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Water Quality Index

Water quality monitoring networks collect thousands of chemical, physical, and biological measurements of water each year. Individually, these data do not give us an indication of trends in water quality over time and across geographic areas. A concise, quick way to draw meaning from these numbers is needed. Water quality indices provide a way to distill thousands of records of environmental data into meaningful values that indicate the health of water resources and create a yardstick for measuring and assessing water quality.

Iowa Water Quality Index

The IWQI was created to adequately reflect the water quality conditions of Iowa. This geographically specific index ensures that Iowa’s unique geographical characteristics will be properly reflected in the water quality index.

In Iowa, this WQI is calculated by using nine common water quality parameters (dissolved oxygen, E. coli bacteria, 5-day BOD, total phosphorus, nitrate + nitrite as N, total detected pesticides, pH, total dissolved solids, and total suspended solids). Values range from 0 – 100 and streams are classified as very poor (0-25), poor (25-50), fair (50-70), good (70-90), or excellent (90-100). WQIs were calculated on the streams monitored monthly as part of Iowa’s Ambient Water Monitoring Program.

Streams in Iowa show seasonal Iowa WQI patterns. For the majority of streams, water quality is poor during the spring, followed by a decline in water quality during the summer months when the number of streams in the very poor category increases, while the number of streams in the poor category remains relatively the same. Water quality is at its best during the fall and winter months, with nearly 59% of the streams classified in the fair, good, and excellent categories.







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