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Tidal fluctuations in a multi-unit coastal aquifer
Journal of Hydrology 580 (2020) 124222
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Research papers
Tidal fluctuations in a multi-unit coastal aquifer a,⁎
a
Y. Ratner-Narovlansky , Y. Weinstein , Y. Yechieli a b c
T
b,c
Department of Geography and Environment, Bar-Ilan University, Ramat-Gan 52900, Israel Geological Survey of Israel, 30 Malkhe Israel St, Jerusalem 95501, Israel Ben Gurion University, Sede Boker, Israel
A R T I C LE I N FO
A B S T R A C T
This manuscript was handled by Corrado Corradini, Editor-in-Chief, with the assistance of V.E.A. Post, Associate Editor
Groundwater tidal fluctuations were studied in a multi-layered sandy coastal aquifer at Dor Bay, Israel, 35 m from shore. The aquifer at the site is ca. 50 m thick, and it consists of a superficial phreatic unit, underlain by two confined units, separated by 1–3 m thick clay layers. While the top of the deep unit is at 30 m below sea level, the top of the intermediate unit is irregular, and it is exposed in a nearby onshore ridge, as well as in the bay. The phreatic unit showed negligible tidal amplitudes, while the two deeper units showed relatively strong amplitudes, with those in the deeper unit exceeding those in the shallow unit (4-month average Agw/Asw of 0.72 and 0.53). Using Li and Jiao’s (2001) solution, we showed that the deeper unit is in hydraulic contact with the sea at 100–400 m from shore, which should be due to the termination or breaching of the deeper clay. This is probably the result of a buried ridge, similar to the submerged calcareous sandstone ridge that exists today in the same location. The weaker amplitudes in the intermediate unit are not in accordance with its closer to shore exposure. We suggest that this is a consequence of the nearby onshore exposure of this unit, which suppresses the tidal signal.
Keywords: Confined aquifer Phreatic aquifer Tidal fluctuations
1. Introduction The interaction of coastal aquifers with the sea is of high concern, both in relation with seawater intrusion and the salinization of aquifers (e.g. Bear et al., 1999; Werner et al., 2013) and regarding the direct discharge of groundwater and their carried solutes and contaminants to the sea (i.e. SGD; e.g. Burnett et al., 2003; Slomp and Van Cappellen, 2004; Moore, 2010; Rodellas et al., 2015). While the connection of the phreatic aquifers and the sea is often apparent and evident at the shore, deep confined aquifers may extend far away from shore, under the continental shelf, often retaining large volume of terrestrial fresh water (e.g. Kohout et al., 1977; Post et al., 2013). These aquifers are usually shown or assumed to be connected with the sea offshore (e.g. Johnston, 1983; Kooi and Groen 2001; Morrissey et al., 2010), which was also the initial working assumption for deep (tens to 200 m), confined subaquifers along the Israeli coast (Kapuler and Bear, 1970). However, Kolton (1988) presented geological evidence for seawards facies changes to low permeability sediments that prevent hydraulic connection with the sea, which was also supported by old ages (> 10 kyr) of fresh water in these deep units (Yechieli et al., 2009). Kafri and Goldman (2006) showed by time domain electromagnetic (TDEM) study that the water in the deep units along the Israeli coast exhibits a patchy pattern of salinity, i.e. there are frequent spatial changes from ⁎
fresh to saline groundwater. This was interpreted as reflecting on an alternating pattern of hydraulic connection of these units with the sea. In this paper, we tackle the question of connection with the sea through tidal fluctuations. Groundwater table often exhibits tidal fluctuations, which could either be caused by ocean tides (seawater ingression into the aquifer or loading on confined units) or by earth tides. The extent of these fluctuations was extensively studied in order to determine aquifer hydraulic properties (e.g. Carr and van der Kamp, 1969; Hsieh et al., 1987; Rojstaczer and Riley, 1990; Merritt, 2004). In coastal aquifers, tidal fluctuations were also used in order to determine the type and location of their contact with seawater (e.g. Li and Chen, 1991; Cheng et al., 2004). Jacob (1950) analyzed the simple case of a confined aquifer unit exposed to seawater infiltration at the coastline, while van der Kamp (1972) analyzed the other extreme, where the unit continues beneath seafloor to infinite distance, therefore tidal stress is applied only by loading. Later on, papers focused on the combination of the above, namely applications to cases, where the confined unit is exposed at the seafloor at a certain distance from shore (e.g. Li and Chen, 1991), or when the confining unit is leaky or breached by a submarine spring (e.g. Jiao and Tang, 1999; Li and Jiao, 2001; Kim et al., 2003; Li et al., 2007a). In all these cases, solutions were developed, where both
Corresponding author. E-mail address: [email protected] (Y. Weinstein).
https://doi.org/10.1016/j.jhydrol.2019.124222 Received 4 June 2019; Received in revised form 24 September 2019; Accepted 9 October 2019 Available online 17 October 2019 0022-1694/ © 2019 Elsevier B.V. All rights reserved.
Journal of Hydrology 580 (2020) 124222
Y. Ratner-Narovlansky, et al.
physical infiltration and loading stress were considered. In general, the higher the hydraulic conductivity and the lower the specific storage, the stronger the groundwater tidal amplitudes are. It was also shown that with a long offshore roof extension, one can assume an infinite extension, and that with a relatively short extension, one can use Jacob’s solution, with the coast line ‘displaced’ to where the roof ends (Li and Jiao, 2001). The effect of a leaky roof (semi-permeable) is in general of decreasing groundwater table amplitudes with increasing roof permeability (leakage), although in the low leakage end (low hydraulic conductivity), there is first an increase in amplitude with increasing leakage, and then pattern changes to a decreasing amplitude (Li and Jiao, 2001). In recent years, there were several studies of multi-layered aquifers, mainly of the phreatic-semipermeable-confined configuration (e.g. Guo et al., 2007; Xia et al., 2007; Chuang and Yeh, 2011). Nevertheless, multi-confined aquifers were hardly studied (e.g. Bresciani et al., 2015a,b). This paper presents research into a complexed coastal aquifer, which includes both a shallow, partly confined unit that is exposed to seawater infiltration close to shore, and a deep, fully confined unit, whose connection with the sea is not observed. Both the effect of partial confinement in the shallow unit and the connection of the deep unit with the sea are investigated by means of tidal fluctuations, with the advantage of the two units sharing the same lithology and of having very close-to-shore observation boreholes. 1.1. Study site and hydrogeology The study site is located at Dor Bay, the southern Carmel coast, northern Israel (Fig. 1), where a Quaternary coastal aquifer about 50 m
Fig. 2. Geological section of the coastal Pleistocene aquifer at Dor Bay. The marl at the bottom is of Cretaceous age.
thick is underlain by Cretaceous marls. The Quaternary aquifer is subdivided into three sub-aquifers, separated by thin (< 1 to 3 m thick) clay units, including a few m thick phreatic Holocene sand unit, which is underlain by two confined units of mixed calcareous sandstone and sand, B and C (Fig. 2). At 30–40 m from the bay, the roofs of the
Fig. 1. Location map and a map of Dor Bay showing the sites of some of the boreholes discussed in this study. 2
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Fig. 3. E-W hydrogeological cross section of the southern Carmel coast at the Dor Bay area.
3. Results: Dor Bay levels and tidal fluctuations
confined units are at 4–5 and 29–31 mbsl, and their thicknesses are 23 and 18 m, respectively (Fig. 2). The top of unit B is highly irregular, forming ridges and depressions, with the latter covered by a shallow clay unit and the phreatic sand unit A (Fig. 3). At the study site, unit B is partly exposed close to the shore (20–30 m offshore in the bay, as well as along an E-W ridge that borders the bay just north of the site, Fig. 1), and is fully exposed 120–150 m offshore, where it creates a submerged N-S ridge parallel to shore (Figs. 1 and 3). The offshore extension of the clay layer that separates unit B and unit C, i.e. the location where unit C’s roof terminates and is exposed to seawater infiltration, is unknown. In this paper, we discuss the progression of the tidal signal in both units, and use it to address the question of where does unit C connect with the sea. In both confined units, a fresh-saline water interface was identified, separating top slightly brackish water (EC= < 5 mS cm−1) from bottom saline water. We note that the salinity of unit C bottom water is similar to that of seawater (55 mS cm−1), while in unit B the conductivity of bottom water is < 40 mS cm−1.
Groundwater level in unit C was on average 54 and 58 cm higher than in units B and A, respectively (averages of 106, 52 and 48 cm above sea level during May-Sept 2012, respectively, Fig. 4a and App. 1), which suggests that units B and C are hydraulically disconnected. Groundwater level in both units nicely correlated with the Hadera IOLR station sea level, and showed typical bi-daily, daily and bi-monthly tidal frequencies, with clear dominance of the first in the confined units (Figs. 4a-b and 5). This was superimposed on long-term trends, e.g. low levels during May through June (App. 1), and level decrease between July and September 2012 (Fig. 4a). Groundwater bi-daily fluctuations were significantly weaker than those observed in sea level, with those in C stronger than in B. The MaySept average of the bi-daily amplitudes were 6.21 and 4.55 cm at frequency of 1.934 (Fig. 5), which is 72% and 53% of the Hadera IOLR station sea level amplitudes, respectively. Nevertheless, in both units, level fluctuations were in phase with sea level (Fig. 4b). Unlike the confined units, fluctuations in A (phreatic) were very small (amplitudes of < 1 cm, Fig. 5, 6.6% of sea level amplitudes). In unit B, level at 60 m from shore was slightly higher than at 40 m (average difference of 4.6 cm), and amplitudes were, on average, 44% and 47%, respectively, of the Hadera sea level amplitudes (2009 data, filtered to 1.94 d−1 due to the large non-tidal fluctuations, Fig. 4c).
