Investigating the feasibility of Bioengineering and Hydropedological techniques in controlling shallow water table problem in urban areas

The continuous and frequent occurrence of the Shallow Water Table (SWT) in urban regions aggravates the severity of geotechnical and environmental problems. Exploring possible measures that effectively reduce the negative impact of SWT in urban aquifers is of extreme importance. This research investigated the effectiveness of bioengineering techniques in lowering SWT in the framework of hydropedological factors (soil structure in the vadose zone, where most pore water fluxes take place). The methodology includes physical models (sand tank experiments) and field-scale studies. A total of nine sand tanks were utilized, segmented into three distinct groups, to investigate the reduction of the water table through different mechanisms: (1) evapotranspiration drawdown by soil water uptake by the roots of vegetation (specifically Reeds), (2) evaporation by bare homogeneous topsoil, and (3) evaporative “soil siphoning” in tanks that were made as “smart composites” with a fine-textured vertical “moisture chimney”. The dynamics of SWT were monitored over 6 months (March–September 2023). Each tank measures 100 cm in length, 70 cm in height, and 15 cm in width. All tanks were packed with two horizontal soil layers (sand and clay) to simulate a perched aquifer, common prototypes of which were explored in Muscat, Oman. In the siphoning experiment, a small trench was made and packed with silt loam soil. This trench extended the entire thickness of the aquifer to enhance capillarity and, hence, evaporation. Analysis of the results showed that the drawdown of the water table ranged from 75% to 300% (winter to summer seasons) in the tanks containing plants (Reeds) compared with the control tanks. In addition, the SWT in the tanks with “soil siphons” was reduced in the range of 22%-46% compared with the control tanks. Another experiment with Reeds was conducted on a larger field scale using Concrete Agricultural Basins (CABs) with dimensions of 1000 cm length, 200 cm width, and 60 cm depth. The experiment spanned three months (June-September 2023) and aimed to investigate the impact of Reeds on SWT levels in larger-scale 3D pore water dynamics (in sand tank experiments flows were 2D). Overall, the large-scale experiment showed that over the three months, the evapotranspiration from Reeds reduced the water level by 16.7%, 66.7%, and 116.7% more than evaporation from bare soil during the first, second, and third months, respectively.This investigation highlights the significant influence of bioengineering through phreatophytic Reeds, seasonal variations of weather conditions, and the hydropedology of the root zone on checking the SWT levels. The influence of fine-textured soil lenses, strata, and engineered soil siphons in controlling water levels was studied. While the presence of Reeds plays a crucial role in influencing water levels, seasonal fluctuations, usually modeled by ET0-ETc, also contribute, with drastic differences between summer and winter. The investigated drainage techniques are ecologically and environmentally benign: no electricity or fuel is used for the reduction of waterlogging because only soil capillarity and solar energy maintain the processes of evaporation and transpiration; the Reeds’ biomass, accumulated during the ecoengineering process, additionally intercepts and sequesters CO2.