5.0 Effects of Timber Harvesting on Water Quality of the Goshen Swamp
Scott H. Ensign and Michael A. Mallin
5.1 Introduction
Coastal North Carolina contains numerous blackwater creeks and rivers with
extensive riparian swamp forests. In contrast to upland areas, the environmental effect of
silviculture in these lowland habitats has not been as well-described (Shepard 1994). Data
gathered by the Lower Cape Fear River Program (LCFRP) at the Goshen Swamp provides an
examination of the effects of timber harvest on water quality in a Coastal Plain
blackwater stream.
In the United States, sedimentation is the most important water quality
concern related to forestry practices (Binkley and Brown 1993). A review of Eastern forest
practices concluded that timber harvesting leads to minor changes in stream nutrient
concentration (Corbett et al. 1978). These changes in stream water chemistry have been
attributable to alterations of biogeochemical cycles in the harvested watershed (Lynch and
Corbett 1990). A review by Shepard (1994) of logging in wetlands concluded that
silviculture has temporary effects on water quality, with water quality parameters
returning to pre-harvest levels within periods of months to several years. The impact of
silviculture in lowland sites is relatively less compared with upland areas where the
potential for erosion is higher due to increased slope of watersheds (Shepard 1994).
However, silviculture’s effects on water quality (especially nutrient concentrations)
are very site specific.
Clearcutting is the most common method used in floodplain hardwood
management (Walbridge and Lockaby 1994). Decreased evapotranspiration following
clearcutting results in rising water tables in poorly drained soils (Shepard 1994; Riekerk
1983; Beasley and Granillo 1988). Consequently, logging increases storm water flows and
allows overland sheet flow (Beasley and Granillo 1988) leading to increase annual water
yields and peak discharge rates (Beasley and Granillo 1988; Lebo and Hermann 1998).
According to simulations done by Richardson and McCarthy (1994), land clearing of a
peat-based pocosin wetland will increase annual runoff by 13%.
In 1990, the North Carolina Division of Forest Resources (NCDFR)
established nine Performance Standards that all forestry activities must comply with in
order to remain exempt from the Sedimentation Pollution Control Act (Deal and Hamilton
1989). The first of these standards (Rule 15 NCAC 1J .0201) describes the Streamside
Management Zone (SMZ) that must be established and maintained along stream margins. This
rule requires that the SMZ (referred to in this paper as a buffer zone) be wide enough to
trap any visible sediment resulting from forested site erosion. Though this rule does not
require a minimum SMZ width, the NCDFR suggests a minimum width of 50’ for perennial,
warm water streams with adjacent slopes of 0-5%. While selective cutting is allowed within
this SMZ, the NCDFR recommends that at least 50% of preharvest shade be preserved to avoid
in-stream temperature changes.
The abundance of industrial swine farms in the Goshen Swamp watershed
(119) represents a potential source of nutrient and bacterial enrichment to the Goshen
Swamp. These facilities store hog waste in outdoor lagoons, and periodically spray liquid
waste from the lagoons onto surrounding fields (Burkholder et al. 1997; Mallin et al.
1999). Surface application of organic waste material, especially when coinciding with
heavy rainfall, can contribute nitrogen and phosphorus to adjacent waterways (McLeod and
Hegg 1984). Subsurface drainage of nitrate and phosphorus from surface application of
animal waste material can pollute streams as well (Evans et al 1984). Furthermore,
bacterial contamination of surface waters by spray field runoff has been found to range
from 103-105 organisms/100mL in watersheds containing animal
husbandry operations (Crane et al 1983).
5.2 Site Description
The Goshen Swamp is a blackwater stream system located in Duplin County,
North Carolina. It is a tributary of the Northeast Cape Fear River and drains a watershed
of 185 square miles. Within its watershed are the towns of Faison and Calypso. Land use
includes agriculture, silviculture, and animal husbandry, with an estimated watershed
swine population of >400,000. The LCFRP maintains a water quality monitoring station at
the Highway 11 bridge over the Goshen Swamp, 0.2 miles upstream from the confluence with
the Northeast Cape Fear River. Since February 1996, the LCFRP has monitored 13 parameters
monthly at this station.
