Clarence river basin

Recent mapping of groundwater-dependant ecosystems in the coastal parts of the Clarence Valley has identified wetland clusters and their relationship to geology and landform and the likely water source (NSW Office of Water, 2013). Types of wetlands that have been identified as part of this mapping include, for example, swamps, coastal salt marshes and freshwater wetlands.

In the Clarence river basin, ditch drains are used to drain water from the acid sulfate soil back swamps of the Clarence River floodplain to the estuary. Acid flux dynamics are mainly controlled by the tidally influenced groundwater gradients and by the hydraulic conductivity (K) of the sulfuric horizons, which are highly heterogeneous. Acid flux rates from groundwater seepage are strongly positively correlated to effluent groundwater hydraulic gradients. Direct groundwater seepage is the primary pathway for acid flux in high K and/or steep-gradient areas. High hydraulic conductivity (>120 m/d) in the sulfuric horizons are due to extensive macropores and, when combined with tidal modulation of drain water levels, encourages rapid seepage of acidic groundwater. However, in areas with a low K and/or smaller groundwater gradients, an episodic discharge flux pathway is due to dilute surface runoff following dissolution of sulfide oxidation products accumulated on the soil surface (Johnston et al., 2004a). Floodgate operations can result in complex site-specific interactions between drain water and adjacent groundwater. Johnston et al. (2005) reported that a 4-day floodgate opening event caused recharge of adjacent acid groundwater during the opening phase, raising the potentiometric groundwater level above local low-tide minima. This was followed by tidally modulated drawdown of acid groundwater and enhanced acid export in the period immediately following floodgate closure.

Weirs affect the water head in the drain, with the changed head difference between the drain and the nearby aquifer impacting acid flux from groundwater seepage (Johnston et al., 2004b). Therefore, sound understanding of surface-groundwater interaction processes that control the acid fluxes from shallow groundwater systems that contain acid sulfate soils is of vital importance.

Last updated:
8 January 2018
Thumbnail images of the Clarence-Moreton bioregion

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