2. Methods The main body of level measurements reported in this study was taken during May-Sept 2012 (total of 147 days) in three boreholes drilled 35 m from the sea (30–40 m during high and low tide, respectively). Two of these boreholes are opened to the confined units B and C (Dor-4 and Dor-3, respectively, Figs. 1 and 2) and the third (Dor-5) is opened to the superficial phreatic unit A. We also report about measurements between Feb-Apr 2009, conducted in two other boreholes (P11 and P21), opened to unit B at 40 and 60 m from the sea. Groundwater levels were measured by Eijkelkamp and Solinst minidivers (1-h resolution) Fluctuation time series are shown in Fig. 4a-c (full data could be found in the Supplementary Information) together with sea level, which was measured 2 km offshore, 15 km south of Dor Bay (the Hadera IOLR station). We note that tidal fluctuations in the bay are in phase with those measured in the open sea, as shown in Fig. 4d, although amplitudes are usually slightly stronger in the bay, therefore the AGW/ASW ratios reported below should be regarded as maximum values.
4. Discussion The connection of deep sub-aquifers with the sea along the Israeli coast was doubted by Kolton (1988) and Yechieli et al. (2009). Unit C in Dor Bay, although not very deep (top at 30 m, 40 m inland), could serve as a case study for the existence and location of this connection, which could then be applied to other, deeper units along the coast. Assuming horizontal layering of sub-surface strata, and considering the local bathymetric slope (< 1%), this unit shouldn’t be exposed (i.e. breach the seafloor) closer than 3 km offshore. This will be tested below against the observed tidal amplitudes in this unit. On the other hand, unit B, although confined at the site and seawards, is exposed in a patchy way at about one third of the bay and along the ridge (seaward land extension) that borders the bay on its north (Fig. 1). Tidal signals in this unit are compared in the discussion below with confined, phreatic and leaky aquifer models, to test the 3
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Fig. 4. Time series data of Dor Bay groundwater table, compared with sea level. (a) July-Sept 2012 in units A through C at 35 m from shore (see App. 1 for the full May-Sept data series); (b) enlargement of (a) for a 15-day interval during August 2012; (c) Feb-Apr 2009 in unit B at 40 and 60 m from shore; (d) comparison of sea level at the Hadera IOLR station (15 km south of the study site) and in Dor Bay (May 2006).
B (0.72 and 0.53 of seawater amplitude at 35 m from shore, Fig. 5), and with no lag behind sea level (< 1 h, considering our resolution; Fig. 4b). Below, we investigate the possibility of complete detachment from the sea (i.e. an infinite or far enough offshore extension of the confining layer) against a scenario of hydraulic contact at a certain distance offshore. A solution for an infinite confining layer configuration (loading only
applicability of these models to such an irregular coastal aquifer unit. In this case, fine tuning is allowed due to the availability of three close-soshore (within 60 m) boreholes that intrude and document this unit. 4.1. Submarine roof extension of the deeper confined unit The deep unit (C) at Dor is apparently completely confined, with its top being ~30 m deep at the shore (Fig. 2). Despite of the abovementioned a-priori remote contact with the sea, the tidal signal in unit C is relatively strong, with amplitudes significantly stronger than in unit
1
− πSs x
effect) is presented by van der Kamp (1972): Agw (x ) = 2 Asw Te e kt 0 , where Agw and Asw are groundwater and seawater amplitudes (peak 4
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Fig. 4. (continued)
rock compressibilities (10−7–10−10 m2N−1) yields Agw/Asw ratios of 0.32–0.47, much lower than the value of 0.72 measured in unit C (Table 1). An alternative scenario could be that unit C is blocked at a certain distance from shore by impermeable sediments (i.e. clays), as suggested by some authors for the deeper units of the Israeli coastal aquifer south of our study site (e.g. Kolton, 1988; Kafri and Goldman, 2006). Nevertheless, with this configuration tidal signal would probably look similar to the infinite case or with somewhat weaker amplitudes (e.g. Li et al., 2007b). Therefore, it seems that unit C has a
heads above datum), x is the distance from shore [L], Te is tidal effiα ciency [dimensionless], which is defined as Te = α + θ · β (Jacob, 1950; α is aquifer rock compressibility, β is water compressibility and θ is porosity), k is hydraulic conductivity [LT−1], t0 is tidal period [T], which is 12.37 h, and Ss is specific storage [L−1]. Using this solution for 35 m onshore, with typical k for sandy coastal aquifer of 20 m d−1 (Yechieli et al., 2010; Levanon et al., 2016), reasonable shallow sandstone specific storage of 10−5 cm−1 (e.g. Cheng et al., 2004), water compressibility of 4.8 × 10−10 m2N−1, porosity of 0.1 and a range of 5
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Fig. 5. Frequency analyses (no filtering) of the May-Sept 2012 data from Unit B (Dor 4), Unit C (Dor 3) and sea level.
10−5 m−1 (e.g. Li and Chen, 1991; Batu, 1998; see also the storativity derived by interference recovery test at a coastal site, 6 km south of Dor, Tal et al., 2018), we got a surprisingly close to shore 150 m contact with seawater (Table 1, Fig. 6a). Using higher conductivity (100 m d−1), the distance changes to 380 m offshore (Fig. 7). We also checked an Ss value as low as 10−6 cm−1 (e.g. Domenico and Mifflin, 1965; Batu, 1998), which if used with the higher conductivity of 100 m d−1 (as measured by one of the slug tests in the same aquifer unit at Ma’agan Michael, 6 km south of Dor, Tal et al., 2018) may even yield a contact distance of 1,300 m (Table 1 and Fig. 7). On the other hand, larger Ss (e.g. 10−4 m−1, as used by Tal et al., 2018), will minimize the roof to less than 100 m. Considering the lithology involved (sandstone mixed with loose sand) and the shallow depths, we believe that the Ss value of 10−5 cm−1 is more reasonable, and conclude that unit C is probably exposed to seawater infiltration within 150–400 m from
physical connection with the sea somewhere offshore. Li and Chen (1991) were the first to derive the equations for a confined unit with a roof that extends to a certain distance offshore. We used an updated version, that of Li and Jiao (2001), to find the roof offshore extension. In their solution, simplified for a no-leakage case, the confined unit groundwater amplitude is Agw (x ) = Asw Ce e−ax , where Agw and Asw are groundwater and seawater amplitudes,
Ce =
λ 2 2
(R + ) 1
+ I12
is
the
comprehensive
tidal
efficiency,
a = πSTt0 is the confined aquifer tidal propagation parameter, 1 R1 = e−aL (1 − λ )cos (aL)]+ 2 e−2aLλ cos(2aL) I1 = e−aL (1 − λ ) and 1
sin(aL) + 2 e−2aLλ sin(2aL) . S is storativity [dimensionless], T is transmissivity [L2T−1], λ = Te and L is the confining layer offshore extension [L]). Using hydraulic conductivity of 20 m d−1 and a specific storage of 6
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Table 1 Calculated tidal amplitude (Agw/Asw) for the confined unit C, 35 m from shore, at Dor Bay. Observed Onshore distance Conductivity Sp. Storage Diffusivity Roof length Thickness Storativity Transmissivity Tidal efficiency
Tidal propagation Angular velocity
Tidal period [T] aq′ rock compressibility Water compressibility Porosity
35 K Ss K/Ss L b S T Te n Ce a w l R1 I1 t0 a(m2/N) β(m2/N) θ
m 0.72 m day−1 −1 m m2day−1 m m Dimensionless m2day−1 Dimensionless Dimensionless m−1 Day−1 Dimensionless Dimensionless Dimensionless Day m2N m2N Dimensionless
Infinite
Infinite
Finite
Finite
Finite
Finite
Finite
0.47 20 10−5 2 × 106 Infinite
0.32 20 10−5 2 × 106 Infinite
1 1.75 × 10−3
6.76 × 10−1 1.75 × 10−3
0.72 20 10−4 2 × 105 25 18 1.8 × 10−3 360 9.53 × 10−1
0.72 20 10−5 2 × 106 155 18 1.8 × 10−4 360 9.53 × 10−1
0.72 20 10−6 2 × 107 570 18 1.8 × 10−5 360 9.53 × 10−1
0.72 100 10−5 1 × 107 380 18 1.8 × 10−4 1800 9.53 × 10−1
0.72 100 10−6 1 × 108 1300 18 1.8 × 10−5 1800 9.53 × 10−1
0.871 5.52 × 10−3 12.189 9.53 × 10−1 3.88 × 10−1 1.04 × 10−1 0.515 9.75 × 10−10 4.8 × 10−10 0.1
0.764 1.75 × 10−3 12.189 9.53 × 10−1 2.72 × 10−1 1.52 × 10−1 0.515 9.75 × 10−10 4.8 × 10−10 0.1
0.732 5.52 × 10−4 12.189 9.53 × 10−1 2.38 × 10−1 1.60 × 10−1 0.515 9.75 × 10−10 4.8 × 10−10 0.1
0.745 7.81 × 10−4 12.189 9.53 × 10−1 2.52 × 10−1 1.57 × 10−1 0.515 9.75 × 10−10 4.8 × 10−10 0.1
0.728 2.47E−04 12.189 9.53E−01 2.33E−01 1.61E−01 0.515 9.75 × 10−10 4.8 × 10−10 0.1
12.189
12.189
0.515 1.00 × 10−7 4.8 × 10−10 0.1
0.515 1.00 × 10−10 4.8 × 10−10 0.1
Since unit B is apparently exposed next to shore, we first attempted to apply the solution of Jacob (1950) for a confined unit with no off-
shore. Identical numbers were derived also when assuming that the shore extends to the end of the roof, using the simple solution of Jacob (1950) for a confined unit that terminates at the shore, and calculating for the
− πSs x
