distribution of uranium and thorium radioelements in...

JAKU: Earth Sci., Vol. 23, No. 2, pp: 149-168 (2012 A.D. / 1433 A.H.)

DOI: 10.4197 / Ear. 23-2.8

149

Distribution of Uranium and Thorium Radioelements in

Subsurface Pleistocene-Holocene Sediments of the Nile

Delta, Egypt

Yehia H. Dawood and Hamdy Hamed Abd El-Naby

Faculty of Earth Sciences, King Abdulaziz Univ., Jeddah, KSA

[email protected]

Received: 12/12/2011 Accepted: 24/6/2012

Abstract. Collaborative techniques were used to investigate the textural and mineralogical characteristics as well as uranium and thorium contents of the Pleistocene-Holocene subsurface sediments of the onshore area of northern Nile Delta. Fluctuation of water flow during the formation of these sediments resulted in particle sizes variability over time. Approximately 25% of the samples are classified into sand, 20% clayey silt, 18% silty sand, 15% sandy silt and 15% sand-silt-clay. The average uranium content of these sediments is 3.7 ppm and that of thorium is 7.83 ppm. The ranges of Th/U ratio (1.03-4.74) possibly reflect vertical variability based on changes in textures, mineralogical composition and organic matter contents. This variation points to interactions between fluvial, brackish and freshwater regimes, which would be expected to typify a delta setting. Geochemical behaviour of both uranium and thorium is regarded as the essential factor controlling the distribution of the two elements. Zircon, monazite and apatite are the main accessory minerals hosting uranium and thorium. However sorptive uptake of uranium by sediments of relatively high organic matter contents have also played a significant role in uranium adsorption to the sediments.

Keywords: Nile Delta; Uranium; Thorium; Heavy minerals; Subsurface sediments.

Introduction

The naturally occurring radioelements are present in different

concentrations in sedimentary rocks. The fate and transport properties of

150 Y. H. Dawood, H. H. Abd El-Naby

contaminant metals such as uranium and thorium in soils and sediments

are crucial for monitoring and mitigate the dispersion effects of these

elements deep in the subsurface. Different factors affect the distribution

of uranium and thorium in the subsurface sediments including source of

clastic supply to the sedimentary basin, groundwater chemistry,

microbial activity that can affect uranium mobility through direct or

indirect redox changes (Wilkins et al., 2006). In oxic environments,

uranium is typically present as UVI

, in the form of uranyl with two tightly

bound oxygen atoms (UO22+

) that are often relatively mobile. In contrast,

under anoxic conditions uranium is generally present as UIV

, in the form

of hardly soluble mineral phases such as uraninite (UO2), and is,

therefore, considered to be relatively immobile (Langmuir, 1978).

Conversely, thorium is unaffected by redox conditions and remains

insoluble as Th+4

. Understanding uranium speciation in groundwater and

uranium adsorption, precipitation and coprecipitation with soils,

sediments, organic matter such as humic substances, and microorganisms

are all key aspects towards understanding the transport properties of

uranium within the subsurface. Several studies were concerned with the

mineralogy of subsurface sediments of the Nile Delta (e.g. Amer, 1974;

Zaghloul et al., 1980; and El Sisi et al., 1996). Zaghloul et al. (1980)

concluded that the older sediments of Sidi Salem and Abu Madi

Formations are relatively rich in stable minerals compared to the younger

sediments of Kafr El Sheikh and younger formations. The Late

Quaternary subsurface stratigraphy of the northern delta consists of the

older alluvial sand and stiff mud unconformably overlain by shallow

marine to coastal transgressive sand which, in turn, is unconformably

overlain by a variable sequence of Holocene deltaic sand, silt, and mud

(Stanley and Warne, 1993).

The main aim of the present study is to recognize the relation

between distribution of uranium and thorium radioelements in the

Pleistocene-Holocene subsurface sediments of northern Nile Delta and

their mineralogical composition.

