Particle Tracking in Groundwater Modeling
Courtesy of USGS
Groundwater models are sometimes used not only to evaluate groundwater levels and aquifer responses to stresses, but also as a backdrop for evaluating flow paths, concentration mass balances, and advection and dispersion of solutes within the groundwater. Particle tracking is the mechanism used in conjunction with groundwater flow models to evaluate these factors. One significant advantage of particle tracking capabilities in a groundwater model is that it can be used as a forward-tracking mechanism (estimating future migration of groundwater and/or solutes) or a backward-tracking mechanism (identifying past sources of contamination).
Particle tracking does not consider the physical and chemical makeup of the water, a fate and transport and/or geochemical model is required to achieve those purposes. However, particle tracking is useful to understand and may need to be evaluated for many reasons. For example, in a system where wells need to be installed to remediate contamination, particles may be tracked within a model to estimate how effective certain well placements are in capturing contaminant plumes. Particle tracking is also useful when a model can be used to assess aquifer storage and recovery (ASR) [see LWS 5/12/2020 blog]. Related to ASR, particle traces within a groundwater model can be useful to determine the areal extent of recharge within a system for the dual purposes of demonstrating dominion and control over the recharged water and also siting the recovery wells such that production optimizes the capture of the recharged water.
By initially using a groundwater flow model that is capable of particle tracking, testing of well placement can be done virtually, prior to building the physical system, so as to guide where wells should be installed for optimal capture, whether it is for recovery of recharged water or to optimize control of contaminants that are migrating through a groundwater system. In an historic groundwater contamination scenario where there are already known contaminants migrating through the system, a model can be used to trace particles backward in time from the identified point(s) where contamination has been identified, such as a river or municipal production wellfield, to identify the source, or sources, of potential contamination. Additionally, forward particle tracking evaluations can also be made to estimate how long it may take for water from the identified source to reach other sensitive or protected water supplies if not controlled.
LWS has recently completed a project using particle tracking to identify the historic sources of contamination that have been identified at multiple municipal production wellfields. A MODFLOW model of the groundwater basin, coupled with MODPATH to provide particle tracking, was used to identify the historic sources of contamination. Figure 1 shows a screen shot of the particle tracking that, due to the heterogeneous nature of the sediments, varying layering of water-bearing strata with depths, and the presence of faults, shows multiple groundwater flow paths. These particle tracking paths not only show the directions of flow but also the timeframe to move from the wellfield when the contaminants were first identified back to the ground surface where the contaminants were sourced. These particle tracking analyses were vital in identifying potentially-responsible parties related to the historic contamination.
Figure 1: Particle tracking, backward to where particles entered the water table.
While particle tracking does not determine whether the water itself contains specific constituents that could potentially cause injury to a system, particle tracking does provide essential information as it evaluates the average timing and location of water through time and space. If the groundwater that is being tracked indicates contamination that has, or could, reach a water source that needs to be protected, then additional modeling can be conducted to identify the fate and transport of specific constituents. Assumptions about the contamination can be used based on the particle traces to further evaluate the sources and extent of contamination, either with analytical equations or through advection and dispersion modeling using MT3D, or other programs that can handle chemical reaction equations like PHAST. Watch for an upcoming LWS blog describing the basics of fate and transport groundwater modeling.
LWS has a groundwater modeling team experienced with groundwater flow, particle tracking, and fate and transport modeling. Please do not hesitate to contact LWS for questions regarding fate and transport modeling to characterize contaminant movement, as well as groundwater remediation strategies.
Bruce Lytle, P.E., President, LWS: bruce@lytlewater.com
Chris Fehn, P.E., P.G., Senior Project Engineer: chris@lytlewater.com
Anna Elgqvist, E.I., Senior Engineer: anna@lytlewater.com
