Quoted from: http://www.weap21.org/webhelp/Overview_Q2K.htm 

Surface water quality can be modeled using the US EPA model QUAL2K, version 2.07. QUAL2K, developed by Dr. Steve Chapra and his grad students at Tufts University, provides for much more detailed water quality modeling than WEAP--including diurnal simulations and modeling of nitrogen, phosphorous, sedimentation, algae, pH and pathogens. QUAL2K consists of an Excel workbook (QUAL2K.xls) that provides the front-end to the model and a Fortran executable (Q2KFortran2_04.exe) that runs the calculations. You may use the workbook to create and edit QUAL2K datasets (saved in files with the extension .q2k), run the model, and view the results, in tables and graphs. The full details of QUAL2K and its use are beyond the scope of this document. Refer to the QUAL2K User Guide (Q2KDocv2_04.pdf) for full details of the model and Excel interface.

In order to link QUAL2K to WEAP, you will first need to prepare and calibrate a QUAL2K file outside of WEAP, using the QUAL2K interface (an Excel spreadsheet, provided in the WEAP directory). Because QUAL2K is so detailed, preparation of an initial QUAL2K data file will entail significant effort.

The following description comes from the official QUAL2K page on the US EPA website: http://www.epa.gov/ATHENS/wwqtsc/html/qual2k.html

QUAL2K (or Q2K) is a river and stream water quality model that is intended to represent a modernized version of the QUAL2E (or Q2E) model (Brown and Barnwell 1987). Q2K is similar to Q2E in the following respects:

One dimensional. The channel is well-mixed vertically and laterally.

• Steady state hydraulics. Non-uniform, steady flow is simulated.

• Diurnal heat budget. The heat budget and temperature are simulated as a function of meteorology on a diurnal time scale.

• Diurnal water-quality kinetics. All water quality variables are simulated on a diurnal time scale.

• Heat and mass inputs. Point and non-point loads and abstractions are simulated.

The QUAL2K framework includes the following new elements:

Software Environment and Interface. Q2K is implemented within the Microsoft Windows environment. It is programmed in the Windows macro language: Visual Basic for Applications (VBA). Excel is used as the graphical user interface.

• Model segmentation. Q2E segments the system into river reaches comprised of equally spaced elements. In contrast, Q2K uses unequally-spaced reaches. In addition, multiple loadings and abstractions can be input to any reach.

• Carbonaceous BOD speciation. Q2K uses two forms of carbonaceous BOD to represent organic carbon. These forms are a slowly oxidizing form (slow CBOD) and a rapidly oxidizing form (fast CBOD). In addition, non-living particulate organic matter (detritus) is simulated. This detrital material is composed of particulate carbon, nitrogen and phosphorus in a fixed stoichiometry.

• Anoxia. Q2K accommodates anoxia by reducing oxidation reactions to zero at low oxygen levels. In addition, denitrification is modeled as a first-order reaction that becomes pronounced at low oxygen concentrations.

• Sediment-water interactions. Sediment-water fluxes of dissolved oxygen and nutrients are simulated internally rather than being prescribed. That is, oxygen (SOD) and nutrient fluxes are simulated as a function of settling particulate organic matter, reactions within the sediments, and the concentrations of soluble forms in the overlying waters.

• Bottom algae. The model explicitly simulates attached bottom algae.

• Light extinction. Light extinction is calculated as a function of algae, detritus and inorganic solids.

• pH. Both alkalinity and total inorganic carbon are simulated. The river’s pH is then simulated based on these two quantities.

• Pathogens. A generic pathogen is simulated. Pathogen removal is determined as a function of temperature, light, and settling.

For full details of QUAL2K, please see the QUAL2K Users Manual (located in the WEAP Program folder, in Adobe Acrobat format): Q2KDocv2_11b8.pdf