Seasonal changes in hydrochemistry along a toposequence of texture-contrast soils

Ameliorative strategies are urgently required in some agricultural catchments in southern Australia to reduce the loss of potential contaminants to streams. However, a better understanding of where the contaminants are generated on hillslopes, their forms, and the pathways through which they are transported were required. Thus, seasonal changes in the quantities and forms of several chemical species were measured in both vertical and lateral flow pathways at 4 sites along a toposequence in the Mt Lofty Ranges, South Australia. Instrumentation was installed to measure and quantify overland flow and throughflow, and porous-wick samplers were installed at 2 depths to study the chemistry of leachate. Neutron moisture meter access tubes were installed to measure seasonal changes in soil water content with depth as this influences chemical concentrations and mobility. In years of average to below average annual rainfall, throughflow was the most important transport pathway for contaminants. However, it was expected that overland flow will be the dominant transport pathway when annual rainfall is above about 550 mm. Changes in water content of the texture-contrast soils was caused by seasonal rainfall causing periodic saturation, by waterlogging, groundwater, or both. This affected the type and form of contaminant. For example, Na and Cl concentrations were generally only large (800 and 1500 mg/L, respectively) on the lower slopes but in the wettest seasons their concentrations increased at depth on mid-slopes due to the influence of shallow saline groundwater. These chemicals then leached when groundwater levels subsided. The results suggest that ameliorative strategies to reduce agricultural contaminants should target the transport pathways specific to each chemical species, at the point (or points) in the landscape where they are generated.