Save Our Streams

Chemical Overall Water Quality Index

To determine the Overall Water Quality Index at a stream site, nine tests are performed. These include: dissolved oxygen, fecal coliform, pH, reactive phosphate, nitrate, chloride, turbidity and total dissolved solids. After completing these tests, the results are recorded on a weighting curve chart where a numerical value is obtained for each test result. The numerical value is multiplied by a weighting factor for each test. The weighting factor for each parameter varies according to its impact on water quality. Each of the nine resulting numbers (numerical value x weighting factor) are added to attain the Water Quality Index (WQI). Since the WQI is a number based on 100, it is then assigned to a range: excellent (90-100), good (70-90), fair (50-70), poor (25-50).

Macroinvertebrate Water Quality Rating

The Water Quality Rating Rating is initiated by identifying and then categorizing each of the macroinvertebrates collected into one of three groups, according to their known response to pollution: sensitive, somewhat sensitive or tolerant. The number of each organism is converted to one of these letter codes: A =1-9; B = 10-99; C = >100. This data istransfered to a form outlining the three aforementioned groups. The number of letters in each group is added and multiplied by the appropriate index number: sensitive = 3; somewhat sensitive = 2; tolerant = 1. The three products are added to determine the Macroinvertebrate Water Quality Rating: excellent (>22); good (17-22); fair (11-16); poor <11). The perceived diversity of the macroinvertebrate sample, not so much their numbers in each group, is the important indicator of water quality at the stream site.