Specific Examples of Trend Analysis Using the Oregon Water Quality Index 

The following are examples of water quality trend analyses utilizing the Oregon Water Quality Index (OWQI). The OWQI relies on data generated from routine ambient water quality monitoring in order to analyze trends over long time periods. The ambient water quality monitoring network is designed to measure cumulative impacts from point and non-point sources in a variety of conditions. Locally, these conditions range from protected, pristine rivers such as the Sandy to significantly impacted rivers such as the Tualatin. Raw analytical results for eight carefully selected parameters are converted into subindices of common units (10-100, worst case to ideal). The OWQI is calculated by combining these subindices. The Seasonal-Kendall test (using WQHYDRO) was applied to OWQI results. This test takes into account seasonal variability of water quality, so any trend in water quality detected is significant. Confidence levels are computed for each trend. When trends or problematic results are noticed, one can step back from the OWQI score and examine subindex scores for the appropriate sample events. This can help determine which factors are influencing water quality while eliminating the task of wading through tables of analytical results.

Point Source Effects

The South Umpqua River receives nonpoint and, more significantly, point source pollution, mainly from municipal wastewater treatment facilities. The Melrose Road Bridge is the most-downstream site on the laboratory's South Umpqua ambient water quality monitoring network. The site is immediately downstream of the Roseburg Sewage Treatment Plant (STP) and has historically been impacted by high levels of phosphates and fecal coliforms and problems associated with eutrophication and high temperature. This site typically shows the poorest water quality of all ambient monitoring sites in the Umpqua basin. Over the last decade, improvements have been made in STP's in the South Umpqua subbasin, while efforts have been made to better understand which factors affect water quality in the South Umpqua River. Trend analysis (Figure 1) shows that water quality has improved on the order of ten to fifteen water quality index points in ten years. One could conclude that water quality improvement programs have resulted in a general improvement in water quality.

Figure 1 - Trend Analysis Results for South Umpqua River

Nonpoint Source Effects

Ambient monitoring of the Umpqua basin occurs eight times per year and the North Umpqua River typically exhibits the best water quality conditions encountered during these monitoring runs. The Garden Valley Road Bridge site is the only ambient monitoring site on the North Umpqua River. This site is located near the mouth of the river and there are no significant point sources on the North Umpqua River. After finding a general improvement in water quality at the downstream end of the South Umpqua River (Figure 1), it was interesting to find that water quality had declined in the North Umpqua River during the same time period (Figure 2). Although not significantly impacted by point source pollution, the North Umpqua River is showing the effects of nonpoint source pollution. Water Quality Index results have been influenced by excursions in biochemical oxygen demand (BOD), total phosphates, and fecal coliform. One could conclude by comparing these results that while control of point sources has significantly improved water quality, nonpoint sources have continued to contribute to the degradation of water quality in the Umpqua basin.

Figure 2 - Trend Analysis Results for North Umpqua River

Figure 3 - Trend Analysis of Tualatin River: Ten Year Period

Changing conditions

Figure 3 presents the application of a Seasonal-Kendall test to the Tualatin River at Boones Ferry Road. Like the South Umpqua River at Melrose Road sampling site, the Tualatin River at Boones Ferry Road is the most downstream site for the ambient water quality monitoring network in the basin. It is also directly downstream from a sewage treatment plant, this being at Durham. Figure 3 indicates that water quality has significantly improved over the last eleven years, but a closer look at this figure reveals two trends: a downward trend during the first half of this period, followed by an upward trend. The following figures explore these trends.

Figure 4 - Trend Analysis of Tualatin River: First Five Years

Figure 4 indicates that water quality deteriorated at the rate of 1.25 WQI points per year. The data points at the bottom of the chart are associated with consistently high levels of phosphates and fecal coliforms. By the beginning of 1990, however, conditions began to improve for the Tualatin River (Figure 5).

Figure 5 - Trend Analysis of Tualatin River: Last Five Years

Starting in mid 1989, the Unified Sewerage Agency began to take steps to remove nutrients from STP effluents as per the Total Maximum Daily Load instruction from Oregon Department of Environmental Quality. The Rock Creek STP began removing nitrogen-related nutrients in August 1989 and phosphorous in August 1990. A basin-wide phosphate detergent ban was instituted in February 1991. Nitrogenous and phosphorous nutrients were consistently removed by the Durham STP by 1994. As previously mentioned, this monitoring site is located directly downstream of the Durham STP. Therefore, while the Durham STP is the predominant factor in water quality at this site, water quality index scores also represent the cumulative impacts of sources upstream. While the water quality index scores show some improvement likely due to the earlier improvements at the Rock Creek STP located upstream and the effect of the phosphate detergent ban, the greatest improvement in OWQI scores begins in 1994. OWQI scores improved an average of about six points per year during this period. One could conclude that changes in water quality management in the Tualatin basin have been positive. It is likely that further improvements in nonpoint source related issues will continue to improve the quality of water in the Tualatin basin.

OWQI as an Indicator

In the past, "Number of river miles assessed meeting standards" had served as a benchmark of performance for water quality programs. This number was tied to the total amount of monitoring done, which was influenced by the number of special monitoring studies performed. These studies are typically concentrated, for good reason, in areas where water quality degradation is a concern. This created a benchmark weighted towards the impacted waters of our state and tends to give the impression that water quality is generally degrading despite our attempts to improve it. Use of the Oregon Water Quality Index for benchmark measurement is tied to key indicator sites routinely monitored by the laboratory, representing the range of water quality found throughout the state. The OWQI can be used to communicate trends in water quality and factors affecting water quality. The Oregon Water Quality Index will more adequately measure the progress (or lack of progress) made by water quality management practices.

Written by Curtis Cude, Oregon Department of Environmental Quality