Science of the Total Environment 433 (2012) 427–433

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Science of the Total Environment

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Estimation of submarine groundwater discharge and associated nutrient fluxes in ,

Chun Ming Lee a, Jiu Jimmy Jiao a,⁎, Xin Luo a, Willard S. Moore b a Department of Earth Sciences, The University of Hong Kong, Hong Kong, China b Department of Earth and Ocean Sciences, University of South Carolina, Columbia, SC 29208, USA

article info abstract

Article history: Tolo Harbour, located in the northeastern part of Hong Kong's , China, has a high frequency of algal Received 24 April 2012 blooms and red tides. An attempt was made to first quantify the submarine groundwater discharge (SGD) into Received in revised form 20 June 2012 Tolo Harbour using 226Ra, and then to estimate the nutrient fluxes into the Harbour by this pathway. The total Accepted 21 June 2012 − SGD was estimated to be 8.28×106 m3 d 1, while the fresh submarine groundwater discharge (FSGD) was esti- Available online 20 July 2012 mated to be 2.31×105 m3 d−1. This showed that a large amount of SGD was contributed by recirculated seawater rather than fresh groundwater in the Harbour. Using the SGD and groundwater nutrient information around Tolo Keywords: 6 −1 4 −1 Submarine groundwater discharge Harbour, the nutrient loading through SGD was estimated to be 1.1×10 mol d for DIN, 1.4×10 mol d for 3−– 6 −1 – fi Nutrients PO4 Pand1.4×10 mol d for SiO2 Si, which was much more signi cant than its counterpart through the Radium isotope river discharge. Despite uncertainties in the estimation, the nutrient loading to Tolo Harbour by SGD is clearly sig- Groundwater nificant. Thus, the current efforts for management of red tides in Tolo Harbour have to be reviewed and control of Estuary groundwater contamination is obviously required. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Tolo Harbour in Hong Kong has experienced frequent algal blooms in the last 30 years. The government implemented the Tolo Harbour Submarine groundwater discharge (SGD) is now widely recognized Action Plan (THAP) in 1987 to reduce the nutrient loading and to im- as an important pathway for material transport to the marine environ- prove the water quality of the Harbour. A series of actions was taken ment (Kim and Swarzenski, 2010; Moore, 2010; Mulligan and Charette, under THAP with the aid of the Tolo Harbour Effluent Export Scheme 2009). SGD includes both fresh groundwater and recirculated seawater. in 1995; the scheme diverted the treated effluent to the Victoria Harbour Recirculated seawater may have a different composition compared with rather than to the Tolo Harbour. Upon the completion of THAP, 83% of the original seawater because material exchange may occur during the the biochemical oxygen demand (BOD) and 82% of the total nitrogen recirculation of seawater in the aquifer. The contribution of SGD to (TN) load from point sources have been eliminated (Xu et al., 2004a); coastal waters has been largely neglected because it was generally be- however, Lie et al. (2011) showed that the total phytoplankton density lieved that the flux was very small compared to that from rivers. How- in different locations of Tolo Harbour from 1988 to 2008 had increased ever, a number of studies have shown that SGD flux is comparable to, or from ~4000 to ~25,000 cell mL−1. The reduction of point source nutri- even higher than, river flux in some places (Beck et al., 2007; Hwang et ent loading did not correlate well with the fluctuation of red tide inci- al., 2010; Swarzenski et al., 2007). Moreover, nutrients in groundwater dents and phytoplankton density in the Harbour. Hu et al. (2001) are typically higher than in riverine water, implying that even a small suggested that contaminated marine sediment could contribute to the flux of groundwater may contribute a large proportion of nutrients to nutrient loading to the Harbour and estimated the maximum release 3− coastal environments (Lee et al., 2009; Santos et al., 2010; Street et al., rate of orthophosphate phosphorus (PO4 –P) and ammonia nitrogen −2 −1 −2 −1 2008); and estimation of nutrient fluxes through SGD into other bays (NH3–N) to be 484 μmol m d and 1471 μmol m d ,respec- was done previously (Kim et al., 2011; Su et al., 2011; Swarzenski et tively. Furthermore, Tse and Jiao (2008) demonstrated that the nutrient al., 2007). The large nutrient fluxes through SGD may possibly control loadings through SGD in the Plover Cove, which is one of the coves in coastal N:P ratio and cause shifts of nutrient limitation of phytoplankton Tolo Harbour, were 1241–2599 mol d−1 for total inorganic nitrogen −1 3 −1 growth (Slomp and Van Cappellen, 2004). (TIN), 27.8–58.2 mol d for PO4–Pand185.9–388.5 mol d for silica silicon (SiO2–Si). This paper reports an estimate of the SGD flux using the radium 226 fl ⁎ Corresponding author. Tel.: +852 28578246; fax: +852 25176912. isotope, Ra,fortheentireToloHarbour.Nutrient uxes into the E-mail address: [email protected] (J.J. Jiao). Harbour through the SGD pathway are estimated using nutrient

