Study of Water Pollution and Bottom Mud Elution of Koya Pond

ABSTRACT To resolve problems of water reservoirs such as deterioration of scenic beauty, reduction in aquatic organisms, and odors resulting from advanced nutrient enrichment, and to promote activation of multiple functions of water reservoirs, we carried out field investigations of water quality and bottom mud at Koya Pond, where dredging work was performed as a water quality improvement measure. We attempted to calculate the elusion rate of nutrient salts from bottom mud using a simulated in situ method. Results showed that water pollution is considerable during summer, and that pollution occurs even after dredging, the COD is high at water temperatures greater than 30°C, and T-N and T-P are high at water temperature around 25°C under anaerobic conditions because the elusion of bottom mud is considered to be great. The elusion rate of T-N under aerobic conditions is 1.12–1.33 times higher than that under anaerobic condition, and elusion rate of T-P under anaerobic conditions is 3.01–7.73 times higher than that under aerobic conditions

Study of Water Pollution and Bottom Mud Elution of Koya Pond

Yasuhiko WADA*, Yasuhiro HEIKE* and Nariaki WADA**

* Civil & Environmental Engineering, Faculty of Engineering, Kansai University, Yamate

3-3-35, Suita, Osaka, 564-8680, JAPAN (E-mail: ywada@ipcku.kansai-u.ac.jp)

** Graduate School of Engineering, Kobe University, 1-1, Rokkodai, Nada-ku, Kobe

657-8501, JAPAN

ABSTRACT

To resolve problems of water reservoirs such as deterioration of scenic beauty, reduction in aquatic organisms, and odors resulting from advanced nutrient enrichment, and to promote activation of multiple functions of water reservoirs, we carried out field investigations of water quality and bottom mud at Koya Pond, where dredging work was performed as a water quality improvement measure We attempted to calculate the elusion rate of nutrient salts from bottom mud using a simulated in situ method Results showed that water pollution is considerable during summer, and that pollution occurs even after dredging, the COD is high at water temperatures greater than 30°C, and T-N and T-P are high at water temperature around 25°C under anaerobic conditions because the elusion of bottom mud is considered to be great The elusion rate of T-N under aerobic conditions is 1.12–1.33 times higher than that under anaerobic condition, and elusion rate of T-P under anaerobic conditions is 3.01–7.73 times higher than that under aerobic conditions

Keywords: dredging effects, nutrient enrichment, elusion rate, nutrient salts, bottom mud, water quality

INTRODUCTION

For their use as valuable waterfront in urban areas, utilization of multiple functions of water reservoirs has been cited For the utilization of multiple functions of water reservoirs, municipal governments have established diversified conservation plans However, as one problem that hinders development of multiple functions, water pollution caused by nutrient enrichment is mentioned Koya Pond, which we investigate in this study, suffers from water pollution because the pond is a place of contact of people with wild birds People routinely feed birds there Simultaneous pursuit for protection of wild birds, contact with them, and conservation of water quality poses an important but complex problem Studies presented so far have undertaken analyses of water quality1),2),3) and bottom mud of lakes and marshes4),5),6),7),

cleaning measures8),9),10), and calculation of inflow and outflow loads11),12),13) However, none have brought meaningful improvements of water quality environments, suggesting that further investigations are required Since the causes of nutrient enrichment of ponds, lakes, and marshes are widespread, it is hardly possible to improve water quality using a single measure In recent years, dredging has been attempted in some cases as a measure used in closed water areas where nutrient enrichment is promoted Although studies on the effects of cleaning after dredging on water quality have been disclosed, many dredging works were performed for limited water areas but not covering the entire water area Few reports describe cases in which the whole pond is dredged and assessments are then made An argument exists regarding the effects of dredging

on cleaning: its effects can not be appreciated unconditionally For that reason, further investigations are necessary14)

Address correspondence to Yasuhiko WADA , Civil & Environmental Engineering, Faculty of

Engineering, Kansai University, E-mail: ywada@ipcku.kansai-u.ac.jp

For the current study, a field investigation was carried out at Koya Pond, where dredging was performed throughout the pond, water quality was evaluated,bottom mud samples were taken, and the degree of pollution of water and bottom mud was identified after dredging Elusion tests were carried out to identify the influences of polluted bottom mud on water quality and water quality improvement effects after dredging were also studied

