, 2010, Hirsch and De Cicco, 2014).

How Does the WRTDS Model Work? The WRTDS model uses a sparse set of discrete water-quality observations combined with a continuous daily discharge record to estimate concentration on days for which no water-quality data are available. Daily concentration and load estimates are then aggregated to monthly and annual time scales. An algorithm is then applied to estimate the trend in "flow-normalized load," namely a trend that minimizes the confounding effect of any concurrent trend in discharge. Confidence in the flow-normalized trend is assigned through application of likelihood analyses using bootstrapped replicates (Hirsch and others, 2015). Detailed comparative studies by Chesapeake Bay River Input Monitoring (RIM) team staff (Moyer and others, 2012; Chanat and others, 2015) have documented that WRTDS performs better than regression-based approaches used historically.

Why Are Trends Flow-Normalized? Observed water-quality loads are highly influenced by streamflow and season. Trends are adjusted for flow and season to minimize the influence of these potentially confounding factors. This process is referred to as "flow-normalization," and is described further in Hirsch and others, (2010). Flow-normalized trends help scientists evaluate changes in load resulting from changing sources, delays associated with storage or transport of historical inputs, and (or) implemented management actions.

How Are Trends in Loads Identified? Identified trends are based on the results of likelihood analyses using bootstrapped replicates (Hirsch and others, 2015). As an example, for a given site and constituent, reported positive (or negative) trends having likelihood estimates of at least 0.67 mean that positive (or negative) trends were evident in about two-thirds of the bootstrapped replicates for that site and (or) constituent.

What Trends Are Computed? For stations having water-quality records beginning prior to 1990, trends in load are computed for both the period of record and for the most recent 10 years. For stations having records beginning after 1990, only 10-year trends are computed. All data available, including data collected prior to 2007, are used to estimate 10-year trends.

Load and Trend Summaries

Summary of Nitrogen, Phosphorus, and Suspended-Sediment Loads and Trends Measured at the Chesapeake Bay Nontidal Network Stations: Water Year 2016 Update: 

Prepared by Douglas L. Moyer and Joel D. Blomquist, U.S. Geological Survey, December 13, 2017

Changes in nitrogen, phosphorus, and suspended-sediment loads in rivers across the Chesapeake Bay watershed have been calculated using monitoring data from 115 stations that are part of the Non-tidal monitoring network (NTN) (Moyer and others, 2017). Constituent loads are calculated with at least five years of monitoring data and trends are reported after at least ten years of data collection. Data collection began in 1985 at nine of these locations, referred to as River Input Monitoring (RIM) stations, where loads are delivered directly to tidal waters. These results are used to help assess efforts to decrease nutrient and sediment loads being delivered to the bay. Additional information for each monitoring station is available through the USGS Chesapeake Bay Nontidal Web site that provides State, Federal, and local partners, as well as the general public, ready access to a wide range of data for nutrient and sediment conditions across the Chesapeake Bay watershed. Results from two time periods are reported in this summary: a long-term time period (1985-2016), and short-term time period (2007-2016). All annual results are based on a water year which extends from October 1 to September 30.

The results are summarized for:

  1. Loads delivered directly to the tidal waters for the most recent year (Water Year 2016); specifically, the combined load from the nine River Input Monitoring (RIM) stations
  2. Trends in loads at the RIM stations over the long- and short-time periods
  3. Patterns in loads and trends at the 115 NTN monitoring stations in the bay watershed (that are part of the Chesapeake Bay Program (CBP) NTN) over the short-term period

What are the patterns in loads delivered to tidal waters from the RIM stations? The USGS combined the load results from the nine RIM stations shown in Chesapeake Bay River Input Summary Figure 1 to quantify the total nitrogen, phosphorus, and suspended-sediment loads delivered from the watershed to tidal waters. Together, the nine RIM stations reflect loads delivered from 78 percent of its 64,000-square-mile watershed.

River flow and loads to tidal waters:

The Chesapeake Bay Program uses the RIM loads and estimates loads from the remaining unmonitored areas to compute a total nutrient and sediment load to the bay.

