FLOOD10133

• County: Statewide
• Community: State of North Carolina
• Basin: Statewide
• Title: Two Months of Flooding in Eastern North Carolina, September-October 1999: Hydrologic, Water-Quality, and Geologic Effects of Hurricanes Dennis, Floyd and Irene
• Date: 1/1/2000
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

NITROGEN AND
PHOSPHORUS IN
SURFACE WATERS

Eutrophication of freshwater streams
generally results from an excessive
amount of phosphorus in the system,
whereas excessive nitrogen is most often
the cause of eutrophication in estuaries.
In order to prevent nuisance plant growth
in streams, the U.S. Environmental Protec-
tion Agency (USEPA) has established a
desired goal of 0.1 mg/L of total phospho-
rus concentrations in freshwaters not
flowing directly into reservoirs or lakes
(U.S. Environmental Protection Agency,
1986a), but currently there is no national
recommendation for total nitrogen concen-
trations in either freshwater or coastal
systems. The USEPA is leading efforts to
develop regional criteria for both phospho-
rus and nitrogen in surface waters across
the Nation.
Nitrate is generally the primary form of
nitrogen dissolved in water. Ammonia is
commonly associated with point sources
and is converted to nitrate or nitrogen gas
in aerated water. Organic nitrogen, mostly
from plant material, can compose a large
part of the total (dissolved and particulate)
nitrogen concentration in a stream. Total
phosphorus in streams consists of phos-
phates that do not dissolve readily, but are
commonly attached to soil particles, and
orthophosphates, which are readily dis-
solved and easily assimilated by aquatic
plants,
In general, nitrogen and phosphorus
concentrations are most elevated in small
streams that drain basins with large pro-
portions of urban or agricultural land use
(U.S. Geological Survey, 1999). Point-
_. source discharges are a major source of
"nitrate to the Tar and Neuse Rivers, but
ground water is an important source of
phosphorus to many streams in eastern
North Carolina (Spruill and others, 1998).
Nutrient concentrations in streams that
drain to the Albemarle and Pamlico
Sounds were high relative to 19 other large
. drainage basins studied by the USGS dur-
ing 1992-95 (Spruill and others, 1998).

concentrations at Fort Barnwell may have been the result of
greater inundation of land areas downstream from Kinston,
resulting in higher organic nitrogen loadings. Previous measure-
ments of organic nitrogen made at Fort Barnwell during
less-than-nonnal flows in April and July 1999 were lower-
measuring 0.45 and 0,37 mg/L, respectively; however, nitrate
concentrations in April and July were an order of magnitude
higher.
Ammonia concentrations in the Cape Fear River (site 40),
Northeast Cape Fear River (site 43), and Lumber River (site 45)
ranged from 0.025 mg/L at site 45 to 0.246 mg/L at site 43, The
highest ammonia concentration (0.246 mg/L) measured among
all of the flood samples (table 6) was in the Northeast Cape Fear
River and occurred 6 days following the flood peak. Nitrate con-
centrations at these three southeast North Carolina sites ranged
from 0.015 mg/L at site 45 to 0.330 mg/L at site 40. Median
organic nitrogen concentrations at sites 40, 43, and 45 were sim-
ilar to median flood-sample concentrations measured in the Tar
and Neuse Rivers-about 0.7 mg/L-although the maximum
concentration in the Northeast Cape Fear River was 1.1 mg/L.
Dissolved organic carbon (DOC) concentrations ranged from
8.4 to 16 mg/L as carbon (C) in the Tar, Neuse, and Cape Fear
Rivers, and 17 to 25 mg/L in the carbon-rich, coastal Northeast
Cape Fear and Lumber Rivers. These DOC concentrations in
floodwaters generally exceeded the highest DOC concentrations
previously measured by the USGS in the Tar and Neuse Rivers
during the 1990's (table 8).
Floodwaters had two primary effects on nitrogen, phosphorus,
and organic carbon. First, floodwaters resulted in tremendous
dilution ofthese constituents. For example, the maximum ammo-
nia concentration at Tarboro (site 14) was 0,051 mglL on
September 21 when the flow was 61,000 fi3/s. If the same mass
of ammonia was present at the long-tenn September mean flow
of 1,220 fi3/s, then the ammonia concentration would have been
2.55 mg/L, or a concentration about 10 times greater than was
measured during 1990-99 at the site (table 8). Despite this dilu-
tion, maximum concentrations of ammonia, organic nitrogen,
total phosphorus, and orthophosphorus were similar to 1990-99
median concentrations, Only nitrate levels were low compared to
1990-99 concentrations. The second effect of floodwaters was to
export large amounts of organic matter from the floodplains to the
streams, as indicated by the high organic nitrogen, DOC, and
suspended organic carbon concentrations. The long-term effects
of these large loads of organic matter on coastal waters remains
to be seen.

Bacteria

Floodwaters from Hurricane Floyd inundated animal waste
lagoons in eastern North Carolina, killed tens of thousands of

Two Months of Flooding in Eastern North Carolina, Septcmber--Octobcr 1999