강동한
(Dong-Han Kang)
*
사자드라자 우말
(Raja Umer Sajjad)
**
김극태
(Keuktae Kim)
*
이창희
(Chang-Hee Lee)
**†
Copyright © 2016, Korean Society on Water Environment
Key words
Eutrophication, Land-use, Sewer system, Stormwater runoff
1. Introduction
Non-point source (NPS) pollution not only occurs at various development sites, but also in everyday surroundings,
including urban and industrialized areas, agricultural regions, forests, roads, rivers
and streams. Changes in rivers, lakes and aquifer water quality are strongly related
to surrounding land-use and land cover (LULC) (Bolstad and Swank, 1997) and the type of land-use usually determines the kinds and amounts of contaminants
that flow into receiving water bodies (Moss, 1998). The type of water contamination is directly related to anthropogenic activities
and non-point or point source, which can be quantified in terms of the population
density and land-use type in the watershed (Elliott and Sorrell, 2002). The fact that non-point source pollution fluctuates according to precipitation causes
difficulty in setting up management measures. It has long been recognized that the
pollutant build-up and wash-off processes are influenced by rainfall and catchment
characteristics (Kim et al., 2007; Liu et al., 2013). However, it is difficult to identify the characteristics of stormwater runoff, especially
from mix land-use catchments with inadequate sewer system coverage.
In Korea, more than 70% of annual rainfall occur during the summer monsoon period.
Consequently, reservoirs have been created for water management and > 90% of Korean
water supplies use surface water from rivers and reservoirs (Park et al., 2009). The Paldang reservoir plays an important role in supplying drinking water to Seoul
(the capital of South Korea) and the surrounding regions. It includes several multi-purpose
dams which control the hydrological and water quality characteristics of the river
system. High intensity rainfalls have been occurring frequently causing high fluctuations
in total effluent load in Paldang reservoir. Water quality degradation, particularly
eutrophication issues in the Paldang reservoir over the last few decades has been
continually reported (Kim et al., 1995; Shin et al., 2000; Youn et al., 2010) and stormwater runoff from various areas such as urban, agricultural, and industrial
regions is considered to be a major source of water quality degradation.
While managing stormwater runoff, the land use of a catchment area plays an important
role in the runoff process. Land use types can dictate or correlate with many other
factors such as ground cover, soil type, topography and even rainfall patterns when
looking at a much larger scale. Several researchers have shown that there are strong
relationships between the land use of a particular site and the runoff volume and
characteristics (Ying et al., 2009). In this study stormwater runoff was monitored and characterized from seven different
counties within Paldang watershed, according to land use and population density. Several
monitoring sites were selected within each county based on sewer system as some areas
within each county is connected to the public sewer system while others have individual
or no sewer system.
The primary purpose of this study was to characterize the stormwater runoff according
to land use type and sewer system facility in Paldang watershed. All sites were grouped
into two groups; those connected to the public sewer system (referred as PSS group) and others with individual or no sewer system (referred as ISS group). The comparison of both groups was made in terms of event mean concentrations
(EMCs), unit effluent load, first flush intensity and correlation with hydrological parameters
and land use percentage. This study provides baseline information to identify priority
management areas and role of a sewer system in order to combat non-point source pollution
in Paldang reservoir.
2. Materials and Methods
2.1. Study Area
The Paldang reservoir is one of Korea’s major reservoirs, and one from which most
people in Seoul and the surrounding areas receive drinking water. The watershed of
Paldang reservoir covers 20,459 km2 comprising the portion of 3 provinces, 11 cities and 18 counties. Monitoring sites
were selected evenly at 7 special countermeasure counties, according to land-use and
population density. A total of 76 monitoring sites were selected within 7 counties
in Paldang watershed (Figure 1). Geographic information system (GIS) was used to determine the land-use characteristics within each county (Gyeonggi-do Provincial Government, 2010). The average land-use percentage of the study area and the hydrological characteristics
of monitored events is presented in Table 1.
