Pump and treat optimization schemes for nitrate removal from a rural basin aquifer

Lake Karla aquifer, with extent of 500 km², is located at the eastern part of the most cultivated region of Greece, Thessaly, where water demanding crops prevail. The last three decades is under a status of quantitative and qualitative degradation because of the long-term intense agricultural activities, the lack of any significant surface water body and of an organized irrigation network. About 80% of the pumped groundwater is used for irrigation. According to Lake Karla recreation project fifty wells have been established at the southern part of the aquifer in order to cover the drinking water needs for the nearby city of Volos. This area is characterized by the greatest depletion of aquifer’s water table and by high values of nitrate concentrations in groundwater resources.  The area is one of the seven vulnerable zones of Greece, with respect to nitrogen pollution from agricultural run-off, according to the requirements of the Nitrates Directive (91/676/EEC).

The study proposes a pump and treat optimization method for nitrates removal from the supply wells area with the use of a simulation-optimization modelling system through two scenarios regarding nitrate fertilization of crops during the remediation period:

The classification of crop types at the study area of aquifer is implemented with the use of a Geographical Information System (GIS). Nitrate leaching to aquifer is achieved through water infiltration from surface to subsurface system and has been estimated to 40% of nitrogen fertilizer application. The simulation-optimization modelling system applied, consists of a groundwater simulation model (MODFLOW), a groundwater transport and a dispersion simulation model (MT3DMS) and a management model (GWM). The optimization problem targets to the minimization of the operation cost of pump and treat wells, subject to plume stabilization through reversing the hydraulic heads slope and to keeping the values of nitrate concentration in the supply wells area lower than the threshold of 25 mg/L introduced by Nitrates Directive. Pump and treat wells are proposed to be located at the boundary of the urban supply wells area, where the plume crosses it.  The historical period is from 1995 to 2007, while the remediation period starts on 2007.

The results indicate that, for the first scenario, the pump and treat wells must operate through the whole remediation period (2007-2017) since great volumes of nitrate pollutants continuously leach to aquifer concluding to high operation costs equal to 372.47 thousands of euros. On the contrary, for the second scenario, the remediation period is decreased to four years resulting to an operational cost lower than the half of the additional of the first scenario equal to 147.09 thousands of euros. These results highlight the importance of the full compliance with Nitrates Directive requirements by the farmers of rural basins, where the groundwater resources are used for potable use; otherwise any remediation design will be costly.