Beginning May 27, 1998 and concluding on September 11, 1998, 130 acres
of the Goshen Swamp watershed were logged immediately upstream of the LCFRP sampling
station (T. Tew, Corbett Timber Co., pers. comm.). This bottomland forest was comprised of
gum (Nyssa sp.; Liquidambar sp.), oak (Quercus sp.), pine (Pinus
sp.), and cypress (Taxodium sp.) (D. Robinson, Corbett Timber Co.,pers. comm.). The
Fish and Wildlife Service National Wetland Inventory categorizes the majority of the site
as palustrine, forested, broad leaved deciduous, and partially flooded. Timber was
harvested using a track cutter and shovel logger. Road construction was minimal (one
single ditch road was extended into the tract), and was not located adjacent to surface
waters. A 30 foot vegetated buffer was preserved on either riverbank. All best management
practices were observed, and no violations were found by North Carolina Forest Service
personnel (R. Darden,N.C. Forest Service, per. comm.).
5.3 Methods
Temperature, pH, specific conductance, dissolved oxygen and
turbidity were
measured monthly with a YSI 6820 sonde linked to a YSI 610D. Probes were calibrated the
morning of sampling, and proper calibration was verified upon return from the field.
Samples for nutrient and biological analyses were
collected mid-stream from the Highway 11 bridge. Orthophosphate-P (PO4-3) was analyzed
from filtered samples using a Technicon AutoAnalyzer (Clindus Technologies, Paramus, New
Jersey, USA). Ammonium-N (NH4+), nitrate/nitrite-N (NOx-), total phosphorus (TP), and
total Kjeldahl nitrogen (TKN) were analyzed using Standard Methods (APHA 1995). Total
nitrogen (TN) was computed as the sum of TKN plus NO3-. Total suspended solids (TSS) were
assessed using Standard Methods (APHA 1995). Fecal coliform bacteria were enumerated using
membrane filtration (APHA 1995) and chlorophyll a was quantified using the fluorometric
method of Welschmeyer (1994). Statistical
analyses were performed comparing 12 months of pre-harvest data (June 1997- May 1998)
against 12 months of harvest/post-harvest data (June 1998- May 1999). The year June 1997-
May 1998 represents a valuable baseline of data for comparison, as no hurricanes influence
this data set, though March of 1998 had unusually elevated rainfall. Each data set was
tested for normality using the SAS univariate procedure (Schlotzhauer and Littell 1987).
The parameters TSS, turbidity, fecal coliforms, PO4-3, and NH4+ were normalized by log
transformation. A two-way t-test was used to test for significant difference between the
parametric data temperature, pH, specific conductivity, dissolved oxygen, total suspended
solids, turbidity, fecal coliforms, PO4-3, and NH4+. A Wilcoxon rank sum test was used to
test for significant difference between non-parametric data TKN, TN, chlorophyll a, NO3-,
TP.
5.4 Results and Discussion
Physical parameters
Stream temperature did not show a statistically significant increase
after timber harvest began (Table 5.1,
Fig. 5.1). In other studies, temperature of streams within clearcut watersheds was shown
to increase due to increased exposure of surface waters to direct sunlight as a result of
clearing (Corbett et al 1978). However, use of a buffer zone has proven to protect streams
from temperature rise of more then 2o C (Binkley 1993).
Post harvest pH values were significantly lower than pre harvest pH
values (Table 5.1,
Fig. 5.1). This finding
contrasts with research done in coastal Florida that showed an increase from pH 3.9 to pH
4.2 the year following harvesting (Fisher 1981). Similarly, research done in coastal South
Carolina showed an increase in pH relative to a control in drainage waters of timbered
land (Askew 1986). Clearcutting of an east Texas forest had no significant effect on pH
(Blackburn and Wood 1990).
