Wetland Biogeochemistry Techniques - Wetland Techniques - page 394

Potential acid sulfate soils form under continuously anaerobic conditions and are
commonly found in coastal wetlands. When these wetlands are drained, exposure of
the soil to O 2 results in the formation of H 2 SO 4 creating very acidic conditions
(pH

2) and often releasing toxic quantities of Fe, aluminum and heavy metals
such as arsenic. Because of the toxic effects of free S 2 and the potential for
detrimental effects upon oxidation of FeS 2 , the rapid assessment of soluble S 2 in
soil pore water is valuable to wetland scientists and managers dealing with estuarine
systems. Below we present two approaches to this assessment.

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7.7.2 Measurement of Soluble Sulfide in Marsh Pore Water

7.7.2.1 Sippers and Peepers

Sulfide in pore water is usually measured using two basic methods that collect pore
water in the field with sippers or peepers and analysis of water S 2 content in the
lab using an ion-selective electrode and a set of standards (Eaton et al. 2005 ).
Pore water extractors or “sippers” are inserted into the marsh soil, suction is applied
and the pore water is collected in a syringe (e.g., Marsh et al. 2005 ; Keller
et al. 2009 ). This approach is relatively rapid, but provides poor resolution as the
sample is drawn from a soil volume of uncertain dimensions. A second approach
uses equilibrium dialysis samplers or “peepers” (Hesslein 1976 ). In this method, a
device containing a vertical series of chambers is filled with deoxygenated, distilled
water, covered with a semi-permeable membrane, inserted into the marsh soil, and
allowed to equilibrate. When the soluble constituents in the pore water reach
equilibrium with those in the chambers, the device is extracted and the water in
the chambers is analyzed in the laboratory. Both sippers and peepers are used to
obtain vertical profiles of pore water chemistry within the sediment column.
By virtue of their placement in the sediment, sippers allow for some vertical
segregation of data. Peepers are designed for the collection of discrete water
samples at a smaller spatial resolution by preventing vertical mixing of adjacent
water masses. Peepers have superior vertical resolution to the sippers (1-2 cm
vs. 5-10 cm), but are limited by a relatively long equilibration period (usually
1 week or longer) (Teasdale et al. 1995 ).

7.7.2.2 IRIS Panels

IRIS tubes were developed to assess the presence of reducing conditions in soils
(Castenson and Rabenhorst 2006 ; Jenkinson and Franzmeier 2006 ) (see Techniques
to Measure Eh/Assess Reducing Conditions below). IRIS tubes are PVC tubes
coated with an Fe oxide paint. When inserted into soil, the paint dissolves when
reduced exposing the bare PVC pipe. It was inadvertently discovered that when
IRIS tubes were exposed to S 2 , Fe monosulfide (FeS) coatings formed on the tubes