Chapter 7 Water Control Zone

Water Quality in 2001 Compliance with Water Quality Objectives

7.1 The Mirs Bay Water Control Zone 7.4 The Mirs Bay WCZ has had an (WCZ) has a very good and uniform water excellent record of Water Quality quality, with low turbidity, E.coli, 5-day Objective (WQO) compliance in the last

Biochemical Oxygen Demand (BOD5) and 10 years (Figure 7.1). Full compliance with nutrients. The only exception is the the WQOs for dissolved oxygen, total ( Hoi), a inorganic nitrogen and unionised ammonia semi-enclosed bay with restricted tidal was again achieved in 2001. flow. Station MM1 at Starling Inlet has higher bacterial counts, suspended solids, 7.5 The secondary contact recreation inorganic nutrients and chlorophyll-a than subzones: (MM2), Long the rest of the WCZ. A summary of the Harbour (MM6) and 2001 water quality data of the Mirs Bay (MM7), fully complied with the E.coli WCZ is shown in Tables D8 and D9 of objective in 2001 (Figure 7.1), indicating Appendix D. their excellent bacteriological quality.

7.2 Similar to other marine waters in Long-term Water Quality Trends the territory, the dissolved oxygen (DO) in Mirs Bay was found to decrease in 2001. In general, the drop was more noticeable in 7.6 The water quality in the Mirs Bay the northern and more enclosed parts of the WCZ has remained stable over the years. bay. The reduction of DO was less than The two notable trends detected include: 0.4mg/L in the southern part of the bay a) a general decrease in total phosphorus; (MM8, MM13-16); 0.8-1.0mg/L in the and b) increases in chlorophyll-a in the northern part (MM3-MM6, MM17) and northern and southern parts of the WCZ 1.1-1.4mg/L around Starling Inlet (MM1) (Table 7.1 and Figure 7.2). and Crooked Island (MM2 and MM7). 7.7 To maintain the good water quality 7.3 The annual mean E.coli at Starling in Mirs Bay, the Government would Inlet (MM1) showed a 60% increase in continue with the provision of sewerage to 2001, reaching 320 cfu/100mL – the unsewered villages in the catchment second highest in record next to that in according to the North District Sewerage 1997. In parallel, ammonia nitrogen also Master Plan. increased by nearly 20% in 2001. So far the water quality deterioration was mostly confined to the Starling Inlet.

海 Marine Water Quality in in 2001 7 - 1

Chapter 7 Mirs Bay Water Control Zone

Dissolved Oxygen (DO) Dissolved Oxygen (DO) (bottom) (depth-averaged)

100% 100% WQO WQO MM1 MM1

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

100% 100% WQO WQO MM2 MM2

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

100% 100% WQO WQO MM3 MM3

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

100% 100% WQO WQO MM4 MM4

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

100% 100% WQO WQO MM5 MM5

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

100% WQO 100% WQO MM6 MM6

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

100% 100% WQO WQO MM7 MM7

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

Shenzhen N Dissolved Oxygen (DO) 1. Bottom 100% WQO MM2 MM3 WQO : 90% sample with bottom DO > 2 mg/L MM1 MM4 % sample with bottom DO > 2 mg/L 0% MM7 MM5 2. Depth-averaged

WQO : 90% sample with depth-averaged 100% WQO DO > 4 mg/L MM6 % sample with depth-averaged 0% DO > 4 mg/L

Non-compliance Sha Tin

Figure 7.1 Level of compliance with key water quality objectives in the Mirs Bay WCZ

海 Marine Water Quality in Hong Kong in 2001 7 - 2 Chapter 7 Mirs Bay Water Control Zone

Dissolved Oxygen (DO) Dissolved Oxygen (DO) (bottom) (depth-averaged)

100% WQO 100% WQO MM8 MM8

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year 100% 100% WQO WQO MM13 MM13

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

100% WQO 100% WQO MM14 MM14

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year 100% 100% WQO WQO MM15 MM15