Specific conductance showed a statistically significant (p<0.05)
increase after logging (Table 5.1,
Fig. 5.1). This measurement likely reflects an increase in ions (Ca2+, Mg2+,
Na+, Cl-, etc.) released from disturbed forest soils. Increased soil
temperature may accelerate microbial decomposition (Walbridge and Lockaby 1994; Corbett et
al 1978). The ions produced by this decomposition may be a source of increased
conductivity to the water column. Riekerk (1983) found a significant increase (relative to
control) in K+ and Ca2+ after a clearcut. In an east Texas clearcut,
no significant change in conductivity was found (Blackburn and Wood 1990).
Dissolved oxygen at the Goshen Swamp station after timber harvest
showed a similar seasonal pattern to previous years: increased winter levels of dissolved
oxygen and depressed summer levels. However, dissolved oxygen dropped from 6.9 before the
clearcut to 0.4 immediately after the clearcut (Fig. 5.1) most likely due to a biochemical
oxygen demand (BOD) load of organic material released as a result of forest disturbance.
Logging debris entering a stream in Quebec and Oregon was shown to depress dissolved
oxygen in this manner (Binkley 1993). Elevated BOD within a harvested watershed relative
to a downstream control site has been documented in coastal Alabama (Lockaby 1994). Algal
blooms occurring in the Goshen Swamp in August 1998 and June 1999 became a BOD source as
they senesced, and resulted in low dissolved oxygen in September 1998 and July 1999.
Logging in the Goshen Swamp watershed had a profound, though ephemeral,
effect on turbidity and total suspended solids. Rainfall in the 24 hours preceding
sampling during June, July, and August 1998 was likely responsible for flushing suspended
solids form the denuded landscape and raising turbidity. In June, the month of
clearcutting activity, turbidity was twice the North Carolina state standard of 50 NTU.
Total suspended solids also registered an unprecedented high in June 1998, declining to
pre-harvest level by November (Fig. 5.1). Interestingly, the extreme rainfall associated
with Hurricane Bonnie (late August, 1998) did not have a notable effect on either
turbidity or total suspended solids (Fig. 5.1). The regrowth of scrub and brush may have
stabilized the soil so that it withstood erosion caused by this storm event. In other
research, mean annual sediment losses have been shown to be significantly greater than in
a control watershed for two years following a clear cut in the Alabama coastal plain
(Beasley and Granillo 1988). Fisher (1981) found TSS concentration following harvest in
coastal Florida was 137mg/L compared to a control site of 6 mg/L, while another study of
clearcutting in Florida showed harvest activities to significantly increase suspended
sediments (Riekerk 1983).
Nutrients
One month after logging began at Goshen Swamp, TN and TKN
(predominantly organic nitrogen) increased to a 40-month high and stayed elevated for 4
months before returning to baseline levels (Fig.
5.2). This increase in TN/TKN may have
resulted from alteration of several watershed processes following clearcutting. First,
disturbance of the forest floor during harvest activities allowed for the wash-down of
particulate organic matter during rain events, consequently increasing nitrogen levels.
However, little is known about the effects of soil compaction by logging equipment on N
and P retention in a logged watershed (Walbridge and Lockaby 1994). A second watershed
source of N and P is from increased decomposition of organic matter. An increase in soil
temperature in a denuded watershed (due to increase light penetration to the forest floor)
with high soil moisture content can increase decomposition rates and increase nutrient
release to soils (Walbridge and Lockaby 1994; Corbett et al. 1978). Third, as the
landscape is cleared of vegetation, there is reduced uptake of the watershed nutrient load
by plants, resulting in increased nutrient export (Walbridge and Lockaby 1994).
