A Pipe pathway A transport pathway element that is intended to represent a feature that essentially behaves as a fluid conduit, such as a fracture. Pipes are primarily used for simualting matrix diffusion in fractured groundwater pathways. is intended to represent a feature which essentially behaves as a fluid conduit. Mass enters at one end of the conduit, advects through (and disperses within) the conduit, and then exits at the other end of the conduit.
Aquifer and Pipe pathways can be used for many of the same purposes. Depending on the system to be simulated, however, one is typically preferred over the other.
Unlike Cell pathways, Pipe pathways can contain only a single fluid medium. They can, however, contain solid media Materials (such as water, sand, clay, air) that constitute (are contained within) transport pathways. GoldSim provides two types of elements for defining media: Fluids and Solids. which can impact transport (e.g., by modifying the porosity of the pathway and/or acting to sorb and hence retard species The chemical (or non-chemical, such as bacterial or viral) constituents that are stored and transported through an environmental system in a contaminant transport model. In GoldSim, the Species element defines all of the contaminant species being simulated (and their properties).).
Pipe pathways use a Laplace transform A method for solving certain types of differential equations that involves transforming the equations into an algebraic form that can be readily solved, and then using an inverse transform to retrieve the solutions of the original problem. approach to provide analytical solutions to a broad range of advectively-dominated transport problems involving one-dimensional advection, longitudinal dispersion, retardation, decay and ingrowth, and exchanges with immobile storage zones (e.g., matrix diffusion).
Two types of simple contaminant retardation processes can be represented within a Pipe pathway:
- equilibrium partitioning between the fluid in the pathway and a user-specified infill material; and
- equilibrium partitioning between the fluid in the pathway and a user-specified coating material (around the perimeter of the pathway).
In addition to these linear retardation mechanisms mentioned above, Pipe pathways can also represent interchanges with two types of immobile storage zones along the length of the pathway:
- matrix diffusion zones, in which the transfer rate into and out of the zone is proportional to the concentration difference and the diffusive properties of the zone; and
- a "stagnant" dispersive zone, in which the transfer rate into and out of the zone is proportional to the concentration gradient and the flow rate in the pathway.
Finally, suspended Solids (e.g., colloids) can be specified to be present in a Pipe. These Solids are assumed to be advected and dispersed along the Pipe, but are not subject to retardation processes or interactions with storage zones. Species which partition onto the suspended Solids are transported with them as they move through the Pipe.
The geometry of the pathway is defined by specifying a length, a cross-sectional area, and a perimeter. Conceptually, the pathway may be straight or curved. It simply represents a stream tube for the advecting medium.
Pipe pathways may be used for simulating processes such as vertical transport through an unsaturated (vadose) zone, and horizontal transport in aquifers, rivers, channels and pipelines. They are particularly valuable for simulating transport through fractured rock masses.