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Water Quality Status and Trends

Water Quality Status and Trends Methodology

Maryland status and trends information is determined from data collected as part of the Department of Natural Resources' Chesapeake Bay Water and Habitat Quality Monitoring Program. Water quality samples are collected once or twice a month (depending on season) from 72 tidal stations and 51 non-tidal stations in the Maryland tributaries. Status is a measure of current condition (most recent three years) at a station compared to scientifically-based threshold values. Each station is given a ranking of "GOOD," "FAIR," or "POOR." Trends are a measure of how the system has been changing over time, either improving or degrading.

Total nitrogen (N) levels used in the analysis are based on data from the surface-mixed layer (i.e., surface and above pycnocline samples, or surface only samples at shallow stations).

Total phosphorus (P) levels used in the analysis are based on data from the surface-mixed layer (i.e., surface and above pycnocline samples, or surface only samples at shallow stations).

Abundance of algae is estimated by measuring active chlorophyll a from surface mixed layer samples.

Summer bottom dissolved oxygen is measured directly. The dissolved oxygen data used in this analysis are from samples taken during the months of June to September in bottom samples only. No dissolved oxygen measurements are taken in non-tidal or other shallow areas.

Total suspended solids (TSS) is measured directly from mixed-surface layer samples.

Water clarity is measured using Secchi disk depth.

For further information on parameters we measure please visit Our Monitoring Explained page.

Current Status

Status is a measure of current condition (most recent three years) at a station compared to a threshold value; most of the measurements have threshold values that are dependent on the salinity zone for the station. Based on this comparison, the station is given a ranking of "GOOD", "FAIR," or "POOR."

Water Quality Components & Threshold Values

    SALINITY ZONES (ppt)
  Status Levels Tidal Fresh
0 - 0.5
Oligohaline
>0.5 - 5
Mesohaline
>5 - 18
Polyhaline
>18

Total Nitrogen1
(mg/L)
good ≤0.9 ≤0.9 ≤0.6 ≤0.5
fair 0.9 - ≤1.8 0.9 - ≤1.6 0.6 - ≤1.0 0.5 - ≤0.8
poor >1.8 >1.6 >1.0 >0.8

Total Phosphorus1
(mg/L)
good ≤0.06 ≤0.07 ≤0.04 ≤0.05
fair 0.06 - ≤0.13 0.07 - ≤0.15 0.04 - ≤0.08 0.05 - ≤0.09
poor >0.13 >0.15 >0.08 >0.09

Sediment2
[Total Suspended Solids]
(mg/L)
good ≤15
fair 15 - ≤19.5
poor
 
>19.5
*same for all salinity zones

Algal Levels2
[Chlorophyll a]
(µg/L)
good ≤15
fair 15 - ≤19.5
poor >19.5
*same for all salinity zones

Water Clarity2
[Secchi Depth]
(m)
good ≥0.725 ≥0.970
fair 0.508 - ≤0.725
0.679 - ≤0.970
poor <0.508
*same for tidal fresh & oligohaline
<0.679
*same for meso- & polyhaline

Bottom Dissolved Oxygen3
[Summer]
(mg/L)
good >5.5 ≥5.0
fair <5.5 - 3.0 <5.0 - 3.0
poor <3.0

<3.0
*same for oligo-, meso-, and polyhaline

[1] EcoCheck. (2011). Sampling and data analysis protocols for mid-Atlantic tidal and tributary indicators. E. C. Wicks, M.L. Andreychek, R.H. Kelsey, S.L. Powell. IAN Press, Cambridge, Maryland, USA. Pages 26, 30. Good includes categories 4 and 5, Fair includes categories 2 and 3, and Poor includes categories 0 and 1.

[2] United States Environmental Protection Agency for the Chesapeake Bay Program. (2000). Chesapeake Bay Submerged Aquatic Vegetation Water Quality and Habitat-Based Requirements and Restoration Targets: A Second Technical Synthesis. Page iv. Sediment, Chlorophyll and Clarity thresholds are based on levels that meet habitat requirements for SAV by salinity zone. Fair levels are within 30% of the habitat requirement. Poor levels fail the habitat requirements.

[3] United States Environmental Protection Agency (2003). Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries. EPA 903-R-02-002. Page xv. Open water 30-day criteria used.



Long-term trends

Trends are a measure of how the system has been changing over time, either improving or degrading. If the trend is not statistically significant at α = 0.05, “no trend” is reported. If the trend is significant, an arrow indicates the direction of the trend (increasing or decreasing concentration). The color of the arrow indicates whether it is a good or bad trend; green arrows indicate an improving trend, and red arrows indicate a degrading trend.

Trends are used to evaluate effectiveness and track progress of management actions to reduce nutrient and sediment pollution. Tidal water quality information is linked with other monitoring information (shallow-water monitoring, benthic monitoring, SAV monitoring, non-tidal monitoring) and examined in the context of the whole to gain a more comprehensive understanding of water quality processes and the relationship between water quality and living resources.

Trend analyses were done to account for the differences in weather from one year to another; these are called 'flow adjusted' trends. Flow adjusted trend analysis uses daily flow measurements collected by the United States Geologic Service (USGS) stream gages to 'adjust' the analysis for flow impacts. Trend tests are conducted using an R statistical package developed by the Chesapeake Bay Program and partners. Trend tests are completed using a Generalized Additive Models (GAMs) approach. The methods used are described in the following documents available online at Chesapeake Bay Program Integrated Trends Analysis Team:

Murphy, R.R., E. Perry, J. Harcum, J. Keisman. 2019. A Generalized Additive Model approach to evaluating water quality: Chesapeake Bay case study. Environmental Modeling and Software 118:1-13.

Murphy, R.R. and E. Perry. 2018 (Draft) Methods for Application of Generalized Additive Models (GAMs) for Water Quality Trends in Tidal Waters of Chesapeake Bay

The R package for tidal trends, called 'BayTrends', was developed through coordinated efforts at the Chesapeake Bay Program and is available on the Comprehensive R Archive Network (CRAN). This BayTrends package is loaded into the R statistical program using RStudio software and calls on many other pre-written and specialty written packages and programs. The entire data record is used for the model fit procedure. GAM4 model was used for flow adjusted analysis trends when no intervention was required through the long-term data record. When method changes or lab changes were determined to be statisically important, a GAM5 model with a single intervention was used to account for these changes in the data record. The trend result is presented as the difference between the 1999-2000 and the 2018-2019 mean values. Version 1.2.1 of BayTrends was used for the 1999-2019 trends analysis

For more details on the field sampling, laboratory analysis, or statistical methods, visit the Monitoring News and Notes page to view our Quality Assurance Project Plan (QAPP).

Find out more details on the statistical methods used in analyzing status and trends, and the history of those methods over time.