Geologic Setting of the Nile Delta

The Nile Delta covers nearly 60,000 sq kms of the northern part of

Egypt. It has, in general, a featureless surface with a northward slope,

except for some limited topographic features such as the Khatatba

Distribution of Uranium and Thorium Radioelements in … 151

positive structural and topographic features, and the westward Wadi El-

Natrun negative element. Generally, no outcrops occur on the Delta

surface, being mainly covered with recent mud and alluvial deposits

(Fig. 1), and also with some sand accumulations known as the Turtle-

backs. At least seven distributaries flowed across the Nile Delta and

discharged into the Mediterranean at various times during the Middle to

Late Holocene (Toussoun, 1922; Stanley and Warne, 1993). Five of these

distributaries became no longer active, leaving only Damietta and

Rosetta branches. The structural setting of the Nile Delta occupies a key

position within the plate tectonic development of the eastern

Mediterranean and the Levant. It lies on the northern margin of the

African plate, which extends from the subduction zone adjacent to the

Cretan and Cyprus arcs, to the Red Sea where it is drifted apart from the

Arabian plate.

Fig. 1. Location map of the study area.


The subsurface stratigraphic column of the Nile Delta region, after

Ismail et al. (2010), is given in Table 1. The Nile Delta basin contains a

thick sequence of Neogene-Quaternary clastics. According to Stanley and

Warne (1993), the Late Quaternary subsurface stratigraphy of the

northern delta consists of, from bottom to top: alluvial sand and stiff mud

(older than ~12 ka) unconformably (separated by a hiatus) overlain by

shallow marine to coastal transgressive sand (~ 12 to 8 ka). This sand is,

in turn, unconformably overlain by a variable sequence of Holocene

deltaic sand, silt, and mud as old as ~7.5 ka. The overall architecture and

specific environments of deposition within these three sequences record

the interplay of factors that controlled the evolution of the Nile delta.

These factors include sea-level changes, subsidence, climate oscillations,

and sedimentary processes.

The Pleistocene-Holocene subsurface sequence of the Nile Delta is

composed of three main formations namely; Baltim, Mit Ghamr and

Bilqas. Baltim Formation of Early Pleistocene overlies El Wastani

Formation (Late Pliocene) and underlies Mit Ghamr Formation

(Pleistocene). Baltim Formation is composed of intercalations of clay,

sand, shale, with thin bioclastic limestones with numerous Planktonic

foraminiferal species and several benthic species (Ismail et al., 2010).

Based on palaeontological studies, this formation has been dated as Early

Pleistocene (Deibis et al., 1986) and from Pleistocene to Recent (Ismail

et al., 2010).

Table 1. The subsurface stratigraphic column of the Nile Delta (after Ismail et al., 2010).

Age

Formation

Description

Holocene

Bilqas Sand and clays interbeds

Pleistocene Mit Ghamr Clay, sand and silt intercalations

Baltim Clay with sand and argillaceous limestone intrebeds, locally lignitic and silty

Late Pliocene El Wastani Intercalations of sand, shale, clay, with some dolomite and limestone

Middle Pliocene Kafr El Sheikh shale-clay intercalations with some minor occurrences of sands, siltstones, argillaceous limestones, and dolomites

Early Pliocene Abu Madi Intercalations of sand, clay, and shale

Late Miocene

Rosetta Sand with occurrences of anhydrite and clay

Qawaseem sand to sandstone

With a few interbeds of clay.

Sidi Salem intercalations of shale, sand, clay, with rare occurrences of white, bioclastic limestone.


Mit Ghamr Formation overlies Baltim Formation and underlies

Bilqas Formation. The thickness of this formation decreases northward

(Ismail et al., 2010). This formation is composed of intercalations of

clay, sand, and silt, with some limestone. Ismail (1984) dated this

formation as Late Pliocene to Quaternary, while Azzam (1994) ascribed

it to the Pleistocene age. Moreover, Badran (1996) dated it as Late

Pliocene to Pleistocene in age, and it was later dated as Pleistocene to

Recent (Ismail et al., 2010). This formation grades into the overlying

Bilqas Formation by the increase of interbedded clays with sands, rich in

peat, with a coastal or lagoonal fauna (Ismail et al., 2010). Bilqas

Formation constitutes the top basin fill with coastal sands and deposits

from the Nile floods. This formation covers the whole Delta region.

However, it is difficult to differentiate it from the underlying Mit Ghamr

Formation (Ismail et al., 2010).This formation is composed of sand

interbedded with clay rich in molluscan fragments. The clays contain

plant remains and carbonaceous matter. It is dated as Holocene by

Badran (1996). Azzam (1994) mentioned that during the Holocene, a

marine transgression covered most of the Northern Delta area and gave

rise to a few metres of marine sediments capped by agricultural soil. Abu

El Enein (1990) ascribed this formation to the same age. Based on the

faunal association, this formation is dated as being Pleistocene to Recent

in age (Ismail et al., 2010). The Holocene rocks are represented by the

intermittent marine transgressions that give rise to a few metres of marine

sediments.

