A water-quality index (WQI) was developed from
historical data for streams in the Atlanta region. The WQI was
derived from percentile ranks of individual water-quality
parameter values at each stream by normalizing the
constituent ranks for values from all sites in the area for the
period from 1990 to 1995. WQIs were developed primarily for
nutrients and nutrient-related parameters, because data for
metals, organics (pesticides and herbicides), biological
conditions, and suspended sediment generally were
unavailable. Average WQI of the individual parameter WQIs
for sites in the region ranged from 0.26 (good quality) to 0.86
(poor quality), and increased downstream of known nutrient
sources. Annual average site WQI decreased at most
long-term monitoring sites from 1986 to 1995. Temporal
trends, in part, reflect effects of a drought in the late 1980's
and normal to higher-than-normal rainfall and runoff in the
1990's. For several sites, particularly in the northern part of
the region where major development is ongoing, WQI
increased dramatically from 1994 to 1995. Interannual WQI
variability typically was less than spatial variability. Average
annual site WQI for individual parameters correlated with
annual hydrologic characteristics, particularly precipitation
amount and water yield, reflecting the effect of dilution on
individual water-quality parameter values.