What are the trends in loads delivered to tidal waters from the RIM stations? Trends in loads from the nine RIM stations are flow-normalized to integrate out the year-to-year variability in river flow; by doing so, changes in nitrogen, phosphorus, and suspended-sediment loads resulting from changing sources, delays associated with storage and transport of historical inputs, and (or) implemented management actions are identified. Changes in loads for nitrogen, phosphorus, and suspended sediment are provided for two time periods: 1985-2016 (long term) and 2007-2016 (short term) (Chesapeake Bay River Input Summary Table 1). Loads that are lower in the end year than the start year are classified as improving conditions, while loads that are higher in the end year than the start year are classified as degrading conditions. Loads are classified as having no trend if there is not a discernable difference between start and years.

Changes in total nitrogen loads

  • Long-term trends in total nitrogen loads indicate improving conditions at 6 stations (Susquehanna River at Conowingo, Maryland; Potomac River at Washington DC; James River at Cartersville, Virginia; Rappahannock River at Fredericksburg,Virginia; Mattaponi River near Beulahville, Virginia; Patuxent River near Bowie, Maryland). The Choptank River near Greensboro, Maryland is the only station whose data indicate degrading conditions. Data from the Appomattox River at Matoaca, Virginia and the Pamunkey River near Hanover, Virginia indicate no trend.
  • Short-term trends in total nitrogen loads indicate improving conditions at 4 stations (Potomac River at Washington DC; James River at Cartersville, Virginia; Rappahannock River at Fredericksburg,Virginia; Patuxent River near Bowie, Maryland), and degrading conditions at 5 stations (Susquehanna River at Conowingo, Maryland; Appomattox River at Matoaca, Virginia; Pamunkey River near Hanover,Virginia; Mattaponi River near Beulahville, Virginia; Choptank River near Greensboro, Maryland).

Changes in total phosphorus loads

  • Long-term trends in total phosphorus loads indicate improving conditions at 3 stations (Potomac River at Washington DC; James River at Cartersville, Virginia; Patuxent River near Bowie, Maryland), degrading conditions at 5 stations (Susquehanna River at Conowingo, Maryland; Rappahannock River at Fredericksburg,Virginia; Appomattox River at Matoaca, Virginia; Pamunkey River near Hanover, Virginia; Choptank River near Greensboro, Maryland), and no trend at 1 station (Mattaponi River near Beulahville, Virginia).
  • Short-term trends in total phosphorus loads indicate improving conditions at 1 station (Patuxent River near Bowie, Maryland), degrading conditions at 5 stations (Susquehanna River at Conowingo, Maryland; Potomac River at Washington DC; Appomattox River at Matoaca, Virginia; Mattaponi River near Beulahville, Virginia; Choptank River near Greensboro, Maryland), and no trend at 3 stations (James River at Cartersville, Virginia; Rappahannock River at Fredericksburg,Virginia; Pamunkey River near Hanover, Virginia).

Changes in suspended-sediment loads

  • Long-term trends in suspended-sediment loads indicate improving conditions at only 3 stations (Potomac River at Washington DC; Patuxent River near Bowie, Maryland; Choptank River near Greensboro, Maryland), degrading conditions at 4 stations (Susquehanna River at Conowingo, Maryland; James River at Cartersville, Virginia; Rappahannock River at Fredericksburg, Virginia; Pamunkey River near Hanover, Virginia), and no trend in conditions at 2 stations (Appomattox River at Matoaca, Virginia; Mattaponi River near Beulahville, Virginia).
  • Short-term trends in suspended-sediment loads indicate improving conditions at only 1 station (James River at Cartersville, Virginia), degrading conditions at 4 stations (Appomattox River at Matoaca, Virginia; Pamunkey River near Hanover, Virginia; Patuxent River near Bowie, Maryland; Choptank River near Greensboro, Maryland), and no trend at 4 stations (Susquehanna River at Conowingo, Maryland; Potomac River at Washington DC; Rappahannock River at Fredericksburg, Virginia; Mattaponi River near Beulahville, Virginia).