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

100% WQO 100% WQO MM16 MM16

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year 100% 100% WQO WQO MM17 MM17

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year 100% 100% WQO WQO MM19 MM19

0% 0% 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

MM17 Tai Po

N

Dissolved Oxygen (DO)

MM16 1. Bottom 100% WQO WQO : 90% sample with bottom DO > 2 mg/L Sai Kung % sample with bottom DO > 2 mg/L 0% 2. Depth-averaged

MM15 WQO : 90% sample with depth-averaged 100% WQO DO > 4 mg/L

% sample with depth-averaged 0% DO > 4 mg/L

MM19 MM14 Non-compliance No data ; monitoring has not started

MM8

MM13

Figure 7.1 Level of compliance with key water quality objectives in the Mirs Bay WCZ (continued) 海 Marine Water Quality in Hong Kong in 2001 7 - 3 Chapter 7 Mirs Bay Water Control Zone

Total Inorganic Nitrogen (TIN) Unionised Ammonia (NH3-N) (depth-averaged) (depth-averaged) TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM1 MM1 0.1

0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM2 0.1 MM2 0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM3 0.1 MM3

0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM4 0.1 MM4 0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L)

0.021 WQO 0.3 WQO 0.2 MM5 0.1 MM5

0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM6 MM6 0.1 0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM7 0.1 MM7 0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

Total Inorganic Nitrogen (TIN) N Shenzhen TIN (mg/L) WQO : annual mean for depth-averaged TIN < 0.3 mg/L 0.3 WQO MM2 MM3 MM1 MM4 annual mean for depth-averaged TIN 0

MM7 MM5 Unionised Ammonia (NH3-N) NH -N (mg/L) WQO : annual mean for depth-averaged 3 WQO NH3-N < 0.021 mg/L 0.021 MM6 annual mean for depth-averaged NH3-N 0

Sha Tin Non-compliance

Figure 7.1 Level of compliance with key water quality objectives in the Mirs Bay WCZ (continued) 海 Marine Water Quality in Hong Kong in 2001 7 - 4 Chapter 7 Mirs Bay Water Control Zone

Total Inorganic Nitrogen (TIN) Unionised Ammonia (NH3-N) (depth-averaged) (depth-averaged) TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM8 0.1 MM8 0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM13 0.1 MM13

0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM14 0.1 MM14 0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM15 0.1 MM15

0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM16 0.1 MM16 0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM17 0.1 MM17 0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year TIN (mg/L) NH3-N (mg/L) 0.021 WQO 0.3 WQO 0.2 MM19 0.1 MM19

0 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

MM17 Tai Po

N

Total Inorganic Nitrogen (TIN)

TIN (mg/L) MM16 WQO : annual mean for depth-averaged TIN < 0.3 mg/L 0.3 WQO Sai Kung annual mean for depth-averaged TIN 0

Unionised Ammonia (NH3-N) MM15 NH -N (mg/L) WQO : annual mean for depth-averaged 3 WQO NH3-N < 0.021 mg/L 0.021

annual mean for depth-averaged NH3-N 0

MM19 MM14 Non-compliance No data ; monitoring has not started

MM8

MM13

Figure 7.1 Level of compliance with key water quality objectives in the Mirs Bay WCZ (continued) 海 Marine Water Quality in Hong Kong in 2001 7 - 5 Chapter 7 Mirs Bay Water Control Zone

E. coli (annual geometric mean)

5 5

MM2 MM7 (cfu/100mL) 0 (cfu/100mL) 0 92 93 94 95 96 97 98 99 00 01 92 93 94 95 96 97 98 99 00 01 Year Year

5

MM6

(cfu/100mL) 0 92 93 94 95 96 97 98 99 00 01 Year

Shenzhen N

E. coli MM2

MM7 WQO : Annual geometric mean E. coli < 610 cfu/100mL

Tai Po 610 WQO annual geometric mean E. coli (cfu/100mL) MM6 0

Non-compliance Sai Kung

Secondary contact recreation subzone

Figure 7.1 Level of compliance with key water quality objectives in the Mirs Bay WCZ (continued) 海 Marine Water Quality in Hong Kong in 2001 7 - 6 Chapter 7 Mirs Bay Water Control Zone