MATERIALS AND METHODS

Outline of the target area9)

The current study specifically examines Koya Pond, situated in Itami City, Hyougo prefecture The outline of Koya Pond park is presented in Table 1 Koya Pond has an area of about 9.9 ha and boundary length of about 1.6 km It is used by residents as a place of recreation and relaxation It is well known as one of the greatest wintering spots of migrating birds in the

Kansai district The vicinity is designated as a wildlife sanctuary

Deterioration of water quality was noticed from the late 1970s to the early 1980s At present, nutrient enrichment is accelerated, as represented by mass generation of blue-green algae in summer Water pollution poses a problem

Causes for contamination of Koya Pond

At present, Koya Pond is a closed water area with no river system flowing into it The problem arises when water in the pond is not circulated sufficiently Well water is supplied to the pond to ensure that it is filled; the water contains nutrient salts such as nitrogen and phosphorus In addition, food given to water birds and the birds’ excrement are deposited there The nutrient salts eluted from these materials contribute to the proliferation of phytoplankton such as blue-green algae The vicious cycle is repeated in the sense that these materials settled at the bottom of the pond, mud is deposited, and nutrient salts are eluted again from the sludge15) Figure 1 illustrates the causes for the pollution of Koya Pond Sakata et al.16) point out that great cormorants formed a large colony on an artificial island in the pond Thereby, a large quantity

of nitrogen and phosphorus were brought into the island via their excrement, which is entirely responsible for the deterioration of the pond’s water quality As described above, factors for deterioration of the Koya Pond water quality have become complex and are more aggravated than ever before

Measures for water quality purification of Koya Pond

Table 1 Outline of Koya Pond park

Park area 27.8 ha Opened 1968

Primary facilities

Natural pond, Water storage pond, Feeding pond, Insectary, Tree area, Grassy plaza, Multi-purpose plaza, Strolling road, etc

Water area About 7.9 ha* Island Japanese island type (1.4 ha*)

Pond capacity About 100,000

m3*

Water depth Average 1.29 m*

Purpose of utilization

Irrigation water, Water control, Access to water (Maintaining ecology, Landscaping)

* Pond water height: Set to T.P + 24.6 m

(Control water level)

Water pollution has been exacerbated continually in Koya Pond; moreover, the loads from various pollution sources, as presented in Fig 1, are complexly related Regarding anti-nutrient

enrichment, various measures have been taken for water quality purification in the Koya Pond

Table 2 lists down the water quality purification measures carried out in Koya Pond

Since the opening of Koya Pond Park, excrement and bait residues of water birds have been accumulating at the bottom of the pond, thereby creating sludge In 1995, Itami City performed

a survey to verify the floating mud thickness Results showed that the general distribution of thickness of the floating mud is 0.0–0.4 m at the north of the island in the pond, and about 0.4–0.6 m at south and east of the island in the pond, except for the observation bridge for wild birds and the edge of the pond Overall, it was estimated that the amount of sedimentary mud in the pond was 30,000 m3 17) Furthermore, because no firm bottom is known, dredging of the pond is not possible Subsequently, nutrient enrichment of the pond water promotes generation

of blue-green algae during summer As countermeasures, reeds and water hyacinths, which support and improve water purification functions, were planted along the bank to protect the pond and induce natural purification Water from the well that is used for filling the pond contains nitrogen and phosphorus Therefore, direct supply to the pond should be avoided A small stream, with a gravel purification device provided upstream, was installed A plant purification device commonly used as a biotope is provided downstream for well water filtration

Table 2 Water quality purification measures carried out at Koya Pond (As of fiscal 2007)

Fiscal 1998* Maintenance of feeding pond

Fiscal 1999 Re-maintenance of water channel of grassy plaza

Fiscal 2000 Maintenance of bank protection, Feeding pond ~ Matsugaoka entrance

Fiscal 2001 Maintenance of water channel along the Koya Pond center

Fiscal 2002 Maintenance of bank protection, Matsugaoka entrance ~ Shallow well

Fiscal 2003 Re-maintenance of water channel along insectary

Fiscal 2004 Re-maintenance of water channel along insectary

Fiscal 2005 Dredging work (Entire water area)