0048-9697/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2012.06.073 428 C.M. Lee et al. / Science of the Total Environment 433 (2012) 427–433 information from the groundwater samples collected around the Observatory (HKO), 2012a). Catchwater, which is a concrete channel Harbour. running along the hillside, is built in Hong Kong to collect surface runoff, which is then directed to reservoirs for water storage. 2. Methodology There are a few notable physical characteristics of the Tolo Harbour: (1) the bottlenecked coastline configuration, (2) prevailing easterly 2.1. Study area winds directed toward the Harbour, (3) estimated water residence times in the Harbour ranging from 14.4 to 38 days (Choi and Lee, Tolo Harbour is located in the northeastern part of Hong Kong, 2004), averaging 28 days (Lee and Arega, 1999) and (4) average tidal which is a semi-closed bay connected to Mirs Bay with a narrow out- current velocities of 0.04–0.08 m s−1 and 0.08 m s−1 in the Harbour let called . It has a surface area of 50 km2 with a large and Channel, respectively (Xu et al., 2004b). With all these natural fac- catchment area of 160 km2 and a shoreline of 82 km. The catchment tors, pollutants cannot be effectively removed and Tolo Harbour is con- is bound by a series of mountain blocks including Wang Leng sidered to be naturally eutrophic. (311 m), Wong Leng (639 m), Tai Mo Shan (957 m), Beacon Hill Semi-diurnal tides are dominant in Hong Kong. There are two high (436 m), Tate's Cairn (577 m), Pyramid Hill (536 m), Kai Kung Shan tides and two low tides on most of the days in a month. Thus, the har- (399 m) and Shek Uk Shan (481 m) (Fig. 1.1). bour circulation is predominantly driven by tidal flushing and the The water depth of the Harbour varies from 5 to 15 m, while along Harbour can be classified as a vertically fully mixed estuary (Chau, the Channel the depth varies from 10 to 21 m. At the south of Pak Sha 2007). Tau, is the deepest point (25 m) among the Harbour and Channel; here the Holocene alluvium is exposed with the absence or thinning of marine sediments. A reservoir is located at the north of Tolo Channel, 2.2. Sampling and chemical analysis which stores mainly river water imported from Mainland China and is believed to have limited hydraulic connection with the seawater in Samplings of seawater (1 m below surface) and river water were the Harbour. The mean sea level is 1.45 m above the Chart Datum conducted on 26–27 May 2011 and 28 June 2011 respectively, while (mCD), with an average tidal range of 1.06 m in 2010 (Hong Kong groundwater was sampled from April 2011 to August 2011. Different Observatory (HKO), 2011a). There are six rivers entering Tolo Harbour volumes of seawater (~50 L), river water (10 L) and groundwater including River, Shan Liu Stream, Shing Mun River, (3.8–10 L) were collected for Ra preconcentration. Filtration using River, Stream and Tung Tsz Stream, with a total annual dis- 1 μm filter was done before the preconcentration for seawater in charge rate of 4.50×107 m3 (Environmental Protection Department order to remove suspended particulates. The water samples were

