Hydrogeology Journal (2015) 23: 277–286 DOI 10.1007/s10040-014-1201-6
Reconstructed chloride concentration profiles below the seabed in Hong Kong (China) and their implications for offshore groundwater resources Jiu Jimmy Jiao & Lei Shi & Xingxing Kuang & Chun Ming Lee & Wyss W.-S. Yim & Shouye Yang Abstract Offshore hydrogeology has been much less studied compared to onshore hydrogeology. The marine Quaternary system in Hong Kong (China) consists of interlayers of aquitards and aquifers and was part of the Pearl River Delta when the sea level was low before the Holocene. Core samples from six offshore boreholes were collected to measure the chloride concentration in the system by adding deionized water. A method was proposed to convert the sediment chloride into that of the original pore water. A one-dimensional sedimentationtransport model was developed to simulate the historical conservative transport of the reconstructed pore-water chloride. The model integrates present knowledge of stratigraphy and the historical evolution of the geological system. The chloride concentration profiles show that the chloride decreases from an average of 13,800 mg/L in the first marine unit to an average of 5,620 mg/L in the first aquifer. At the bottom of one borehole, the concentration is only 1,420 mg/L. The numerical model shows that the vertical chloride distribution is due to diffusion-controlled downward migration of seawater. The second marine unit obstructs the downward migration, indicating its low permeability and good aquitard integrity. The relatively fresh or brackish water in deep aquifers protected by the Received: 5 June 2014 / Accepted: 15 October 2014 Published online: 12 November 2014 * Springer-Verlag Berlin Heidelberg 2014 Electronic supplementary material The online version of this article (doi:10.1007/s10040-014-1201-6) contains supplementary material, which is available to authorized users.
J. J. Jiao ()) : L. Shi : X. Kuang : C. M. Lee : W. W. Yim Department of Earth Sciences, The University of Hong Kong, Hong Kong, China e-mail:
[email protected] Tel.: (852) 2857 8246 L. Shi Shenzhen Water Resources Planning and Design Institute, Shenzhen, China S. Yang State Key Laboratory of Marine Geology, Tongji University, Shanghai, 200092, China
aquitard has the potential to be used as drinking water following some treatment, or at least as raw water with much cheaper desalinization compared with using seawater. The methodology and findings in this study are instructional for other coastal areas with similar geology and history in the South China Sea. Keywords Chloride . Sedimentation . Numerical modeling . Offshore groundwater . China
Introduction Traditionally, hydrogeologists have focused on onshore groundwater. Offshore hydrogeological studies are difficult because of the expensive field investigations and offshore drilling that are involved. It has been speculated that there must be fresh or relatively fresh groundwater in offshore aquifers in many parts of the world because usable water resources have been revealed by submarine springs and occasional boreholes that are a few to tens of kilometers away from the present coastline (Kohout 1966; Swarzenski et al. 2001; Clendenon 2009). The studies focusing on offshore groundwater, however, have started only in the last approximately 10 years (Kooi and Groen 2001; Person et al. 2003; Cohen et al. 2010; Post et al. 2013) and the study sites are all located in the coastal seas of the Atlantic Ocean, especially in the United States (Cohen et al. 2010) and European countries (Edmunds and Milne 2001; Lofi et al. 2013). Three conceptual models (Cohen et al. 2010) were summarized to explain the formation of the emplacement of freshwater below the current seabed. The fresh or relatively fresh water was formed during the Pleistocene sea-level low stands from aquifers that received paleo- or modern terrestrial water recharges, or sub-ice-sheet recharges (Pope and Gordon 1999; Kooi and Groen 2001; Person et al. 2003). In many cases, the presence of a near-offshore freshwater tongue off the coastline can be explained by the present-day terrestrial recharge to confined aquifers that outcrop on land (Kooi and Groen 2001). In this case, the groundwater below the seabed is just an offshore extension of the modern terrestrial groundwater system. In some other