The source rocks which provide sediments to the Nile River and

Delta are characterized by markedly different geological terranes. An

average of 84% of the flow in the main Nile is derived from Ethiopia

during summer floods and 16% is introduced from the central Africa

Lake Plateau (Said, 1981; Stanley et al., 1988). The dominant influence

of the Ethiopian Plateau terrains is recorded in heavy mineral data from

Pleistocene and Holocene core sections in the promontories of the Nile

Delta (Stanley et al., 1988) and in offshore deposits on the Egyptian shelf

(Stanley et al., 1979). Furthermore, heavy mineral suites in modern

coastal sands along the delta margin are characterized by Ethiopian

components (El Fishawi and Molnar, 1985).


Sampling and Analytical Procedures

Forty ditch cutting samples were collected from one borehole,

located at 20 Km SE of the Burullus Lake of northern Nile Delta area

(Fig. 1). Depth interval from surface ranges between 1-40 m. The color

of these sediments is variable depending on the mineralogical

composition and organic matter contents. The color ranges from light

gray, brown to black. The sample analyses were performed using

different analytical procedures available at the laboratories of Geology

Department of Ain Shams University, Faculty of Earth Sciences of King

Abdul Aziz University and the laboratories of XRAL, USA. The

collected samples were dried at 100 °C in an oven for 3 days and then

split into three parts for laboratory investigations. The investigations

include determination of organic matters, carbonates, heavy minerals,

uranium and thorium contents for the whole sample.

The first portion of the sample was weighed and then treated by

warm 0.2 N HCl in order to calculate the carbonate percentage. The

insoluble residue was wet sieved in order to separate sand size fractions

for the purpose of heavy minerals separation. After dryness, the

percentage of the sand fractions was calculated. The heavy minerals were

separated using bromoform. Microscopic investigation, Back Scattered

Electron imaging (BSE) and Energy Dispersive X-ray Spectrometry for

some separated heavy mineral grains were performed using JEOL JXA-

8200 Electron Probe Micro Analyzer. The clay deflocculation in the

separated -63µm size fraction was obtained through repeated

centrifugation. After transferring the suspended sediments to a graduated

glass cylinder and vigorously shaking, the silt and clay percentages were

determined using the pipette method.

The second portion of the sample was used to measure the

percentage of organic matter contents. A clean and desiccated crucible

was weighed before putting in a few grams of dried samples. The

combined mass was measured and the crucible was then put in a muffle

furnace at temperature of 450oC for 8 hrs. The crucible was removed and

allowed to dry in desiccator. The crucible is then re-weighed and the

organic matter content is calculated.

The third portion of each sample was crushed in a jaw crusher and

powdered to 200-mesh size by ball milling. Uranium and thorium

contents were measured by Instrumental Neutron Activation Analysis


(INAA). The irradiated samples are allowed to decay for approximately 7

days, and are counted on a high-resolution coaxial germanium detector

for up to 2000 s. All spectra are collected on a Canberra Series 90 multi-

channel analyzer. Reference materials were analyzed in each batch of

samples to monitor analytical accuracy. Precision was also determined by

analysis of sample duplicates. Both precision and accuracy were

generally within 10%.

Results

Composition of Pleistocene-Holocene Subsurface Sediments

The percentages of organic matters, carbonates, sand, silt and clay

size fractions in the borehole sediments are shown in Table 2. The

organic matter contents range between 3.28 and 15% with an average of

8.77%. The carbonate fraction is represented mainly by molluscan

fragments of different sizes. It constitutes a significant proportion of the

light minerals in the sediments with an average content of about 4.92%.

The sand size fractions of the borehole sediments constitute about

39.51%, in average. Conversely, the averages of silt and clay size

fractions are 32.2 and 14.6%, respectively (Table 2). The sand-silt-clay

ternary diagram of Krumbein and Sloss (1963) shows wide distribution

of samples in seven different fields. This plot classifies around 25% of

the samples into sand, 20% clayey silt, 18% silty sand, 15% sandy silt

and 15% sand-silt-clay (Fig. 2).

