Numerical and analytical assessment of stormwater infiltration via vadose zone wells and infiltration trenches

Jose David Henao Casas1, Fritz Kalwa1, Marc Walther2, Randolf Rausch3
1 Department of Hydrosciences, Technische Universität Dresden
2 Department of Hydrosciences, Technische Universität Dresden - Helmholtz-Centre for Environmental Research, UFZ Leipzig
3 Department of Geo-sciences, Technische Universität Darmstadt

9.10 in Artificial and natural groundwater recharge (co-organized by IAH-D)

26.03.2020, 17:15-17:30, Händel-Saal

A solution commonly used in drylands to cope with water scarcity is the use of stormwater, which is often collected in surface basins and subsequently stored in shallow aquifers via infiltration. Common problems with this stormwater harvesting system are high evaporation rates, and salinisation, among others. These problems are a consequence of low infiltration rates, which are caused by clogging layers in the topsoil and the presence of a thick vadose zone. This study aims to increase the groundwater recharge rates in stormwater harvesting systems by bridging the vadose zone and the low permeable upper layers of the soil profile. The efficiency of vadose zone wells and infiltration trenches is tested using analytical equations, numerical models and conducting a sensitivity analysis. Dams built in the channel of wadis (i.e. ephemeral streams) are selected as a study case to construct the numerical simulation of well- and trench infiltration. A comparison with the analytical solutions proves the adequate setup of the numerical models. The modelling demonstrated that the vadose zone wells and the trenches bridge the vadose zone and the clogging layer in terms of the time at which recharge starts and the quantity of water recharged. By implementing these solutions, recharge begins between 24 and 58 times faster than the infiltration from the surface of the reservoir. The sensitivity analysis showed that the length of the well had the highest impact on the well recharge. In the case of the trench, the initial position of the groundwater table and the depth of trench showed sensitivity. In terms of recharge quantity, the well has the best performance. During a year, it can infiltrate up to 16.4 times more water than the surface of the reservoir, and between 3.4 and 8.3 more than the trench. Moreover, the well can yield the highest cumulative recharge per dollar and possibly the highest recharge when there are limitations of the available area. Based on the recharge rates per unit of volume, for a fixed cost, it is better to have several trenches with a short lateral and vertical extension (e.g. around 0.5 m and 1.5, respectively), than fewer trenches with longer dimensions. For the well, the optimum ratio between recharge quantity and construction cost is obtained with several wells of small diameter and long length. The methods investigated here increased infiltration rates significantly, and therefore, provide meaningful options to enhance aquifer water storage in the Kingdom of Saudi Arabia and beyond.

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