shore roof extension: Agw (x)=Asw e kt0 . In this solution, the ratio Agw/ Asw determines the aquifer diffusivity [L2T−1] = k/Ss. Ratios of 0.53–0.47 at 35–40 m from the sea (Dor 4 and P11) imply relatively low diffusivity values of ~18,000 m2day−1 (Table 2). However, the Agw/ Asw ratio of 0.44 at 60 m from shore (i.e. relatively small decrease between 40 and 60 m) implies larger diffusivity for the same unit (> 30,000 m2day−1). Moreover, both diffusivity values are very low, and could be achieved only by very low hydraulic conductivity (k ≪ 1 m day−1, Table 2) or by high specific storage (~10−3 m−1). These values are very different from the values we used for unit C (diffusivity of 2 × 10−6 m2day−1, Table 1), which shares the same lithology with B, and are in large disagreement with tests conducted on these rocks (K’s > 20 m/day, specific storage < 10−4 m−1, Tal et al., 2018). Together, this could suggest that although partly exposed close to shore, the actual contact with seawater that affects the signal occurs farther offshore, namely: although unit B is partly exposed in the bay (Fig. 1), it may still be treated as a confined layer under the sea. Alternatively, we tried extending the roof of unit B offshore and used Li and Jiao’s (2001), as with the above discussion of unit C. Since the two confined units share similar lithology and both are shallow, we used similar hydraulic properties to those chosen for C, i.e. k = 20 m d−1 and Ss = 10−5 m−1. With these values, the desired Agw/Asw (0.53 at 35 m inland) is not attained until the roof is extended more than 300 m offshore (Table 2, Fig. 7), which is not in accordance with the actual observations (full exposures at the submerged ridge ≤200 m, Fig. 1) and with the above conclusion about the closer exposure of the underlying unit C. Therefore, we assume that the lower amplitudes in unit B, compared with C, is due to the lower confinement of the first (e.g. Trefry and Johnston, 1998). Several papers developed models for a leaky aquifer (e.g. Jiao and Tang, 1999; Li and Jiao, 2001; Jeng et al., 2002; Xia et al., 2007), which assume that the confining layer is semi-permeable. Although leakage is not exactly the case at Dor Bay, where the actual case is of ‘patchy exposure’, it could still give an idea about the roof offshore extension, i.e. where is the main or average hydraulic contact with seawater. The equation developed by Li and Jiao (2001) for this
− πSs x
‘onshore distance’ of the borehole: A gw (x)=A sw e kt 0 . We note for future studies that two boreholes at different distance from shore could assist in deriving both roof distance offshore and aquifer hydraulic properties (see Fig. 6a). We suggest that, unlike the above ad hoc assumption of a farther offshore termination, the confining clay that separates unit B and C terminates beneath or not far from the submerged N-S ridge (Fig. 1), similar to the shallower clay layer that separates the phreatic sand (A) from unit B (Fig. 8). The sandstone ridge is a fossilized Pleistocene dune, and the shallow clay was deposited in coastal marshes that abounded in the topographic depressions between the dunes (Sivan et al., 2003), which in turn were covered by Holocene sands. This is probably also the case with the deeper unit, which likewise consists of a calcareous sandstone that was apparently formed under similar environmental conditions. Accordingly, it is suggested that the submerged ridge (Fig. 1) is the locus of succession of older ridges, and that also in the past (prior to unit B deposition) the clay was bounded to a topographic depression at the same location (Fig. 8). As a result, in the ridge area there is a direct hydraulic contact between the sandstone units B and C. Whether or not the deep clay resumes seawards of the ridge should be further studied. Nevertheless, the implications of this closeto-shore contact are very important to seawater intrusion and aquifer management. We note that the above calculations were based on tidal amplitudes from the more open sea conditions of the Hadera IOLR station (15 km to the south), while the amplitudes inside the bay were probably slightly stronger (Fig. 4d). Nevertheless, using the stronger bay level amplitudes will produce a lower Agw/Asw, which will result in a roof termination slightly farther offshore (> 150 m). Considering the bay’s width of ca. 150 m, this favors the use of the ‘open sea’ amplitudes. 4.2. Tidal fluctuation in a partially confined unit Although exposed nearby, groundwater in unit B clearly behaves the confined way in our observation boreholes, with amplitudes much more similar and in phase with those found in the confined unit C than in the phreatic unit A (Fig. 4a-b). Therefore, below we investigate into the confinement of this unit, using the advantage of its lithological similarity to unit C.
configuration
is: k1 b1
Agw (x) = Asw Ce e−pax ,
where
p=
1 + u2 + u ,
u=Ls /ωS , Ls = is the specific leakage through the confining layer [T−1], ω = 2π /t0 [T−1], k1 is the hydraulic conductivity of the confining layer [LT−1] and b1 is its thickness [L]. Also here, we used the same parameters chosen for unit C, i.e. k = 20 m d−1 and Ss = 10−5 7
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Fig. 7. Calculated groundwater-seawater amplitude ratios using various hydraulic conductivity and specific storage values, compared with those observed in units B and C at 35 m inland.
again is hard to accommodate with the shorter termination of the underlying unit C’s roof. Using different hydraulic properties (e.g. lower conductivity or larger specific storage for both units) may change roof length, but always results with unit B extending farther offshore than the deeper unit C. To conclude, our observations at different onshore distances do not accommodate with neither full confinement nor leaky roof for unit B. Accordingly, we suggest that the reason for the relatively low amplitudes in this unit, while not big difference between 40 and 60 m from shore, is due to its onshore exposure. This happens along a N-S ridge just 150 m inland of the boreholes, but probably more important, along the ridge extending in E-W direction, just 50 m northwest of the observation boreholes (Fig. 1). This large phreatic area that extends seawards from the observation boreholes probably restrains the tidal signal (e.g. Nielsen, 1990). On the other hand, the confinement between the shore and the boreholes still maintains a prominent tidal signal (in phase with sea level fluctuations), unlike the pattern observed in the phreatic unit A (Fig. 4a-b). We are not aware of any analytical solutions relating to such a configuration. Townley (1995) did develop equations that solved for onshore no flow boundary conditions, but in this case the boundary was farther offshore from the observation point and not seawards, as is the main exposure at Dor Bay. The above is supported by observations from a coastal site, 6 km south of Dor Bay (Tal et al., 2017, 2018), which shares a very similar stratigraphy. At this site, a similar unit B is well confined until 300 m offshore and > 300 m inland. Accordingly, and unlike Dor Bay, amplitudes in unit B are stronger than in unit C, which is probably the combined result of unit B’s confined configuration and of unit C’s roof extending farther offshore. We conclude that the partly phreatic nature of unit B at Dor Bay results in the damping of its tidal amplitudes. The above discussion of unit B suggests that coastal exposures of shallow confined units may have significant impact on groundwater tidal signal, which is similar to that developed in an offshore leaky aquifers. This could result in wrong conclusions about either the offshore extension or the hydraulic parameters of the studied aquifer. Distinction between the two could be inferred by investigation of the tidal signal along a close-to-shore transect of observation boreholes.
Fig. 6. Sensitivity analysis of tidal signal intensity (groundwater/sea level amplitude ratio) in the aquifer as a function of piezometer distance from the sea (inland). (a) Unit C; different curves are for different seaward distances and different specific storage (m−1), while conductivity is assumed 20 m d−1; average amplitude ratio observed in this unit at 35 m is also shown. (b) Unit B; different curves are for different seaward distances (m) and for different conductivities (m d−1) of the confining roof (left and right values in parentheses, respectively), with conductivity and specific storage of unit B assumed 20 m d−1 and 10−5 m−1, respectively. The average amplitude ratios measured at 40 and 60 m from sea (P11 and P21, respectively) during 2009 are also shown. Note that the small difference between the signal at the two boreholes could not be accommodated with large leakage, and that the derived distance of the roof is significantly larger than that calculated for the underlying unit C (> 300 and 150 m, respectively).
m−1. Amplitudes were then tuned by playing with the hydraulic conductivity of the unknown, apparent semi-permeable layer k1. Although this leaky scenario indeed allowed simulating the amplitude in unit B (Table 2), the calculated difference between the amplitudes at 40 and 60 m in the 2009 measurements (Fig. 3c) was way too large, if roof terminates between shore and the submerged ridge (0–150 m, Table 2), as in unit C (above, Table 1). In general, the higher the leakage of the roof (i.e. conductivity of the confining unit), the larger the difference between onshore observation points should be (see Fig. 6b). The amplitude difference between 40 and 60 m could be achieved only with a leaky roof extending to > 300 m from shore (Table 2, Fig. 6b), but this
5. Summary and conclusions – Tidal data from next to shore boreholes provided insight into the 8
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Fig. 8. Suggested hydrogeological cross section at the Dor Bay, highlighting the possibility that a fossil ridge of unit C is buried beneath the exposed unit B ridge.
