Environmental Engineering Reference
In-Depth Information
sample (5-10 g) is placed into a biological oxygen demand (BOD) bottle (250 ml)
containing a stir bar. An additional soil sub-sample is weighed, dried, and
re-weighed to determine water content. The bottle is placed on a magnetic stirrer
and filled with deionized distilled water while stirring continuously. The contents
are stirred for 15 min and dissolved O
2
(DO) is measured with an O
2
electrode.
The bottle is sealed with a glass stopper and incubated in the dark for 24 h at 25
C.
Then, DO is again measured under continuous stirring. The analysis should be
repeated with smaller sample size if DO levels decrease by more than 50 %
(U.S. EPA
2008
). Soil O
2
demand is calculated as follows:
SOD mg
ð
=
kg- hour
Þ ¼
½
ð
DO
t0
;
mg
=
L
Þ
ð
DO
t24
;
mg
=
L
Þ
ð
water volume
;
L
Þ=
dry weight of soil
;
kg
where: DO
t0
and DO
t24
¼
DO at time 0 and DO after the 24 h incubation period,
respectively.
7.9.3 Bulk Density
7.9.3.1 Overview
To convert from SOM (or soil N, P, etc.) on a percentage basis to a weight basis, the
bulk density (dry weight per unit volume) of the soil must be determined. This entails
extracting a known volume of soil and determining the dry weight. Obtaining
an accurate estimate of the volume of soil is the most difficult step in this exercise.
Bulk density can be determined on intact soil cores obtained at known depth and
volume (volume is determined from core diameter and depth), soil clods pulled from
a pit (volume is determined by water displacement in the lab), or on unconsolidated
materials obtained through excavation (volume obtained by filling the excavation
hole with water). The soil is weighed after drying at 105
C to a constant weight.
Bulk density (g/cm
3
) is calculated as “dry weight (g)/volume (cm
3
)”.
Depending on the intended use of the bulk density values, the contribution of
rocks in the sample to sample weight and volume may or may not be accounted for.
In most cases, the objective is to determine the bulk density of “fine earth”
representing particles with diameters less than 2 mm, so coarser material is removed
by screening. Mineral particles larger than very coarse sand (
2.0 mm) are consid-
ered rock fragments. If the bulk density value is to be used to extrapolate another
parameter value to a soil volume or area basis, rocks are left in the sample.
Otherwise, rocks in the sample should be accounted for by removing them, weighing
them, and determining their total volume through water displacement. The rocks are
placed in a graduated cylinder containing a known volume of water. The change in
water volume after adding the rocks equals the volume of the rocks. The weight
and volume of the rocks is then subtracted from the total sample values before
calculating bulk density.
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