ENVIRONMENTAL PROCESS COMPUTER CODES AND MODELS
Load Estimator (LOADEST) is a FORTRAN program developed by USGS researcher and CWEST member Rob Runkel and two USGS colleagues for estimating constituent loads in streams and rivers. According to the LOADEST documentation, “given a time series of streamflow, additional data variables, and constituent concentration, LOADEST assists the user in developing a regression model for the estimation of constituent load (calibration). Explanatory variables within the regression model include various functions of streamflow, decimal time, and additional user-specified data variables. The formulated regression model then is used to estimate loads over a user-specified time interval (estimation). Mean load estimates, standard errors, and 95 percent confidence intervals are developed on a monthly and (or) seasonal basis.”
The One-Dimensional Transport with Equilibrium Chemistry (OTEQ) computer model was developed by USGS scientist and CWEST researcher, Dr. Rob Runkel. It is used to characterize the fate and transport of waterborne solutes in streams and rivers. As explained in the OTEQ documentation, “the model is formed by coupling a solute transport model with a chemical equilibrium submodel. The solute transport model is based on OTIS, a model that considers the physical processes of advection, dispersion, lateral inflow, and transient storage. The equilibrium submodel is based on MINTEQ, a model that considers the speciation and complexation of aqueous species, acid-base reactions, precipitation/dissolution, and sorption.”
The One-Dimensional Transport with Inflow and Storage (OTIS) computer model was developed by USGS researcher and CWEST member, Dr. Rob Runkel, to characterize the fate and transport of water-borne solutes in streams and rivers. For conservative (non-reactive) solutes, four processes can be simulated – advection, dispersion, inflow, and transient storage. For nonconservative (reactive) solutes, two additional processes can be simulated – sorption and first order decay. Data for the model can be obtained by running field-scale tracer experiments.
PHREEQC is a computer code developed by the USGS that can be used for simulating chemical reactions and transport processes in natural or polluted water, in laboratory experiments, or in industrial processes. The program is based on equilibrium chemistry of aqueous solutions interacting with minerals, gases, solid solutions, exchangers, and sorption surfaces, which accounts for the original acronym — pH-REdox-EQuilibrium. The program has evolved to include the capability to model kinetic reactions and 1D (one-dimensional) transport. PHREEQC is written in the C and C++ programming languages. Several CWEST researchers at the USGS’s Marine Street location in Boulder use PHREEQC and can provide assistance in employing the model and the code.
WATEQ4F is a computer code developed by USGS researcher Dr. James Ball and USGS researcher and CWEST member Dr. Kirk Nordstrom to determine chemical speciation of major, trace, and redox elements in natural waters. The code uses field measurements of temperature, pH, Eh, dissolved oxygen and alkalinity, and the chemical analysis of a water sample as input and calculates the distribution of aqueous species, ion activities, and mineral saturation indices that indicate the tendency of a water to dissolve or precipitate a set of minerals (see Drever, 1988; Nordstrom and Munoz, 1994). The code uses the ion-association model and assumes homogeneous aqueous phase equilibria, except for redox species. Equilibrium with respect to mineral solubilities is not assumed. The program results are used primarily to examine the tendency of a water to reach mineral solubility equilibria as a constraint on interpreting the chemistry of natural waters. A recent update includes an electrical conductivity calculation that can be compared to the measured conductivity for QA/QC (McCleskey et al, 2010). Results from the WATEQ4F code can be used as input for other programs such as PHREEQC to model geochemical reactions.