A numerical groundwater model for Urânia (SP, Brazil)

This work is part of a project aiming to determine the origins of the nitrate contamination in the urban area of Urânia (SP, Brazil) and the objective is to make a conceptual and numerical model of the groundwater flow system which will improve the understanding of the groundwater circulation in the area. The conceptual model is based on data from earlier studies in the area complemented with data collected in the field. The model area is delimited by streams and water dividers attribut-able to topographic highs. The upper boundary of the model domain is the water table and the bedrock is an impermeable lower boundary. Two geological formations are considered in the model: a sandstone aquifer and a layer of fractured basaltic rock underneath. It is assumed that the hydraulic conductivity of the sandstone aquifer is isotropic in the plane and varies gradually from its lowest value in the north of the area to its highest value in the south. The hydraulic conductivity of the fractured basalt is estimated to be two orders of magnitude lower than for the sandstone. Groundwater is assumed to flow from topographic highs towards the streams. No evidence of a deep groundwater flow underneath streams has been found and it is thus assumed that there is no groundwater movement to or from the model domain. The software Visual MODFLOW is used for the numerical model. A grid is made with 106 columns, 97 rows and five layers, where the four top layers represents the sandstone aquifer and the bottom layer represents the fractured basalt. The streams of the conceptual model are simulated as drain boundaries. To get the model to converge and in order to get reasonable flow directions, constant head boundaries are located in layers 2 – 5 in cells below drain boundaries. Static head values and stream flows have been used as quantitative calibration targets whereas groundwater flow directions and the distribution of hydraulic conductivities have been used as qualitative calibration targets. A combination of automatic and manual trial-and-error calibration have been performed, where hydraulic conductivities and recharge have been automatically optimized, while drain conductance, constant head elevation and the number and location of zones with different hydraulic conductivities have been modified manually. An effort has been made to calibrate the model when pumping of public wells in Urânia is included, but no calibrated model has been obtained for the pumping scenario. After calibration, the sensitivity of the model for selected parameters has been analysed. This study shows that groundwater circulation in Urânia is more complex than was previously assumed and more data needs to be collected in order to obtain a thorough understanding of the flow system. The horizontal hydraulic conductivity of the Adamantina aquifer increases from north to south, at least within the urban area of Urânia, but more data is needed to evaluate the extent of this variation. Calibration data also need to be improved. Boundary conditions are crucial for the reliability of the model and are at present imperfectly under-stood. Vertical groundwater movement need to be better understood in order to obtain a numerical model calibrated for pumping of public wells. The data collection should also be extended to the rural part of the model area. If the model should be used for evaluation of contaminant transport in the future, the results will not be reliable unless present doubts about the groundwater circulation are removed and more precise data is collected.Validerat; 20101217 (root