Fiscal 2006 Drilling of new shallow well and maintenance of water channel

*1998.4.1～1999.3.31

Fig 1 Causes of contamination of Koya Pond

Excrement and remains of dead birds

Well water

Nutrient salts (nitrogen and phosphorus)

Phytoplankton such as blue-green algae

Soil particles and organic matter

Generation

Sedimentation Elution

Polluted sludge on pond bottom

Vicious cycle

Excrement and remains of dead birds

Well water

Nutrient salts (nitrogen and phosphorus)

Phytoplankton such as blue-green algae

Soil particles and organic matter

Generation

Sedimentation Elution

Polluted sludge on pond bottom

Vicious cycle

A partition dam is provided at the boundary of the park pond to make it independent as a feeding pond so that food given to birds might not spread throughout the pond bottom18)

As a Koya Pond purification measure, a full-scale dredging plan covering sediment mud that is responsible for water quality pollution was established; substantial removal was started from the 1st of July 2004 and completed at the 1st of September 2005

Outline of bottom mud dredging

Dredging was performed from the 1st of July ,2004 to the 1st of September ,2005 The depth dredged was 44 cm on the average (nominal average 35 cm) and the total amount of polluted sludge dredged was 32,398 m3 (nominal value 30,000 m3) This work is unique in the sense that all water was dredged thoroughly, although the depth of dredging is deeper at the south of the pond, where much floating mud exists

The constraint was that draining of the pond water was not possible so to promote dredging,

a combination of a barge and a mud pumping backhoe was used to improve the working efficiency and to keep the planned schedule steadily In addition, a vacuum suction and pressurized feeding system was adopted to suppress odors and turbidity and to eliminate their influences on water birds and irrigation water The dredged bottom mud was processed through recycling and a high-pressure thin-layer dehydration system The dehydrated cake that resulted from processing (17,300 m3) was recycled as a filling material for green spaces

Outline of investigation

Field investigations of Koya Pond were undertaken twice a month from July to November

2007 Investigations were made during fine weather Field investigations included water sampling and water level measurement at the natural pond and feeding pond, at the flow inlet and flow outlet of the pond, in addition to sampling of mud in the pond (including samples for elusion test)

Koya Pond is divided into a natural pond and a feeding pond by a partition dam installed as

a nutrient load reduction measures

Regarding the inflow water source to Koya Pond, along with shallow wells at two points and deep wells at five points, one inflow from a rainwater drainage system exists from which water is introduced only when rainfall exceeds 8 mm/h The water catchment area is 20.07 ha It

is considered that the inflow load from the rainwater drainage system at rainfall is not great Agricultural land or the like accounts for 34.5% A drain outlet is provided in three locations

at the southern part of the pond

Outline of points of investigation19)

Fig 2 depicts sampling points for water quality analysis and amounts of inflow and outflow Fig 3 shows sampling points of bottom mud

Points of water sampling at inflow are natural pond (St 1, St 2, St 3, St 4), feeding pond (St 5), deep wells at park feeding sites (A1, A2), Muko River shallow well (B1, B2), deep well

at northeastern part of the park (C), and deep well at the eastern part of the park (D) The weather was fine on all investigation days The inflow from the rainwater drainage system (E) was not observed and water sampling was not performed

A water-level meter was installed around Chayatoi (b1) and the water level of the pond on the investigation day was determined using this meter Points of sampling of mud in the pond are St A, St B, St C, St D, and St E

The storage capacity of Koya Pond is 94,000m3 (superficial area is 9.9 hectare, the average depth is 0.95 m) The amount of inflow water per day is 4,500 m3/day, and the average residence time of water in Koya Pond is 20.88 days

Method of sampling

Water sampling was done directly using a plastic water container For the feeding pond (St 5), water sampling was done from the observation bridge for wild birds using a canvas bucket Regarding mud sampling (for bottom mud analysis), the mud surface layer was taken using

an Ekman-Birge bottom sampler, stirred sufficiently using a shovel, and then put into a sealed bag