Declaration of Competing Interest
relations of a multi-unit coastal aquifer with the sea. Both confined units share the same lithology, which facilitated constraining their hydraulic properties. – The strong tidal signal (an amplitude > 70% that of seawater) in the deeper (> 30 m) unit (C) cannot be supported only by loading; rather, it is a clear indication of hydraulic connection with seawater not far from shore. – Based on equations developed by Li and Jiao (2001), we calculated a relatively short (150–400 m) termination of unit C’s confining roof, which is unexpected, considering its roof’s depth, and which is highly important to the aquifer management. This probably reflects on paleo coastal geomorphology, e.g. the location of a buried ridge that marked the seaward edge of the confining clay layer. – Coastal exposures of the shallower unit B result in weaker tidal amplitudes than in the deeper unit C. Common solutions used for either completely confined or for leaky aquifers could not be adequately applied to this unit. This highlights the importance of characterizing coastal exposures of confined aquifers prior to the application of various leaky aquifer models.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements This paper is dedicated to the memory of Haim Hemo, whose assistance in the field was invaluable, and sadly passed away during the preparation of this manuscript. We wish to express our gratitude to Yehuda Shalem, Iyad Swaed, Halel Lutsky, and Adi Tal, who helped with the field work. We also thank Elad Levanon and Steve Brenner for their assistance with analyzing the time series data. This project was supported by the Israel Water Authority [grant number 4500962408] and by MERC USAID [grant number M29-073].
Table 2 Calculated tidal amplitude (Agw/Asw) for unit B, with a leaky and non-leaky roof.
35 m from shore 40 m from shore 60 m from shore Roof offshore B Hydraulic conductivity Specific storage Diffusivity Aquifer thickness Storativity Transmissivity Tidal propagation param'
Angular velocity Tidal period Tidal efficiency aquifer-rock compressibility Water ompressibility Porosity
Agw/Asw Agw/Asw Agw/Asw L K Ss K/Ss b S T Ce a p u Ls λ R1 I1 k1 b1 w t0 Te a(m2/N) β(m2/N) n
Observed
Jacob
No leakage
No leakage
Leaking roof
Leaking roof
Leaking roof
Leaking roof
0.53 0.47 0.44
0.62 0.58 0.44 0 0.32 10−5 32,000
0.73 0.72 0.69 150 20 10−5 2 × 106 23 2.3 × 10−4 460 0.771 0.002
0.53 0.53 0.51 350 20 10−5 2 × 106 23 2.3 × 10−4 460 0.565 0.002
9.53 × 10−1 2.79 × 10−1 1.51 × 10−1
9.53 × 10−1 6.88 × 10−2 1.47 × 10−1
12.18938 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
12.18938 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
0.53 0.49 0.34 0 20 10−5 2 × 106 23 2.3 × 10−4 460 1.000 0.002 10.34 53.50 0.15 9.53 × 10−1 5.23 × 10−1 1 0.3 2 12.19 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
0.53 0.51 0.41 150 20 10−5 2 × 106 23 2.3 × 10−4 460 0.771 0.002 5.97 17.83 0.05 9.53 × 10−1 2.79 × 10−1 1.51 × 10−1 0.1 2 12.19 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
0.60 0.57 0.49 150 20 10−5 2 × 106 23 2.3 × 10−4 460 0.771 0.002 4.23 8.92 0.025 9.53 × 10−1 2.79 × 10−1 1.51 × 10−1 0.05 2 12.19 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
0.54 0.53 0.49 300 20 10−5 2 × 106 23 2.3 × 10−4 460 0.605 0.002 1.96 1.78 0.005 9.53 × 10−1 1.08 × 10−1 1.59 × 10−1 0.01 2 12.19 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
m (m/day) (1/m) m2day−1 (m) Dimensionless m2/day Dimensionless (1/m)
Specific leakage Dimensionless Dimensionless Dimensionless
(1/day) Day Dimensionless
0.515
9
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Appendix A. Supplementary data
Hydrol. 246, 19–35. Levanon, E., Shalev, E., Yechieli, Y., Gvirtzman, H., 2016. Fluctuations of fresh-saline water interface and of water table induced by sea tides in unconfined aquifers. Adv. Water Resour. 96, 34–42. Li, G., Chen, C., 1991. Determining the length of confined aquifer roof extending under the sea by the tidal method. J. Hydrol. 123, 97–104. Li, H., Jiao, J.J., 2001. Tide-induced groundwater fluctuation in a coastal leaky confined aquifer system extending under the sea. Water Resour. Res. 37 (5), 1165–1171. Li, H., Li, L., Lockington, D., Boufadel, M.C., Li, G., 2007a. Modelling tidal signals enhanced by a submarine spring in a coastal confined aquifer extending under the sea. Adv. Water Resour. 30, 1046–1052. Li, H., Li, G., Cheng, J., Boufadel, M.C., 2007b. Tide-induced head fluctuations in a confined aquifer with sediment covering its outlet at the sea floor. Water Resour. Res. 43, W03404. https://doi.org/10.1029/2005WR004724. Merritt, M.L., 2004. Estimating Hydraulic Properties of the Floridan Aquifer System by Analysis of Earth-Tide, Ocean-Tide, and Barometric Effects, Collier and Hendry Counties, Florida. U.S. Geological Survey Water-Resources Investigations Report 03–4267. Moore, W.S., 2010. The effect of submarine groundwater discharge on the ocean. Ann. Rev. Mar. Sci. 2, 59–88. Morrissey, S.K., Clark, J.F., Bennett, M., Richardson, E., Stute, M., 2010. Groundwater reorganization in the Floridan aquifer following Holocene sea-level rise. Nat. Geosci. 3, 683–687. Nielsen, P., 1990. Tidal dynamics at water table in beaches. Water Resour. Res. 26, 2127–2134. Post, V.E.A., Groen, J., Kooi, H., Person, M., Ge, S., Edmunds, W.M., 2013. Offshore fresh groundwater reserves as a global phenomenon. Nature 504, 71–78. Rodellas, V., Garcia-Orellana, J., Masqué, P., Feldman, M., Weinstein, Y., 2015. Submarine Groundwater Discharge: a major source of nutrients to the Mediterranean Sea. Proc. Natl. Acad. Sci. 112 (13), 3926–3930. Rojstaczer, S., Riley, F.S., 1990. Response of the water level in a well to earth tides and atmospheric loading under unconfined conditions. Water Resour. Res. 26 (8), 1803–1817. Sivan, D., Eliyahu, D., Raban, A., 2003. Late Pleistocene to Holocene wetlands now covered by sand, along the Carmel Coast, Israel, and their relation to human settlement: An example from Dor. J. Coastal Res. 20, 97–110. Slomp, C.P., Van Cappellen, P., 2004. Nutrient inputs to the coastal ocean through submarine groundwater discharge: Controls and potential impact. J. Hydrol. 295, 64–86. Tal, A., Weinstein, Y., Yechieli, Y., Borisover, M., 2017. The influence of fish ponds and salinization on groundwater quality in the multi-layer coastal aquifer system in Israel. J. Hydrol. 551, 768–783. Tal, A., Weinstein, Y., Walman, S., Goldman, M., Yechieli, Y., 2018. The interrelations between a multi-layered coastal aquifer, a surface reservoir (fish ponds) and the sea. Water 10, 1426. https://doi.org/10.3390/w10101426. Townley, L.R., 1995. The response of aquifers to periodic forcing. Adv. Water Resour. 18 (3), 125–146. Trefry, M.G., Johnston, C.D., 1998. Pumping test analysis for a tidally forced aquifer. Ground Water 36 (3), 427–433. van der Kamp, G., 1972. Tidal fluctuations in a confined aquifer extending under the sea. Int. Geol. Congr. 24, 101–106. Werner, A.D., Bakker, M., Post, V.E.A., Vandenbohede, A., Lu, C., Ataie-Ashtiani, B., Simmons, C.T., Barry, D.A., 2013. Seawater intrusion processes, investigation and management: Recent advances and future challenges. Adv. Water Resour. 51, 3–26. Xia, Y., Li, H.L., Boufadel, M.C., Guo, Q., Li, G., 2007. Tidal wave propagation in a coastal aquifer: Effects of leakages through its submarine outlet-capping and offshore roof. J. Hydrol. 337, 249–257. Yechieli, Y., Kafri, U., Sivan, O., 2009. The inter-relationship between coastal sub-aquifers and the Mediterranean Sea, deduced from radioactive isotopes analysis. Hydrogeol. J. 17, 265–274. https://doi.org/10.1007/s10040-008-0329-7. Yechieli, Y., Shalev, E., Wollman, S., Kiro, Y., Kafri, U., 2010. Response of the Mediterranean and Dead Sea coastal aquifers to sea level variations. Water Resour. Res. 46, W12550. https://doi.org/10.1029/2009WR00870.