(EPD), 2012). The mean annual rainfall is 2030 mm (Hong Kong passed through ~30 g of MnO2-fiber to preconcentrate Ra with a

Fig. 1. Map of the Tolo Harbour. The triangles represent the mountain peaks. The circles represent the groundwater sampling stations. C.M. Lee et al. / Science of the Total Environment 433 (2012) 427–433 429

flow rate below 1 L min−1 in order to ensure complete Ra adsorption harbour always has a higher 226Ra activity than an open sea due to on the fiber. MnO2-fibers were produced according to Moore (1976). the dilution effect when the bay water mixes with the open sea water. To estimate Ra release from suspended riverine particles, two The highest activities were observed near river mouths of the Shing samples of river water were collected separately in each river; re- Mun River (29.17 dpm 100 L−1), with Tai Po River agent grade sodium chloride (Sigma-Aldrich) was added to one of (21.74 dpm 100 L−1) and Shan Liu Stream (22.62 dpm 100 L−1). The the samples until the salinity was similar to the seawater salinity. activity remained >20 dpm 100 L−1 near the Lam Tsuen River and This is because river water may have a large amount of suspended Shing Mun River. In the center of the Harbour and along Tolo Channel, particles with radium adsorbed on them and high salinity releases the activity ranged from 11.66 to 19.93 dpm 100 L−1, while the open the particle-bound radium, which then dissolves in the water. sea activity ranged from 4.97 to 10.47 dpm 100 L−1. The activity de- 226Ra was determined using radon-in-air monitor as proposed by creased from the Harbour to open sea, which signified the enrichment Kim et al. (2001).Thisisanindirectmeasurementof226Ra based on sec- of radium inside the Harbour. ular equilibrium between 226Ra and 222Rn. The determination of 226Ra However, some anomalies were found such as the depressions at the in this study is based on Kim et al. (2001) with some modifications in mouth of Tung Tsz Stream (9.59–9.86 dpm 100 L−1), at the Three order to improve the results. The modifications are (1) longer fiber in- Fathom Cove (1.58–2.02 dpm 100 L−1) and at the mouth of the Plover cubation time (>19 d) for reaching secular equilibrium between Cove (7.70 dpm 100 L−1). It is unusual for seawater with values less 226Ra and 222Rn, (2) longer time for purging the RAD7 system prior to than 7 dpm 100 L−1, the anomalies may be due to the biological removal analysis (~24 h) to reduce the background noise and (3) longer mea- of 226Ra (Shannon and Cherry, 1971) since phytoplankton density is al- surement time (18 h) to reduce the errors. The analytical uncertainties ways high in the Harbour. The relationship between 226Ra and salinity of this study are 25% for background and 7% for fiber (n=55). was shown in Fig. 3. River and seawater samples formed a cluster at Groundwater from wells, spring and coast was collected for nutrient low and high salinities respectively with a similar range of 226Ra activi- 226 analysis. The nutrients including ammonia nitrogen (NH3–N), nitrate ties, but groundwater samples had a relatively large range of Ra activ- − − 226 nitrogen (NO3 –N), nitrite nitrogen (NO2 –N), phosphate phosphorus ities from fresh to saline samples. This showed a prominent Ra 3− (PO4 –P) and silica silicon (SiO2–Si) were analyzed by ammonia pho- enrichment in seawater by groundwater end-member rather than river tometer (HANNA HI93733), spectrophotometer (HACH DR2800) and end-member. flow injection system in the School of Biological Sciences at the Univer- sity of Hong Kong (Lachat Instruments, 1996, 2003). In addition, dis- 3.2. Radium budget solved inorganic nitrogen (DIN) was based on calculation from other nitrogenous parameters. Desorption of radium from seabed sediment (Gdes) occurs once the adsorbed radium reaches water with higher salinity, the radium 2.3. Radium budget tends to desorb and then readily dissolves in water. Regeneration of radium in the sediment is governed by (Moore, 1996), In a defined system (i.e. bay) with an assumed steady state, the  −λ gain of radium is equal to the loss (Moore et al., 2006). Generally ¼ − Rat ð Þ ARa ATh 1 e 3 speaking, enrichment of radium isotopes in coastal waters has to be supported by potential sources including (1) desorption from seabed 226 230 λ sediment, (2) diffusion from seabed sediment, (3) erosion of terres- where ARa is the Ra activity, ATh is the Th activity, Ra is the 226 trial sediment, (4) river supply and (5) SGD (Moore, 1996). Radium decay constant of Ra and t is time. Assuming t =280 d, which loss is due to (1) tidal mixing between the bay water and open sea is 10 times that of the average residence time of Tolo Harbour, 226 water and (2) decay of the radium isotope (Moore et al., 2006). The only 0.03% of the desorbed Ra can be regenerated. This implies 226 mass balance equation is: that the generation of Ra from the desorption of seabed sedi- ment is a very slow process and it is insignificant in the 226Ra en- þ þ þ þ ¼ þ ð Þ Gdes Gdiff Gero Griv GSGD Lmix Ldec 1 richment to Tolo Harbour; this is due to the difference between the half life of 226Ra and a relatively short residence time of the where Gdes is the desorption of radium from the seabed sediment, Gdiff Harbour. Therefore this term can be neglected in the radium balancing is the diffusion of radium from the seabed sediment, Gero is the radi- equation. um gain from the erosion of the terrestrial sediment, Griv is the radi- Erosion of terrestrial sediment (Gero) is regarded as one of the poten- 226 fi um from the river, GSGD is the radium from the SGD, Lmix is the tial sources of Ra enrichment to coastal water, but it is only signi - mixing loss of radium due to tidal exchange between the water of cant when the weather is stormy (Moore, 1996). There was no rainfall the bay and open sea and Ldec is the decay of the radium. Detailed ex- during the days of seawater sampling. Therefore, it is believed that planations for each term are given in Section 3.2. most of the radium enrichment to the Harbour are not due to erosion

