Influence of Hydraulic Conductivity and Wellbore Design in the Fate and Transport of Nitrate in Multi-aquifer Systems
- PDF / 911,485 Bytes
- 12 Pages / 439.37 x 666.142 pts Page_size
- 41 Downloads / 170 Views
Influence of Hydraulic Conductivity and Wellbore Design in the Fate and Transport of Nitrate in Multi-aquifer Systems Amanda Mejía · Eduardo Cassiraga · Andrés Sahuquillo
Received: 5 April 2011 / Accepted: 16 January 2012 / Published online: 28 January 2012 © International Association for Mathematical Geosciences 2012
Abstract Nitrate concentrations in multi-aquifer systems are heavily affected by the presence of wellbores (active or abandoned) that are screened in several aquifers. The spatial variability of hydraulic conductivity in the confining layers has also an important impact on the concentrations. A synthetic three-dimensional flow and transport exercise was carried in a multi-aquifer system consisting of two aquifers separated by an aquitard in which 100 vertical wellbores had been drilled. To model the wellbores and the flow and transport connection between aquifers that they may induce, we assign a high vertical hydraulic conductivity and a low effective porosity to the cell blocks including the wells. With these parameters, a solute will travel quickly from one aquifer to the other without being stored in the well itself. The wellbores will act as preferential pathways, and the solute will move quickly between aquifers according to the hydrodynamic conditions. Not considering these preferential pathways could induce erroneous interpretations of the solute distribution in an aquifer. We also noted that when there are vertical wellbores that connect aquifers in a multiaquifer system, low conductivity in the aquitard enhances the flow of solute through the wellbores. Time-varying pumping rates induce important fluctuations in nitrate concentrations; therefore, any estimate of the water quality of the aquifer will depend on the moment when the data has been recorded. Consequently, concentration maps obtained by interpolation of point samples are seldom a good indicator of the chemical status of groundwater bodies; alternatively, we recommend complementing the usual interpolated maps with numerical models to gain a true understanding of the spatial distribution of the solute concentration. Keywords Nitrate · Wellbores · Aquifer · Groundwater · Fast pathways · Abandoned wells A. Mejía () · E. Cassiraga · A. Sahuquillo Group of Hydrogeology, Department of Hydraulic and Environmental Engineering, Universitat Politècnica de València, Camino de Vera s/n, 46022 Valencia, Spain e-mail: [email protected]
228
Math Geosci (2012) 44:227–238
1 Introduction Nitrate is one of the most widespread groundwater contaminants and becomes a risk for human health when its concentration exceeds 50 mg/l NO− 3 . In addition, it is a molecule that could originate other more toxic compounds (Spalding and Exner 1993; Wolfe and Patz 2002). Recently, many of the deep aquifers in the Spanish Mediterranean coast have displayed an increase in nitrate concentrations. We argue that this increase in concentration is mainly due to the vertical connection in multi-aquifer systems through abandoned wells or through active wells that ar
Data Loading...