The mineralogical composition of the clay fraction in the subsurface

sediments of the Nile Delta were previously identified as

montmorillonite, chlorite, mixed-layer illite-montmorillonite, illite and

kaolinite (Fayed and Hassan, 1970) and as smectite, kaolinite and illite

(Stanley et al., 1998). Conversely, the separated clay fraction of the Nile

River streambed sediments is dominated by kaolinite, illite and

montmorillonite (Dawood, 2010).

The microscopic investigation of the heavy mineral fractions

showed the presence of the characteristic River Nile assemblage

including amphiboles, pyroxenes, biotite, muscovite, iron oxides,

tourmaline, zircon, monazite, apatite, titanite, kyanite, sillimanite,

andalusite, staurolite and garnet (Shukri, 1950). The percentage of heavy

fractions range between 0.01 and 7.52 % with an average of 2.43%

(Table 2).


Table 2. Sedimentological data of the borehole sediments of the Nile Delta.

S. N.

Sand

Total Light

minerals %Heavy

minerals %Total

Sand% Silt % Clay %

Organic matters % Carbonates %

1 65.76 3.31 69.07 23.66 2.21 4.15 0.91 100 2 72.51 7.52 80.03 1.71 7.05 8.05 3.15 99.99 3 74.27 7.14 81.42 1.32 6.60 9.72 0.94 100 4 59.73 3.97 63.70 19.26 6.08 7.36 3.58 99.98 5 47.47 1.87 49.34 0.88 30.14 9.32 10.32 100 6 33.76 2.82 36.57 27.31 20.16 11.8 4.16 100 7 39.73 3.65 43.38 28.86 13.23 11.74 2.79 100 8 28.14 0.01 28.15 39.65 20.55 8.2 3.45 100 9 40.73 1.68 42.41 22.25 20.37 14.08 0.89 100 10 6.34 0.08 6.42 59.12 10.46 7.59 16.4 99.99 11 17.75 0.40 18.15 48.49 17.59 8.48 7.28 99.99 12 2.58 0.01 2.59 61.92 15.46 9.09 10.93 99.99 13 1.78 0.03 1.81 58.66 22.93 10.7 5.9 100 14 65.22 2.67 67.88 10.29 9.06 9.35 3.42 100 15 58.45 3.94 62.39 21.11 8.47 3.56 4.47 100 16 68.54 2.65 71.19 5.15 8.05 5.09 10.52 100 17 70.16 7.90 78.05 3.51 5.06 10.93 2.43 99.98 18 75.32 2.33 77.65 10.45 5.17 5.78 0.94 99.99 19 72.10 4.25 76.35 13.21 3.90 5.6 0.92 99.98 20 60.79 0.09 60.88 0.47 22.01 11.63 4.98 99.97 21 52.93 3.92 56.85 15.68 5.24 10.08 12.13 99.98 22 54.16 3.69 57.86 21.34 14.70 5 1.08 99.98 23 45.77 5.12 50.89 16.06 15.30 13.29 4.44 99.98 24 60.68 5.19 65.87 0.43 13.00 6.01 14.69 100 25 75.68 3.27 78.95 4.98 7.57 6.23 2.26 99.99 26 8.71 0.06 8.77 65.28 14.38 8.99 2.58 100 27 33.40 0.28 33.68 27.12 26.21 11.22 1.77 100 28 7.57 0.06 7.63 49.07 28.29 13.9 1.11 99.99 29 5.59 0.05 5.64 46.51 31.26 10.86 5.7 99.97 30 19.18 2.75 21.93 44.07 14.12 7.75 12.12 99.99 31 5.68 1.02 6.70 71.82 14.28 3.28 3.9 99.98 32 0.47 0.02 0.49 58.60 20.01 14.58 6.32 100 33 0.39 2.01 2.40 66.62 18.97 8.94 3.04 99.97 34 13.06 1.16 14.22 61.07 12.94 10.56 1.21 100 35 0.19 4.31 4.50 55.06 25.47 12.45 2.51 99.99 36 25.67 1.51 27.17 55.81 12.15 3.4 1.47 100 37 29.96 1.65 31.60 51.05 7.90 4.23 5.2 99.98 38 28.18 2.22 30.40 47.03 8.23 7.13 7.18 99.97 39 20.67 1.57 22.24 30.79 23.94 15 8.02 99.99 40 34.16 0.85 35.01 42.15 15.64 5.55 1.65 100

Ave. 37.08 2.43 39.51 32.20 14.60 8.77 4.92 99.99


Fig. 2. Triangular plot of the Nile Delta subsurface sediments (after Krumbein and Sloss,

1963) .