The term GSGD can be calculated from Eq. (1) if all other terms are of terrestrial sediment, it should be attributed to other sources. In addi- 226 known. If the radium concentration is known, the volumetric flow tion, the decay term (Ldec) can be neglected in this study because Ra rate SGD can be estimated as: has a long half-life of 1600 years which is much longer than the residence time in Tolo Harbour (i.e. 28 days). A similar approach was used by Moore et al. (2006). ¼ GSGD ð Þ Q SGD 2 fi Ragw After neglecting the above three terms, Eq. (1) can be modi ed to,

G ¼ L −G −G ð4Þ where QSGD is the SGD flux and Ragw is the radium activity in SGD mix diff riv groundwater. The mass balance of radium in Tolo Harbour is only due to the

3. Result and discussion river, SGD and tidal mixing loss. GSGD is equal to the difference be- tween Lmix and the sum of Gdiff and Griv. 3.1. Spatial distribution of 226Ra

3.2.1. Tidal mixing loss of radium (Lmix) A total of 28 seawater samples were measured for 226Ra activity, Radium in the harbour water is lost from mixing (tidal exchange) these ranged from 1.58 to 29.17 dpm 100 L−1 (Fig. 2). In general, a with the open sea water. This loss term is based on the return flow 430 C.M. Lee et al. / Science of the Total Environment 433 (2012) 427–433

Fig. 2. Spatial distribution of 226Ra in the Tolo Harbour.

