Modeling of transient water table response to managed aquifer recharge: a lagoon in Muscat, Oman

Abstract

Natural and anthropogenic conditions in the Gulf countries lead to substantial changes in groundwater (GW) dynamics. GW mound under a lagoon supplied by treated wastewater (TWW) from a municipal sewage treatment plant (STP) in Muscat, Oman, is studied using analytical and numerical modeling techniques. The water table mound in an unconfined aquifer, laterally bounded by the sea, receives recharge from a lagoon fed by a STP in a seasonally varying regime. The onset and duration of the filling period and, correspondingly, infiltration from the lagoon bed is dictated by the wastewater processing and TWW use. Following the period of recharge-induced growth, the GW mound decays when recharge ceases, and the void space above the water table becomes available for future injection in the next annual cycle of infiltration and TWW subsurface storage. The Green-Ampt model application illustrates that the first phase of the process (vertical infiltration with front propagating from the lagoon bed to the regional flat water table) is typically short (several hours). The second phase, when TWW gets in a direct hydraulic contact with GW, is the longest and with a mound spreading to hundreds of meters during winter months. No analytical solutions to this moving boundary-value problem are known and approximate models, based on the Boussinesq approximation and its linearizations, are utilized. The Hantush-type drawup of the water table in the vicinity of the lagoon is studied by the Watson analytical solution to the diffusion equation for a vertically averaged hydraulic head in an unbounded domain, with an instantaneous jump of the head over a disk-shaped zone. This drawup is compared with MODFLOW simulations, which take into account a finite size of the flow domain (catchment) and discharge into the sea through a constant-head segment. The Mishra-Guyonnet’s formula assumes a spatially uniform flux from the lagoon bed (disk in a model plain) into a growing mound and is used for calculation of the total discharge, which progressively decreases from the early stages of the Green-Ampt infiltration to a theoretically zero value if the second phase lasts indefinitely long. We assess characteristic times and phases of the mound dynamics during the rise and fall stages using regional hydrogeological parameters, site geometry, and typical patterns of wastewater disposal. Sensitivity analysis of the locus of the phreatic surface at a given instance and location is carried out, with storativity, hydraulic conductivity, undisturbed aquifer thickness and lagoon radius as input variables. For an example of an aquifer of saturated depth of 40 m, undisturbed vadose zone thickness of 10 m, saturated hydraulic conductivity of 10 m/day, effective porosity of 0.2, a circular lagoon size of 90 m and water depth there of 1 m we found that the infiltration front propagates to the water table in less than 0.2 days; after 40 days of a continuous mound growth in a hypothetical piezometer located 400 m from the lagoon the water table drawup was in the range of 2.0–2.7 m if the aquifer hydraulic parameters and lagoon size are varied 20 % from the average value. This approach will permit to plan more detailed site investigation, facilitate management decisions on the rate of water release to the lagoon, and foresee waterlogged areas.