Fig. 1.
Sampling locations in the Paldang reservoir watershed
Table 1.
Summary statistics of rainfall and land use type in study area
2.2. Sampling Strategy and Water Quality Parameters Analysis
Pollutant concentrations, rainfall, and flow were measured for the entire storm duration.
Among the 76 sites selected for storm water monitoring in Paldang watershed, 48 were
connected to the public sewer system (PSS group) whereas 28 sites has no or individual sewer system (ISS group). Stormwater runoff monitoring was carried out from April 2009 to November
2011. A total of 26 storm events were monitored throughout the monitoring period with
storm duration ranged between 2 to 29.9 hours. Similarly, rainfall intensity ranged
between 0.4 to 11mm/hr and the rainfall depth ranged between 2 to 104 mm for all monitored
events. The hydrological characteristics of monitored events do not show any considerable
variation within both groups. The antecedent dry days (ADD) were determined as the number of days following the cessation of measurable rain.
In this study, minimum ADD were selected to be ≥ 2 and ranged between 2 to 12 days in both PSS and ISS group sites. Each site was monitored 5-6 times during wet weather and 12-15 grab
samples were acquired during each event based on optimal sampling frequencies described
by Leecaster et al. (2001). Samples were taken at 5 to 15 min intervals in the initial hour of the storm event,
and then at 60 min interval during receding flow. The shortest acceptable sampling
duration for each event was based on the principle of sampling from the beginning
of the runoff until it recovered to its original flow. Meteorological information
for each rainfall event was obtained from the regional office of the National Weather
Service in Korea. Rainfall data were also measured through an automated rainfall gauge.
Automated flow meter was installed at the outlet of the catchment area, and discharge
was calculated based on velocity, depth of water, and width of the channel. Two liters
of each sample were collected in polyethylene bottles, and transported to the laboratory
and refrigerated at 4℃ until analysis. Water quality parameters including, chemical
oxygen demand (COD), biological oxygen demand (BOD), suspended solids (SS), total nitrogen (TN), and total phosphorous (TP) were analyzed according to Korean standard methods.
2.3. Data Analysis
Some stormwater management strategies are based on the assumption that there is a
first flush of pollutants at the start of each event. The first flush of pollutants
can be visualized by a hydro-pollutograph, in which an X-Y plot of pollutant mass
(M) against volume (V) is developed. On this plot a cumulative M-V line and a bisector
line (45° line) are drawn and first flush occurred when M-V line lies above the bisector
line. In this study first flush mass and volume is calculated by the difference of
M-V line and bisector line. The highest difference point between M-V and bisector
line means the first flush amount for BMPs (Best Management Practices). In this study first flush characteristics were compared
between PSS and ISS group sites in order to quantify the effluent load percentage corresponds to initial
runoff.
Similarly EMCs and pollutant loads were also estimated from different sites occupying variable land
use type percentage to evaluate the stormwater runoff quantity and quality characteristics
of the monitored events. EMC is appropriate for evaluating the effects of stormwater runoff on receiving waters
and is often used as a single index to characterize concentrations. It can be expressed
as:
Where Ct is the pollutant concentration at time t and Qt is the storm water discharge at time t. M is the pollutant mass and V is the runoff volume during the storm event. EMC can be used in water quality management and concentration control for a catchment
as it reflects the water quality of the runoff.
The runoff coefficients could be calculated for the storm event measured by taking
into account the total rainfall and total catchment area and is an important factor
in calculating the pollutant load. The generated load mechanism of watershed development
was estimated by using the EMCs criteria, which requires flow rate, pollutant concentration, and time data. Both
EMCs and load values were then compared according to sites grouping. To analyze the impact
of hydrological parameters and land use percentage on EMCs and pollutant loads, a Pearson’s correlation coefficient matrix (MS Excel, 2010)
was adopted to perform a bivariate analysis of storm water quality parameters. Pearson
correlation coefficient is a measure of the correlation between two variables, giving
a value between +1 and -1 inclusive. The two-sided test method was chosen for significance
level at a p value < 0.05.
