Hydrology, biogeochemistry and management of drained coastal acid sulfate soil backswamps in the Lower Clarence River floodplain
Funding / collaborators:
Land and Water Australia, NSW DPI, SRDC, Fisheries Research and Development Corporation, Southern Cross University, Clarence Valley County Council.
Drainage systems on coastal floodplains have greatly increased the rate of acidity entering creeks and estuaries from acid sulfate soils. They have degraded floodplain wetlands and led to major changes in their hydrology and vegetation. This study examined key hydrological and biogeochemical processes controlling the water quality characteristics of artificially drained acid sulfate soil wetlands on a coastal floodplain in eastern Australia. Drainage fluxes of acidity and deoxygenating compounds were quantified. The effects of coupled biogeochemical cycling of Fe-S-C on the geochemistry of surface waters were investigated. Alteration of groundwater and soil geochemistry due to encroachment of Melaleuca quinquenervia was also examined. The effects of a weir at reducing acid export were quantified. The impacts of tidal exchange within drains upon acid export, drain water quality, saline ground water seepage and ground water behaviour were also assessed.
Dr Scott Johnston, Dr Peter Slavich, Phil Hirst
Outcomes / key findings:
- After a major flood artificial drainage transferred anoxic surface waters from ASS backswamps to the estuary well beyond what would have occurred naturally. This drainage made a substantial contribution to the magnitude of a deoxygenation event in the Clarence River estuary. Anaerobic decomposition of flood intolerant pasture species coupled with Fe and S redox transformations was a dominant process affecting the geochemistry of backswamp floodwaters.
- Deoxygenation processes, redox transformations and acidification in ASS backswamp surface waters are controlled by complex interactions between hydrology, soils and vegetation. The pool of labile vegetative carbon and the concentration of acidic solutes in surface soils have major effects on redox conditions and surface water acidity in ASS backswamps. Therefore, drainage induced changes to backswamp vegetation communities has substantial long term implications for surface water quality.
- Encroachment of Melaleuca quinquenervia in an ASS backswamp was found to have caused large increases in soil and groundwater acidity. This is likely to enhance acid flux loads by increasing the acidity of surface waters and groundwater seepage.
- The acid flux of drained ASS backswamps is highly dynamic on an hourly, daily and seasonal basis. Acid flux rates can vary greatly between backswamps and are strongly influenced by sulfuric horizon hydraulic properties and effluent groundwater gradients. Some ASS backswamps have acid soil horizons with very high saturated hydraulic conductivity (Ksat) associated with macropores. Such sites can have very high rates of acid flux via groundwater seepage to the drainage system. Tidal variations in drain water levels are important as they regulate the magnitude of the effluent groundwater gradients which drive this seepage. An 'acid export window' concept was developed to explain observed acid export behaviour. The position of backswamp water levels relative to backswamp surface elevations and local tidal minima in bisecting drains exerts a dominant control on acid export.
- Acid groundwater seepage was reduced by about 65 - 70% by using a weir to prevent tidal draw down of drain water levels and reduce effluent hydraulic gradients.
- Floodgate opening and tidal exchange improved drain water pH and dissolved oxygen concentrations. However, there are limitations and complexities associated with short duration floodgate openings which limit its efficacy as a stand-alone acid management strategy. ASS backswamps with very high Ksat also have substantial risk of experiencing saline intrusion into shallow groundwater if floodgates are opened.
Johnston S.G., Slavich P.G., Hirst P., (2005). The effects of controlled tidal exchange on improving drainage water quality in acid sulfate soil backswamps. Agricultural Water Management 73, 87-111.
Johnston S.G., Slavich P.G., Hirst P., (2005). Opening floodgates in coastal floodplain drains: effects on tidal forcing and lateral transport of solutes in adjacent groundwater. Agricultural Water Management 74, 23-46.
Johnston S.G., Slavich P.G., Hirst P., (2005). Changes in surface water quality after inundation of acid sulfate soils with different vegetation cover. Australian Journal of Soil Research 43, 1-12.
Johnston S.G., Slavich P.G., Hirst P., (2004). The acid flux dynamics of two artificial drains in acid sulfate soil backswamps on the Clarence River floodplain, Australia. Australian Journal of Soil Research 42 (5), 623-637.
Johnston S.G., Slavich P.G., Hirst P., (2004). The effects of a weir on reducing acid flux from a drained coastal acid sulfate soil backswamp. Agricultural Water Management 69, 43-67.
Johnston S.G., Slavich P.G., Hirst P., (2003). Alteration of groundwater and sediment geochemistry in a sulfidic backswamp due to Melaleuca quinquenervia encroachment. Australian Journal of Soil Research 14, 1343-1367.
Johnston S.G., Slavich P., Sullivan L.A., Hirst P., (2003). Artificial drainage of floodwaters from sulfidic backswamps: effects on deoxygenation in an Australian estuary. Marine and Freshwater Research 54, 781-795.