Fig 2 Water sampling points

Fig 3 Mud sampling points

N

N B1

C

b1

b1

a1

St.3 St.4

St.5

St.5

Natural pond

Feeding pond

N

N

0 m 100 m m

0 m 100 m m

0 m 100 m m

0 m 100 m m

E

B1

C B2

B2

b1

b1

A1

b2 A2

A2

D

D

a1

St.3 St.4

Natural pond

Feeding pond

St.C

St.A

St.B

St.D St.E St.C

St.A

St.B

St.D St.E

0

00m 0 m m m 100 100 m m m

00m 0 m m m 100 100 m m m

00m 0 m m m 100 100 m m m

00m m m m 100 100 m m m

St.5

b2

N

N

A, B, C, D, E; Position of inflow load

a, b ; Position of outflow load

Method of analysis

The following method was used for the analysis of water in the pond and bottom mud samples Variables such as water temperature, air temperature, and mud temperature were measured onsite Other analyses such as DO, pH, COD, SS,T-N,T-P were done in the laboratory

The DO was monitored using the diaphragm electrode method and pH was monitored using the glass electrode method Each of the COD, SS, T-N, and T-P was analyzed in conformity to the river water quality test method20)

For bottom mud samples, analyses of all variables (CODsed, Ignition loss (I.L.), drying loss, moisture content, and grain size test) except for mud temperature were performed in the laboratory

For CODsed, I.L and drying loss, the bottom mud examination method21) was based from literature as well as for grain size tests, the soil test––basics and guidance22)–– For COD, T-N, and T-P, the dissolved forms of COD (D-COD), total nitrogen (D-TN) and total phosphorus (D-TP) were analyzed using the filtrate after filtration

RESULTS AND DISCUSSION

Results of water quality test

Fig 4 shows the field investigation results on water quality Fig 5 illustrates the results of the dissolved state concentrations of COD, total nitrogen (T-N), particulate total phosphorus (P-TP), and dissolved total phosphorus (D-TP) in 2007

The obtained results were compared to environmental criteria (C, V)23) for lakes and marshes Measurements performed on samples from the natural pond (July 18) were excluded because the water sampling method used was different

Chemical oxygen demand (COD)

COD in the natural pond was in a range of 20.0–33.7 mg/L, which was apparently greater than the 8.0 mg/L specified in the environmental criteria for lakes and marshes, type C It was considered that contamination by organic matter was excessive in this reservoir

In general, values of the suspension state are high, which is considered to be affected by SS They generally remain at the same level from August, to November when changes in water quality in the natural pond of this fiscal year were reviewed

The COD concentrations of the inflow at every point were greater than 5 mg/L, which was specified in the environmental criteria for lakes and marshes, type B

Suspended solids (SS)

The levels of SS in the natural pond were in the range of 20–80 mg/L Values in August were particularly high, which was considered to be attributable to the fact that the air temperature was high and generation of phytoplankton was great When changes in water quality in the natural pond of this fiscal year were reviewed, although values at the beginning of August and October were high, it was noticed that they generally remain at the same level in September, October, and November This is considered to be due to the fact that generation of phytoplankton is suppressed as the water temperature decreases

The SS concentrations in the inflow were generally 15 mg/L, which was specified in the environmental criteria for lakes and marshes, type B

Total nitrogen (T-N)

The T-N in the natural pond was 1.0–2.4 mg/L, which is higher than the 1.0 mg/L specified in the environmental criteria for lakes and marshes, type V Values in August were particularly high, which was considered to be a result of the influence of SS or the like because values in the suspension state were generally high Observation of changes in the water quality of the natural pond reveals that, although levels as high as 1.9 mg/L were detected at the end of September, it