U-Th Contents of the of Pleistocene-Holocene Subsurface Sediments

The concentrations of uranium and thorium in the Pleistocene-

Holocene subsurface sediments are given in Table 3, The measured

values are on a dry weight basis. The uranium contents of these

sediments range between 1 and 5.4 ppm with an average of 3.7 ppm and

that of thorium is in the range of 2.8-16.1 ppm with an average of 7.83

ppm. The Th/U ratio ranges between 1.03 and 4.74 with an average value

of 2.19.

Accessory minerals hosting uranium and thorium are common in the

studied sediments and represented mainly by zircon, monazite and

apatite. The percentages of these minerals were determined by counting

about 400 grains per slide. The zircon percentages range between 1.96

and 8.67 with an average of 5.42, monazite ranges between 0.44 and 1.97

with an average of 1.12 and apatite ranges between 0.1 and 0.41 with an

average of 0.23. Back Scattered Electron Image (BSI) of separated zircon

and apatite grains from the studied sediments at 1.5 non-magnetic


fraction is shown in Fig. 3a, while Fig. 3b shows a BSI of a fractured

monazite grain separated at 1.5 magnetic fraction.

Table 3. U and Th contents of the borehole sediments of the Nile Delta.

S. N. U (ppm) Th (ppm) Th/U S. N. U (ppm) Th (ppm) Th/U 1 4.9 13 2.65 21 4.8 7.4 1.54 2 5.4 13.3 2.46 22 3.5 8.8 2.51 3 4.3 15 3.49 23 5 15.7 3.14 4 5.2 15.2 2.92 24 3.4 16.1 4.74 5 2.5 6.1 2.44 25 2.5 6.3 2.52 6 4.2 5.3 1.26 26 3.8 3.9 1.03 7 5.2 15.2 2.92 27 4.2 4.4 1.05 8 3.2 5.4 1.69 28 4.7 5 1.06 9 3.3 13.3 4.03 29 2.1 5.6 2.67 10 2.7 3.1 1.15 30 4 5.4 1.35 11 3.7 4.5 1.22 31 1 4.3 4.30 12 2.1 6 2.86 32 4 8.4 2.10 13 3.9 5.2 1.33 33 5 6.6 1.32 14 4.2 4.7 1.12 34 5.2 7.7 1.48 15 2.9 7.6 2.62 35 4.6 9.5 2.07 16 2.3 3.5 1.52 36 2.1 3.7 1.76 17 5 12.8 2.56 37 1 3.2 3.20 18 3.5 4.6 1.31 38 4.4 12.7 2.89 19 3.4 10.4 3.06 39 5.2 7.4 1.42 20 3.8 4.1 1.08 40 1.7 2.8 1.65 Ave. 3.7 7.83 2.19

Ave.: average of 40 samples.

Fig. 3. Back Scattered Electron Images (BSI) a) separated zircon grains (dark grey) and

well rounded apatite grains (white). b) separated fractured monazite grain.

Discussion

The composition of the Pleistocene-Holocene subsurface sediments

of the northern Nile Delta is dominated by sand size fraction followed by

silt and clay sizes, respectively (Table 2 and Fig. 2). Data of the grain


size analysis of the studied sediments indicates that the grain sizes are

most likely changed over time. Fluctuation of water flow is the main

cause for such changes. For example, an increase in silt at about 26-40m

is followed by an increase in sand at about 14-25m. This shift in

composition may reflect input of finer sediments in the deeper part

during large flood period (Fig. 4).

Fig.4. Vertical distribution of particle sizes in the studied sediments.

The contents of organic matter and clay fraction in these sediments

are remarkably higher than the corresponding constituents in the Nile

River streambed sediments (Dawood, 2010). Figure 5 shows a significant


positive correlation between organic matters and clay fractions in the

investigated sediments. On the other hand, carbonates, composed

predominantly of shell fragments of different sizes, are lower in the

subsurface sediments compared to the streambed sediments (Dawood,

2010).

Fig. 5. Bivariant plot between clay fraction and organic matter contents.

The heavy mineral varieties of these sediments reflect basically the

composition of the rocks in the drainage basin. Amphiboles, pyroxenes,

kyanite, sillimanite, andalusite and ilmenite seem to be derived from the

Ethiopian and Central African Highlands (Foucault and Stanley, 1989).

