Environmental Engineering Reference
In-Depth Information
incorporation of published data with field data. Apparent strengths and weaknesses
of the various approaches, and wetland scenarios that would preclude the use or
compromise the accuracy of a given technique are addressed.
7.1 Overview of Techniques
Biogeochemistry is the scientific discipline that addresses the biological, chemical,
physical, and geological processes that govern the composition of the natural
environment. Particular emphasis is placed on the study of the cycles of chemical
elements such as carbon (C), nitrogen (N), and phosphorous (P) which are critical to
biological activity. Biogeochemical assays may measure a specific elemental pool
(e.g., soil organic carbon), determine the rate of a pathway (e.g., denitrification), or
address a surrogate of a biogeochemical process or an elemental pool. The surrogate
approach is popular for rapid assessment to characterize ecosystem health, functional
capacity, nutrient loading, or water quality. In each case the practitioner must be
aware of the exact nature of the parameter in question as well as limitations to the
method. Attempts to quantify individual pools of C or N at best, produce representa-
tive estimates. On a wetland scale, it is not realistic to believe that the pool can be
quantified with 100 % certainty. There is too much variability in the field and input
sources which cannot be completely accounted for. Accuracy is compromised due to
precision limits inherent to the technique and due to field variability. Results are often
expressed on a per area basis (e.g., m
2
). Extrapolation of the values to a larger
spatial area or to represent an entire wetland further increases the error. Therefore, the
practitioner should consider these methods to be estimates. They are most useful for
comparing wetlands, not for deriving absolute values. Also, the wetland concept
encompasses a wide variety of ecosystems. So these techniques will be most reliable
when comparing wetlands within a given class (e.g., piedmont slope wetlands). This
chapter is not meant to be all inclusive. We have chosen to emphasize the cycles C,
N, P, sulfur (S), manganese (Mn) and iron (Fe). Since many of these processes are
microbially mediated or there is an exchange between the water column and the soil,
there is inherent overlap with other chapters.
Some of these techniques will not be appropriate for all types of wetlands,
particularly with respect to hydroperiod class. Nitrification levels will be difficult
to detect and quantify in a permanently-inundated freshwater marsh as nitrification
is an aerobic process. However, nitrification certainly could be measured in a
seasonally saturated mineral soil flat as long as the measurements are not taken
during a wet phase. Conversely, methane (CH
4
) emissions could be detected in a
marsh but not in a mineral soil flat. In addition, because some of these processes are
strictly aerobic and others are strictly anaerobic, care must be taken to determine the
time of season to run a field assay as the target process may not be occurring at
detectable levels. This is primarily an issue with seasonally saturated wetlands
where the practitioner must take into account seasonal variability in hydrologic
conditions.
