Waste load allocation by integrated GMS modeling and economic evaluation for nitrate reduction in Varamin aquifer

Groundwater is a dependable freshwater resource in arid and semi-arid areas where its quality management is essential. However, untreated wastewater is a risk to safe water supply from aquifers. Wastewater treatment plants (WWTPs) can reduce pollutants, but their impact on groundwater quality versus their operating costs requires case studies. This research uses the two modules of the groundwater modeling system (GMS) to simulate the Varamin Plain, south-eastern Tehran, Iran. The MODFLOW and MT3D were used for groundwater quantity and quality modeling, respectively. Through these modules, the effectiveness of two WWTPs (Pakdasht and Varamin) in six waste load allocation (WLA) scenarios was compared based on nitrate reduction in 3-year and 10-year periods. The construction and operating costs of each WLA scenario were also calculated. The best WLA is a scenario with the lowest marginal cost. Therefore, constructing Varamin WWTP with 25% nitrogen removal was the selected option. Here, the average nitrate concentration in the aquifer is reduced from 28.4 mg/L (±4.1) to less than 25 mg/L (±2.4) with an annual marginal cost of 8 M$.L/mg. It implies that constructing more WWTPs or tertiary units for nitrate removal is not recommended as they do not add significant nitrate abatement in groundwater regarding the related costs.