fi factor and the tidal prism of a de ned tidal area (Moore, pers. 3.2.2. Diffusion of radium from seabed sediment (Gdiff) comm.), Diffusion of material under steady state is governed by Fick's first law of diffusion. Schulz (2006) described the calculation of diffusive P flux in the sediment by the following equation, L ¼ ½226Ra −226Ra þ bð226Ra −226Ra Þ ð5Þ mix T bay sea sea bay ∂C J ¼ −ϕ D ð7Þ sed sed ∂ where P is the tidal prism, T is the tidal period, b is the return flow fac- x tor, 226Ra is the average 226Ra activity in the Harbour and Channel bay where J is the diffusive flux, ϕ is the porosity, D is the diffusivity and 226Ra is the average 226Ra activity in the open sea. The follow- sed sed sea and ∂C/∂x is the concentration gradient. The diffusivity for the diffu- ing equation is used to calculate the tidal prism (Barber, 2003), sive flux in the sediment is based on the diffusivity in free solution of seawater but modified by considering tortuosity of the sediments ¼ ð Þ P H A 6 (Schulz, 2006), where H is the tidal range and A is the surface area of the Harbour ¼ Dsw ð Þ Dsed 8 and Channel. In Tolo Harbour, H =0.73 m in May 2011 (Hong θ2 Kong Observatory (HKO), 2011b)andA=50 km2 yielding a tidal 7 3 where Dsw is the diffusivity in free solution of seawater and θ is the prism of 3.65×10 m . The return flow factor is equal to (1− R0) where R is the tidal exchange ratio (Sheldon and Alber, 2006). tortuosity. Tortuosity is the degree of deviation around particles and 0 fi Lee and Arega (1999) calculated the tidal exchange ratio of Tolo is estimated by the porosity and measurement of speci c electrical Harbour at 0.7, so the return flow factor of the Harbour is 0.3. The resistivity in pore water and sediment. However, Boudreau (1997) 226 226 showed another accurate calculation of tortuosity based on porosity tidal period is 0.517 day. Rabay and Rasea are estimated to be −1 only, 14.87 and 7.05 dpm 100 L , respectively. Finally, Lmix is calculated to be 3.86×109 dpm d−1.  θ2 ¼ 1− ln ϕ2 ð9Þ

The porosity (ϕ) in the Plover Cove area is 0.41 (Tse and Jiao, 2+ −10 2 −1 2008) and Dsw of Ra at 25 °C is found to be 8.28×10 m s (Boudreau, 1997). ∂C/∂x is based on the coastal groundwater samples in Stations 1, 3 −1 −1 and 11, which yields −0.45 dpm 100 L cm (Fig. 4). Then, Jsed is −2 −1 calculated to be 0.00474 dpm m d . Gdiff is the multiplication of 2 Jsed and the bottom area of the Harbour is assumed to be 50 km . 5 −1 Therefore, Gdiff is calculated to be 2.37×10 dpm d .

3.2.3. River supply of radium (Griv) Enrichment of 226Ra from rivers is composed by two parts which are the water-soluble portion and particle-bound portion. Li et al. (1977) discovered that the desorption of 226Ra from particles oc- curred once the particles encountered brackish water. In this study, Fig. 3. A plot of 226Ra versus salinity for all samples. The solid line represents the linear relationship of 226Ra and salinity for groundwater. The dashed line represents the two samples were taken from the freshwater section of each river, mixing line between river and seawater samples. salt was added to one of the samples until salinity was similar to C.M. Lee et al. / Science of the Total Environment 433 (2012) 427–433 431

Table 2 226Ra activities of different groundwater samples around the Tolo Harbour.

Station Date Depth (cm) Salinity (ppt) 226Ra (dpm 100 L−1)