What are the patterns in loads and trends across the nontidal monitoring network (2007-16)? The USGS computes load and trend results from the NTN to display (1) the range in loads of nitrogen, phosphorus, and suspended sediment; and (2) the trends in these loads. The majority of the NTN sites whose data were used for the analysis had data collected since 2007 (Chesapeake Bay River Input Summary Figure 6 and Chesapeake Bay River Input Summary Table 2). To facilitate the comparison of loads and trends between sites, load results from each NTN station are normalized by the respective drainage area to present the results as per-acre loads (also known as yield). The total number of NTN stations analyzed for total nitrogen, total phosphorus, and suspended-sediment load and trends varies because of the length of record and because of the presence or absence of targeted water-quality samples collected during stormflow conditions (see Chanat and others, 2015).

Patterns in total nitrogen loads:

  • Average annual total nitrogen loads, 2007-2016, range from 1.19 to 30.0 pounds per acre (lb/acre; Chesapeake Bay River Input Summary Figure 7) with a combined average load over this period of 6.97 lb/acre.
  • Half of the NTN stations are improving; while, 31 percent are degrading and the remainder are showing no trends. 
    • 43 of 86 (50 percent) stations have improving trends, with load reductions ranging from 0.13 to 6.46 lb/acre.
    • 27 of 86 (31 percent) stations have degrading trends, with load increases ranging from 0.03 to 0.79 lb/acre.
    • 16 of 86 (19 percent) show no trends.

Patterns in total phosphorus loads:

  • Average annual total phosphorus loads, 2007-2016, range from 0.12 to 2.01 lb/acre (Chesapeake Bay River Input Summary Figure 8) with a combined average load over this period of 0.48 lb/acre.
  • Just over one-third of the NTN station are improving; while, a quarter of the stations are degrading and the remainder are showing no trends.
    • 25 of 66 (38 percent) stations have improving trends, with load reductions ranging from 0.02 to 0.53 lb/acre.
    • 17 of 66 (26 percent) stations have degrading trends, with load increases ranging from 0.01 to 0.59 lb/acre.
    • 24 of 66 (36 percent) have no trends.

Patterns in suspended-sediment loads:

  • Average annual suspended-sediment loads, 2007-2016, range from 19 to 1,670 lb/acre (Chesapeake Bay River Input Summary Figure 9) with a combined average load over this period of 401 lb/acre.
  • Less than a quarter of the NTN stations are improving; while, 37 percent are degrading and the remainder are showing no trends.
    • 12 of 65 (20 percent) stations have improving trends, with load decreases ranging from 11.2 to 706 lb/acre.
    • 24 of 65 (37 percent) stations have degrading trends, with load increases ranging from 1.24 to 395 lb/acre.
    • 29 of 65 (43 percent) have no trends.

The Chesapeake Nontidal Monitoring Network and Role of USGS: The Chesapeake Bay Nontidal Water-Quality Monitoring Network is a partnership implemented among the States in the watershed, the U.S. Environmental Protection Agency, the USGS, and the Susquehanna River Basin Commission. A network of monitoring stations has been established and is sampled using standardized protocols and quality-assurance procedures designed to measure pollutant loads and changes in pollutant loads over time. The initial network formed around 1985 with coordinated monitoring at the nine RIM stations. In 2004, the CBP formalized the network, and a period of expansion followed. In 2010 and 2011, the network was further expanded to address the needs of the Total Maximum Daily Load (TMDL). The network currently has 115 sites designed to measure changes in nitrogen, phosphorus, and suspended sediment in the Chesapeake Bay watershed. Through this partnership, nitrogen, phosphorus, and suspended-sediment loads and trends are determined based on (1) continuous streamflow monitoring, (2) extensive water-quality sampling, and (3) advanced statistical analysis. The USGS computes the loads and trends and present this information through this Web site.

Learn more about water-quality loads and trends:

Glossary

anthropogenic - resulting from human activity 

atmospheric deposition - includes wet precipitation such as snow, rain, or hail, as well as dry deposition from airborne particles like smoke and dust. 

base-flow conditions - normal streamflow conditions; that is, without recent input from precipitation. 

censored data - data concentrations reported at less than a specified reporting limit or analytical limit. 

concentration - the mass of a chemical per volume of water - commonly expressed as milligrams per liter (mg/L), or micrograms per liter (ug/L). 

dissolved inorganic phosphate (DIP) - P00671 -- Dissolved inorganic phosphate (mg/L as P) 

dissolved nitrite-plus-nitrate nitrogen (DNO23) - P00631 -- Dissolved nitrite-plus-nitrate nitrogen (mg/L as N) 

ESTIMATOR - a multiple regression model used to quantify fluxes and determine trends in concentration. 

