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# River and Stream Water Quality Model (QUAL2K)        

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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. 

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Technical Support Center fact sheet (PDF, 2 pp., 733 KB, about PDF)

TMDL fact sheet (PDF, 2 pp., 697 KB, about PDF) 

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