Modeling the spatial and temporal patterns of infiltration and recharge in a phreatic non-fractured coastal aquifer

The accurate estimation of groundwater recharge is the key and a necessity for sustainable use of groundwater resources. This is especially true for phreatic coastal aquifers as the balance between the fresh and the saline water is more fragile. Excluding the "one time reserve", the safe yield of a groundwater system must not accede, on the average, the average recharge. [Further, in most cases some flushing of the aquifer must also be kept to prevent salination and accumulation of solutes beyond an acceptable level]. Recharge is typically estimated using large scale mass balance models based on water and/or isotopes. The two methods do provide, when correctly applied, a good rough estimation of the groundwater recharge. However, the estimation of recharge is lacking the understanding of the recharge process and its spatial and temporal patterns, significant primarily for the understanding of surface based aquifer pollution. Therefore, there is no practical ability to influence recharge other than attempts to increase rainfall. In this research we focus on matrix flow and transport in the vadose zone. Specifically, we will examine through numerical modeling two different aspects of groundwater recharge: a) the way natural large scale (order of km) inhomogeneities, typical in many coastal aquifers, in the vadose zone alter flow and transport patterns; b) the conditions for which surface infiltration becomes recharge. In the case of large-scale inhomogeneities, large vadose zone bodies can act as a resistance or as an enhancement to the flow, depending on the size, shape, orientation, hydraulic and transport properties of a single inhomogeneity in contras to the background properties and, perhaps more important, the scale of recharge fluxes. In the infiltration case, in arid conditions most infiltration (and for that matter also irrigation water) is returned to the atmosphere by direct evaporation or by evapo-transpiration. Therefore the focusing of surface water by micro-depressions and by subsurface bending of micro-layering may be crucial for matrix based recharge.