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cases, the offshore groundwater may be paleogroundwater that was emplaced during the Pleistocene sea-level low stands that somehow escaped salinization during the Holocene sea-level rise. The groundwater was isolated from the modern recharge system and can be regarded as “fossil” groundwater. Paleo-groundwater is usually brackish. Contours of salinity or chloride are used to depict the freshwater distributions below the seabed. Because direct pore-water data in offshore formations are extremely rare, the distributions are largely based on sparse data and/or sparsely constrained process models (Cohen et al. 2010). Offshore hydrogeological studies along the AsiaPacific ocean, similar to the Atlantic ocean, are extremely limited, although freshwater has been found from submarine springs in coastal areas in China (Qu ∼1670), Japan (Marui 2003), and Korea (Kim et al. 2011). The first offshore hydrogeological survey aiming to identify usable submarine groundwater in China is the geophysical survey around the Shengsi Islands, Zhejiang Province (Wang 1994) which are ∼100 km away from the mouth of Yangtze River. The study suggested that an extensive freshwater body was located in the aquifers below the depth of 140–155 m. In Japan, the chloride concentrations and stable chlorine isotopic ratios were obtained from pore waters extracted from cores in a borehole approximately 350 m offshore near Kyushu (Tokunaga et al. 2011). The chloride concentrations were higher than 16,400 mg/L at depths shallower than 1.5 m below the seabed and gradually decreased downwards to less than 250 mg/L at depths lower than 7.7 m below the seabed. A one-dimensional (1D) diffusion-sedimentation model was applied to explain both the chloride and the stable chlorine isotopic ratios. It was concluded that the groundwater below the inter-tidal zone constitutes a part of the present-day active groundwater system, while that below the sea bottom moves extremely slowly. Hong Kong is located to the east of the Pearl River System, which flows into the South China Sea. Approximately 85 % of the land in Hong Kong is formed by hard volcanic and granitic rocks. There is not much sedimentary rock; therefore, there is not a regionally extensive aquifer. Hong Kong lacks water resources and has to import approximately 70 % of its drinking water from the East River, one of the three major tributaries of the Pearl River System located in Guangdong Province, China. The water arrives via a 83-km closed aqueduct. The spectacular industrial and population growth of cities along and near the river catchment has made them become both major polluters and competing consumers of the river water. To find another source of drinking water for Hong Kong, the Hong Kong Government has begun to examine desalination as a supply alternative. It is hypothesized that submarine groundwater below the seabed may be another possible source of water for Hong Kong. This report presents a preliminary study of offshore hydrogeology in Hong Kong. Although there is not much sedimentary rock onshore, there are thick Hydrogeology Journal (2015) 23: 277–286
sedimentary units below the seabed. A key step in this study is to measure and model the chloride concentration of the pore water in the offshore formations. On land, observation wells, or piezometers, can be installed in formations, and groundwater samples can be collected directly (Freeze and Cherry 1979; Jiao et al. 2010), but this is difficult in the offshore environment. If core samples are available and the water is well preserved, a centrifuge machine or a squeezer can be used to extract pore water (Robertson et al. 1996; Tokunaga et al. 2011). In many cases, boreholes were drilled for other purposes years prior, and the cores had become dry. Although other soil chemicals cannot represent the original cores below the seabed any more, the chloride concentration can still reflect reasonably well the chloride concentration in the original cores due to its conservative nature. Soil chloride can be estimated by adding deionized water to soil samples and then measuring the chloride content of the solution (Carter 1993). However, for a groundwater study, the chloride concentration in the original pore water of the sediments is needed. A simple method is proposed here to convert the chloride in the soil samples to chloride concentration in the original pore water in the sediments below the seabed. Cores from five boreholes drilled over different time periods in the offshore areas of Hong Kong were collected. The chloride was extracted from the cores to reconstruct the chloride profiles below the seabed using a simple method. A 1D numerical model is used to simulate the chloride profile from the cores of the bestkept hole with the least contamination. The implication of these profiles on possible offshore groundwater resource is discussed.
