Seasonal and stratigraphic controls in coastal floodplain soils

Abstract

Soils of the Macleay River coastal floodplain are developed in a relatively thin veneer of alluvium which is underlain by extensive estuarine deposits representing the postglacial rise of sea level. Soil particle size, water table, salt, and pH distributions relate to catenary variations from levee to back swamp and acid aeration products derived from the underlying estuarine beds. Groundwater data from 50 swampland locations for the period 1962 to 1968 showed strong seasonal trends. At 39 sites, water-table level correlated closely and positively (P < 0.05) with rainfall summed for the 6 months (log R6) prior to the month of sampling. Groundwater salinity at 20 sites correlated negatively (P < 0.05) with water-table level. Surface soil salinity up to 7.8% and groundwater pH as low as 2 were recorded during the 7-year period. Aeration experiments showed that reserves of estuarine sediment occurred in certain swamplands which were capable of further acid production. Programmes of swamp drainage need to avoid exposure of these reserves. A system of shallow drains, set no deeper than the upper limit of the estuarine deposits, could be used both to remove excess flood water from back swamps and to remove salts produced by aeration of the estuarine materials.