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Using GIS to Examine the Role of Land Cover in the Bacterial Contamination of Swashes
1
Abstract
Bacterial contamination of surface waters from non-point sources is an
issue of increasing concern. Critically important riparian and estaurine
environments are often the receiving body for stormwater as it
accumulates and picks up contaminants while moving through a
drainage basin. As a result substantial research has been conducted
with the goal of better understanding the role of stormwater as a means
of pollutant transport. This study examines land cover characteristics
within a watershed, a commonly cited contributing factor to water
quality degradation. By characterizing landcover in the subwatersheds
and comparing it to E. coli densities, correlations between increased
development and poor water quality are examined.
Coastal Carolina University Department of Coastal Marine and Wetland Studies
Methods
The influence of upstream development and increased impervious cover
within a watershed are examined by comparing land cover data generated
within a GIS and water quality data collected in the summer of 2012.
Comparisons between the two swashes are also made using a 3 year data
set, ranging from 2010 to the present, collected by Coastal Carolina
University's Environmental Quality Lab.
Watershed Landuse
Dogwood Swash
Withers Swash
Background
Nonpoint sources of pollution (NPS), such as those
transported via stormwater, are of increasing concern for
receiving waters worldwide. The Clean Water Act of 1972 has
largely been effective in regulating point source pollutant
discharges in the US but has only come to consider the
aggregate effects of many, smaller, nonpoint sources since a
1987 addition to the legislation. The Municipal Separate Storm
Sewer (MS4) program is a key portion of this act, requiring
cities and construction sites to account their runoff (US EPA
2000). As a result there has also been considerable research
into the factors affecting NPS pollution of receiving waters.
Two of the major factors identified consitently in the literature
as having a negative affect on water quality are an increasing
population and the associate increase in impervious surface
due to development (Mallin et al 2000). These factors are
investigated both within a single watershed and between two
watersheds in coastal South Carolina.
Study Area
1
Kyle Curtis
Results
Regression
analysis
comparing
percent
developed/commercial land cover and E. coli concentrations
shows that increasing impervious surface does correlate
with higher bacteria values in the Withers Swash watershed.
Percent residential land use was also compared but did not
significantly correlate with E. coli levels.
Population was also examined as a possible predictor of
increased bacterial contamination.
While increasing
population within a watershed did correlate with higher
bacteria levels the relationship was also relatively weak.
Results for these analyses can be found in the Results
Table.
Results Table
3,200,000
Withers Swash Land Class
Land Use
3,161,781.567
2,599,109.068
813,158.666
65,537.138
5,952.889
68,635.932
3,000,000
2,800,000
2,600,000
2,400,000
2,200,000
2,000,000
1,800,000
Area (m^2)
1,600,000
1,400,000
1,200,000
1,000,000
800,000
600,000
400,000
200,000
0
Residential
Undeveloped
Class
Pond
Stream
Swash
Populations
within
each
subwatershed of the Withers
Swash drainage area are
determined by summarizing
US Census block population
units.
Blockpop units represent the
smallest geographic unit used
by the US Census Bureau for
calculating the total population
of an area, rather than
sampling households.
Residential
Developed
Undeveloped
Pond
Stream
Swash
2,000,000
1,900,000
1,800,000
1,700,000
1,600,000
1,500,000
1,400,000
1,300,000
1,200,000
1,100,000
Area (m^2)
1,000,000
900,000
800,000
700,000
600,000
500,000
400,000
300,000
200,000
100,000
Dogwood Swash Land Cover
Land Use
1,958,827.114
704,050.561
1,052,243.279
132,019.105
11,393.755
84,392.681
Residential
Developed
Undeveloped
Pond
Stream
Swash
Withers Swash Land Use/Bacteria
Comparison
Between Swash Comparisons
Fecal Indicator Bacteria (cfu/100ml)
Fecal Indicator Bacteria (cfu/100ml)
r2
P
% Developed
0.42
0.046
% Residental ￿
0.129
0.383
Subwatershed
Populaton￿
0.38
0.047
* Bacteria data collected Summer 2012
Dogwood
Swash
Withers
Swash
Populaton￿
% Developed % Residental ￿ Median FIB
3429
17.86
49.68
33
8414
41.80
46.28
1646
* Bacteria data generated by the CCU Environmental
Quality Lab and Volunteer Monitoring Program
Conclusions
Residential
Undeveloped
Class
Pond
Stream
Swash
Withers Swash Subwatershed Boundaries and Blockpop Units
Ü
0
0.5
Kilometers
1
2
Subwatershed Boundaries
Block Population Units
Subwatershed Boundaries
Understanding those factors that contribute to bacterial contamination of surface
waters has importance from an economic as well as environmental standpoint.
Receiving waters for polluted runoff are often areas of recreation, fisheries, or
drinking water sources, all of significant economic importance.
These
waterbodies also serve as critical habitat in many cases and warrant effective
managment regardless of the reason.
Results of this study support those found in the body of literature which suggests
that bacterial impairment will coincided with increased population and
development in an area. While the results presented here do support that, their
correlation coefficients are not as strong as those found in previous studies. It is
possible this results from the comparatively smaller sample size for both
bacteria (n=98 across 14 sites) and that subwatershed comparisons could only
be made within a single swash drainage basin (Withers Swash). Despite these
limitations it does seem that the relationship is exhibited using regression
analyses as well as qualitatively comparing percent impervious cover and
population with bacteria values over a longer data set between swashes.
Acknowledgements
Data for this project has been provided by the following sources: Horry Co., ESRI, Coastal Carolina University Environmental Quality Lab, and Waccamaw Watershed Academy
Volunteer Water Quality Monitoring Program.
Cited References:
U.S. Environmental Protection Agency (2000) NPDES Stormwater Guidance Document. EPA-821/R-97/004
Mallin MA, Williams KE, Esham EC, Lowe RP (2000) Effect of human development on bacteriological water quality in coastal watersheds. Ecol App 10:1047-1058
A big thanks to Dr. Wright and the 612 class for all their help.