Effects of intraborehole flow on groundwater age distribution

Environmental tracers are used to estimate groundwater ages and travel times, but the strongly heterogeneous nature of many subsurface environments can cause mixing between waters of highly disparate ages, adding additional complexity to the age-estimation process. Mixing may be exacerbated by the presence of wells because long open intervals or long screens with openings at multiple depths can transport water and solutes rapidly over a large vertical distance. The effect of intraborehole flow on groundwater age was examined numerically using direct age transport simulation coupled with the Multi-Node Well Package of MODFLOW. Ages in a homogeneous, anisotropic aquifer reached a predevelopment steady state possessing strong depth dependence. A nonpumping multi-node well was then introduced in one of three locations within the system. In all three cases, vertical transport along the well resulted in substantial changes in age distributions within the system. After a pumping well was added near the nonpumping multi-node well, ages were further perturbed by a flow reversal in the nonpumping multi-node well. Results indicated that intraborehole flow can substantially alter groundwater ages, but the effects are highly dependent on local or regional flow conditions and may change with time.RésuméLes traceurs environnementaux sont habituellement utilisés pour estimer les âges des eaux souterraines et les temps de résidence. Cependant, la nature hautement hétérogène de nombreux environnements souterrains peut engendrer des mélanges entre des eaux d’âges très disparates, complexifiant par-là même le processus d’estimation des âges. La présence de puits peut exacerber le phénomène de mélange : de longues sections en trou nu ou crépinées exploitant plusieurs niveaux productifs distincts peuvent transporter rapidement l’eau et les solutés sur une grande distance verticale. Les conséquences de flux intra-forages sur les ages des eaux souterraines ont été étudiées numériquement en couplant les simulations directes de temps de résidence avec le “Multi-Node Well Package” de MODFLOW. Dans un aquifère homogène et anisotrope, les âges ont atteint un régime permanent étroitement dépendant de la profondeur. Un puits multinœud au repos a ensuite été inséré dans l’une des trois zones du système. Dans les trois cas, les transports verticaux par le puits ont entraîné des modifications substantielles des distributions des âges dans le système. Enfin, après ajout d’un puits en pompage à proximité du puits au repos, les âges ont été perturbé davantage, par une inversion du flux dans le puits au repos. Les résultats ont montré que les flux intra-forages peuvent modifier substantiellement les âges des eaux souterraines, mais leurs effets sont hautement dépendants des conditions locales ou régionales d’écoulement, et peuvent de surcroît changer dans le temps.ResumenLos trazadores ambientales pueden usarse para estimar el tiempo de viaje y las edades del agua subterránea pero la naturaleza fuertemente heterogénea de muchos ambientes subsuperficiales puede causar mezcla entre aguas de edades altamente dispares, añadiendo complejidad adicional al proceso de estimación de edades. La mezcla puede exacerbarse por la presencia de pozos debido a que intervalos largos abiertos o mallas largas con aberturas a profundidades múltiples pueden transportar agua y solutos rápidamente a lo largo de grandes distancias verticales. Se examinó numéricamente el efecto de flujo entre pozos en la edad del agua subterránea usando la simulación de transporte de edades directa acompañada con el Paquete de Pozos Multi-Nodo de MODFLOW. Las edades en un acuífero anisotrópico y homogéneo alcanzaron un régimen de predesarrollo permanente con fuerte dependencia de la profundidad. Luego se introdujo un sistema de pozos multi-nodo sin bombeo en uno de tres lugares dentro del sistema. En todos los tres casos, el transporte vertical a lo largo del pozo dio por resultado cambios substanciales en las distribuciones de edad dentro del sistema. Después de que se añadió un pozo de bombeo cerca del pozo multi-nodo sin bombeo se perturbaron las edades aún más por inversión del flujo en el pozo multi-nodo sin bombeo. Los resultados indican que el flujo entre los pozos puede alterar substancialmente las edades del agua subterránea pero que los efectos son altamente dependientes en las condiciones de flujo regional o local y pueden cambiar con el tiempo.

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