Background of the study and geology of the study area Brief summary of the onshore and offshore geology in Hong Kong Onshore geology and hydrogeology are well studied in Hong Kong (e.g. Fletcher 1997; Jiao et al. 2006). Mesozoic volcanic and plutonic rocks are the dominant rock types in Hong Kong, which account for about 85 % of rock outcrop on land. Locally colluvium overlies decomposed volcanic and granite rocks. Groundwater mainly exists in the colluvium and the fracture zones between the decomposed rock and the relatively fresh rock. Extensive studies of geology have provided detailed information on the Quaternary geological formations in the offshore areas of Hong Kong. Two geological models have been proposed to describe the offshore stratigraphy of the Quaternary sediments in Hong Kong. One is based mainly on sequence stratigraphic interpretations of seismic data (Fyfe et al. 2000) and the other is based on the recognition of five marine/terrestrial cycles in both continuous and discontinuous offshore boreholes of the Quaternary succession of Hong Kong (Yim 1994). In the present study, the latter offshore Quaternary geological DOI 10.1007/s10040-014-1201-6
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model is adopted (Table 1). According to the model, the marine unit is denoted M, and the terrestrial unit is denoted T. The fully developed Quaternary stratigraphy has five marine and five terrestrial units. The sequence from young to old units is as follows: M1, T1, M2, T2, …, M5, and T5. The upper two marine units (M1 and M2) were formed during the marine transgression in the Holocene and the last interglacial period, respectively. Between them is the first terrestrial unit T1, which was formed in the last glacial period, when the seabed was exposed to the land from ∼30 to ∼8 ky BP (Yim 1994). Unit T1 can be sandy fluvial deposits or clayed silt weathered from the in situ materials. Below M2 is the second terrestrial unit T2, which consists of sand and gravel and was formed in the second to last glacial period. Usually the first four units are well recognized due to extensive age data and are consistent with the onshore geology of the Pearl River Delta to the west of Hong Kong (Zong et al. 2009; Jiao et al. 2010), but the identification of the sedimentary units below T2 has great uncertainties because age data are scarce. M1 and M2 are aquitards, and T2 is an aquifer (Jiao et al. 2010). T1 can be sandy fluvial deposits or clayed silt weathered from the in situ materials during the last glacial period and becomes a local aquifer when it is sandy, also it is possible that T1 may be missing. In shallow seas, T2 is the basal aquifer, and granite and volcanic rocks underlay it; however, in deeper seas or paleo river channels, the stratigraphy can be very thick, and three more marine units and three more terrestrial units may exist (Yim 1994). The stratigraphy varies considerably in the seabed depending on the paleotopography. Borehole BH2 is approximately 45 m deep and dominated by fine materials, but borehole A5/2 is approximately 100 m deep and has sand and gravel layers up to 50 m thick (Fyfe et al. 2000) that form good aquifers (Fig. 1). There is no study on the offshore hydrogeology of these units, but the hydrogeology of the onshore extension of these units in the Pearl River Delta has been extensively studied (Jiao et al. 2010; Wang and Jiao 2012; Wang et al. 2012). The interlayered formations of the marine and terrestrial units were associated with the sea-level changes in the Quaternary (Yim 1994). The sea level around Hong Kong has fluctuated significantly in recent geological time, and the shallow seabed has been exposed as the ground surface a few times. At approximately 10,000 years BP, the sea level was at least 60 m below the current sea level, and the paleo-coastal line was most likely over 100 km southeast of the current coastline (Shackleton 1987). At that time, Hong Kong was part of an extended Pearl River flood plain. In the interglacial periods such as at the present time, the global sea level rose to its present level and higher, and marine sediments were deposited. It is possible that the aquifers that are now below the current seabed around Hong Kong were once exposed above the paleo sea level and may still preserve Hydrogeology Journal (2015) 23: 277–286
fresh groundwater or brackish water with salinity that is much lower than the current seawater.
Cores used in this study Cores from six offshore boreholes available to the authors were collected for this study. The holes were drilled by vibracoring, a method of rapidly retrieving continuous, relatively undisturbed core samples from sediments. The depth of seawater in the drilling site was less than 15 m below the principal datum (the Hong Kong reference datum is approximately 1.23 m below mean sea level). These holes were drilled by Quaternary geologists and civil engineers to understand the sedimentary geology and engineering properties of the seabed. All boreholes were located offshore around Lantau Island in Hong Kong (Fig. 1). Boreholes N20E14, N21E15 and N23E17 were drilled in the Yam O reclamation area in 2002 (Chui 2004). The drilling site used to be offshore but was later reclaimed. Borehole BH2 was drilled in West Lamma Channel, east of Cheung Chau Island in 2001. The cores from these four holes were stored at room temperature and air dried for many years before being used for this study. Boreholes HKUV11 and HKUV15 were drilled in 2007 and are about 1.4 and 0.6 km away from coast of the southwest of Lantau Island, respectively. The cores of these two holes were kept in capped PVC pipes and stored in a freezer, so pore water was relatively well preserved. However, the cores in HKUV11 were used for various sedimentological studies and left at room temperature for a few days before being taken for chloride studies. Borehole BH2 was the deepest among the six and has been extensively studied by Quaternary geologists (Yang et al. 2008).
