Relation of land use to streamflow and water quality at selected sites in the city of Charlotte
and Mecklenburg County, North Carolina, 1993-98
Water-Resources Investigations Report 99-4180
By J.D. Bales, J.C. Weaver and J.B. Robinson
Full Report (PDF, 104 pages, 43 KB)
Abstract
Streamflow and water-quality data were collected at nine sites in the
city of Charlotte and Mecklenburg County, North Carolina, during 1993-
97. Six of the basins drained areas having relatively homogeneous land
use and were less than 0.3 square mile in size; the other three basins
had mixed land use. Atmospheric wet-deposition data were collected in
three of the basins during 1997-98.
Streamflow yield varied by a factor of six among the sites, despite the
fact that sites were in close proximity to one another. The lowest yield
occurred in a residential basin having no curbs and gutters. The
variability in mean flow from these small, relatively homogeneous
basins is much greater than is found in streams draining basins that
are 10 square miles in size or larger. The ratio of runoff to rainfall
in the developing basin appears to have increased during the study
period.
Low-flow suspended-sediment concentrations in the study basins were
about the same magnitude as median stormflow concentrations in Piedmont
agricultural basins. Sediment concentrations were higher in the mixed
land-use basins and in the developing basin. Median suspended-sediment
concentrations in these basins generally were an order of magnitude
greater than median concentrations in the other five basins, which had
stable land use.
Some of the highest total nitrogen concentrations occurred in residential
basins. Total nitrogen concentrations detected in this study were about
twice as high as concentrations in small Piedmont streams affected by
agriculture and urbanization. Most of the total nitrogen consisted of
organic nitrogen at all of the sites except in two residential land-
use basins. The high ammonia content of lawn fertilizer may explain the
higher ammonia concentration in stormflow from residential basins.
The two basins with the highest median suspended-sediment concentrations
also had the highest total phosphorus concentrations. Median total
phosphorus concentrations measured in this study were several times
greater than median concentrations in small Piedmont streams but almost
an order of magnitude less than total phosphorus concentrations in
Charlotte streams during the late 1970's.
Bacteria concentrations are not correlated to streamflow. The highest
bacteria levels were found in "first-flush" samples. Higher fecal
coliform concentrations were associated with residential land use.
Chromium, copper, lead, and zinc occurred at all sites in concentrations
that exceeded the North Carolina ambient water-quality standards. The
median chromium concentration in the developing basin was more than
double the median concentration at any other site. As with chromium,
the maximum copper concentration in the developing basin was almost an
order of magnitude greater than maximum concentrations at other sites.
The highest zinc concentration also occurred in the developing basin.
Samples were analyzed for 121 organic compounds and 57 volatile organic
compounds. Forty-five organic compounds and seven volatile organic
compounds were detected. At least five compounds were detected at all
sites, and 15 or more compounds were detected at all sites except two
mixed land-use basins. Atrazine, carbaryl, and metolachlor were detected
at eight sites, and 90 percent of all samples had measurable amounts
of atrazine. About 60 percent of the samples had detectable levels of
carbaryl and metolachlor. Diazinon and malathion were measured in
samples from seven sites, and methyl parathion, chlorpyrifos, alachlor,
and 2,4-D were detected at four or more sites. The fewest compounds
were detected in the larger, mixed land-use basins. Residential basins
and the developing basin had the greatest number of detections of organic
compounds.
The pH of wet atmospheric deposition in three Charlotte basins was more
variable than the pH measured at a National Atmospheric Deposition
Program (NADP)site in Rowan County. Summer pH values were significantly
lower than pH measured during the remainder of the year, probably as a
result of poorer air quality and different weather patterns during the
summer.
Concentrations of ammonia and nitrate at the Charlotte sites generally
were lower than those measured at the NADP site. Summer concentrations
of ammonia and nitrate at both the Charlotte and the NADP sites were
significantly greater than concentrations measured during the remainder
of the year, again probably reflecting poorer summertime air-quality
conditions.
Sediment yields at the nine sites ranged from 77 tons per square mile
per year in a residential basin to 4,700 tons per square mile per year
at the developing basin. Residential areas that have been built-out for
several years and industrial areas appear, in general, to have the
lowest sediment yields for the Charlotte study sites.
Average annual yields of total nitrogen loads ranged from about 1.7
tons per square mile to 6.6 tons per square mile. Average annual total
phosphorus yields for all sites except the developing basin were less
than 1.4 tons per square mile. Phosphorus yield at the developing
basin was 13.4 tons per square mile per year.
Biochemical oxygen demand loading in 1993 from all of the permitted
watewater-treatment facilities in Charlotte and Mecklenburg County was
about 1.5 tons per day or 548 tons per year. Converting this point-
source loading to an annual yield for the 528 square-mile area of
Mecklenburg County is equivalent to 1.03 tons per square mile per
year, or a yield much lower than any of the yields measured at the
nine study sites. In other words, biochemical oxygen demand loading
from nonpoint sources in Mecklenburg County probably exceeds loading
from all point sources by a large amount.
Loads and average annual yields were computed for five metals--chromium,
copper, lead, nickel, and zinc. The highest annual average yields for
all five of these metals were in the developing basin, which also had
the highest annual average suspended-sediment yield of all the sites.
Estimated wet-deposition watershed loadings suggest that atmospheric
deposition may be an important source of some metals, including chromium,
copper, lead, and zinc, in Charlotte stormwater.
Stormwater from residential land-use basins has higher concentrations
of total nitrogen, fecal coliform bacteria, and organic compounds than
do other land-use types. Reductions in suspended-sediment concentrations
should generally result in reduced export of phosphorus and metals.
Stable land uses, such as industrial areas and built-out residential
basins, have lower sediment concentrations in stormwater than do mixed
land-use and developing basins. Finally, atmospheric deposition may be
an important source of nitrogen and some metals in Charlotte stormwater.
Citation:
Bales, J.D., Weaver, J.C., and Robinson, J.B., 1999, Relation of land use to streamflow and water quality at selected sites in the city of Charlotte and Mecklenburg County, North Carolina, 1993-98: U.S. Geological Survey Water-Resources Investigations Report 99-4180, 95 p.
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U.S. Geological Survey
3916 Sunset Ridge Road
Raleigh, North Carolina 27607
(919) 571-4000
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