Environmental Engineering Reference
In-Depth Information
volumes are purged, where a “casing volume” is defined as the volume of water
standing in the well (Barackman and Brusseau
2004
). In some cases, recharge of
the well is so slow that the water sample must be collected a day after purging.
A number of portable battery driven pumps are available for purging and sampling
the wells. These pump the water through tubing inserted into the well. To avoid
contamination, deionized water should be pumped through the tubing between
samples. This method is not appropriate for all wetlands, and it should not be
used to compare different types of wetlands. For seasonally saturated wetlands,
there may not be sufficient well water to sample during parts of the year.
7.6 Phosphorous
7.6.1 Overview
Phosphorous is generally considered to be the major limiting nutrient in most
freshwater wetlands (Mitsch and Gosselink
2000
) determining the rate of primary
production and decomposition. However, when present at very high concentrations,
P stimulates macrophyte and algae growth and the resulting increase in the rates of
decay of the vegetation depletes O
2
levels in the water. Wetlands represent both a
source of P and a sink for P. Furthermore, an individual wetland can switch from
a P sink to a P source as its trophic state changes. The trophic state of an aquatic
ecosystem is defined by its rate of primary production, the concentration of
nutrients and minerals and the rate of supply of organic matter (determined by
the balance of primary production and decomposition) (Correll
1998
). Oligotrophic
systems have low rates and concentrations, while eutrophic conditions exist when
the ecosystem is over-enriched with nutrients and minerals. As the flux of P into the
wetland increases, primary production and trophic state increase while dissolved O
2
and biodiversity decrease (Correll
1998
). These changes in ecosystem processes
interfere with the wetland's ability to retain P and wetland soils can become
saturated with orthophosphate (OP), after which P may be released to the water
column resulting in eutrophication of receiving surface waters (Champagne
2008
;
Vepraskas and Faulkner
2001
).
The major pool of P in natural wetlands is found in the soil sediments (fixed
mineral P) and the litter (organic P) comprising approximately 80-90 % of the
P within the wetland (Champagne
2008
; Richardson and Craft
1993
; Vepraskas and
Faulkner
2001
). The remaining fraction of P is found in the water column and the
pore water as soluble OP, dissolved organic P and total dissolved P (Vepraskas and
Faulkner
2001
). Phosphorus in wetlands can be found as soluble and insoluble
complexes in both organic and inorganic forms. The principle inorganic form is
OP, which is present in the pH-dependent ionic species; H
2
PO
4
(2
<
pH
<
7),
HPO
4
2
(8
12) and PO
4
3
(pH
12) (Kadlec and Knight
1996
). However,
OP can react with soil constituents to form both insoluble and soluble compounds. In
acidic soils (4-7 pH) under aerobic conditions, OP forms insoluble precipitates with
<
pH
<
>