Stratigraphy of the boreholes All the cores used here have been studied by Quaternary geologists and civil engineers for their depositional environment, sedimentological structure and engineering properties. These studies have provided important geological background information for this study. The cores in the Yam O reclamation area (Fig. 1) were presented in two geological cross sections A–A′ and B–B′ (Chui 2004), as shown in Fig. 2. The two marine units (M1 and M2) overlay the terrestrial alluvial unit T2 in this area. The terrestrial unit T1 formed in the last interglacial period does not exist, most likely due to weathering and erosion processes. Below unit T2, completely decomposed granite ended the Quaternary stratigraphy. The cross sections in Fig. 2 are typical in the shallow marine areas of Hong Kong. In the deeper sea or the sea areas with deep paleo river valleys, some of the units may be thicker (e.g., BH2 in Fig. 3), and units older than T2 may be found (Yim 1994). An extensive description of the stratigraphy of BH2 was given by Yang et al. (2008). This hole has five marine units and three terrestrial units. The Holocene unit M1 is mostly composed of soft, grey, clayey silt with rare shell fragments. The accelerator mass spectrometry carbon-14 dating results DOI 10.1007/s10040-014-1201-6
280 Table 1 Summary of Quaternary marine and terrestrial units identified on the continental shelf of Hong Konga Stratigraphic unit
Estimate age (ky BP)
Oxygen-isotope stage
Age
Estimated maximum thickness (m)
M1 T1 M2 T2 M3 T3 M4 T4 M5 T5
440
1 2 5 6 7 8 9 10 11 12
Holocene Last glacial Last interglacial Second last glacial Second last interglacial Third last glacial Third last interglacial Fourth last glacial Fourth last interglacial Fifth last glacial
21.5 6.5 15.7 9.5 13 7.3 14.2 6 5.5 7
a
Based on Yim (1994), Yim et al. (2002), and Zong et al. (2009)
yielded the core sample age of 8,350±45 years BP at 10.7 m, which is near the bottom of unit M1. The average sedimentation rate of unit M1 was estimated to be approximately 1.28 m/ky, which is calculated as the total sediment thickness divided by the sedimentation age. The consolidation of the sediments was ignored, so this rate may be underestimated for the sediments near the bottom of this unit, which has been slightly consolidated. Unit M2 in BH2 was mainly composed of soft to firm, grey, homogeneous clayey silt with rare shell fragments and humus layers and mottles. Unit T2 was composed of unconsolidated, medium-to-coarse-grained sand and gravel, and the sand content was higher than 90 %. Marine unit M3 comes into sharp contact with the overlying unit T2 and primarily consists of firm, mottled grey and brown clayey silt. Terrestrial unit T3 is mainly composed of brown sandy silt, with gravels occurring at the top, constituting an upward-coarsening sequence. Marine unit M4 is primarily composed of firm, mottled grey and brown clayey silt at the top 1 m, with grey silt in the middle-lower part, and dull yellow to bluish grey sandy silt at the bottom. Marine unit
M5 comprises a distinct desiccated crust at the top, which is characterized by stiff, mottled, yellowish brown clayey silt with humus layers. The bottom unit T5 consists of firm, locally mottled, yellowish brown and white-grey clayey silt. The bottom of the borehole is underlain disconformably by regolith that is completely composed of weathered granite, which constitutes the base of the Quaternary strata on the inner shelf of Hong Kong.