Table 7.1 Results of the Seasonal Kendall Test for trends in water quality parameters measured in the Mirs Bay Water Control Zone, 1986 - 2001 Monitoring Station MM1 MM2 MM3 MM4 MM5 MM6 MM7 MM8 Monitoring Period 1991 1991 1991 1991 1991 1991 1991 1991 I I I I I I I I 2001 2001 2001 2001 2001 2001 2001 2001 Parameter Water Depth Temperature (°C) Surface ------Middle ------Bottom - - - ì ì - - - Average ì ------Salinity Surface î - - - - - î - Middle î î - - - î - - Bottom î î - î - - - - Average î - - - î - î - Dissolved Oxygen (mg/L) Surface î î î î î î î - Middle ------Bottom ------Average î ------Dissolved Oxygen (%) Surface î î - î - - - - Middle ------Bottom ------Average ------pH Surface ------Middle ------Bottom ------Average ------Secchi disc depth (m) - î ------Turbidity (NTU) Surface ì ì ì ì ì ì ì ì Middle ì ì ì ì ì ì ì - Bottom ì ì ì ì ì ì ì ì Average ì ì ì ì ì ì ì ì Suspended Solids (mg/L) Surface ------Middle ------Bottom ------Average ------Total volatile solids (mg/L) Surface ------Middle ------Bottom ------Average - - - î - - - - 5-day Biochemical Oxygen Demand (mg/L) Surface - - - - - î - - Middle - - - - - î - - Bottom - - î - - - - - Average - - - - - î - - Ammonia nitrogen (mg/L) Surface ------Middle ------Bottom ------Average ------Nitrite nitrogen (mg/L) Surface ------Middle ------Bottom ------Average ------Nitrate nitrogen (mg/L) Surface ------Middle ------Bottom ------Average ------Total inorganic nitrogen (mg/L) Surface ------î - Middle ------Bottom ------Average ------Total Kjeldahl nitrogen (mg/L) Surface ------Middle ------Bottom ------î Average ------î Total nitrogen (mg/L) Surface ------Middle ------Bottom ------î Average ------Orthophosphate phosphorus (mg/L) Surface î - - - - î - - Middle ------Bottom ------Average ------Total phosphorus (mg/L) Surface î î î î î î î î Middle î î î î î î î î Bottom - î î î î î î î Average î î î î î î î î Silica (mg/L) Surface - î - - - - î - Middle î î - - - - î - Bottom ------Average - î ------Chlorophyll-a (µg/L) Surface ì ì ì ì ì ì ì ì Middle ì ì ì ì ì ì ì ì Bottom - ì ì ì ì ì ì ì Average ì ì ì ì ì ì ì ì E.coli (cfu/100mL) Surface ------Middle ------Bottom ì ------Average ------Faecal coliforms (cfu/100ml) Surface ì ì ------Middle ì ì ------Bottom ì ------Average ì ì ------

Note 1. Results of the Seasonal Kendall Test shown are statistically significant at p < 0.05 2. – indicates no significant trend is detected 3. ì represents a significant increase over time 4. î represents a significant decrease over time 5. Test applied to past 16 years’ data from each monitoring station unless stated otherwise

海 Marine Water Quality in Hong Kong in 2001 7 - 7 Chapter 7 Mirs Bay Water Control Zone

