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A Guide to Water Quality Terms
Water temperature gives us an indication of how water is warming with summer season and will affect many
of the other measured parameters. For example, higher water temperatures are generally associated with
lower dissolved oxygen levels and increased occurrence of algal growth.
Salinity is a measure of salt. The waterways around the Chesapeake Bay are brackish with historical values in
Spa Creek varying from just under 7 ppt through greater than 11 ppt. Salinity can be affected by temperature
and rain. Runoff after a heavy rain can decrease salinity significantly. Salinity also generally increases as water
temperature increases.
Conductivity is a measure the ability of the water to conduct electricity or heat. We measure in mSiemens
per Celsius with value corrected to 25 degrees Celsius for all measurements. This value is directly correlated
with salinity but measures total ions. Ions commonly found in water include calcium, magnesium, potassium
and sodium cations and bicarbonate, carbonate, chloride, nitrate, and sulfate anions.
Secchi Depth is a measure of water clarity. We measure in cm. Clarity is important for submerged aquatic
vegetation survival. The higher the Secchi depth, the more light penetration occurs. A recommended Secchi
depth for SAV growth can vary depending on water depth and salinity. For the Severn River, a Secchi Depth of
100 cm (1 m) is the cutoff, although in the shallower water in the creeks, that value may be lower.
Dissolved oxygen is measured in mg/L. A standard cutoff for DO is 5 mg/L. Dissolved oxygen is necessary for
the survival of many marine animals. When DO falls below 5 mg/L the area is considered degraded. Values
below 2 mg/L are considered very hypoxic and below is 0.2 mg/L is considered anoxic. Temperature, wind,
and algal blooms will affect DO.
pH is a measure of hydrogen ions in water. A range of 6.5 – 8 is considered healthy for the marine life in our
rivers. Runoff, car exhaust, acid rain can all affect pH.
Total Suspended Solids is measured as mg/L and values below 15 mg/L are needed for SAV growth and
Chlorophyll A is measured as micrograms/L. The Chesapeake Bay Program has determined that chlorophyll A
concentration below 15 µg/L are required for SAV growth. Things like algal blooms will significantly increase
levels, which in turn affect clarity and DO levels.
Nitrogen and Phosphorous - both of these nutrients are measured in µg/L. There are multiple forms of each
that can be measured. These assays will be conducted at the Chesapeake Bio labs in Solomon’s, the total
value of each, which includes multiple forms, will be measured. High levels of either contribute significantly
to algal blooms. The Chesapeake Bay Program has set a high limit of 20 µg/L of phosphorous and 150 µg/L of
Enterococci is measured as colony-forming units/ 100 ml are indicators of fecal contamination by warmblooded animals including birds and mammals. The bacteria that fall into this group are not themselves,
typically infectious, but their presence is strongly correlated with the presence of other bacteria that can
cause both gastrointestinal and skin infections. The high limit for water that is regularly used for swimming is
104 cfu/ 100 ml. In addition, the county, and many other agencies, recommend not swimming in the water
for 48 hours after a rain event of 0.5 inches or more, especially for those that are immune-suppressed and
those with open wounds.