Monazite, zircon and apatite are most probably of local sources, i.e. the

Red Sea Hills and Aswan granites (Dawood and Abd El Naby, 2007).

Detrital zircon, apatite and monazite are considered as the major uranium

and thorium bearing minerals in these sediments. Uranium is associated

with zirconium and hafnium in zircon and sometimes with thorium and

cerium in monazite. In addition, uranium in the form of uranyl ion

(UO2)2+

commonly replaces Ca in the structure of apatite (Abd El Naby

and Dawood, 2008). Thorium is a main constituent in monazite, Th-rich

monazite and thorite. Thorite inclusions in monazite were reported by

Dawood and Abd El-Naby (2007) in the Mediterranean beach sediments

derived by the River Nile. Based on the microscopic investigation of

different magnetic fractions, zircon is more abundant than both monazite

and apatite (Fig. 3a, b).


While thorium is not soluble in near surface environments, dissolved

uranium is found in most natural water at very low concentration, where

the estimated worldwide average for dissolved uranium in rivers ranges

between 0.3 and 0.6 ppb (Scott, 1982). This value increases significantly

to be abnormally high in some cases due to chemical weathering of

uraniferous rocks such as in Platte River of the North American High

Plains Region where uranium content reaches up to 31.7 ppb (Snow and

Spalding, 1994). The main factors controlling uranium concentration in

river water are the prevailing Eh–pH condition (Scott, 1982) and

availability of HCO3− complex, where uranyl-carbonate plays a crucial

role in uranium migration (Langmuir, 1978; Mangini et al., 1979; and

Gorman-Lewis et al., 2008). Conversely, the uranium and thorium

contents of the Delta sediments depend, in general, on the mineralogical

forms of these two elements in the catchment area. River runoff delivers

uranium to the coastal zone and delta areas where interactions between

dissolved and particulate phases can act to remove from, or add to,

dissolved uranium in the water (e.g. Barnes and Cochran, 1993). The

average values of uranium and thorium in the subsurface sediments of the

northern Nile Delta are 3.7 and 7.83 ppm, respectively (Table 3), which

are comparable with the corresponding values in the majority of the

World Rivers. For example, the average uranium in the streambed

sediments of the River Nile is 2.06 ppm and that of thorium 6.58 ppm,

whereas uranium and thorium averages in the sediments of the rivers

draining into Gulf of Mexico are 2.48 and 9.2 ppm, respectively (Scott,

1982, Dawood, 2010).

Uranium and thorium in these sediments are most likely found

incorporated into minerals of detrital origin. In addition, uranium could

be adsorbed directly from river water onto clay minerals or organic

debris, removed from river water to sediments directly if the reducing

condition exists (Durrance, 1986). The interrelationships between

different components of the investigated sediments and both uranium and

thorium is shown in the correlation matrix (Table 4).

Thorium concentrations are strongly correlated with heavy mineral

contents (R=0.71) (Table 4, Fig. 6a). This correlation is obvious in the

vertical distribution of both variables (Fig. 7). The shaded rectangles in

Fig. 7 demonstrate the intervals of high correlation between thorium and

heavy mineral contents (2-26m and 34-40m). At the depth of 32m,

thorium shows a remarkable increase with increasing organic matter


contents reflecting a local thorium biosorption in the studied column.

Picardo et al. (2009) observed an increase in thorium biosorption with

increasing bed depth. Conversely, uranium while being less significantly

correlated with heavy minerals content (R=0.4), it shows a higher

positive correlation with organic matter contents (R=0.55) (Table 4, Fig.

6a, b). The (8-27m) depth interval marks this positive correlation (Fig. 7).

Samples of relatively low uranium, low organic contents and high heavy

minerals (e.g. sample 30) were microscopically investigated in detail.

These samples showed predominance of heavy mineral varieties of low

uranium contents such as amphiboles, pyroxenes and garnet. Based on

grain counting process, the ratios of (Am+Grt+Px)/(Zrn+Mnz+Ap) in

these samples are high, ranging from 5.2 to 7.3. Despite that, clays are

considered as a potential sorbent of uranium and the ability of

montmorillonite to sorb uranyl ions affects the fate and transport of

uranium (Chisholm-Brause et al., 2001; and Drot et al., 2007). Clays do

not show any particular correlation with uranium or thorium in the Nile

Delta subsurface sediments indicating minor role for radioelements

adsorption by clays (Table 4).