Additionally, changes in forest hydrology and vegetation will alter microbial pathways of
nitrification and denitrification, further affecting watershed nutrient dynamics
(Walbridge and Lockaby 1994). In other research, Lebo
and Hermann (1998) noted a slight increase in TN following harvest of a coastal pine
forest in North Carolina. Research in western Florida shows that NO3-, TN, and NH4+ were
elevated over a control the year following harvest of a lowland tract (Fisher 1981).
Water column NO3- was minimal for the 7 months after logging. This
reflects both a normal summertime trend for the Goshen Swamp, and increased uptake by
algae during a July/August bloom (Fig. 5.2
and 5.3). Furthermore, anoxic conditions during
the summer of 1998 force the conversion of NO3- to NH4+. NO3- was significantly lower for
the year following clearcutting compared with the previous year. This finding agrees with
research done in coastal South Carolina that showed a decrease in NO3- in logged areas
compared with control areas (Askew 1986). In contrast, NO3- was significantly increased
for 1 year after an east Texas clearcut (Blackburn and Wood 1990).
Ammonium-N reached very high levels on three separate occasions after
logging (Fig. 5.2). These NH4+ peaks contributed to a statistically significant increase
for the year following the clearcut (Table 5.1). We hypothesized that an algal bloom in
August used up much of the NH4+ in the water column during that month. Several possible
sources of NH4+ include decreased plant uptake in the denuded watershed and influxes of
NH4+ bound to clay particles (Walbridge and Lockaby 1994). Another possible source of NH4+
is from atmospheric deposition. Lebo and Hermann (1998) found that nitrogen and phosphorus
from atmospheric deposition was partially retained in a harvested watershed in eastern
North Carolina. Data from this study contrasts with the significant decrease in ammonia
after a clearcut in Florida (Riekirk 1983).
The month logging commenced, TP began a four month rise that peaked in
October 1998 at 700 mg/L (six times the pre-logging average). Orthophosphate was elevated
only in September and October of 1998. The low inorganic phosphate level (relative to TP)
is most likely due to increased nutrient demand by stream phytoplankton during this time,
as evidenced by high chlorophyll a values (Fig. 5.3). As the algal bloom senesced in
September, orthophosphate levels began to rise and reach a peak in October. Sources of
phosphate after logging are similar to those mentioned for nitrogen: decreased uptake by
vegetation in the denuded watershed and influx of particulate matter harboring phosphorus
(Walbridge and Lockaby 1994). In other research, Fisher (1981) found phosphate and TP to
be greater than a control site the year after a harvest in a west Florida lowland.
However, in coastal Alabama, phosphate was not shown to increase significantly in a
clearcut compared with an upstream and downstream control (Lockaby 1994). Lebo and Hermann
(1994) found slight increases in TP concentration after harvest of a coastal pine forest
in North Carolina.
A three year maximum in TN, TKN, TP, PO4-3, and NH4+ occurred in
October 1998, one month after Hurricane Bonnie. It is likely that the nutrient peaks
measured in October 1998 are due to an accumulation of organic and inorganic material
flushed from the watershed during the heavy rainfall of Hurricane Bonnie in late August
1998. Intensive livestock operation sprayfields in the Goshen Swamp watershed are a
possible source of nutrients (especially after heavy rainfall and consequent overland
sheet flow of runoff water- see Introduction), however, it is not possible to discern what
portion of the nutrient load originated there. The slow stream flow and dendritic stream
channel of the Goshen Swamp may have delayed the downstream movement of this nutrient-
enriched mass of water, influencing the October samples.
Biological Parameters
Chlorophyll a concentrations displayed unusually high values in July
(75 m g/L) and August (166 m g/L), followed by a decrease to normal levels following
Hurricane Bonnie (Fig. 5.3). The peak in algal biomass following the clearcut may be due
in part to the increase in solar irradiance to the water as the tree canopy was reduced,
even though a buffer zone was left along the creek (Corbett et al. 1978). It was also
encouraged by an increase in inorganic nutrients due to less uptake in the denuded
watershed. The increased NH4+ following the clearcut (Fig.
