Environmental Engineering Reference
In-Depth Information
evaluated by the comparison of the results of this check sample on a day-to-day
basis. The pooled standard deviation of the check sample over many days and
analyses gives an evaluation of the precision of the method over time.
Accuracy measures the bias in a measurement and can be defined as the degree
of agreement of a measurement, X, with an accepted or true value, T. It is usually
expressed as the difference between the two values, or as a percentage of the
reference value 100 (X
T)/T. Accuracy of laboratory measurements are usually
defined as percent recoveries of the analyte of interest from matrix spikes, or spike
reference material introduced into selected samples of a particular matrix, or by the
use of appropriate internationally certified materials. For many projects, percent
recoveries of the spiked samples and the laboratory control standards are set at
80-120 %.
The method detection limit is the analyte concentration derived from the
method that yields a signal which is large enough to be considered significantly
different from the blank with a statistical 99 % probability. The method detection
limit is determined by analyzing reagent water fortified at a concentration consid-
ered to be two to three times the estimated detection limit. At least seven
replicates of this fortified blank are analyzed by the same procedure followed in
the determination of unknown samples. The MDL is then calculated using the
equation MSDL
¼
(t)
(S), where t
¼
3.14 (for seven replicates) and S
¼
the
standard deviation of the replicate analysis.
Completeness refers to the percentage of valid data received from actual
analyses performed in the laboratory. Completeness (C) is calculated as follows:
C
¼
100 (V/T); where V
¼
number of measurements judged valid, and T
¼
total
number of measurements.
7.3 Background
7.3.1 Characteristics of Wetlands That Promote
Biogeochemical Processes
Wetlands are diverse ecosystems and variation is found in topographic position
(e.g., slope vs. depression), substrate (organic soils or mineral soils), plant commu-
nity composition, dominant water source, and hydroperiod. Each of these
characteristics affects biogeochemical cycles. One characteristic common to all
wetlands is the presence of a water table close to the soil surface for at least part of
the growing season. The shallow water table leads first to anaerobic soil conditions
and then to reduced soil conditions. A number of biogeochemical pathways proceed
only under anaerobiosis or reducing conditions. These pathways play a greater role
in biogeochemical cycles in wetlands than in uplands. Many freshwater wetlands
display significant temporal variability in water table depth so that anaerobic or
reduced soil conditions are present for only a portion of the growing season. In these
