SOP SOD Measurements
Revision Date: July 12, 2005
Page 1
I. Introduction
This standard operating procedure (SOP) is applicable to the collection of representative
sediment oxygen demand (SOD) data from streams, rivers, lakes, ponds, lagoons, and surface
impoundments. These typically applicable procedures have been adapted from the Ohio EPA
Sediment Sampling Guide and Methodologies (OEPA, 2001), and may be varied or changed as
required, dependent upon site conditions or equipment and procedural limitations. The actual
procedures used should be documented in the field notes, especially if changes are made.
In order to collect representative SOD data, the hydrology and morphometry of a stream or
impoundment should be determined prior to sampling. This will aid in determining appropriate
sampling locations (see Section II).
SOD is measured using a dark chamber (resembling a large, inverted bowl) that isolates a known
area of sediment and a known volume of water. A pump and tubing are used to form a closed
system loop to circulate the volume of water over the area of sediment and ensure complete
mixing. A dissolved oxygen (DO) probe in the chamber provides a continuous display of the DO
concentration inside the chamber, which is recorded every five minutes for two hours or until the
DO drops by 2 mg/L.
By using a dark chamber, photosynthesis does not affect the DO of the water in the chamber, and
respiration and SOD are the only influences in the DO chamber. The effects of respiration are
quantified by filling a blank SOD chamber or dark bottle with a known volume of water from the
same location as the measurement chamber and measuring the DO at the beginning and end of
the SOD test. The change in DO in the blank chamber or dark bottle provides an estimate of the
amount of DO consumed by algal respiration in the water column.
The rate of change of DO in the chamber is determined by plotting the DO recorded in the
chamber every five minutes. A regression analysis is then performed on the dataset. The rate of
change of DO in the chamber is equal to the slope of the regression. The respiration rate
measured in the dark bottle is subtracted from this rate. The corrected value is then divided by
the area of the underlying sediment, resulting in an SOD value expressed as grams of oxygen
consumed per square meter per day (g/m2/day) at the ambient temperature. To provide for
standardization, temperatures are usually corrected to 20 degrees Celsius using a temperature
correction factor.
II. Site Selection
SOD should be evaluated when any of the following conditions exist:
♦ Reaches having extensive low velocity pools (less than 0.25 fps).
♦ Reaches having diurnal DO swings greater than 100%.
♦ Reaches having extensive sludge deposits.
Sites should be selected based on a field evaluation that includes:
♦ Stream velocity; less than 0.25 fps (Velz, 1970), i.e., pools.
♦ Discharger location.