5.2) was a likely source for
nutrients for the bloom. Increased flow following the rains of Hurricane Bonnie probably
flushed out the algal bloom, replacing it with a BOD load of organic matter, causing
dissolved oxygen impoverishment. There is evidence that the clearcut had a more long-term
effect on stream phytoplankton. In June 1999 chlorophyll a concentration was 60 mg/L and
in August 1999 was 85 mg/L (the North Carolina water quality standard for chlorophyll a is
40 m g/L). The August phytoplankton bloom consisted mainly of the blue-green alga
Microcystis aeruginosa, accompanied by euglenoids (Euglena and Phacus species). In the
three years prior to the clearcut the highest chlorophyll a concentration found at this
location was 30 mg/L in July 1997. We suspect that reduced forest canopy contributed to
the summer 1998 and 1999 blooms by increasing light penetration to the water.
Following logging, fecal coliform bacterial counts increased
dramatically for a four month period (Fig. 5.3), reaching an unprecedented level of 23,400
colony-forming units (CFU)/100 mL (120 times the N.C. state standard for human contact
waters). The increased storm flow following rain events (see Introduction) may be
responsible for flushing waste material of forest animals from the denuded landscape.
Additionally, warm-blooded animals may have been driven to the buffer zone during and
after harvest activities causing increased coliform export from these streamside areas.
The overland sheet-flow expected after the heavy rains of Hurricane Bonnie in late August
1998 may have transported fecal coliform bacteria into the Goshen Swamp, contributing to
the September peak of 23,900 CFU/100 mL. Nine months after timber operations concluded at
the Goshen Swamp, fecal coliforms reached 657 CFU/100 mL, followed in July 1999 by 3,510
CFU/100 mL. These measurements represent a recent and persistent problem of microbial
pollution at the Goshen Swamp sampling station.
5.5 Conclusions
Hurricane Bonnie impacted the North Carolina coast on August 26-28, 1998,
three months after logging commenced at the Goshen Swamp. This weather system influenced
the September samples with heavy overland runoff. The two closest United States Geological
Survey rainfall gauging stations, Warsaw, N.C. and Clinton, N.C. received 8.5’’
and 5.1’’ of rain respectively, in the 8 days prior to the September sampling.
In contrast to the detrimental effect Hurricane Bonnie had on Goshen Swamp water quality
is the lack of response shown following Hurricane Fran in 1996 (see Fig. 5.1, 5.2, and
5.3). Fran, which dumped an equivalent amount of rainfall over the area, did not
significantly influence any of the physical, chemical, or biologic parameters measured
(Mallin et al. 1999). This contrast demonstrates that an intact, riparian wetland forest
can assimilate the nutrient and microbiological loading associated with heavy rainfall,
whereas a clearcut watershed will allow water quality degradation after storm events. The
portion of this post-hurricane pollutant loading that is due to watershed biogeochemical
processes or land use (i.e. intensive livestock operations) is unknown.
The data presented in this paper indicates that logging activities in
the Goshen Swamp led to both short term (5 months) and longer term (>1 year) effects on
stream water quality. Short-term impacts were increases in TSS, turbidity, total nitrogen
and total phosphorus. Long-term impacts include stream phytoplankton blooms caused by
relatively high nutrient levels combined with increased solar irradiance via tree canopy
decrease. Additionally, fecal coliform bacterial counts show recurrent high levels, due
possibly to soil disruption and/or watershed land uses. The 30-foot wooded riparian buffer
employed at the clearcut site was insufficient to prevent water quality degradation of the
creek following logging.
5.6 Acknowledgements
For funding support we thank the Lower Cape Fear River Program and the
Water Resources Research Institute of the University of North Carolina (Project No.
70171). For field and laboratory assistance we thank J. Cook, J. Johnson, M. McIver, D.
Parsons, C. Shank and A. Skeen. Helpful information was provided by Tommy Tew and David
Robinson of the Corbett Timber Company.
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