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jhydrol.2019.124222. References Batu, V., 1998. Aquifer Hydraulics: A Comprehensive Guide to Hydrogeologic Data Analysis. John Wiley & Sons, New York, pp. 727p. Bear, J., Cheng, A.H.D., Sorek, S., Ouazar, D., Herrera, I. (Eds.), 1999. Seawater Intrusion in Coastal Aquifers: Concepts, Methods, and Practices. Kluwer Academic Publishers, Dordrecht, The Netherlands; Boston, MA, USA; London, UK. Bresciani, E., Batelaan, O., Banks, E. W., et al. 2015. Assessment of Adelaide Plains Groundwater Resources: Summary Report. Goyder Institute for Water Research Technical Report Series No. 15/31, Adelaide, South Australia. Bresciani, E., Batelaan, O., Banks, E.W., et al. 2015. Assessment of Adelaide Plains Groundwater Resources: Appendices Part I – Field and Desktop Investigations. Goyder Institute for Water Research Technical Report Series No. 15/32, Adelaide, South Australia. Burnett, W.C., Bokuniewicz, H., Huettel, M., Moore, W.S., Taniguchi, M., 2003. Groundwater and pore water inputs to the coastal zone. Biogeochemistry 66, 3–33. Carr, P.A., van der Kamp, G., 1969. Determining aquifer characteristics by the tidal method. Water Resour. Res. 5 (5), 1023–1031. Cheng, J., Chen, C., Ji, M., 2004. Determination of aquifer roof extending under the sea from variable-density flow modelling of groundwater response to tidal loading: case study of the Jahe River Basin, Shandong Province, China. Hydrogeol. J. 12, 408–423. Chuang, M.H., Yeh, H.D., 2011. A generalized solution for groundwater head fluctuation in a tidal leaky aquifer system. J. Earth Syst. Sci. 120 (6), 1055–1066. Domenico, P.A., Mifflin, M.D., 1965. Water from low-permeability sediments and land subsidence. Water Resour. Res. 1 (4), 563–576. Guo, Q., Li, H., Boufadel, M., Xia, Y., Li, G., 2007. Tide-induced groundwater head fluctuation in coastal multi-layered aquifer systems with a submarine outlet-capping. Adv. Water Resour. 30 (8), 1746–1755. Hsieh, P.A., Bredehoeft, J.D., Farr, J.M., 1987. Determination of aquifer transmissivity from earth tide analysis. Water Resour. Res. 23 (10), 1824–1832. Jacob, C.E., 1950. Flow of groundwater. In: Rouse, H. (Ed.), Engineering Hydraulics. John Wiley, Hoboken N.J., pp. 321–386. Jiao, J.J., Tang, Z., 1999. An analytical solution of groundwater response to tidal fluctuation in a leaky confined aquifer. Water Resour. Res. 35 (3), 747–751. Jeng, D.S., Li, L., Barry, D.A., 2002. Analytical solution for tidal propagation in a coupled semi- confined/phreatic coastal aquifer. Adv. Water Resour. 25 (5), 577–584. Johnston, R.H., 1983. The saltwater-freshwater interface in the Tertiary limestone aquifer, southeast Atlantic outer continental shelf of the U.S.A. J. Hydrol. 61, 239–249. Kafri, U., Goldman, M., 2006. Are the lower subaquifers of the Mediterranean coastal aquifer blocked to seawater intrusion? Results of a TDEM (time domain electromagnetic) study. Israel J. Earth Sci. 55, 55–68. Kapuler, Y., Bear, J. 1970. Numerical solution to the movement of the interface in a multilayered coastal aquifer. Tahal, Water Planning for Israel Rep. 01/74/70 (in Hebrew). Kim, K.-Y., Kim, Y., Lee, C.-W., Woo, N.-C., 2003. Analysis of groundwater response to tidal effect in a finite leaky confined coastal aquifer considering hydraulic head at source bed. Geosci. J. 7 (2), 169–178. Kohout, F.A., Bothner, M.H., Manheim, F.T., 1977. Fresh ground water stored in aquifers under the Continental Shelf: implications from a deep test, Nantucket island, Massachusetts. J. Am. Water Resour. Assoc. 13 (2), 373–386. Kolton, Y., 1988. Examination of the connection between seawater in the Pleistocene aquifer in the Mediterranean shelf of central Israel. Tahal, Water Planning for Israel Rep. 01/88/31 (in Hebrew). Kooi, H., Groen, J., 2001. Offshore continuation of coastal groundwater systems: predictions using sharp-interface approximations and variable-density flow modelling. J.
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Research papers
Tidal fluctuations in a multi-unit coastal aquifer a,⁎
a
Y. Ratner-Narovlansky , Y. Weinstein , Y. Yechieli a b c
T
b,c
Department of Geography and Environment, Bar-Ilan University, Ramat-Gan 52900, Israel Geological Survey of Israel, 30 Malkhe Israel St, Jerusalem 95501, Israel Ben Gurion University, Sede Boker, Israel
A R T I C LE I N FO
A B S T R A C T
This manuscript was handled by Corrado Corradini, Editor-in-Chief, with the assistance of V.E.A. Post, Associate Editor
Groundwater tidal fluctuations were studied in a multi-layered sandy coastal aquifer at Dor Bay, Israel, 35 m from shore. The aquifer at the site is ca. 50 m thick, and it consists of a superficial phreatic unit, underlain by two confined units, separated by 1–3 m thick clay layers. While the top of the deep unit is at 30 m below sea level, the top of the intermediate unit is irregular, and it is exposed in a nearby onshore ridge, as well as in the bay. The phreatic unit showed negligible tidal amplitudes, while the two deeper units showed relatively strong amplitudes, with those in the deeper unit exceeding those in the shallow unit (4-month average Agw/Asw of 0.72 and 0.53). Using Li and Jiao’s (2001) solution, we showed that the deeper unit is in hydraulic contact with the sea at 100–400 m from shore, which should be due to the termination or breaching of the deeper clay. This is probably the result of a buried ridge, similar to the submerged calcareous sandstone ridge that exists today in the same location. The weaker amplitudes in the intermediate unit are not in accordance with its closer to shore exposure. We suggest that this is a consequence of the nearby onshore exposure of this unit, which suppresses the tidal signal.
Keywords: Confined aquifer Phreatic aquifer Tidal fluctuations
1. Introduction The interaction of coastal aquifers with the sea is of high concern, both in relation with seawater intrusion and the salinization of aquifers (e.g. Bear et al., 1999; Werner et al., 2013) and regarding the direct discharge of groundwater and their carried solutes and contaminants to the sea (i.e. SGD; e.g. Burnett et al., 2003; Slomp and Van Cappellen, 2004; Moore, 2010; Rodellas et al., 2015). While the connection of the phreatic aquifers and the sea is often apparent and evident at the shore, deep confined aquifers may extend far away from shore, under the continental shelf, often retaining large volume of terrestrial fresh water (e.g. Kohout et al., 1977; Post et al., 2013). These aquifers are usually shown or assumed to be connected with the sea offshore (e.g. Johnston, 1983; Kooi and Groen 2001; Morrissey et al., 2010), which was also the initial working assumption for deep (tens to 200 m), confined subaquifers along the Israeli coast (Kapuler and Bear, 1970). However, Kolton (1988) presented geological evidence for seawards facies changes to low permeability sediments that prevent hydraulic connection with the sea, which was also supported by old ages (> 10 kyr) of fresh water in these deep units (Yechieli et al., 2009). Kafri and Goldman (2006) showed by time domain electromagnetic (TDEM) study that the water in the deep units along the Israeli coast exhibits a patchy pattern of salinity, i.e. there are frequent spatial changes from ⁎
fresh to saline groundwater. This was interpreted as reflecting on an alternating pattern of hydraulic connection of these units with the sea. In this paper, we tackle the question of connection with the sea through tidal fluctuations. Groundwater table often exhibits tidal fluctuations, which could either be caused by ocean tides (seawater ingression into the aquifer or loading on confined units) or by earth tides. The extent of these fluctuations was extensively studied in order to determine aquifer hydraulic properties (e.g. Carr and van der Kamp, 1969; Hsieh et al., 1987; Rojstaczer and Riley, 1990; Merritt, 2004). In coastal aquifers, tidal fluctuations were also used in order to determine the type and location of their contact with seawater (e.g. Li and Chen, 1991; Cheng et al., 2004). Jacob (1950) analyzed the simple case of a confined aquifer unit exposed to seawater infiltration at the coastline, while van der Kamp (1972) analyzed the other extreme, where the unit continues beneath seafloor to infinite distance, therefore tidal stress is applied only by loading. Later on, papers focused on the combination of the above, namely applications to cases, where the confined unit is exposed at the seafloor at a certain distance from shore (e.g. Li and Chen, 1991), or when the confining unit is leaky or breached by a submarine spring (e.g. Jiao and Tang, 1999; Li and Jiao, 2001; Kim et al., 2003; Li et al., 2007a). In all these cases, solutions were developed, where both
Corresponding author. E-mail address: [email protected] (Y. Weinstein).
https://doi.org/10.1016/j.jhydrol.2019.124222 Received 4 June 2019; Received in revised form 24 September 2019; Accepted 9 October 2019 Available online 17 October 2019 0022-1694/ © 2019 Elsevier B.V. All rights reserved.
