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In acid-mine drainage and acid-sulfate soil environments, the cycling of Fe and As are often linked to the formation and fate of schwertmannite (Fe(8)O(8)(OH)(8-2x)(SO(4))(x)). When schwertmannite-rich material is subjected to near-neutral Fe(III)-reducing conditions (e.g., in reflooded acid-sulfate soils or mining-lake sediments), the resulting Fe(II) can catalyze transformation of schwertmannite to goethite. This work examines the effects of arsenic(V) and arsenic(III) on the Fe(II)-catalyzed transformation of schwertmannite and investigates the associated consequences of this mineral transformation for arsenic mobilization. A series of 9-day anoxic transformation experiments were conducted with synthetic schwertmannite and various additions of Fe(II), As(III), and As(V). X-ray diffraction (XRD) and Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy demonstrated that, in the absence of Fe(II), schwertmannite persisted as the dominant mineral phase. Under arsenic-free conditions, 10 mM Fe(II) catalyzed rapid and complete transformation of schwertmannite to goethite. However, the magnitude of Fe(II)-catalyzed transformation decreased to 72% in the presence of 1 mM As(III) and to only 6% in the presence of 1 mM As(V). This partial Fe(II)-catalyzed transformation of As(III)-sorbed schwertmannite did not cause considerable As(III) desorption. In contrast, the formation of goethite via partial transformation of As(III)- and As(V)-sorbed schwertmannite significantly decreased arsenic mobilization under Fe(III)-reducing conditions. This implies that the Fe(II)-catalyzed transformation of schwertmannite to goethite may help to stabilize solid-phase arsenic and retard its subsequent release to groundwater.

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. (2010) Arsenic mobilisation in a seawater inundated acid sulfate soil. Environmental Science and Technology 44 (6), [1968][1969][1970][1971][1972][1973] 1 Original article published as: Johnston S.G., Keene A.F., Burton E.D., Bush R.T., Sullivan L.A., McElnea A.E., Ahern C.R., Smith C.D., Powell B. (2010) Arsenic mobilisation in a seawater inundated acid sulfate soil. Environmental Science and Technology 44 (6), 1968-1973

Casuarina cunninghamiana Miq. is an important rheophytic tree in New South Wales, Australia because it is fast growing and can tolerate flood disturbance. Widden Brook is an active sand-bed stream that has widened substantially since initial European settlement in the early 1800s and is characterized by high flood variability and multi-decadal periods of alternating high and low flood frequency, called flood-and drought-dominated regimes. Channel contraction by bench formation is currently occurring. Conversion of coarse-grained point bars to benches is an important process of channel contraction. When point bars grow to a height where suspended sediment is first deposited to thicknesses of at least 50 mm by sub-bankfull floods, rapid establishment of C. cunninghamiana occurs. As the trees grow they partially block bankside flows, thereby locally reducing flow velocity and inducing further deposition on the benches. Such synergistic relationships between bar height and inundation, fine-grained sediment deposition, tree establishment and the development of a bankside low current velocity zone are fundamental for bench development. Size-class frequency data demonstrate that C. cunninghamiana on the benches consists of pure even-aged stands with most trees clustering near the average diameter. Two benches have similar size class frequency distributions but a third has significantly smaller trees. Recruitment on benches is episodic, may occur in areas open to grazing and is dependent on favourable conditions that allow tree survival. These favourable conditions include high seed availability, low levels of competition, deposition of fine sediments and adequate moisture for tree growth. Although C. cunninghamiana germinates on bars, seedlings are eliminated by prolonged inundation or flood scour and do not reach maturity. Recurring catastrophic floods or a sequence of large floods in rapid succession episodically destroy benches by substantial channel widening and initiate a new phase of bar and bench development. A conceptual model of the conversion of point bars to benches by thick mud deposition and C. cunninghamiana recruitment has been developed for sand-bed streams draining similar sandstone catchments. Figure 4. Channel cross sections and bench sediments of (A) upstream right bank bench, (B) upstream unvegetated section (sand splay) of downstream right bank bench, and (C) downstream section of downstream right bank bench.Figure 5. Bivariate plots of graphic mean size versus (A) inclusive graphic standard deviation and (B) inclusive graphic skewness, showing that the bench nucleus sediments are similar to the adjacent bed and bar sediments.Figure 6. Size-class (DBH) distributions of live (white) and dead (black) stems of C. cunninghamiana on (A) upstream right bank bench, n = 510; (B) downstream right bank bench, n = 275; (C) left bank bench, n = 139.Figure 7. Model of bench development and growth from a point bar, and bench destruction by catastrophic floods or a sequence of large floods in rapid succession.

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