The system modelled by PICNIC-TD consists of a network of one-dimensional transport legs, with each leg having the potential for advection, dispersion, diffusion and matrix diffusion. The structure of this network may be based on analysis of a continuum flow model or a discrete fracture network model, or it may be created directly by the user. In terms of the network, the basic concepts are legs and junctions. Every leg has an upstream end and a downstream end, each corresponding to a particular junction. Junctions can have several incoming and outgoing legs. Some junctions have no incoming legs, but are connected to a source term. Some junctions have no outgoing legs, but produce a result.
Two main aspects distinguish PICNIC from other codes. These are relevant mainly to its use in modelling transport through fractured media.
- The transport paths may be in contact with so-called matrix zones of variable geometry and heterogeneous properties that contain stagnant pore water.
- Different types of transport paths in contact with rock matrix can be combined in series or in parallel to mimic a network of fractures with heterogeneous properties with respect to flow, matrix diffusion, and sorption.
Thus, PICNIC has the capability to describe transport influenced by heterogeneities of the geosphere at two different scales. These are the local scale, where matrix diffusion is an important feature, and the larger scale, where the network of transport paths between sources and the output location may significantly influence the breakthrough of solutes.