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Water-Quality Loads and Trends at Nontidal Monitoring Stations in the Chesapeake Bay Watershed

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

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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 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

  • Estimated annual-mean streamflow entering the Chesapeake Bay in 2016 was 71,600 cfs, about 19 percent (15,100 cfs) below the long-term (1937-2016) annual-mean streamflow of 78,476 cfs (Figure 2).
  • In 2016, the combined loads from the nine RIM stations were as follows:
    • Total nitrogen (TN): 157 million pounds (Mlb), 50 Mlb less than the long-term average of 207 Mlb for 1985-2016 (Figure 3).
    • Total phosphorus (TP): 9.32 Mlb, 4.16 Mlb less than the long-term average of 13.48 Mlb for 1985-2016 (Figure 4).
    • Suspended sediment: 2.51 million tons (Mton), 2.25 Mton less than the long-term average of 4.76 (Mtons) for 1985-2016 ((Figure 5).

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) (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 the majority of the stations, including the four largest rivers. The Choptank River is the only station whose data indicate degrading conditions. Data from the Appomattox and Pamunkey Rivers indicate no trend.
  • Short-term trends in total nitrogen loads indicate improving conditions at 4 stations and degrading conditions at 5 stations.

Changes in total phosphorus loads

  • Long-term trends in total phosphorus loads indicate improving conditions at only 3 stations and degrading conditions at another 5 stations. Data from the Mattaponi River indicate no trend.
  • Short-term trends in total phosphorus loads indicate improving conditions at only the Patuxent station, degrading conditions at 5 stations, and no trend at the 3 remaining stations.

Changes in suspended-sediment loads

  • Long-term trends in suspended-sediment loads indicate improving conditions at only 3 stations, degrading conditions at 4 stations, and no trend in conditions at 2 stations.
  • Short-term trends in suspended-sediment loads indicate improving conditions at only the James station; degrading conditions at 4 stations, and no trend at the remaining 4 stations.
Table 1

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 (Figure 6 and 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; 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 (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 (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.

Additional Information

  • Maps of the load and trend in load results are available as:
  • Tabular results for each station are available in the Downloads section of the navigation menu on this Web site.

USGS Contacts

Doug Moyer, Richmond, VA, Joel Blomquist, Baltimore MD,

For more information on USGS Chesapeake Bay Studies

Scott Phillips, or visit https://chesapeake.usgs.gov/

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