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Water-quality indices as measures of reservoir water quality in
North Carolina
Abstract - American Water Resources Association
By Jerad D. Bales
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 water bodies 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 30 sets of measurements were collected
during a 7-year period from 1991 to 1997 at each of 3 reservoirs in the Cape Fear
River Basin. Furthermore, about 20 sets of measurements were collected during
1990-95 at each of 4 reservoirs in the Neuse River Basin. Measurements generally
were collected 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 in the indices. 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 Fusilier WQI and the NCTSI were not significantly
correlated.
The WQI's varied both seasonally at a site, and from location to location and with a
reservoir. Within reservoir WQI variations were as large as variations among
reservoirs. 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.
Citation:
Bales, J.D., 2001, Water-quality indices as measures of reservoir water quality
in
North Carolina [abs.], in Warwick, J.J., ed., Proceedings of the AWRA Annual
Spring Speciality Conference on Water Quality Monitoring and Modeling, April
30-May 2, 2001, San Antonio, Tex.: American Water Resources Association, p. 15.
For more information, contact |
To order printed copies, contact |
North Carolina Water Science Center
U.S. Geological Survey
3916 Sunset Ridge Road
Raleigh, North Carolina 27607
(919) 571-4000
E-mail
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Available from the American Water Resources Association or J.J. Warwick,
Professor and Chair, Department of Environmental Engineering Sciences, University
of FLorida, 217 A.P. Black Hall, Gainesville, FL 32611-6450; telephone
352-392-0841; fax 352-392-3076; e-mail warwick@ufl.edu
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