＜natural pond＞

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

7/18 8/8 8/29 9/19 10/10 10/31 11/21

T-COD P-COD D-COD

＜natural pond＞

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

7/18 8/8 8/29 9/19 10/10 10/31 11/21

T-P P-TP D-TP

＜natural pond＞

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

7/18 8/8 8/29 9/19 10/10 10/31 11/21

T-N P-TN D-TN

Fig 5 Concentration of COD, T-N and T-P in natural pond (2007)

water temperature（℃）

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

7/18 8/5 8/23 9/10 9/28 10/16 11/3 11/21

natural pond

A2 water course

B2 water course

C water course

COD（mg/L）

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0

7/18 8/5 8/23 9/10 9/28 10/16 11/3 11/21

natural pond A2 water course B2 water course

C water course

SS（mg/L）

0

20

40

60

80

100

120

140

160

180

200

7/18 8/5 8/23 9/10 9/28 10/16 11/3 11/21

natural pond A2 water course B2 water course

C water course

T-N（mg/L）

0.00 1.00 2.00 3.00 4.00 5.00 6.00

7/18 8/5 8/23 9/10 9/28 10/16 11/3 11/21

natural pond A2 water course B2 water course

C water course

T-P（mg/L）

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

7/18 8/5 8/23 9/10 9/28 10/16 11/3 11/21

natural pond A2 water course B2 water course

C water course

z

DO（mg/L）

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

7/18 8/5 8/23 9/10 9/28 10/16 11/3 11/21

natural pond A2 water course B2 water course

C water course

Fig 4 Water quality of natural pond at inflow (2007)

was observed that after August, they generally remained at the same level during September, October, and November

The inflow water generally had T-N values of 0.4–1.2 mg/L In particular, A1, A2, and D were greater than 1.0 mg/L specified in the environmental criteria for lakes and marshes, type V, and were considered to be greatly responsible for the nutrient enrichment of Koya Pond

Total phosphorus (T-P)

The T-P in the natural pond was 0.5–1.67 mg/L, which exceeds the level of 0.1 mg/L used

as reference for the preservation of environments for lakes and marshes The T-P values in July and August were high and values in the suspension state were generally high Therefore, influences of SS are suggested When changes in water quality are reviewed, T-P values in the natural pond in August are high and it can be observed that after August, they generally decrease This could be due to the fact that, as the water temperature decreases, phytoplankton generation was suppressed, and the amount of SS was reduced

The level of T-P in the inflow water was 0.07–2.86 mg/L, which was much greater than the 0.1 mg/L specified in the environmental criteria for lakes and marshes, type V Depending on the point of inflow, they were higher than those in the pond, suggesting that the contamination load by the inflow water is excessive

Dissolved oxygen (DO)

The level of DO in the natural pond was 4.6–13.8 mg/L Although they were low at the beginning of September and October, they are generally greater than 10.0 mg/L, which satisfied the reference value 2.0 mg/L specified in the environmental criteria for lakes and marshes, type

C It was considered that higher DO is attributable to photonic synthesis by phytoplankton The

DO values at the end of October and in November were high (12.1–13.7 mg/L), when changes

in the water quality of this fiscal year were reviewed

As discussed above, regarding water contamination, a tendency of a higher contamination level exists during summer In many cases, measurements exceeded values (C, V) used for preservation of environments, of the environmental criteria for lakes and marshes, indicating that the progression of contamination is substantial In particular, the T-P level, which is used as the index of nutrient enrichment, is high in both natural ponds and inflow water

Results of a survey on bottom mud

Both Fig 6 and Table 3 show results of a survey on the bottom mud performed at the site

It is noteworthy that although mud was sampled on July 24 and then analyzed, those results were excluded because the point of sampling differs slightly from those of other surveys The results of St C on August 7 were also excluded because the point of mud sampling and method

of sampling were different

No standard is available for CODsed and I.L in storage reservoirs, lakes and marshes For the current study, both CODsed and I.L were compared with “Critical value of bottom mud which gives significant influences on water quality” obtained by the Takamatsu Port and Air Department Technology Investigation Office of the Shikoku Regional Development Bureau, Ministry of Land, Infrastructure and Transport through field investigations and indoor tests24)

Grain size distribution

Table 3 represents Grain size distribution (Average Value of Suraey in 2007).Regarding the grain size distribution, silt contents exceeded 90% at all sampling points, except for St C In particular, the silt contents were high at points St A, St B, and St E at the southern part of the pond where the water is deep This is considered to be attributable to the fact that the flow velocity at the southern part of the pond is slow because it is located away from the point of inflow; silt materials being carried by the flow are accumulated here

