Alteration of groundwater and sediment geochemistry in a sulfidic backswamp due to Melaleuca quinquenervia encroachment

Abstract

Extensive encroachment of the native tree species Melaleuca quinquenervia (Cav.) Blake has occurred on a coastal floodplain sulfidic backswamp in eastern Australia. Almost 50% of the open swamp area c. 1870 is now monospecific M. quinquenervia forest. Encroachment has been associated with shortened hydroperiods and land management changes following drainage for agriculture. Large differences to shallow groundwater and sediment geochemistry were observed beneath both individual M. quinquenervia trees and encroaching forests compared to open swamp. Groundwater beneath M. quinquenervia had enhanced titratable acidity and acidic metal cations, increased concentrations of other ionic species (Cl–, SO42–), altered ionic ratios, and increased dissolved organic carbon. Soil beneath M. quinquenervia displayed enhanced accumulation of acidity and soluble ions, with concentration profiles suggesting vertical redistribution towards the surface. Deepening of the sulfide oxidation front in the soil beneath encroached M. quinquenervia suggests that enhanced sulfide oxidation may be occurring. Changes in soil pH, redox potential, and Fe mineral precipitation/dissolution were also evident. These changes appear to be the result of interactions between M. quinquenervia physiology and the unique groundwater and sediment geochemistry of the surrounding sulfidic/sulfuric horizons. Mechanisms to explain the observed changes are discussed along with potential management implications.

Seawater causes rapid trace metal mobilisation in coastal lowland acid sulfate soils: Implications of sea level rise for water quality

“…Enhanced acidity and surface accumulations of acidic metal cations have previously been found beneath tea tree forests (Johnston et al, 2003). Similarly, the high exchangeable Fe concentrations in the SC Melaleuca layer sediments provide a larger pool of Fe for exchange compared to the organic layer extracts from the other sites.…”

“…In these backswamps, the sulfuric horizon is often capped by a thin layer of fluvial sediments and, being close to the ground surface (b1 m), can have a strong influence on surface sediment geochemistry (Johnston et al, 2003;Rosicky et al, 2004) and drainage water quality (Johnston et al, 2004(Johnston et al, , 2005. CLASS backswamp basins are highly susceptible to pulses of saline water caused by saltwater intrusion, storm surge and king tides due to their i) low elevation, ii) coastal proximity, and iii) connectivity to estuarine channels.…”

“…Jarosite mottles were evident at all sites. Previous studies have shown extensive encroachment of tea trees (Melaleuca quinquenervia) in drained backswamps can effect large changes in the surface geochemistry of sediments and shallow groundwater (Johnston et al, 2003). Therefore, an additional acidic organic-rich layer was sampled in a forest dominated by tea trees at the SC site (termed SC Melaleuca).…”

Impact of long-term, saline flooding on condition and reproduction of the clonal wetland tree, Melaleuca ericifolia (Myrtaceae)

“…This is not surprising in terms of salinity, as the estimated field salinity tolerance of M. ericifolia is *40-45 dS m -1 [between 25 and 30 ppm (Bird 1978)]. Although the effect of soil acidity on M. ericifolia has not been investigated, Melaleuca quinquenervia shows extremely high tolerance to the combination of low soil pH and salinity (Johnston et al 2003).…”

Seawater inundation of coastal floodplain sediments: Short-term changes in surface water and sediment geochemistry

“…These backswamp basins are frequently underlain by sulfidic estuarine sediments which contain high concentrations of acidity and available trace metals (Pons, 1973;Pons and Van Breemen, 1982;Dent and Pons, 1995;Johnston et al, 2004;Sullivan et al, 2012). The estuarine sediments are usually capped by a thin layer of fluvial sediments, and hence, can have a strong influence on surface water, groundwater and drainage water geochemistry (Pons, 1973;Pons and Van Breemen, 1982;Sammut et al, 1996;Johnston et al, 2003;Johnston et al, 2005a). The watertable on coastal lowlands such as floodplains has been artificially lowered by extensive constructed drainage networks (Sammut et al, 1996;Minh et al, 1997;Boman et al, 2008).…”