Table 7.1 Results of the Seasonal Kendall Test for trends in water quality parameters measured in (continued) the Mirs Bay Water Control Zone, 1986 - 2001 Monitoring Station MM13 MM14 MM15 MM16 MM17 Monitoring Period 1991 1994 1994 1994 1986 I I I I I 2001 2001 2001 2001 2001 Parameter Water Depth Temperature (°C) Surface - - - - - Middle - - - - ì Bottom - - - - - Average - - - - - Salinity Surface - - - - - Middle - - - - - Bottom - - î - - Average - - - - - Dissolved Oxygen (mg/L) Surface - - - - - Middle - - - - - Bottom - - - - - Average - - - - - Dissolved Oxygen (%) Surface - - - - - Middle - - - - - Bottom - - - - ì Average - - - - ì pH Surface - - - - - Middle - - - - - Bottom - - - - - Average - - - - - Secchi disc depth (m) - - - - - Turbidity (NTU) Surface - ì ì ì ì Middle ì ì ì ì ì Bottom ì - ì ì ì Average ì ì ì ì ì Suspended Solids (mg/L) Surface - - - - î Middle - - - - î Bottom - - - - - Average - - - - î Total volatile solids (mg/L) Surface - - - - î Middle - - - - î Bottom - - - - î Average - - - - î 5-day Biochemical Oxygen Demand (mg/L) Surface - - - - - Middle - - - - - Bottom - - - - - Average - - - - - Ammonia nitrogen (mg/L) Surface - - - - - Middle - - - - - Bottom - - - - - Average - - - - - Nitrite nitrogen (mg/L) Surface - - - - - Middle - - - - - Bottom - - - - - Average - - - - - Nitrate nitrogen (mg/L) Surface - - - - - Middle - - - - - Bottom - - - - - Average - - ì ì - Total inorganic nitrogen (mg/L) Surface - - - - - Middle - - - - - Bottom - - - - - Average - - - - - Total Kjeldahl nitrogen (mg/L) Surface - - - - - Middle î - - - - Bottom - - - - - Average - - - - - Total nitrogen (mg/L) Surface - - - - - Middle - - - - - Bottom - - - - - Average - - - - - Orthophosphate phosphorus (mg/L) Surface - - - - - Middle - - - - ì Bottom - î - - - Average - - - - ì Total phosphorus (mg/L) Surface î î î î - Middle î î î î - Bottom î î î î - Average î î î î - Silica (mg/L) Surface - - - - - Middle - - - - - Bottom - - - - - Average - - - - - Chlorophyll-a (µg/L) Surface ì - - - - Middle ì - - ì - Bottom ì - - - - Average ì ì - - - E.coli (cfu/100mL) Surface - - - - - Middle - - - - - Bottom - - - - - Average - - - - - Faecal coliforms (cfu/100ml) Surface - - - - - Middle - - - - - Bottom - - - - - Average - - - - - Note 1. Results of the Seasonal Kendall Test shown are statistically significant at p < 0.05 2. – indicates no significant trend is detected 3. ì represents a significant increase over time 4. î represents a significant decrease over time 5. Test applied to past 16 years’ data from each monitoring station unless stated otherwise 6. MM19 has one year’s data only, which is insufficient to perform seasonal Kendall Test

海 Marine Water Quality in Hong Kong in 2001 7 - 8 Chapter 7 Mirs Bay Water Control Zone

Increasing trend in depth-averaged Chlorophyll-a at MM7, 1991 - 2001

10 ----- Kendall-Theil Robust Line : CHY = -0.2771 + (0.0233) Month

8 g/L) µ ( 6 a -

4 Chlorophyll

2

0 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Decreasing trend in depth-averaged Total Phosphorus at MM7, 1991 - 2001

0.3 ----- Kendall-Theil Robust Line : TP = 0.0651 + (-0.0003) Month

0.25

(mg/L) 0.2

0.15

0.1 Total Phosphorus 0.05

0 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Decreasing trend in depth-averaged Total Phosphorus at MM13, 1991 - 2001

0.3

----- Kendall-Theil Robust Line : TP = 0.0633 + (-0.0004) Month 0.25

0.2 (mg/L)

0.15

0.1 Total Phosphorus 0.05

0 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Year

Figure 7.2 Marine water quality trends in the Mirs Bay WCZ (based on the Seasonal Kendall Test significant at p<0.05)

海 Marine Water Quality in Hong Kong in 2001 7 - 9