MODPATH Tracking of Flow Paths from Groundwater Contamination Sites
By Darrel Dunn, Ph.D., PG, Hydrogeologist
Purpose and Scope of Groundwater Flow Path Web Page
The purpose of this non-technical webpage is to demonstrate the use of the United States Geological Survey's MODPATH computer program to track groundwater flow paths from sites in a MODFLOW model. The MODFLOW model described in the Surficial Sediment MODFLOW Model webpage is used for the demonstration. MODPATH has extensive capabilities that may be used for describing and illustrating groundwater flow systems. This webpage is limited to showing flow paths and rate of groundwater movement from particular sites in the model using MODPATH, and to showing the flow throughout the model using STREAMPLOT.
MODPATH uses output files produced by a MODFLOW model to track positions of hypothetical particles placed at points in the simulated groundwater system. The particles are assumed to move with the groundwater. MODPATH calculates positions of a particle at successive times since the placement of the particle. Consequently, it provides information on where contaminated groundwater will go and how fast it will get there. It does not, however, directly provide contaminant concentrations or the effect of dispersion of the contaminants as they move through the system.
Figure 1 demonstrates the use of MODPATH in conjunction with the aforementioned MODFLOW model. It shows paths from four sites in the model. Particles were placed at the center of cells with model row-column locations of (10,30) (16,30) (22,22) and (4,24) for paths 1, 2, 3, and 4, respectively. The dots show the positions of the particles in one-year increments as they move in the general direction of the water table gradient. The particle that follows Path 1 exits the model at the southeast corner after 18.5 years. Particles that follow paths 2, 3, and 4 reach the stream in 12.5, 9.0, and 43.7 years, respectively. The inflection of the last four points in Path 4 illustrates the tendency of groundwater flow to be diverted through more conductive parts of the system. See Figure 3 in the Surficial Sediment MODFLOW Model webpage, where the high saturated thickness there represents a more conductive part of the system. The closely spaced annual positions of the particles in Path 1 and Path 4 illustrate slow movement through low permeable silty sand in the northern part of the model. See Figure 4 in the Surficial Sediment MODFLOW Model webpage.
Figure 1. Particle paths from four sites in the Surficial Sediment MODFLOW model.
STREAMPLOT is a module in Matplotlib. Matplotlib is a publicly distributed library of computer programs for creating graphs and plots. It is not USGS software. However, STREAMPLOT is capable of using MODFLOW output to produce a map representing groundwater flow direction and speed. Figure 2 is a STREAMPLOT map of the same Surficial Sediment MODFLOW Model providing the MODPATH results in Figure 1. The lines in the STREAMPLOT map show the direction of groundwater flow, and the colors indicate the speed of the flow. Consequently, the map characterizes the groundwater flow throughout the model. STREAMPLOT does not track particles like MODPATH. Rather, it calculates groundwater speed and direction along the lines shown. Some lines terminate in the interior of the map so the density of the lines does not become to great for a legible map.
Figure 1. STREAMPLOT representation of the Surficial Sediment MODFLOW model.
Posted May 23, 2019
Revised May 31, 2019