Chloride measurements and reconstructed pore water chloride concentration When the cores were available for this study, the samples were no longer dry or fresh. The chemicals in the soil samples were not reliable, except for the conservative chemical chloride. Two methods were used to obtain the chloride concentration profiles in the boreholes. For HKUV11 and HKUV15 cores, which were still wet, the cores were centrifuged directly to obtain pore water samples for the chloride analyses. For the cores in the
Fig. 1 Locations of the study area and the boreholes Hydrogeology Journal (2015) 23: 277–286
DOI 10.1007/s10040-014-1201-6
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soil. The sampling interval from the cores for the chloride analyses was approximately 1 m.
Chloride measurements Chloride measurement in boreholes of N20E14, N21E15, N23E17, and BH2 Because the cores from these four holes were stored at room temperature and air dried for many years before being used for this study, a modified extraction procedure (Carter 1993) was used to extract the chloride from the samples after being air-dried. At each sampling depth, 30 g of soil sample was collected and placed into labeled beakers. It was recommended (Carter 1993) that the deionized water should be twice the mass of the soil sample; therefore, 60 ml of deionized water was added to each beaker. The soil-water mixture was stirred with a glass rod and then covered with parafilm. After shaking for 1 h, the mixture was centrifuged for 30 min to separate the solution and solid in the mixture. Then, the centrifuged solution was filtered using 0.45-μm vacuum filter paper. The filtered solution is referred to as the extracted solution hereafter. Ion chromatography (IC) was used to measure the chloride concentration of the extracted solution. Before measuring the solution, a set of standard solutions were prepared to calibrate the IC. The extracted water was usually salty and was diluted 10 times to avoid damaging the IC machine.
Chloride measurement in HKUV11 and HKUV15 Cores of these two holes were well preserved in a freezer and had relatively fresh pore water. A centrifuge machine was used to directly extract pore water from the core samples. Each sample had a weight of 150– 250 g. After centrifugation for approximately 30 min at 3,500 rpm, the sample yielded 5–25 ml of water. Parallel experiments were conducted using the cores from HKUV15 to compare the chloride concentration estimated from direct pore-water measurement and the extraction method by adding deionized water to the airdry samples.
Fig. 2 Geological cross-sections A–A′ and B–B′ in the Yam O reclamation area (modified from Chui 2004). The depth was calculated as meters below sea level (mbsl). CDG means completely decomposed granite
other boreholes, the samples were air-dried, and then deionized water was added to extract the chloride from the Hydrogeology Journal (2015) 23: 277–286
Reconstruction of the pore-water chloride from the soil chloride After the chloride in the extracted solution is measured, the chloride concentration in the pore water of the original sediments can be reconstructed if the water content is known. The water content (θ) is defined here as the mass of the pore water (mpw) divided by the mass of the pore water plus the mass of the dry soil samples (ms), or θ= mpw/(mpw +ms). If the total mass of the chloride in the soil sample is mCl, which is assumed to be fully extractable from the dry soil using deionized water, and the volume of the original DOI 10.1007/s10040-014-1201-6
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Fig. 3 Chloride concentration profiles in N20E14, N21E15, N23E17, HKUV11 and BH2. All the profiles were reconstructed based on sediment chloride and water content, except HKUV11, which was based on direct pore water extracted from the core samples using a centrifuge machine. For HKUV15, chloride concentrations were measured using both wet (open circles) and dry (solid circles) soils. No stratigraphic information is available for HKUV15
pore water is Vpw, the solute concentration in the original pore water, C, can be given by mCl ρpw mCl ρpw 2mCl ρpw 1−θ mCl ¼ ¼ ¼ θ θ V pw mpw mdw ms 1−θ ρpw mCl 1−θ 1−θ ¼2 C ext ¼2 θ ρdw V dw θ
C¼
ð1Þ
where mdw is the mass of the deionized water added to the soil sample, Cext is the chloride concentration of the extracted solution measured by IC, and ρpw and ρdw are the density of the pore water and the deionized water, respectively. It is assumed that ρpw and ρdw are approximately the same and equal to 1 g/ml. The maximum error from this simplification is 2.5 % if the sample is right below the seabed and has the density of typical seawater of 1.025 g/ml. Eq. (1) also used the relation of mdw =2ms as the deionized water added to the soil samples in this study was twice the mass of the soil sample. As shown in Eq. (1), the water content is needed to reconstruct the chloride of the original pore water. The authors did not analyze the water content for the cores used in this study because most of the cores were dry. The water content of the offshore Quaternary deposits in Hong Kong has been studied (Yim et al. 2002; Choy 2004), but these studies focused on shallow units. They also used cores in different boreholes and reported that there were not significant spatial changes in the water content of M1 and M2. The water content of 237 samples was studied from M1 down to M4 in borehole WBH1, which is located in the north coastal area of Lantau Island (Fig. 1). Hydrogeology Journal (2015) 23: 277–286