Journal of Hydrology 580 (2020) 124222
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physical infiltration and loading stress were considered. In general, the higher the hydraulic conductivity and the lower the specific storage, the stronger the groundwater tidal amplitudes are. It was also shown that with a long offshore roof extension, one can assume an infinite extension, and that with a relatively short extension, one can use Jacob’s solution, with the coast line ‘displaced’ to where the roof ends (Li and Jiao, 2001). The effect of a leaky roof (semi-permeable) is in general of decreasing groundwater table amplitudes with increasing roof permeability (leakage), although in the low leakage end (low hydraulic conductivity), there is first an increase in amplitude with increasing leakage, and then pattern changes to a decreasing amplitude (Li and Jiao, 2001). In recent years, there were several studies of multi-layered aquifers, mainly of the phreatic-semipermeable-confined configuration (e.g. Guo et al., 2007; Xia et al., 2007; Chuang and Yeh, 2011). Nevertheless, multi-confined aquifers were hardly studied (e.g. Bresciani et al., 2015a,b). This paper presents research into a complexed coastal aquifer, which includes both a shallow, partly confined unit that is exposed to seawater infiltration close to shore, and a deep, fully confined unit, whose connection with the sea is not observed. Both the effect of partial confinement in the shallow unit and the connection of the deep unit with the sea are investigated by means of tidal fluctuations, with the advantage of the two units sharing the same lithology and of having very close-to-shore observation boreholes. 1.1. Study site and hydrogeology The study site is located at Dor Bay, the southern Carmel coast, northern Israel (Fig. 1), where a Quaternary coastal aquifer about 50 m
Fig. 2. Geological section of the coastal Pleistocene aquifer at Dor Bay. The marl at the bottom is of Cretaceous age.
thick is underlain by Cretaceous marls. The Quaternary aquifer is subdivided into three sub-aquifers, separated by thin (< 1 to 3 m thick) clay units, including a few m thick phreatic Holocene sand unit, which is underlain by two confined units of mixed calcareous sandstone and sand, B and C (Fig. 2). At 30–40 m from the bay, the roofs of the
Fig. 1. Location map and a map of Dor Bay showing the sites of some of the boreholes discussed in this study. 2
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Fig. 3. E-W hydrogeological cross section of the southern Carmel coast at the Dor Bay area.
3. Results: Dor Bay levels and tidal fluctuations
confined units are at 4–5 and 29–31 mbsl, and their thicknesses are 23 and 18 m, respectively (Fig. 2). The top of unit B is highly irregular, forming ridges and depressions, with the latter covered by a shallow clay unit and the phreatic sand unit A (Fig. 3). At the study site, unit B is partly exposed close to the shore (20–30 m offshore in the bay, as well as along an E-W ridge that borders the bay just north of the site, Fig. 1), and is fully exposed 120–150 m offshore, where it creates a submerged N-S ridge parallel to shore (Figs. 1 and 3). The offshore extension of the clay layer that separates unit B and unit C, i.e. the location where unit C’s roof terminates and is exposed to seawater infiltration, is unknown. In this paper, we discuss the progression of the tidal signal in both units, and use it to address the question of where does unit C connect with the sea. In both confined units, a fresh-saline water interface was identified, separating top slightly brackish water (EC= < 5 mS cm−1) from bottom saline water. We note that the salinity of unit C bottom water is similar to that of seawater (55 mS cm−1), while in unit B the conductivity of bottom water is < 40 mS cm−1.
Groundwater level in unit C was on average 54 and 58 cm higher than in units B and A, respectively (averages of 106, 52 and 48 cm above sea level during May-Sept 2012, respectively, Fig. 4a and App. 1), which suggests that units B and C are hydraulically disconnected. Groundwater level in both units nicely correlated with the Hadera IOLR station sea level, and showed typical bi-daily, daily and bi-monthly tidal frequencies, with clear dominance of the first in the confined units (Figs. 4a-b and 5). This was superimposed on long-term trends, e.g. low levels during May through June (App. 1), and level decrease between July and September 2012 (Fig. 4a). Groundwater bi-daily fluctuations were significantly weaker than those observed in sea level, with those in C stronger than in B. The MaySept average of the bi-daily amplitudes were 6.21 and 4.55 cm at frequency of 1.934 (Fig. 5), which is 72% and 53% of the Hadera IOLR station sea level amplitudes, respectively. Nevertheless, in both units, level fluctuations were in phase with sea level (Fig. 4b). Unlike the confined units, fluctuations in A (phreatic) were very small (amplitudes of < 1 cm, Fig. 5, 6.6% of sea level amplitudes). In unit B, level at 60 m from shore was slightly higher than at 40 m (average difference of 4.6 cm), and amplitudes were, on average, 44% and 47%, respectively, of the Hadera sea level amplitudes (2009 data, filtered to 1.94 d−1 due to the large non-tidal fluctuations, Fig. 4c).
2. Methods The main body of level measurements reported in this study was taken during May-Sept 2012 (total of 147 days) in three boreholes drilled 35 m from the sea (30–40 m during high and low tide, respectively). Two of these boreholes are opened to the confined units B and C (Dor-4 and Dor-3, respectively, Figs. 1 and 2) and the third (Dor-5) is opened to the superficial phreatic unit A. We also report about measurements between Feb-Apr 2009, conducted in two other boreholes (P11 and P21), opened to unit B at 40 and 60 m from the sea. Groundwater levels were measured by Eijkelkamp and Solinst minidivers (1-h resolution) Fluctuation time series are shown in Fig. 4a-c (full data could be found in the Supplementary Information) together with sea level, which was measured 2 km offshore, 15 km south of Dor Bay (the Hadera IOLR station). We note that tidal fluctuations in the bay are in phase with those measured in the open sea, as shown in Fig. 4d, although amplitudes are usually slightly stronger in the bay, therefore the AGW/ASW ratios reported below should be regarded as maximum values.
4. Discussion The connection of deep sub-aquifers with the sea along the Israeli coast was doubted by Kolton (1988) and Yechieli et al. (2009). Unit C in Dor Bay, although not very deep (top at 30 m, 40 m inland), could serve as a case study for the existence and location of this connection, which could then be applied to other, deeper units along the coast. Assuming horizontal layering of sub-surface strata, and considering the local bathymetric slope (< 1%), this unit shouldn’t be exposed (i.e. breach the seafloor) closer than 3 km offshore. This will be tested below against the observed tidal amplitudes in this unit. On the other hand, unit B, although confined at the site and seawards, is exposed in a patchy way at about one third of the bay and along the ridge (seaward land extension) that borders the bay on its north (Fig. 1). Tidal signals in this unit are compared in the discussion below with confined, phreatic and leaky aquifer models, to test the 3
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Y. Ratner-Narovlansky, et al.
Fig. 4. Time series data of Dor Bay groundwater table, compared with sea level. (a) July-Sept 2012 in units A through C at 35 m from shore (see App. 1 for the full May-Sept data series); (b) enlargement of (a) for a 15-day interval during August 2012; (c) Feb-Apr 2009 in unit B at 40 and 60 m from shore; (d) comparison of sea level at the Hadera IOLR station (15 km south of the study site) and in Dor Bay (May 2006).
B (0.72 and 0.53 of seawater amplitude at 35 m from shore, Fig. 5), and with no lag behind sea level (< 1 h, considering our resolution; Fig. 4b). Below, we investigate the possibility of complete detachment from the sea (i.e. an infinite or far enough offshore extension of the confining layer) against a scenario of hydraulic contact at a certain distance offshore. A solution for an infinite confining layer configuration (loading only
applicability of these models to such an irregular coastal aquifer unit. In this case, fine tuning is allowed due to the availability of three close-soshore (within 60 m) boreholes that intrude and document this unit. 4.1. Submarine roof extension of the deeper confined unit The deep unit (C) at Dor is apparently completely confined, with its top being ~30 m deep at the shore (Fig. 2). Despite of the abovementioned a-priori remote contact with the sea, the tidal signal in unit C is relatively strong, with amplitudes significantly stronger than in unit
1
− πSs x
effect) is presented by van der Kamp (1972): Agw (x ) = 2 Asw Te e kt 0 , where Agw and Asw are groundwater and seawater amplitudes (peak 4
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Fig. 4. (continued)
rock compressibilities (10−7–10−10 m2N−1) yields Agw/Asw ratios of 0.32–0.47, much lower than the value of 0.72 measured in unit C (Table 1). An alternative scenario could be that unit C is blocked at a certain distance from shore by impermeable sediments (i.e. clays), as suggested by some authors for the deeper units of the Israeli coastal aquifer south of our study site (e.g. Kolton, 1988; Kafri and Goldman, 2006). Nevertheless, with this configuration tidal signal would probably look similar to the infinite case or with somewhat weaker amplitudes (e.g. Li et al., 2007b). Therefore, it seems that unit C has a
heads above datum), x is the distance from shore [L], Te is tidal effiα ciency [dimensionless], which is defined as Te = α + θ · β (Jacob, 1950; α is aquifer rock compressibility, β is water compressibility and θ is porosity), k is hydraulic conductivity [LT−1], t0 is tidal period [T], which is 12.37 h, and Ss is specific storage [L−1]. Using this solution for 35 m onshore, with typical k for sandy coastal aquifer of 20 m d−1 (Yechieli et al., 2010; Levanon et al., 2016), reasonable shallow sandstone specific storage of 10−5 cm−1 (e.g. Cheng et al., 2004), water compressibility of 4.8 × 10−10 m2N−1, porosity of 0.1 and a range of 5
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Fig. 5. Frequency analyses (no filtering) of the May-Sept 2012 data from Unit B (Dor 4), Unit C (Dor 3) and sea level.
