Environmental Engineering Reference
In-Depth Information
to nutrient release from the breakdown of organic matter coupled with reduced
plant uptake as the peak growing season begins to trail off. Summer increases in
wetland orthophosphate levels have also been observed and related to possible
release of the nutrients from anoxic sediments (Gli
´
ska-Lewczuk
2009
). Increased
water concentrations of other elements, including potentially toxic metals such as
cadmium, have been reported in some temperate wetlands and have been attributed
to release of these chemicals from sediments due to increasing temperatures and
changing redox conditions in spring (Olivie-Lauquet et al.
2001
).
6.5 The Role of Wetlands in Improving Water Quality
Discussions related to wetlands and water quality often focus on “improving water
quality” as one of the key services wetlands provide and studies on both natural and
created wetlands have demonstrated clear effects on the chemical and physical
characteristics of water moving through these systems (see Barnes et al.
2002
;
O'Geen et al.
2010
; Dotro et al.
2011
for basic examples). Some of the proximate
mechanisms that underlie these water quality effects have been reviewed by Hemond
and Benoit (
1988
)andVerhoevenetal.(
2006
). However, there are a few general
qualifiers that are worth keeping in mind when considering the “water quality”
function of wetlands. First, some wetlands (e.g., some types of vernal pools) have
little influence on water quality due to limited hydrological linkage to other waters
(Rains et al.
2006
). Second, the capacity of wetlands to enhance water quality may
ultimately depend on the area of wetland available relative to the total catchment
area. For example, Verhoeven et al. (
2006
) state that wetlands can significantly affect
catchment water quality if wetland habitat makes up at least 2-7 % of the catchment
area. Finally, in some cases, activities in the surrounding landscape and wetland
alterations can actually lead to wetlands becoming sources of sediments, nutrients,
and toxic chemicals that could actually degrade downstream water quality (Brinson
1988
; Whigham and Jordan
2003
; Verhoeven et al.
2006
).
6.6 General Study Design and Approaches
6.6.1 Study Design
The objectives and questions to be addressed in collecting water quality data from a
wetland should be clearly defined prior to the start of the study since this will inform
the study design and associated statistical analyses. USEPA (
2002a
) discusses some
of the key objectives that should be considered in a wetland monitoring study and
provides guidance on site selection and sampling designs. Common sampling designs
used for wetland water quality monitoring include stratified random sampling,