Excluding the data from the paleo-desiccated crust, which is not found in the samples in this study, the average water content for the different units was: M1: 46.7 %; M2: 36.4 %; M3: 35.4 %, and M4: 30.1 %. These data were used in this study. There was no information for T2 and the other units below M4. T2 is a typical sand and gravel layer, and the water content is hard to preserve. It was assumed that the water content of T2 was approximately equal to a porosity of 30 %. The water content of 30 % was also given to the units below unit M4. After measuring the chloride concentration in the extracted solution and then converting it to the chloride concentration of the pore water using Eq. (1), the pore water chloride concentration profiles of the boreholes were calculated and are shown in Fig. 3, together with the profile from the direct measurement of the pore-water chloride in HKUV11—the data are presented in Table S1 of the electronic supplementary material (ESM).
Discussion of reconstructed chloride concentration profiles The salinity of local seawater usually deviates from the world average due to the proximity of freshwater bodies or melting glaciers, as well as seasons, temperature, depth, and so on. The average concentration of seawater in the study area was 31,200 mg/L (Shin and Thompson 1982), which is lower than the world average because of the significant impact of the Pearl River. The concentration for brackish water ranges from 1,000 to 20,000 mg/L (Freeze and Cherry 1979). The preceding values can be expressed approximately by the chloride concentration after multiplying by 55 % (Cipollina et al. 2009), which results in a chloride concentration of approximately DOI 10.1007/s10040-014-1201-6
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17,100 mg/L for the seawater in the study area, with a range of 550–11,000 mg/L for the brackish water. The reconstructed chloride concentration profiles of cores BH2, N20E14, N21E15, N23E17 and the direct measurement of the pore water in core HKUV11 are presented in Fig. 3. All of them show a decreasing trend with depth. These profiles suggest that the gradual downward migration of the seawater from inside and above the M1 units formed in the Holocene into the underlying units. For cores N20E14, N21E15 and N23E17 in the Yam O reclamation area, the average concentration values in M1, M2, and T2 were ∼12,000, ∼9,650, and ∼5,390 mg/L, respectively. The chloride concentration in M1 and T2 was much lower than the seawater chloride of ∼17,100 mg/L. The chloride in T2 was relatively high compared to that in BH2. In the inter-tidal zone, the marine deposits are in contact with the beach deposits and decomposed igneous rock. There may have been times when the sea level or tide was high, and seawater leaked into the T2 formation through the contact zone, which increased the chloride concentration in T2. These boreholes are very close to the coastline, and the marine units here may be within the discharge zone of the active topography-driven groundwater flow system as concluded in the study of coastal areas in Kyushu, Japan (Tokunaga et al. 2011). As a result, the present-day terrestrial flow dilutes the chloride in the near-shore marine units. In HKUV11, the distribution of chloride concentration also showed a decreasing trend from the maximum of 17,690 mg/L near the seabed to 9,180 mg/L in the basal aquifer. The chloride in unit M1 of HKUV11 ranged from 14,900 to 19,800 mg/L, with an average value of 17,860 mg/L. The chloride concentration at the interface of units M1 and M2 gradually decreased from 18,200 to 9,900 mg/L at the bottom of unit M2, with an average value of ∼15,300 mg/L. In unit T2, the concentration values fell between 9,900 and 14,700 mg/L, with an average of 12,100 mg/L. The concentrations in the cores of HKUV11 were much higher than for the other holes. This was most likely because some of the water was evaporated when the cores were exposed to room temperature for several days before the samples were centrifuged to extract the pore water. BH2 had the longest profile. The chloride concentration in this hole changed from an average concentration of 14,800 mg/L in M1 to ∼3,600 mg/L in T2. At the bottom of BH2, the concentration was only 1,420 mg/L. There was an obvious drop in the chloride concentration across the boundary M2/T2, indicating that the marine unit M2 obstructs the downward migration of the seawater; therefore, this unit must have very low permeability. Other boreholes also showed a similar change near to or at the boundary of M2/T2, although not as obvious as for BH2. Compared to the other holes, BH2 is far from the coastline, and the terrestrial topography-driven flow dominating in the inter-tidal zone may have little impact here. M2 may have much better aquitard integrity. The migration of the seawater into the deep units may be controlled largely by 1D transport. Hydrogeology Journal (2015) 23: 277–286
The results from the parallel experiments in HKUV15 shows that the direct pore-water measurements and the extraction method using dry soil lead to similar chloride concentration profiles (Fig. 3). However, the extraction method using dry soil consistently overestimates the chloride concentration, which suggests that the actual pore water in the system should have lower chloride concentration than estimated for the dry sediments.
