Seasonal changes to hyporheic zone water exchange in a coastal plain drain

H. Mohamadi; C.J. Ocampo; K.R.J. Smettem

Surface water and groundwater (SW-GW) interaction plays a key role in replenishing alluvial aquifers and sustaining the ecology, quality, and quantity of streams and rivers. The interaction, mainly through the hyporheic zone (HZ), also plays a role in the nutrient exchange between terrestrial and aquatic ecosystems. Hyporheic water exchange in many sandy coastal plain drains presents an intermittent hydrological regime, as often shifts occur in their hydraulic functioning from a losing to a gaining stream condition upon the position of the surrounding water table. This work documented the existence and the complex hydrodynamics of HZ water exchange in an artificial drain typical of a coastal plain area (Mayfield drain, Harvey River) in Western Australia (WA), which resulted from a combination of highly responsive water level regime in the drain and a seasonal-transient shallow water table developed in a duplex soil (sand over clay) setting. A novel hydrometric approach using a rugged field camera (water levels on the drain), automatic water level sensors (bores), and a set of temperature sensors (drain?s bed and bank) provided a robust data set to explore vertical water exchange (fluxes) under baseflow and storm event conditions for different hydraulic scenarios (water stages for drain and water table) across the wet season. Water fluxes and direction in the HZ were computed using the one dimensional (1D) heat transport model for pore water (VFLUX) from temperature data, and further verified by a standard hydraulic approach using Darcy?s law and hydrometric data. The results indicated that under baseflow conditions, vertical water flux estimates were directed downwards, of drain water into a shallow sandy layer (< 0.4 m), but with an upward flux from the underneath clay layer (0.4 - 0.7 m). The magnitude and temporal variability of the fluxes corresponded with the drain water stage as increasing downward fluxes due to higher drain stage resulted in decreasing upward fluxes from the clay layer. During storm event conditions, a similar water exchange direction was observed but a substantial increase in the downward flux (by an order of magnitude) of drain water occurred without any change on the upward flux. This dynamics showed a dependency on both mean drain storm water stage and the duration of high flow conditions. These results and the observations of high water table level at the bank indicated that the excess of water into the HZ was mainly mobilized to downstream locations along the drain bed. Further analysis of the gaining and losing water condition over a 620 m drain reach, using flow measurements and water quality surveys, supported VFLUX results. This work identified the presence of a shallow HZ confined vertically by a clay layer (typical feature of duplex soils in the area) and laterally by a high water table in the bank, under both baseflow and stormflow conditions present in the drain. The findings also highlighted the importance of flux estimation using thermal records to complement traditional hydraulic approaches due to lack of conclusive results provided by the latter.

Seasonal changes to hyporheic zone water exchange in a coastal plain drain

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