Chemical oxygen demand in sediment (COD sed)

The CODsed at St A was 33.7–42.0 mg/g, 37.4–44.0 mg/g at St B, and 35.0–42.6 mg/g at St E All are far above 30.0–35.0 mg/g, which the Takamatsu Port and Air Department Technology Investigation office uses as the critical value of bottom mud from which influences on water quality become significant, suggesting that the bottom mud of Koya Pond remains contaminated even after dredging Furthermore, it is known that although the CODsed value at St D was 23.5–24.1 mg/g, which is lower than the said critical value, the degree of contamination is high

In particular, values of CODsed at St A, St B, and St E at the southern part of the pond are high This is considered to be attributable to the fact that the flow velocity at the southern part of the pond is slow because it is located away from the point of inflow, the water depth is great and mud such as remains of phytoplankton easily accumulate Results obtained show that the amount of new mud is greater in deeper points at the southern part of the pond than at the northern part of the pond

Ignition loss (I.L.)

The I.L at St A at the southern part of the pond is 9.1–10.9%, 3.4–10.7% at St B, and 10.8–11.0% at St E, which are higher than two points at northern part of the pond Values of St

Coarse

Medium

Fine sand

Silty sand

Table 3 Grain size distribution (Average value of survey in 2007)

Table 3 Grain size distribution of bottom mud (Average value of 2007 survey)

percentage of water content（%）

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

160.0

180.0

7/24 8/13 9/2 9/22 10/12 11/1 11/21

St.A St.B St.C St.D St.E

CODsed（mg/g）

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0

7/24 8/13 9/2 9/22 10/12 11/1 11/21

St.A St.B St.C St.D St.E

I.L.（%）

0.0 2.0 4.0 6.0 8.0 10.0 12.0

7/24 8/13 9/2 9/22 10/12 11/1 11/21

St.A St.B St.C St.D St.E

Fig 6 Mud of Koya Pond

A, St B, and St E are greater than about 10.0 mg/g, which is used as the critical value, suggesting that the bottom mud of Koya Pond remains contaminated even after dredging This

is considered to be due to the fact that the flow velocity at the southern part of the pond is slow because it is located away from the point of inflow, the water is deep and sediments such as remains of phytoplankton are easily accumulated

As discussed above, there is a tendency for bottom mud contamination, and the contamination degree in summer is higher than that in winter Although the I.L of bottom mud

of Koya Pond is below the average of lakes and marshes (about 13%), progression of contamination is considerable all throughout Moreover, the tendency of a higher degree of contamination is apparent at St A and St B at the southern part of Koya Pond, where the water

is deep In these points, loads brought from Koya Pond basin and loads generated in Koya Pond are readily accumulated, and sediments primarily comprise soft mud (so-called mud); measurements here are higher than those at other points The fine grain content rate is 90% at St

A, St B, and St D, while at St C it is greater than 80%, suggesting that bottom mud mostly comprises silt materials The value at St C suggests that the ratio of fine sand is slightly higher than that of other points

Outline of elusion test

To identify the effects of the removal of bottom mud (by dredging) on the amount of elusion of nutrient salts from bottom mud, elusion tests were carried out using a simulated in situ method Bottom mud used in the experiments was sampled from Koya Pond on September

6, October 10, and November 8, 2007 Fig 7 shows the elusion test apparatus

5cm

Bottom sediment

Exhaust port

Nitrogen gas supply port Sampling tube

Water immediately above

5cm

Bottom sediment

Exhaust port

Nitrogen gas supply port Sampling tube

Water immediately above

Fig 7 Elusion test apparatus

For the sampling of the mud, a polyvinyl chloride pipe (5 cm internal diameter, 150 cm length) was inserted into the bottom mud and mud was sampled without stirring Points of mud sampling (St A – St D) were set to surround the island in the pond

For the bottom mud thus sampled, only the surface layer (20 cm from the surface layer) was left and water was carefully added directly onto it up to 110 cm height taking care not to stir the bottom mud For the water to be used for the elusion test, the surface layer of water was sampled at point St B and filtered using glass fiber filter paper (0.45 μm nominal pore diameter,