Numerical modeling of the chloride concentration As discussed earlier, BH2 had the longest profile. It is the farthest from the land, and the downward migration of the chloride may be approximated using a vertical 1D model. For these reasons, further study was performed to investigate the chloride concentration profile in BH2. A 1D numerical model was used to simulate the chloride distribution in BH2 caused by the downward migration of the chloride in M2 and the units below due to the seawater transgression in the Holocene. Knowledge of the mechanism of historical solute transport in the sediment during transport is critical in understanding the observed pore-water chemistry and will form a basis for understanding the occurrence and properties of the marine groundwater. The modeling also provides information on the integrity of the aquitard and whether the vertical transport of the chloride is diffusion controlled.
Equation The following 1D advection–dispersion equation was used to describe the migration of the chloride (Zheng and Bennett 2002): ∂ ∂C ∂ðvφC Þ ∂C φDe ¼φ ð2Þ − ∂z ∂z ∂z ∂t where C is the solute concentration, De is the effective diffusion coefficient in the sediment, φ is the porosity, v is the average linear pore-water velocity, z is the distance from the seabed, and t is the time since diffusion began. Previous researchers (Beekman et al. 2011; Tokunaga et al. 2011) considered diffusion only and assumed that advection could be ignored. The importance of advection relative to diffusion, however, is site dependent. When the seawater transgressed over the M2 unit and gradually deposited M1, there may have been some hydraulic gradient in addition to diffusion that drove the chloride to migrate downward. Given that there is no evidence to indicate that one can ignore advection, it was retained in the equation. After the parameters of D e and v were estimated by calibrating the model against the chloride data, the DOI 10.1007/s10040-014-1201-6
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importance of the advection term related to v was evaluated by comparing it with De.
was set to be Δt = 12 years. For simplicity, the simulation time was changed to 8,400 years. According to Yang et al. (2008), the length of the model domain is 41.8 m.
Initial and boundary conditions and sedimentation
Choice of parameters
All the units below M1 were assumed to be full of freshwater because these units were exposed on the land for over 80 ky when the sea level was over 100 m below the current sea level. The marine sediments of M1 were deposited under seawater during the Holocene; therefore, the chloride concentration of the pore water in M1 was given the value of seawater. The downward migration of the chloride occurred as soon as the sedimentation in the Holocene began. The constant concentration at the upper boundary for chloride was set to be 17,100 mg/L. There was no information on the initial chloride in the system before the Holocene marine transgression. A constant value of chloride concentration of 4,300 mg/L was assigned to all the units below M1. Such a concentration was obtained from averaging the measured concentrations in the lower part of the profile (20–37 m depth). The simulation time was set to be 8,350 years to cover the entire Holocene period. The sedimentation of M1, with a rate of 1.28 m/ky, was realized through the upper boundary condition. The constant concentration boundary moves up a grid space, Δz, after certain time steps when the product of the accumulated time and the sedimentation rate equals Δz. This is actually a moving boundary problem with the moving rate being equal to the sedimentation rate. The boundary starts to move from the surface of M2 at the beginning of the simulation and finally stops at the surface of M1 when the simulation is completed. The initial condition and boundary conditions can be written as follows:
