Interpretation of WQ data using OWQI

The Oregon Water Quality Index (OWQI) is a single number which expresses water quality by integrating measurements of eight carefully selected water quality parameters (temperature, dissolved oxygen, biochemical oxygen demand, pH, ammonia+nitrate nitrogen, total phosphates, total solids, fecal coliform). The index was developed for the purpose of providing a simple, concise and valid method for expressing the significance of regularly generated laboratory data, and was designed to aid in the assessment of water quality for general recreational uses. Due to resource availability and in order to maintain a manageable, yet representative, index, the OWQI will have certain limitations. The OWQI index cannot determine the quality of water for all uses. Some uses conflict with others. For instance, water quality considerations for agricultural uses are different from considerations for recreational uses. The OWQI cannot provide complete information on water quality. An index provides only a summary of the data. Also, the OWQI cannot evaluate all health hazards. The OWQI can be used to show water quality variation both spatially and temporally. The index allows users to easily interpret data and relate overall water quality variation to variations in specific categories of impairment. The OWQI can identify water quality trends and problem areas. These can be screened out and evaluated in greater detail by direct observation of pertinent data, thus increasing efficiency. Used in this manner, the OWQI provides a basis to evaluate effectiveness of water quality improvement programs and assist in establishing priorities for management purposes.

Water quality indices were first seriously proposed and demonstrated beginning in the 1970s but not widely utilized or accepted by agencies that monitor water quality. Oregon's Department of Environmental Quality developed the original Oregon Water Quality Index in 1980. Use of the index was discontinued because calculating index results in the pre-personal computer era was too labor intensive. The uses and limitations of a water quality index may be misunderstood and the potential of water quality indices for communicating the current status and trends of water quality overlooked. Evaluation of water quality only in terms of raw data can be misleading and confusing for the general public. As a result, it may be difficult for a person interested in water quality to interpret the data to gain an overall understanding of water quality conditions. This may result in faulty conclusions regarding water quality status and management practices. Thus it is difficult to effectively communicate the results from water quality improvement programs when the raw data is confusing. As a solution, a water quality index integrates complex analytical data and generates a single number expressing the degree of impairment of a given water body. This improves communication with the public and increases public awareness of water quality conditions.

The OWQI is calculated in two steps. The raw analytical results for each parameter, having different units of measurement, are transformed into unitless subindex values. These values range from 10 (worst case) to 100 (ideal) depending on that parameter's contribution to water quality impairment. These subindices are combined to give a single water quality index value ranging from 10 to 100. The unweighted harmonic square mean formula used to combine subindices allows the most impacted parameter to impart the greatest influence on the water quality index. This method acknowledges that different water quality parameters will pose differing significance to overall water quality at different times and locations. The formula is sensitive to changing conditions and to significant impacts on water quality.

As an example of how a water quality index communicates information, Table 1 presents application of the Oregon Water Quality Index to rivers throughout the Portland metropolitan area.

Site	Mean OWQI 10/85-9/87	Mean OWQI 10/93-9/95	Trend^a
Sandy R. at Troutdale Bridge	93	93	NST
Clackamas R. at High Rocks (Gladstone)	90	90^b	NST
Mollala R. at Knights Bridge (Canby)	--	87^c	ID
Clatskanie R. at HWY 30 (Clatskanie)	--	82^d	ID
South Yamhill R. at HWY 99W (McMinnville)	79	80	NST
Willamette R. at Hawthorne Bridge (Portland)	72	78	Inc.
North Yamhill R. at Poverty Bend Rd. (McMinnville)	71^e	76	Inc.
Tualatin R. at Rood Bridge (Hillsboro)	71	75	Inc.
Tualatin R. at Boones Ferry Rd. (Durham)	30	57	Inc.

Notes: ID - Insufficient data available. Inc.- Increasing water quality. NST - No significant trend.
a -Trend analysis using Seasonal-Kendall test (WQHYDRO). Results at confidence level of 95% or greater.
b - Mean taken 10/91-9/95.
c - Monitoring started 3/91. Mean taken 3/91-9/95.
d - Monitoring started 10/92. Mean taken 10/92-9/95. e - Mean taken 10/88-9/90

Table 1 compares mean OWQI values, where available, for the first and last two years of the ten year reporting cycle beginning October 1, 1985. Where data is collected quarterly rather than monthly, mean values are computed for four years. A significant finding is that no decreasing trends were found for these sites. Using the OWQI, one can compare water quality as it changes from upstream to downstream. The Tualatin River at Rood Bridge is situated above all major municipal wastewater treatment facilities in the Tualatin Basin. The Tualatin River at Boones Ferry Road is situated just below the furthest downstream facility at Durham. While these sources and others have impacted the Tualatin River in the past, changes in management practices and significant improvements in wastewater treatment are reflected by a marked increase in water quality. One can also compare water quality among rivers. Portland metropolitan area rivers range in quality from the significantly impacted (but improving) Tualatin River to the high quality of the Clackamas and Sandy Rivers. Also interesting is that the relative rankings of these rivers has not changed over this time period. While factors contributing to water quality impairment differ for each river, one could conclude there have been general improvements in water quality throughout the region. It is likely that further improvements in land and water use will continue to improve the quality of waters in the Portland metropolitan area.

The Oregon Water Quality Index indicates impairment of water quality and progress of water quality management practices. It can be used to detect trends over time and trends across river basins. Most importantly, the Oregon Water Quality Index improves comprehension of general water quality issues, communicates water quality status, and illustrates the need for and effectiveness of protective practices.

Written by Curtis Cude, Oregon Department of Environmental Quality Laboratory Division

Interpretation and Communication of Water Quality Data Using the Oregon Water Quality Index