1 04/08/2011 45.0 11.3 55.00 1 04/08/2011 85.0 5.4 69.47 2 04/08/2011 55.0 33.9 60.79 3 28/06/2011 30.0 19.7 17.60 3 09/07/2011 90.0 18.8 72.16 3 04/08/2011 90.0 29.6 69.52 3 04/08/2011 185.0 28.0 119.84 6 28/06/2011 n/aa 0.04 22.00 7 06/07/2011 57.5 9.4 21.12 8 09/07/2011 82.5 25.2 73.92 9 06/07/2011 31.5 0.3 8.14 10 09/07/2011 52.0 0.6 3.74 11 26/07/2011 47.0 33.2 10.42 11 26/07/2011 140.0 34.4 40.53 Average 17.8 46.02 Fig. 4. 226Ra activity profiles in groundwater around the Tolo Harbour. a Not available. the water in Tolo Harbour (i.e. >32 ppt) to trigger the desorption of 226 226 Ra in the suspended particles. The river supply of Ra is calculat- where Pr is the precipitation, QFSGD is the FSGD flux, Qc is the volumetric ed by, flux of catchwater, E is the evapotranspiration and ΔS is the change in 8 3 −1 storage of groundwater. Pr is 3.52×10 m yr which is based on the − ¼ ðÞþð ÞðÞ 11-year average data from HKO (2012a). Q is 4.50×107 m3 yr 1 Griv Q r Rar Q ss Rap 10 r which is based on EPD (2012). Qc is the data obtained from the Water Supplies Department, the 10-year average catchwater flux from 1996 to where Qr is the volumetric flux of river, Rar is the water-soluble 7 3 −1 226 2005 is 2.12×10 m yr . E is the 30-year average data obtained from Ra activity, Qss is the supply rate of total suspended solids and 8 3 −1 226 Hong Kong Observatory (HKO) (2012b), which is 1.70×10 m yr . Rap is the particle-bound Ra activity. Qr and Qss were obtained ΔS can be assumed to be 0 if the estimation is under a long-term period from EPD (2012) as monthly averages over 11 years from 1999 to and groundwater in Hong Kong, which is not a major water resource, is 2011. Table 1 shows the result of river supply of radium which is 7 −1 only used by a small amount of localized villagers. Finally, QFSGD is esti- 5.12×10 dpm d . mated to be 8.41×107 m3 yr−1 or 2.31×105 m3 d−1. Therefore, al- though FSGD contributes only about 0.03% of SGD, it is about 2 times 3.2.4. SGD flux (QSGD) 9 −1 greater than the total river discharge. Based on Eq. (4), GSGD was calculated as 3.81 ×10 dpm d by fl knowing Lmix, Gdiff and Griv.The ux of SGD can be calculated by di- 3.4. SGD-derived nutrient fluxes viding the 226Ra activity in groundwater (Eq. (2)). All groundwater samples were collected around 1 to 100 m away from the coast of Rivers have long been regarded as the most serious sources of pol- Tolo Harbour except one spring water sample which is around 226 lution into Tolo Harbour, discharging large quantities of sewage and 1 km away from the coastline (Table 2). The average Ra activity agricultural animal waste (Hodgkiss and Chan, 1986; Holmes, 1988; −1 fl in groundwater (Ragw) is 46.02 dpm 100 L yielding SGD ux of Hodgkiss and Yim, 1995). On the other hand, seabed sediment was 6 3 −1 −1 fl 8.28×10 m d or 16.6 cm d (per area). The estimated SGD ux also considered to be an important source of nutrients (Hu et al., is about 67 times that of the total river discharge, however, SGD in- 2001; Chau, 2002; Xu et al., 2004b). The contribution of nutrients to cludes both fresh water and recirculated seawater. the Harbour through SGD was entirely ignored. However, studies elsewhere show that, regardless of the origin, SGD can be quantita- 3.3. Water balance tively more important than these two sources because nutrients are expected to be enriched in brackish groundwater (Moore, 1999). SGD includes fresh submarine groundwater discharge (FSGD) and Nutrient loading from land to sea through SGD is regarded as one of saline groundwater discharge. Water balance is used to reveal FSGD the most important pathways as SGD usually has a much larger flux than in Tolo Harbour and hence the proportion of FSGD and saline ground- surface water. Different types of chemical reactions (e.g. denitrification, water discharge in Tolo Harbour can be known. In the catchment be- P precipitation, dissolution of minerals, desorption) may occur during hind Tolo Harbour, the water balance can be represented by the the flow of groundwater and this could enhance or diminish the nutrient following equation, fluxes through SGD. The nutrient loadings through SGD were estimated by multiplying SGD flux with the average nutrient concentrations in ¼ þ þ þ þ Δ ð Þ Pr Q r Q FSGD Q c E S 11 groundwater, assuming no denitrification or P precipitation between

Table 1 River supply of 226Ra.