A multilayer model was used to simulate the chloride concentration profile. The marine units (M1–M5) and the terrestrial units (T2, T3, and T5) have different porosities and effective diffusion coefficients. The values of the two parameter De and v were determined by calibrating the model against the observed chloride concentration profile. The initial values of the parameters were selected based on previous studies on aquitards in similar environments. Volker and van der Molen (1991) simulated the observed chloride concentration profiles below a brackish lagoon, where seawater diffused into the sediments containing freshwater, using a single constant diffusion coefficient of De = 4.6× 10−10 m2/s for all the sedimentary layers. Groen et al. (2000) studied the chloride concentration profiles in paleowater in Tertiary formations at two sites in the coastal plain of Suriname along the Atlantic Ocean. Their 1D diffusive modeling of the chloride concentration profiles led to an effective diffusion coefficient of 7.0×10−10 m2/s. Tokunaga et al. (2011) studied the profiles of chloride concentrations in extracted pore waters from a near-shore borehole using a 1D diffusion-sedimentation model with a diffusion coefficient of 1.0×10−11 m2/s. These studies provided a range of diffusion coefficients from 1.0×10−11 to 7.0×10−10 m2/s. The aforementioned modeling studies on offshore formations did not include an advection term. Some onshore 1D modeling in thick clayed aquitards has been performed (Simpkins and Bradbury 1992; Remenda et al. 1996; Hendry et al. 2000) and provides a range of 0.08 to 1 m per 1,000 years for the average linear velocity. Many runs of the numerical model were conducted with the selected parameters within the preceding ranges for the diffusion coefficient and the average linear velocity.
C ðz; 0Þ ¼ C 0
ð3Þ
Results and discussion C ð0; t Þ ¼ C sea
ð4Þ
C ð∞; t Þ ¼ C 0
ð5Þ
where C0 and Csea are the initial concentrations before the Holocene and the chloride concentration of the seawater, respectively. A block-centered fully implicit finite difference scheme with upstream weighting (Zheng and Bennett 2002) was used to solve Eq. (2) subject to the initial and boundary conditions. The resulting linear matrix system was solved using the Thomas algorithm (Wang and Anderson 1982). The grid size was set to be Δz =0.1 m and the time step Hydrogeology Journal (2015) 23: 277–286
Using the trial and error method, the model was calibrated against the observed chloride concentration profile. The parameters that led to the best fit between the calculated and observed profiles are as follows: De = 2×10−11 m2/s and φ=0.5 for the marine units, De = 3×10−11 m2/s and φ=0.3 for the terrestrial units, and a downward Darcy velocity of 0.05 m/ky, corresponding to v = 0.1 m/ky in the marine units and v = 0.17 m/ky in the terrestrial units. A comparison of the reconstructed chloride concentration profile and the simulated profile using the preceding parameters is shown in Fig. 4. Overall, a reasonable agreement was obtained. It should be noted that the fit for v=0 seems as good as that for v = 0.1 m/ky. The sensitivity of the average linear velocity to the model results was investigated by giving different average linear velocity values and the results are also shown in Fig. 4. It is hard to fit all the data, especially those below T2. It may be possible that the cores at different vertical DOI 10.1007/s10040-014-1201-6
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Fig. 4 Comparison of simulated and reconstructed chloride concentration profiles. The quantity v refers to the average linear pore-water velocity of the marine units
depths were mixed or cross contaminated to a certain degree because the cores had been used for various other analyses. The reconstructed chloride ranged from a few thousand mg/L to almost that of seawater. When the cores with very high and very low chloride are mixed with each other, the chloride from the samples with high chloride may not be underestimated, but the chloride from the samples with low chloride may be significantly overestimated. It is believed that cross contamination of the samples may lead to a much smoother but overestimated profile. If the cores were not contaminated, there may be some residual chloride left in the sediments before the start of the Holocene. To understand if the solute transport is diffusion controlled or not, the Peclet number was calculated. This number determined by the average linear velocity, characteristic length and the diffusion coefficient evaluates the significance of advection relative to diffusion. The characteristic length for the numerical problem was the grid spacing Δz, which was uniformly 0.1 m in this study. Even for the maximum v of 0.5 m/ky, the estimated Peclet number is