10−5 m−1 (e.g. Li and Chen, 1991; Batu, 1998; see also the storativity derived by interference recovery test at a coastal site, 6 km south of Dor, Tal et al., 2018), we got a surprisingly close to shore 150 m contact with seawater (Table 1, Fig. 6a). Using higher conductivity (100 m d−1), the distance changes to 380 m offshore (Fig. 7). We also checked an Ss value as low as 10−6 cm−1 (e.g. Domenico and Mifflin, 1965; Batu, 1998), which if used with the higher conductivity of 100 m d−1 (as measured by one of the slug tests in the same aquifer unit at Ma’agan Michael, 6 km south of Dor, Tal et al., 2018) may even yield a contact distance of 1,300 m (Table 1 and Fig. 7). On the other hand, larger Ss (e.g. 10−4 m−1, as used by Tal et al., 2018), will minimize the roof to less than 100 m. Considering the lithology involved (sandstone mixed with loose sand) and the shallow depths, we believe that the Ss value of 10−5 cm−1 is more reasonable, and conclude that unit C is probably exposed to seawater infiltration within 150–400 m from
physical connection with the sea somewhere offshore. Li and Chen (1991) were the first to derive the equations for a confined unit with a roof that extends to a certain distance offshore. We used an updated version, that of Li and Jiao (2001), to find the roof offshore extension. In their solution, simplified for a no-leakage case, the confined unit groundwater amplitude is Agw (x ) = Asw Ce e−ax , where Agw and Asw are groundwater and seawater amplitudes,
Ce =
λ 2 2
(R + ) 1
+ I12
is
the
comprehensive
tidal
efficiency,
a = πSTt0 is the confined aquifer tidal propagation parameter, 1 R1 = e−aL (1 − λ )cos (aL)]+ 2 e−2aLλ cos(2aL) I1 = e−aL (1 − λ ) and 1
sin(aL) + 2 e−2aLλ sin(2aL) . S is storativity [dimensionless], T is transmissivity [L2T−1], λ = Te and L is the confining layer offshore extension [L]). Using hydraulic conductivity of 20 m d−1 and a specific storage of 6
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Table 1 Calculated tidal amplitude (Agw/Asw) for the confined unit C, 35 m from shore, at Dor Bay. Observed Onshore distance Conductivity Sp. Storage Diffusivity Roof length Thickness Storativity Transmissivity Tidal efficiency
Tidal propagation Angular velocity
Tidal period [T] aq′ rock compressibility Water compressibility Porosity
35 K Ss K/Ss L b S T Te n Ce a w l R1 I1 t0 a(m2/N) β(m2/N) θ
m 0.72 m day−1 −1 m m2day−1 m m Dimensionless m2day−1 Dimensionless Dimensionless m−1 Day−1 Dimensionless Dimensionless Dimensionless Day m2N m2N Dimensionless
Infinite
Infinite
Finite
Finite
Finite
Finite
Finite
0.47 20 10−5 2 × 106 Infinite
0.32 20 10−5 2 × 106 Infinite
1 1.75 × 10−3
6.76 × 10−1 1.75 × 10−3
0.72 20 10−4 2 × 105 25 18 1.8 × 10−3 360 9.53 × 10−1
0.72 20 10−5 2 × 106 155 18 1.8 × 10−4 360 9.53 × 10−1
0.72 20 10−6 2 × 107 570 18 1.8 × 10−5 360 9.53 × 10−1
0.72 100 10−5 1 × 107 380 18 1.8 × 10−4 1800 9.53 × 10−1
0.72 100 10−6 1 × 108 1300 18 1.8 × 10−5 1800 9.53 × 10−1
0.871 5.52 × 10−3 12.189 9.53 × 10−1 3.88 × 10−1 1.04 × 10−1 0.515 9.75 × 10−10 4.8 × 10−10 0.1
0.764 1.75 × 10−3 12.189 9.53 × 10−1 2.72 × 10−1 1.52 × 10−1 0.515 9.75 × 10−10 4.8 × 10−10 0.1
0.732 5.52 × 10−4 12.189 9.53 × 10−1 2.38 × 10−1 1.60 × 10−1 0.515 9.75 × 10−10 4.8 × 10−10 0.1
0.745 7.81 × 10−4 12.189 9.53 × 10−1 2.52 × 10−1 1.57 × 10−1 0.515 9.75 × 10−10 4.8 × 10−10 0.1
0.728 2.47E−04 12.189 9.53E−01 2.33E−01 1.61E−01 0.515 9.75 × 10−10 4.8 × 10−10 0.1
12.189
12.189
0.515 1.00 × 10−7 4.8 × 10−10 0.1
0.515 1.00 × 10−10 4.8 × 10−10 0.1
Since unit B is apparently exposed next to shore, we first attempted to apply the solution of Jacob (1950) for a confined unit with no off-
shore. Identical numbers were derived also when assuming that the shore extends to the end of the roof, using the simple solution of Jacob (1950) for a confined unit that terminates at the shore, and calculating for the
− πSs x
shore roof extension: Agw (x)=Asw e kt0 . In this solution, the ratio Agw/ Asw determines the aquifer diffusivity [L2T−1] = k/Ss. Ratios of 0.53–0.47 at 35–40 m from the sea (Dor 4 and P11) imply relatively low diffusivity values of ~18,000 m2day−1 (Table 2). However, the Agw/ Asw ratio of 0.44 at 60 m from shore (i.e. relatively small decrease between 40 and 60 m) implies larger diffusivity for the same unit (> 30,000 m2day−1). Moreover, both diffusivity values are very low, and could be achieved only by very low hydraulic conductivity (k ≪ 1 m day−1, Table 2) or by high specific storage (~10−3 m−1). These values are very different from the values we used for unit C (diffusivity of 2 × 10−6 m2day−1, Table 1), which shares the same lithology with B, and are in large disagreement with tests conducted on these rocks (K’s > 20 m/day, specific storage < 10−4 m−1, Tal et al., 2018). Together, this could suggest that although partly exposed close to shore, the actual contact with seawater that affects the signal occurs farther offshore, namely: although unit B is partly exposed in the bay (Fig. 1), it may still be treated as a confined layer under the sea. Alternatively, we tried extending the roof of unit B offshore and used Li and Jiao’s (2001), as with the above discussion of unit C. Since the two confined units share similar lithology and both are shallow, we used similar hydraulic properties to those chosen for C, i.e. k = 20 m d−1 and Ss = 10−5 m−1. With these values, the desired Agw/Asw (0.53 at 35 m inland) is not attained until the roof is extended more than 300 m offshore (Table 2, Fig. 7), which is not in accordance with the actual observations (full exposures at the submerged ridge ≤200 m, Fig. 1) and with the above conclusion about the closer exposure of the underlying unit C. Therefore, we assume that the lower amplitudes in unit B, compared with C, is due to the lower confinement of the first (e.g. Trefry and Johnston, 1998). Several papers developed models for a leaky aquifer (e.g. Jiao and Tang, 1999; Li and Jiao, 2001; Jeng et al., 2002; Xia et al., 2007), which assume that the confining layer is semi-permeable. Although leakage is not exactly the case at Dor Bay, where the actual case is of ‘patchy exposure’, it could still give an idea about the roof offshore extension, i.e. where is the main or average hydraulic contact with seawater. The equation developed by Li and Jiao (2001) for this
− πSs x
‘onshore distance’ of the borehole: A gw (x)=A sw e kt 0 . We note for future studies that two boreholes at different distance from shore could assist in deriving both roof distance offshore and aquifer hydraulic properties (see Fig. 6a). We suggest that, unlike the above ad hoc assumption of a farther offshore termination, the confining clay that separates unit B and C terminates beneath or not far from the submerged N-S ridge (Fig. 1), similar to the shallower clay layer that separates the phreatic sand (A) from unit B (Fig. 8). The sandstone ridge is a fossilized Pleistocene dune, and the shallow clay was deposited in coastal marshes that abounded in the topographic depressions between the dunes (Sivan et al., 2003), which in turn were covered by Holocene sands. This is probably also the case with the deeper unit, which likewise consists of a calcareous sandstone that was apparently formed under similar environmental conditions. Accordingly, it is suggested that the submerged ridge (Fig. 1) is the locus of succession of older ridges, and that also in the past (prior to unit B deposition) the clay was bounded to a topographic depression at the same location (Fig. 8). As a result, in the ridge area there is a direct hydraulic contact between the sandstone units B and C. Whether or not the deep clay resumes seawards of the ridge should be further studied. Nevertheless, the implications of this closeto-shore contact are very important to seawater intrusion and aquifer management. We note that the above calculations were based on tidal amplitudes from the more open sea conditions of the Hadera IOLR station (15 km to the south), while the amplitudes inside the bay were probably slightly stronger (Fig. 4d). Nevertheless, using the stronger bay level amplitudes will produce a lower Agw/Asw, which will result in a roof termination slightly farther offshore (> 150 m). Considering the bay’s width of ca. 150 m, this favors the use of the ‘open sea’ amplitudes. 4.2. Tidal fluctuation in a partially confined unit Although exposed nearby, groundwater in unit B clearly behaves the confined way in our observation boreholes, with amplitudes much more similar and in phase with those found in the confined unit C than in the phreatic unit A (Fig. 4a-b). Therefore, below we investigate into the confinement of this unit, using the advantage of its lithological similarity to unit C.