Eutrophication - nutrient enrichment in a stream, lake, or other water body that stimulates excessive growth of algae and other aquatic plants and leads to reduced dissolved oxygen levels. 

Fall Line - the boundary between the Piedmont and Coastal Plain Physiographic Provinces in the Eastern United States. In the area near this boundary, there is a relatively large change in elevation, resulting in waterfalls or rapids in streams or rivers flowing east; thus the name "Fall Line". This line roughly coincides with the area between the tidal and nontidal parts of each river. 

flow-adjusted concentration trends - trends after streamflow has been removed as a factor affecting concentration; allows examination of the effects of management strategies. 

high-flow conditions - above-normal streamflow conditions, as a result of stormflow or release of water. 

homoscedastic - with constant variance throughout a dataset. 

hydrogeologic setting - the geology and the resultant hydrologic conditions associated with an area. 

inorganic - being or composed of matter other than plant or animal. 

load - product of the concentration and streamflow, equivalent to the total amount of a constituent passing that point in the stream, at that time. Usually expressed as kilograms per time or pounds per time. 

multivariate - more than one variable; for the regression model used in this project, variables of time, streamflow, and seasonality are used. 

nonpoint source - a source of natural or man-made chemicals delivered to a stream; the source is distributed over an area and may include runoff from fields, groundwater discharge, and urban runoff. 

nontidal - outside the area where the water level is influenced by tidal fluctuation. A nontidal stream reach may or may not be elevated in salinity. 

nutrients - chemicals necessary for animal and plant life; includes phosphorus and nitrogen. 

organic - of, relating to, or derived from living organisms. 

physiographic provinces - geographic divisions based on landforms, geology, and geologic origin of an area. 

point source - a source of natural or man-made chemicals that can be attributed to a single location, such as a pipe outflow. 

p-value - the probability value, which is the estimated probability that a hypothesis is found to be true when it actually is not. In this study, it is the probability that a trend is found when there is in fact no true trend (that is, that random variation in concentration alone could produce the observed behavior). A lower p-value indicates greater confidence in a given conclusion. 

regression - an examination of the relationship between a response (dependent) variable and the explanatory (independent) variable(s); a measure of the tendency for the expected value of one of these jointly correlated random variables to approach more closely the mean value of its own dataset than that of any of the other variables. 

runoff - Overland flow to a stream that occurs when either (1) the precipitation rate exceeds the infiltration rate through the soil, (2) precipitation reaches soil that is already saturated, or (3) precipitation reaches a surface that is nearly impermeable, such as pavement or bedrock. 

submerged aquatic vegetation (SAV) - a general term referring to the many species of plants that grow in the shallow water of the Chesapeake Bay and its tributaries; these plants provide an important habitat and food source for fish and wildlife. 

suspended material - a general term indicating the concentration of either suspended sediment or suspended solids, depending on the analysis performed. 

tidal - the area where the water level is influenced by tidal fluctuation. A tidal stream reach may or may not be elevated in salinity. 

total nitrogen (TN) - P00600 -- Total nitrogen (mg/L as N). 

total phosphorus (TP) - P00665 -- Total phosphorus (mg/L as P). 

total suspended sediment (SSC) - the concentration of total suspended material carried by a stream as determined by an analysis of an entire collected water sample. P80154 -- SSC (mg/L) see also: total suspended solids. 

trend - the measurable, statistically significant tendency for something (such as, chemical concentrations) to change over time. 

unadjusted concentration trends - trends that account for all factors, including streamflow, that affect concentration; necessary when considering the health of aquatic organisms in the Chesapeake Bay and its tributaries. 

wastewater discharge - water that has been treated to meet State and Federal standards for chemical levels. 

WRTDS - Weighted Regressions on Time, Discharge, and Season � a tool for the determination of nutrient and sediment fluxes and trends using multiple weighted regressions. 

yield - a normalization of load by area, usually expressed as pounds per year per square mile, or kilograms per year per square mile.

Bibliography

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