Based on analyses of numerous rock samples, Adams and Weaver

(1958) demonstrated the usefulness of the Th/U ratios as an indicator of

relatively oxidizing or reducing conditions. Thorium is unaffected by

redox conditions and remains insoluble as Th+4

. Uranium, however,

exists as insoluble U+4

under highly reducing conditions, which leads to

Table 4. Correlation matrix between different components of the subsurface sediments

and both uranium and thorium.

lights heaviesTotal Sands

Silt Clay Organics Carbonates U Th

lights 1.00

heavies 0.67 1.00

Total Sands 1.00 0.71 1.00

Silt -0.92 -0.64 -0.92 1.00

Clay -0.61 -0.55 -0.62 0.31 1.00

Organics -0.32 -0.11 -0.31 0.03 0.60 1.00

Carbonates -0.19 -0.14 -0.19 0.03 0.05 -0.02 1.00

U 0.13 0.40 0.15 -0.21 -0.06 0.55 -0.19 1.00

Th 0.40 0.71 0.43 -0.42 -0.31 0.18 -0.13 0.55 1.00

Value more than ± 0.55 is significant


uranium enrichment in sediments, whereas it exists as soluble U+6

under

oxidizing conditions, leading to uranium loss from sediments. Thus,

Th/U ratios could be used as a proxy for redox conditions of the

depositional environment varying from 0 to 2 in anoxic environments to

7 in strongly oxidizing environments. Lower ratios would indicate

reducing environmental conditions, most commonly marine (Carvalho et

al., 2011), while higher values would be associated with uranium

mobilization through weathering and/or leaching, and would therefore

indicate an oxidizing, possibly terrestrial environment. According to the

diagnostic values suggested by Adams and Weaver (1958), the ranges of

b

Fig. 6. Bivariant plots of: a) U, Th and heavy minerals content, b) organic matters vs. uranium

content in the subsurface sediments of northern Nile Delta.


Th/U ratio (1.03-4.74) of the subsurface sediments of northern Nile Delta

most likely reflect vertical variability based on changes in textures,

mineralogical composition and organic matter contents. A case

demonstrating exchange between fluvial, brackish and freshwater

regimes, which would be expected in delta areas.

Fig. 7. Vertical distributions of uranium (ppm), thorium (ppm), heavy minerals (%) and

organic matters (%) in the in the Pleistocene-Holocene subsurface sediments of the

Nile Delta in Egypt.


Conclusions

Particle size analyses, heavy minerals separation, microscopic

investigation, Back Scattered Electron imaging and Instrumental Neutron

Activation Analysis (INAA) were used to investigate the

sedimentological, mineralogical and uranium-thorium distribution of the

Pleistocene-Holocene subsurface sediments of northern Nile Delta. These

sediments are composed of three main formations namely; Baltim, Mit

Ghamr and Bilqas. The sand size fractions of these subsurface sediments

constitute about 39.51%, in average. Whereas, the averages of silt and

clay size fractions are 32.2 and 14.6%, respectively. About 25% of the

samples are classified into sand, 20% clayey silt, 18% silty sand, 15%

sandy silt and 15% sand-silt-clay. The organic matter contents range

between 3.28 to 15% with an average of 8.77%. The average content of

carbonates is 4.92%. The particle sizes variability of the studied

sediments indicates variation over time based on the fluctuation in water

flow. The average uranium content of these sediments is 3.7 ppm and

that of thorium is 7.83 ppm. The variation in Th/U ratios indicates

vertical inconsistency based on changes in textures, mineralogical

composition and organic matter contents of the subsurface sediments.

Correlation matrix between different components of studied sediments

showed significant positive correlations between organic matters and

clay fractions; thorium and heavy minerals contents and uranium and

organic matter contents. Geochemical behaviour of both uranium and

thorium, frequency of accessory minerals, particularly zircon, monazite

and apatite as well as sorptive uptake of uranium by organic matters are

the essential factors controlling the distributions of the two elements.

Acknowledgment

Laboratory facilities were kindly offered by the Faculty of Science

of Ain Shams University and Faculty of Earth Sciences of King

Abdulaziz University. Uranium and thorium contents were measured by

Instrumental Neutron Activation Analysis (INAA) at XRAL laboratories,

USA.

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