Rivers Qr Rar Qss Rap Griv (×104 m3 d−1) (dpm 100 L−1) (×105 gd−1) (dpm g−1) (×106 dpm d−1)

Lam Tsuen River 2.02 12.10 2.87 8.36 3.99 Shan Liu Stream 0.64 3.74 0.47 0.19 0.16 Shing Mun River 3.74 33.00 5.69 8.51 12.4 Tai Po River 2.20 25.08 1.61 151.80 26.7 Tai Po Kau Stream 0.49 4.62 1.04 11.00 1.26 Tung Tsz Stream 3.24 24.20 4.92 4.50 6.71 Total 12.3 16.6 51.2 432 C.M. Lee et al. / Science of the Total Environment 433 (2012) 427–433

Table 3 Nutrient concentrations of different groundwater samples around the Tolo Harbour.

− − 3− Station Date Depth Salinity NH3–N NO3 –N NO2 –N PO4 –P SiO2–Si DIN (cm) (ppt) (μM) (μM) (μM) (μM) (μM) (μM)

1 04/08/2011 85.0 5.4 27.7 42.8 42.837 0.827 401.5 113.39 3 28/06/2011 30.0 19.7 33.3 71.4 0.643 1.575 78.8 105.30 3 09/07/2011 90.0 18.8 16.6 21.4 0.214 0.154 108.1 38.26 3 04/08/2011 90.0 29.6 27.7 50.0 49.976 0.325 119.9 127.67 3 04/08/2011 185.0 28.0 27.7 35.7 35.697 1.331 170.8 99.11 4 14/04/2011 n/aa n/aa 27.7 185.6 0.428 7.816 167.3 213.77 5 14/04/2011 n/aa n/aa 16.6 114.2 0.785 1.676 222.5 131.65 5 24/05/2011 n/aa n/aa 44.3 199.9 1.571 7.429 36.8 245.82 6 24/05/2011 n/aa n/aa 27.7 35.7 0.286 0.233 364.5 63.70 6 28/06/2011 n/aa n/aa 33.3 85.7 0.286 0.278 292.1 119.22 7 06/07/2011 57.5 9.4 27.7 285.6 0.357 0.585 102.8 313.65 8 09/07/2011 82.5 25.2 44.3 7.1 0.071 1.345 101.6 51.56 9 06/07/2011 31.5 0.3 72.1 28.6 0.214 0.252 69.0 100.84 10 09/07/2011 52.0 0.6 66.5 64.3 10.138 0.794 155.6 140.92 11 26/07/2011 47.0 33.2 49.9 14.3 14.279 0.042 160.9 78.45 11 26/07/2011 140.0 34.4 27.7 28.6 28.558 1.843 128.4 84.83 Average 35.7 79.4 11.6 1.66 167.5 126.8

a Not available. the sampling site and discharge into the Harbour. A total of 16 ground- practices for the management of algal blooms in Hong Kong, in which water samples from Stations 1 to 11 except Station 2, including coastal, nutrient loading through SGD was ignored, have to be reviewed and spring and well groundwater, were collected for the nutrient determina- control of groundwater contamination is obviously required. tion (Table 3). The average concentrations and SGD-derived fluxes of different nutrients were shown in Table 4. Comparing the nutrient Acknowledgments fluxes from SGD and rivers, SGD-derived nutrient fluxes are at least an fi order of magnitude greater than the river-derived ones. A signi cant This study was supported by a grant from the Research Grants amount of nutrient input through SGD to Tolo Harbour is notable. It Council of Hong Kong (HKU 7028/06P). The authors thank the staff has been shown that nutrient inputs through SGD could contribute to of the School of Biological Sciences, HKU who helped with the nutri- algal bloom outbreaks (Hu et al., 2006; Lee and Kim, 2007), which im- ents analysis. The authors also appreciate Ms. Chan, Yi Kei who fi plies that SGD is one of the signi cant nutrient sources utilized by assisted in the boat trips in the Tolo Harbour. phytoplankton.

4. Conclusion References

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