On Modeling Weak Sinks in MODPATH

Abstract

Regional groundwater flow systems often contain both strong sinks and weak sinks. A strong sink extracts water from the entire aquifer depth, while a weak sink lets some water pass underneath or over the actual sink. The numerical groundwater flow model MODFLOW may allow a sink cell to act as a strong or weak sink, hence extracting all water that enters the cell or allowing some of that water to pass. A physical strong sink can be modeled by either a strong sink cell or a weak sink cell, with the latter generally occurring in low-resolution models. Likewise, a physical weak sink may also be represented by either type of sink cell. The representation of weak sinks in the particle tracing code MODPATH is more equivocal than in MODFLOW. With the appropriate parameterization of MODPATH, particle traces and their associated travel times to weak sink streams can be modeled with adequate accuracy, even in single layer models. Weak sink well cells, on the other hand, require special measures as proposed in the literature to generate correct particle traces and individual travel times and hence capture zones. We found that the transit time distributions for well water generally do not require special measures provided aquifer properties are locally homogeneous and the well draws water from the entire aquifer depth, an important observation for determining the response of a well to non-point contaminant inputs.

A comparison of particle-tracking and solute transport methods for simulation of tritium concentrations and groundwater transit times in river water

“…However, it is also noted that the Omori and Whareroa CFDs are virtually identical in shape, while the Kuratau, Waihaha, and Whanganui CFDs tend to have more short and long transit times. This is consistent with observations made by Abrams et al (2013) that watersheds with more partially penetrating streams tend to skew toward shorter and longer transit times with a smaller frequency of intermediate transit times. This implies that the Kuratau, Waihaha, and Whanganui watersheds have a greater incidence of weak sinks; see Gusyev et al (2013).…”

Sensitivity of Simulated Hyporheic Exchange to River Bathymetry: The Steinlach River Test Site

“…Evidently, the decrease in head within Domain B has become a weak sink in the Sloped Bathymetry case, causing particles originating from Domain A to take a longer and more convoluted travel path through the subsurface, thus increasing their transit times. Abrams et al () defines a weak sink as a groundwater discharge boundary condition that lets some water pass underneath it, while a strong sink extracts water from the entire aquifer depth.…”

“…Abrams et al () suggests approaches to account for this weak sink problem for surface water modeling, however, the remaining problem is that “there is no way to know whether a specific particle should discharge to the sink or pass through the [weak sink] cell (Pollock )”. In this modeling study, HGS does not require prescribing the spatial distribution of losing or gaining river reaches a priori.…”

“…Evidently, the decrease in head within Domain B has become a weak sink in the Sloped Bathymetry case, causing particles originating from Domain A to take a longer and more convoluted travel path through the subsurface, thus increasing their transit times. Abrams et al (2013) defines a weak sink as a groundwater discharge boundary condition that lets some water pass underneath it, while a strong sink extracts water from the entire aquifer depth. Abrams et al (2013) suggests approaches to account for this weak sink problem for surface water modeling, however, the remaining problem is that "there is no way to know whether a specific particle should discharge to the sink or pass through the [weak sink] cell (Pollock 1994)".…”

“…Abrams et al (2013) defines a weak sink as a groundwater discharge boundary condition that lets some water pass underneath it, while a strong sink extracts water from the entire aquifer depth. Abrams et al (2013) suggests approaches to account for this weak sink problem for surface water modeling, however, the remaining problem is that "there is no way to know whether a specific particle should discharge to the sink or pass through the [weak sink] cell (Pollock 1994)". In this modeling study, HGS does not require prescribing the spatial distribution of losing or gaining river reaches a priori.…”