A Method for Measuring Sediment Oxygen Demand
  Using a Bench Model Benthic Respirometer
                     by
               Peter M. Nolan
              Arthur F. Johnson
                  May 1979
      U.S.  ENVIRONMENTAL PROTECTION AGENCY
                    REGION I
         NEW ENGLAND REGIONAL LABORATORY
                60 WESTVIEW STREET
         LEXINGTON, MASSACHUSETTS 02173

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           A Method for Measuring Sediment Oxygen Demand
             Using a Bench Model Benthic Respirometer
Introduction

     The apparatus described has been in use at the New England

Regional Laboratory since 1973 and was designed and constructed to

facilitate the measurement of Sediment Oxygen Demand (SOD) in the

laboratory without the expense and time required for in situ techniques

which involve the careful placement of large chambers on the bottom

sediment using SCUBA or other techniques for positioning the equipment

from a boat.  Advantages of the "bench" technique include the ability

to conduct several rate measurements for a given sample for prolonged

periods of time, samples taken from environments with low dissolved

oxygen can be measured, and a variety of substrate types can be used.


Design

     The bench model SOD chamber is constructed of clear 1/4 inch

acrylic plastic and has uniform inside dimensions of 30.5 cm.  The top

is larger (33.0 cm square)  and is fitted with a rubber gasket.  Water

tightness is perfected using a lock down arrangement constructed from

wing nuts, and threaded rod connected in two places on each side of

the chamber with 2.5 cm thickness blocks of acrylic plastic.


     A hole is drilled centrally in the top to accommodate a dissolved

oxygen probe.  A small reservoir 5 cm h X 10 cm 1 X 10 cm w is built

on the top around the hole so that when the probe is in place a water

seal is achieved and air leakage prevented.  Large rubber O rings to
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fit the probe on either side of the top gives added protection




against air leakage.





     Two holes are drilled, one each on opposite sides of the




chamber and threaded to accommodate a 5/8" pipe thread.  'The influent




port is centrally located 7.5 cm from the bottom.  The effluent port




is centrally located 7.5 cm from the top.





     All seams are "glued" using butt joints where the ends to be




joined are soaked in a mixture of 70% Methylene chloride - 30%




Ethylene dichloride until slightly softened and then joined under




pressure.  A bead of silicon rubber cement run along the inside




seams adds strength and water tightness to the chamber.  Figure I is




a schematic of system design.





Operation




     The basic principle of operation of the bench model respirometer




is to circulate a confined volume of water at a controlled rate within




a chamber into which an environmental sediment sample has been placed




to a uniform depth and area.  Oxygen depletion in the water is




monitored for a specified period of. time in order to estimate the




sediment oxygen demand which is the sum total of processes in the




sediment utilizing oxygen.





     In the field, replicate sediment samples are collected by grab -




placed in plastic bags with a minimal amount of disturbance - iced and




returned to the laboratory for analysis.  The sediment is carefully
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placed in the chamber to a uniform depth of 2.5 cm.  Aerated aged




tap water or water from the sample source  (min D.O. 70% saturation)




is gently layered over the sediment to prevent roiling.  A flat




acrylic plastic paddle or stainless steel spatula works well to




disburse water over the sediment and with experience a minimum amount




of disturbance can be achieved.  Before a run, any suspended sediments




are allowed to settle.





     Circulation in the chamber is maintained at a constant rate with




a variable speed peristaltic pump using a closed loop principle.  The




entry port is split into two diffusion nozzles so that mixing is




rapid and total.  The exit port is a single fitting.  Both ports are




connected to the pump by one continuous piece of tygon or equivalent




pump tubing.





     Tests can be conducted at ambient room temperature 20+2°C or at




environmental temperatures found at the sample source providing




temperatures can be held constant.





     Dissolved oxygen is monitored by probe calibrated daily (more




often if required)  for periods of time usually ranging from 4-24 hours




depending on the rate of uptake.  This normally includes a stabiliza-




tion period of 15-60 minutes.  If possible, the D.O. information should




be continuously recorded.





     Light and Dark bottles should be set using incubation water from




the chamber to determine if respiration or photosynthesis attributable
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'to the water has a significant influence on the rate.

     Given the change in dissolved oxygen, time, water volume,
sediment surface area, and correction for respiration or production
if any, SOD results are derived from the following equation:
     SOD gm O2/m2/day = Rbj - Of) - (B^ - BfT|   V
                         (SA)        (t)
     O^ = DO initial mg/1
     Of = DO final mg/1

     BI = DO bottle initial mg/1
     Bf = DO dark bottle final mg/1

     V  = Volume confined water (.0255 m^)
     SA = Sediment area  (.093 m2)

     t  = time/days

     For the final determination of SOD, a rate is obtained by
plotting time vs. oxygen depletion.  The portion of each rate,
where oxygen consumption versus time is constant is used in calculation
of the rate.  Appropriate corrections are made  if respiration or
production is significant in the li^ht and dark bottles.

     Whenever possible, analysis on a minimum of three sample
replicates should be conducted for use statistically and as a means
to determine natural variability among the sediments tested.
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                                                  FIGURE  I

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                                Schematic and Design of Bench Model Sediment
                                         Oxygen Demand Apparatus
                                                                                 1.   Recorder
                                                                                 2.   Dissolved Oxygen Meter & Probe
                                                                                 3.   Bench Model Respirometer
                                                                                 4.   Variable Speed Peristaltic
                                                                                     Pump with Closed Loop and
                                                                                     Tubing
Single pf flnpnt-.ypnr
                                                                     Lid with rubber gasket
                                                                     Water reservoir and D.O. probe port
                                                                     Lock down 2 each side
                                                              • Sediment 2.5
                                                                    Influent port split into 2-way diffuser
                                                                                                                     i
                                                                                                                     in
                                                                                                                     I
    Not To Scale

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