NETPATH is an interactive program for modeling net geochemical mass-balance reactions between an initial and final water along a hydrologic flow path. Alternatively, NETPATH computes the mixing proportion of two initial waters and net geochemical reactions that can account for the observed composition of a final water. The program utilizes previously defined chemical and isotopic data for waters from a hydrochemical system. The calculations are of use in interpreting geochemical reactions, mixing proportions, evaporation and (or) dilution of waters, and mineral mass transfer in the chemical and isotopic evolution of natural and environmental waters.

A mass-balance model is defined as the masses (per kilogram H2O) of a set of plausible minerals and gases that must enter or leave the initial solution in order to define exactly a set of selected elemental, electron transfer, and isotopic constraints observed in a final (evolutionary) water. In hydrochemical systems the number of reacting phases is often larger than the number of constraints necessary to define their composition. The processes of dissolution, precipitation, ion exchange, oxidation/reduction, degradation of organic compounds, incongruent reaction, gas exchange, mixing, evaporation, dilution, isotope fractionation, and isotope exchange can be considered. Geochemical mass-balance reaction models are examined between selected evolutionary waters for every possible combination of the plausible phases that account for the composition of a selected set of chemical and isotopic constraints in the system.

NETPATH solves a set of linear equations that account for conservation of mass, and (optionally) conservation of electrons and selected isotopes to find every subset of the selected phases (every model) that satisfies the chosen constraints. Each mass balance model can be treated as an isotope evolution problem solving isotope mass balance and Rayleigh distillation problems to predict the isotopic composition at the final point on the flow path.

Limitations

Net geochemical mass-balance reactions should be investigated only between evolutionary initial and final waters. NETPATH does not check whether waters are truly evolutionary, hence caution is required in interpreting modeled results. Water analyses are not exactly charge-balanced because of analytical errors or failure to determine all dissolved species. DB offers an option for balancing the charge of the major elements in constructing the NETPATH input file. Other errors in analytical data, even for charge-balanced waters, will be distributed through the calculated mass transfers of all phases containing that element, and probably affecting the mass transfers of other phases in the model. NETPATH does not consider uncertainties in analytical data; therefore, any model requiring precipitation of a phase will not be displayed if the phase has been marked for dissolution only, even if the amount precipitated is very small and within the uncertainties of the analytical data. The effects of hydrodynamic dispersion are not explicitly accounted for in mass-balance modeling.

Input Requirements

Water analyses are entered through the interactive data base program, DB. DB allows editing and processing of the data through the WATEQF equilibrium model, to produce an input file (filename.PAT) to NETPATH. DB (and NETPATH) accept laboratory and field analytical data for waters containing the elements C, S, Ca, Al, Mg, Na, K, Cl, F, Si, Br, B, Ba, Li, Sr, Fe, Mn, N, and P, and the isotopes, C-13, C-14, S-34, Sr-87, D, O-18, N-15, and H-3. Temperature and pH are required for every water analysis, the rest are optional.

Output Options

All files and data used by DB and NETPATH are interactively updated and maintained. DB produces full speciation output for all waters in the format of WATEQF results, maintains the [filename].LON data file of water analyses and produces the [filename].PAT input file to NETPATH. Files for checking charge imbalances in original data and data reports of water analyses are optionally produced from DB. NETPATH automatically updates the minerals data file, NETPATH.DAT, upon normal termination, saves model input files, model result files, and details of isotopic calculations.

SYSTEM REQUIREMENTS Intel 80i86 based computer, 640 Kb RAM, DOS 3.0 or higher; math coprocessor optional.

NETPATH is a public domain program distributed on DOS-formatted disks containing source code, executable image, thermodynamic databases, and example data sets. The documentation includes installation instructions and pertinent USGS reports.

Authors: L.N. Plummer, E.C. Prestemon, and D.L. Parkhurst (U.S. Geological Survey).