Opening floodgates in coastal floodplain drains: effects on tidal forcing and lateral transport of solutes in adjacent groundwater

The effects of opening tidal barriers (floodgates) upon water table levels and lateral transport of solutes adjacent drains was investigated at two sites on a coastal floodplain. The sites had contrasting geomorphology, soil texture and sediment hydraulic properties. The site with lower hydraulic conductivity (0.3-0.9 m day^-1) soils (Romiaka) also had a higher elevation and hydraulic gradients towards the drain. While floodgate opening at Romiaka enhanced the amplitude of pre-existing tidal interaction with adjacent shallow groundwater, altered hydraulic gradients and caused some salt seepage, lateral solute movement from the drain was highly attenuated (<10 m). The site with very high hydraulic conductivity soils (Shark Creek; > 125 m day^-1) had a lower elevation and seasonally fluctuating hydraulic gradients. The introduction of a tidal pressure signal into the drain by opening the floodgate at Shark Creek caused tidal forcing of groundwater over 300 m from the drain. Floodgate opening at this site also caused changes in groundwater hydraulic gradients, leading to incursion of saline drain water into shallow groundwater over 80 m from the drain. Lateral movement of solutes was relatively rapid, due to macropore flow in oxidised acid sulfate soil horizons, and caused substantial changes to shallow groundwater chemical composition. Conversely, when groundwater hydraulic gradients were towards the drain at this site there was substantial lateral outflow of acid groundwater into drains. This study highlights the importance of assessing the hydraulic properties of soils next to drains on coastal floodplains prior to opening floodgates, particularly in acid sulfate soil backswamps, in order to prevent unintended saline intrusion into shallow groundwater.