3. Results and Discussion
3.1. Rainfall and Land Use Characteristics of Monitoring Sites
The statistical summary of monitored rainfall events and land use percentage is shown
in table 1. The study area is classified into four different land use types including impervious
cover, farm land, paddy field, and forest. The geomorphological factors like slope
and population density were also quantified. At PSS group sites, the average impervious land use and population density are higher i.e.;
63.7 % and 67.0 capita/ha as compared to the ISS group which is 19.2 % and 8.6 capita/ha respectively. It shows that PSS group mostly represents the urban area within the watershed. On the other hand, average
forest land-use percentage is considerably higher, i.e.; 52.7 % in the ISS group as compared to 3.7 % in PSS group. Overall, the average watershed area connected to the public sewer system is
far less.
3.2. First Flush Analysis
Initial rainwater interception is one of the effective and popular measure to reduce
runoff pollutant loads if the catchments have high first flush intensity. Generally,
the first flush intensifies as the watershed area gets smaller, the impervious land
area gets larger or the rainfall intensity increases (Lee and Bang, 2000). A Similar trend was observed in this study for most water quality parameters. The
first flush intensity for BOD5, TN, CODcr, and CODmn was observed higher at PSS group sites (Figure 2) with higher impervious cover and smaller watershed area. However, the first flush
intensity for SS and TP was found higher at ISS group sites. This is perceived to be the function of rainfall characteristics and
higher forest land-use percentage within ISS group sites (Table 1). In case of ISS group sites the average rainfall intensity and rainfall depth of monitored events
were comparatively higher. This is particularly noticeable in the case of SS first flush intensity, where runoff flow (function of rainfall intensity) needs sufficient
energy to scour and mobilize the pollutant as well as runoff volume (function of rainfall
depth) to wash out the pollutant. The study carried out by Menacher and Augustin (1992) showed that first rainfall peak is not necessarily sufficient to flush out a sewer
system and that the following flow peaks, when they exist, contribute greatly to the
pollutant mass discharged. However, Bertrand-Krajewski et al. (1998) argued that these conclusions can be different for pollutants other than suspended
solids because of their sources and their transfer conditions. In terms of pollutant
loads, 33 % of the initial flow at PSS group sites contain 40 % to 57 % of all pollutant loads whereas, 43 % of the initial
flow at ISS group sites contain 46 % to 71 % of all pollutant loads. The results imply that intercepting
the first 33 % and 43 % of runoff volume can remove 40 % to 57 % and 46 % to 71 %
of all pollutant loads respectively within PSS and ISS group sites. In case of TP load, which is considered major pollutant causing eutrophication within Paldang reservoir,
59 % of load can be captured during first 43 % of storm runoff by upgrading runoff
interception system. As mentioned in earlier section, the average area connected to
PSS within Paldang watershed is far less compared to ISS (table 1), lack of sewer system within these sites is supposed to accelerates the erosion
of topsoil and the untreated stormwater runoff is constantly being discharged into
the tributaries and subsequently entering the reservoir without any treatment. Therefore,
first flush should be taken into account while selecting management measures such
as a storm runoff interception system to reduce the pollution loads for Paldang reservoir.
The effective rainfall range for designing runoff interception system as BMPs (Best management practices) was found to be 4.5mm and 3.5mm for both PSS and ISS group sites respectively.
Fig. 2.
First flush intensity and load comparison.
Table 2.