configuration
is: k1 b1
Agw (x) = Asw Ce e−pax ,
where
p=
1 + u2 + u ,
u=Ls /ωS , Ls = is the specific leakage through the confining layer [T−1], ω = 2π /t0 [T−1], k1 is the hydraulic conductivity of the confining layer [LT−1] and b1 is its thickness [L]. Also here, we used the same parameters chosen for unit C, i.e. k = 20 m d−1 and Ss = 10−5 7
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Fig. 7. Calculated groundwater-seawater amplitude ratios using various hydraulic conductivity and specific storage values, compared with those observed in units B and C at 35 m inland.
again is hard to accommodate with the shorter termination of the underlying unit C’s roof. Using different hydraulic properties (e.g. lower conductivity or larger specific storage for both units) may change roof length, but always results with unit B extending farther offshore than the deeper unit C. To conclude, our observations at different onshore distances do not accommodate with neither full confinement nor leaky roof for unit B. Accordingly, we suggest that the reason for the relatively low amplitudes in this unit, while not big difference between 40 and 60 m from shore, is due to its onshore exposure. This happens along a N-S ridge just 150 m inland of the boreholes, but probably more important, along the ridge extending in E-W direction, just 50 m northwest of the observation boreholes (Fig. 1). This large phreatic area that extends seawards from the observation boreholes probably restrains the tidal signal (e.g. Nielsen, 1990). On the other hand, the confinement between the shore and the boreholes still maintains a prominent tidal signal (in phase with sea level fluctuations), unlike the pattern observed in the phreatic unit A (Fig. 4a-b). We are not aware of any analytical solutions relating to such a configuration. Townley (1995) did develop equations that solved for onshore no flow boundary conditions, but in this case the boundary was farther offshore from the observation point and not seawards, as is the main exposure at Dor Bay. The above is supported by observations from a coastal site, 6 km south of Dor Bay (Tal et al., 2017, 2018), which shares a very similar stratigraphy. At this site, a similar unit B is well confined until 300 m offshore and > 300 m inland. Accordingly, and unlike Dor Bay, amplitudes in unit B are stronger than in unit C, which is probably the combined result of unit B’s confined configuration and of unit C’s roof extending farther offshore. We conclude that the partly phreatic nature of unit B at Dor Bay results in the damping of its tidal amplitudes. The above discussion of unit B suggests that coastal exposures of shallow confined units may have significant impact on groundwater tidal signal, which is similar to that developed in an offshore leaky aquifers. This could result in wrong conclusions about either the offshore extension or the hydraulic parameters of the studied aquifer. Distinction between the two could be inferred by investigation of the tidal signal along a close-to-shore transect of observation boreholes.
Fig. 6. Sensitivity analysis of tidal signal intensity (groundwater/sea level amplitude ratio) in the aquifer as a function of piezometer distance from the sea (inland). (a) Unit C; different curves are for different seaward distances and different specific storage (m−1), while conductivity is assumed 20 m d−1; average amplitude ratio observed in this unit at 35 m is also shown. (b) Unit B; different curves are for different seaward distances (m) and for different conductivities (m d−1) of the confining roof (left and right values in parentheses, respectively), with conductivity and specific storage of unit B assumed 20 m d−1 and 10−5 m−1, respectively. The average amplitude ratios measured at 40 and 60 m from sea (P11 and P21, respectively) during 2009 are also shown. Note that the small difference between the signal at the two boreholes could not be accommodated with large leakage, and that the derived distance of the roof is significantly larger than that calculated for the underlying unit C (> 300 and 150 m, respectively).
m−1. Amplitudes were then tuned by playing with the hydraulic conductivity of the unknown, apparent semi-permeable layer k1. Although this leaky scenario indeed allowed simulating the amplitude in unit B (Table 2), the calculated difference between the amplitudes at 40 and 60 m in the 2009 measurements (Fig. 3c) was way too large, if roof terminates between shore and the submerged ridge (0–150 m, Table 2), as in unit C (above, Table 1). In general, the higher the leakage of the roof (i.e. conductivity of the confining unit), the larger the difference between onshore observation points should be (see Fig. 6b). The amplitude difference between 40 and 60 m could be achieved only with a leaky roof extending to > 300 m from shore (Table 2, Fig. 6b), but this
5. Summary and conclusions – Tidal data from next to shore boreholes provided insight into the 8
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Fig. 8. Suggested hydrogeological cross section at the Dor Bay, highlighting the possibility that a fossil ridge of unit C is buried beneath the exposed unit B ridge.
Declaration of Competing Interest
relations of a multi-unit coastal aquifer with the sea. Both confined units share the same lithology, which facilitated constraining their hydraulic properties. – The strong tidal signal (an amplitude > 70% that of seawater) in the deeper (> 30 m) unit (C) cannot be supported only by loading; rather, it is a clear indication of hydraulic connection with seawater not far from shore. – Based on equations developed by Li and Jiao (2001), we calculated a relatively short (150–400 m) termination of unit C’s confining roof, which is unexpected, considering its roof’s depth, and which is highly important to the aquifer management. This probably reflects on paleo coastal geomorphology, e.g. the location of a buried ridge that marked the seaward edge of the confining clay layer. – Coastal exposures of the shallower unit B result in weaker tidal amplitudes than in the deeper unit C. Common solutions used for either completely confined or for leaky aquifers could not be adequately applied to this unit. This highlights the importance of characterizing coastal exposures of confined aquifers prior to the application of various leaky aquifer models.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements This paper is dedicated to the memory of Haim Hemo, whose assistance in the field was invaluable, and sadly passed away during the preparation of this manuscript. We wish to express our gratitude to Yehuda Shalem, Iyad Swaed, Halel Lutsky, and Adi Tal, who helped with the field work. We also thank Elad Levanon and Steve Brenner for their assistance with analyzing the time series data. This project was supported by the Israel Water Authority [grant number 4500962408] and by MERC USAID [grant number M29-073].
Table 2 Calculated tidal amplitude (Agw/Asw) for unit B, with a leaky and non-leaky roof.
35 m from shore 40 m from shore 60 m from shore Roof offshore B Hydraulic conductivity Specific storage Diffusivity Aquifer thickness Storativity Transmissivity Tidal propagation param'
Angular velocity Tidal period Tidal efficiency aquifer-rock compressibility Water ompressibility Porosity
Agw/Asw Agw/Asw Agw/Asw L K Ss K/Ss b S T Ce a p u Ls λ R1 I1 k1 b1 w t0 Te a(m2/N) β(m2/N) n
Observed
Jacob
No leakage
No leakage
Leaking roof
Leaking roof
Leaking roof
Leaking roof
0.53 0.47 0.44
0.62 0.58 0.44 0 0.32 10−5 32,000
0.73 0.72 0.69 150 20 10−5 2 × 106 23 2.3 × 10−4 460 0.771 0.002
0.53 0.53 0.51 350 20 10−5 2 × 106 23 2.3 × 10−4 460 0.565 0.002
9.53 × 10−1 2.79 × 10−1 1.51 × 10−1
9.53 × 10−1 6.88 × 10−2 1.47 × 10−1
12.18938 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
12.18938 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
0.53 0.49 0.34 0 20 10−5 2 × 106 23 2.3 × 10−4 460 1.000 0.002 10.34 53.50 0.15 9.53 × 10−1 5.23 × 10−1 1 0.3 2 12.19 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
0.53 0.51 0.41 150 20 10−5 2 × 106 23 2.3 × 10−4 460 0.771 0.002 5.97 17.83 0.05 9.53 × 10−1 2.79 × 10−1 1.51 × 10−1 0.1 2 12.19 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
0.60 0.57 0.49 150 20 10−5 2 × 106 23 2.3 × 10−4 460 0.771 0.002 4.23 8.92 0.025 9.53 × 10−1 2.79 × 10−1 1.51 × 10−1 0.05 2 12.19 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
0.54 0.53 0.49 300 20 10−5 2 × 106 23 2.3 × 10−4 460 0.605 0.002 1.96 1.78 0.005 9.53 × 10−1 1.08 × 10−1 1.59 × 10−1 0.01 2 12.19 0.515 9.53 × 10−1 9.754 × 10−10 4.8 × 10−10 0.1
m (m/day) (1/m) m2day−1 (m) Dimensionless m2/day Dimensionless (1/m)
Specific leakage Dimensionless Dimensionless Dimensionless
(1/day) Day Dimensionless
0.515
9
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Appendix A. Supplementary data
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