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Water-quality indices as measures of reservoir water quality in North Carolina

Conference Proceedings
By Jerad D. Bales and C.J.O. Childress

Abstract

Water-quality indices (WQI's) are used to aggregate results of several types of physical, chemical, and biological measurements into a single indicator of water-quality conditions. WQI's are often used by regulatory and management agencies to classify waterbodies, and may facilitate communication of water-quality conditions to the public.

Data collected from nine North Carolina reservoirs between 1990 and 1997 were used to compare and evaluate trends in two commonly used WQI's, the Fusilier WQI and the North Carolina Trophic State Index (NCTSI). Eighteen sets of measurements were collected at 3 locations within each of 2 reservoirs in the Catawba River Basin during a 15-month period in 1993-94. About thirty sets of measurements were collected during a 7-year period from 1991-97 at each of three reservoirs in the Cape Fear River Basin. Furthermore, about 20 sets of measurements were collected during 1990-95 at each of four reservoirs in the Neuse River Basin. Measurements generally were made at only one location in each of the Cape Fear and Neuse basin reservoirs, with the exceptions of Jordan Lake (3 sites) and Falls Lake (4 sites). Data from all reservoirs were used to assess correlations among component measures of the WQI's and to characterize spatial and seasonal patterns. Data from selected Cape Fear and Neuse River Basin reservoirs were used to compute temporal trends in the WQI's and to compare these trends to individual trends in individual water-quality measures.

The nine component measures of the Fusilier index (water temperature, dissolved oxygen percent saturation, chlorophyll a, secchi disk depth, nitrate, total alkalinity, pH, specific conductance, and total phosphate) were not independent. Water temperature was strongly correlated with dissolved oxygen percent saturation, and nitrate at most sites. Dissolved oxygen percent saturation was highly correlated with pH at many sites, and alkalinity and specific conductance were correlated at all sites in the Catawba reservoirs and most of the other sites. However, with some exceptions, there was no significant correlation among the four measures (secchi disk depth, chlorophyll a, total organic nitrogen, and total phosphorus) used in the NCTSI for any of the sites.

The WQI's varied both seasonally at a site, and from location to location and within a reservoir. Within reservoir WQI variations were as large as variations among reservoirs. The Fusilier WQI and the NCTSI were not significantly correlated. There was better agreement between trends in individual water-quality measures and the NCTSI than with the Fusilier WQI. These results suggest that caution should be used when applying the Fusilier WQI to classify reservoirs or to interpret water-quality conditions in North Carolina.