Correlation of EMCs and pollutant loads with various parameters
3.3. Pollutant Loads and EMCs Comparison
The runoff coefficient calculated for PSS group sites was found to be 0.40 as compared to 0.23 for ISS group sites. In case of ISS group, the EMC for SS, TN, and TP were 1.8, 1.2, and 1.4 times higher than PSS group sites (Figure 3). It is thought to be the function of watershed characteristics such as slope and
the land-use impact within ISS group sites which has higher coverage of forest and paddy fields. The application
of fertilizers and soil erosion in steeper areas is perceived to be greater within
these sites which results in higher EMC values for solids and nutrients. The center for streamside studies (2001), found a strong correlation between TSS and TP under storm conditions, which indicates that the dominant fraction of TP moved through the stream systems in particulate form. Therefore, controlling sediment
will aid in controlling particulate -bound phosphorous. Because of inadequate sewer
network within these sites, the sediment-attached nitrogen and phosphorous could be
dissolved and transported to receiving water body during heavy rainfall event. It
has important implications as Paldang reservoir faces major problem of eutrophication
in recent years (Kim et al. 2014). The higher nutrient concentrations contributed by ISS group sites can be selected as priority management area to combat eutrophication
of Paldang reservoir. On the other hand, organic matter (BOD and COD) showed higher EMC values in PSS group. The PSS group sites are mainly associated with urban area with the higher percentage of impervious
cover. The main sources of organic matter during storm events are restaurants and
food stalls, especially in the commercial catchments. Therefore, higher EMCs regarding organic matter were observed within PSS group sites.
Fig. 3.
Pollutants average EMC and unit load comparison.
However, the pollutant loads results were different compared to EMCs between two groups. The unit load for all pollutants were found higher is PSS group compared to ISS group (Figure 3). This was due to the difference in the drainage area sizes and runoff coefficient.
The relative effluent loads of SS, TN, and TP are higher within the ISS group than that of organics.
3.4. Correlation of EMCs and Pollutant Loads with Different Parameters
The Pearson correlation analysis was carried out in order to determine the relationship
of EMCs and pollutant loads with hydrological parameters as well as with land use percentages
and geomorphological factors. The pollutant load showed relatively higher positive
correlation with hydrological parameters in the ISS group as compared to PSS group. The nutrient load within ISS group sites showed a positive correlation (R2 > 0.6) with rainfall depth and storm duration. It shows that these sites can contribute
elevated nutrient load during heavier storm events and therefore can be targeted as
priority management areas. Overall, rainfall depth and storm duration exhibit positive
R2 values (> 0.4) with almost all water quality parameters. Similarly, impervious land
use percentage and population density also showed relatively higher R2 values (> 0.6) with pollutant loads in ISS group. Generally, the correlation coefficient for pollutant loads was found very
pronounced and higher in the ISS group as compared to PSS group. It implies that sites connected to the public sewer system are less susceptible
to different hydrological and land use variables. The EMCs correlation with hydrological parameters and with land use percentage was not profound
in both groups. Most constituents EMCs showed weak correlation with hydrological parameters as well as with land use percentage.
4. Conclusion
This study focused on characterizing stormwater runoff from the Paldang watershed
area based on land-use type and sewer system coverage. In recent years the water quality
degradation, especially eutrophication problem in Paldang reservoir is extensively
reported. The major sources of excessive nutrients and eutrophication are fertilizers
and animal manure, mostly received from agricultural non-point sources. It was found
that monitoring sites with a higher percentage of forest land-use with no or individual
sewer system coverage generates higher pollutant concentrations for particulate matter
and nutrients. Lack of sewer system within these sites accelerates the erosion of
topsoil and the untreated stormwater runoff is constantly being discharged into the
tributaries and subsequently entering the reservoir without any treatment. Based on
first flush analysis, it was proposed that installation of the runoff interception
system for the first 43 % of storm runoff can capture up to 59 % of nutrient load
within these sites and contribute meaningfully in combating eutrophication problem
in Paldang reservoir. On the contrary, the sites connected to public sewer network
with higher impervious land-use generates higher pollutant concentrations for organic
matter. The results of this study will be helpful for NPS management policy in Paldang reservoir.
Acknowledgements
Preparation of this article was supported by the Korea Environmental Technology and
Industrial Institute, Next Generation Eco Innovation Project (No.413-111-003).
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