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    Water Quality Index

    Grand River and Tributary Sampling Sites

    (select any point on the map below)


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     WQI value              Water quality   Aquatic life  Recreational use


    Poor             Very limited       No body contact

    Fair               Low diversity    Limited contact only

    Average        Some stress     Use with caution

    Good            High diversity    Very few limits

    Excellent      High diversity     Fully usable


    The water quality index (WQI) is a statistic designed to closely approximate state and national WQI's as published on the Internet and elsewhere. The usual factors that enter into its calculation are: the percent saturation of dissolved oxygen, the change in temperature from a reference temperature (usually the most upstream from heat sources), pH, biochemical oxygen demand (BOD), total solids, fecal coliform counts, phosphate, nitrate, and turbidity.

    For each parameter, a Q-statistic is calculated on a scale of 0 to 100, 100 being best. These Q's are subjected to exponentially weighted averaging to produce the final WQI value for the location.



    The reference for the temperature change was chosen to be the temperature of the Grand River at Northland Dr. (our most upstream river location). There were a couple of times when there was too much ice at Northland Dr. to get a sample. On these occasions, the reference temperature was set to zero Celsius. This is likely to be very close to the actual temperature considering the circumstances. The application of the temperature of the Grand River at Northland Dr. is easy to defend as a reference for the down stream river locations. It is harder to defend as a reference for the streams. The rational is that it is the quality of these streams as they impact the Grand River that is of interest. Therefore, using the upstream river reference temperature is also applicable to the streams.

    For calculation of the saturated dissolved oxygen value, a constant barometric pressure of 742 torr was employed throughout. The amount of error that this introduces is negligible. The solubility of oxygen in water changes considerably with the temperature of the water but very little with normal changes in barometric pressure. 742 torr was the average barometric pressure reported on a random sampling of our labs BOD benchsheets.

    Not all of the standard WQI tests are performed on our river samples. Total solids are not done. However, total suspended solids and chloride are measured. The sum of the total suspended solids and the chloride values, as sodium chloride, should give us a reasonably close approximation of the total solids. This sum then is used in place of the total solids value in the WQI.

    Turbidity is also not done. In this case, there is no way to approximate it from the other data. It has simply been omitted and the weighting factors have been adjusted to cover this omission.

    No WQI has been generated where any test was not done or did not turn out except as described above.

    WQI Summary (PDF)