Research Journal of Environmental and Earth Sciences 4(3): 207-214, 2012
ISSN:2041-0492
© Maxwell Scientific Organization, 2012
Submitted: June 09, 2011
Accepted: August 08, 2011
Published: March 01, 2012
Distribution and Chemical Speciation of Some Elements in the Ground
Waters of Oban Area (South-Eastern Nigeria)
A.S. Ekwere and A. Edet
Department of Geology, University of Calabar, Calabar, Nigeria
Abstract: The computer modeling programs, PHREEQC and VISUAL MINTEQ were used to ascertain the
distribution, chemical speciation and mineral saturation indices of groundwater in the Oban massif (SouthEastern Nigeria). The prime objective was to determine the potential risk of groundwater by potentially toxic
Elements. Results reveals Fe, Mn, Ni, Pb, Cd and Cr are distributed as free ions. Oxides and hydroxides of iron
and manganese are predominant, reflective of mineralogy/geology of the crystalline basement. The groundwater
is super-saturated (SI>10), with respect to goethite, hematite, ferrihydrite, jarosite-k, hausmanite, manganite,
pyrochroite and pyrolusite. These species are relatively mobile under the prevailing pH-Eh regime, but total
concentrations are low and within permissible limits for safe water.
Key words: Groundwater, Nigeria, Oban, speciation, saturation index, toxic elements
predominance of isolated hills, largely on the eastern arm
of the massif, with maximum heights of about 1,200 m
above sea level (Ayi, 1987). The hills rise steeply and
abruptly, often dissected by V-shaped valleys and are
thickly forested to the summit of even the highest peaks.
The western flank is more sub-dued topographically.
Drainage within the massif is controlled by
weathered zones, fractured and jointed areas. The massif
is well drained with a network of rivers and associated
streams, actively engaged in erosion of channels (juvenile
stage). The drainage on the massif courses in two
directions: southwards and seaward and northward to join
the upper course of the Cross River in the Ikom
depression (Ekwere, 2010).
The study area is characterized by tropical climate
with two distinct seasons viz wet and dry. The wet season
spans a period of about six months (May to October) and
the dry season lasts from November to April. General
temperature trend for the study area is high with
negligible diurnal and annual variations. The average
monthly temperatures in the area ranged from 26-34ºC
during the period of study.
Mean annual rainfall of about 2,300 mm have been
reported for the area, with annual mean daily relative
humidity and evaporation of 76-86% and 3.85 mm/day
respectively, (Petters et al., 1989). Peaks of precipitation
usually are between the months of June to August, and
vary annually. Regional run-off coefficient of the study
area, are in the order of 0.21-0.61 and are due to
topography and evaporation, (Petters et al., 1989).
Geologically, the Oban Massif is described as being
underlain by highly deformed Precambrian crystalline
basement rocks, mainly migmatites, granites, gneisses and
schists. These rocks exhibit varying degrees of weathering
across the massif. They are intruded by pegmatites,
INTRODUCTION
The geochemical behavior, chemical and isotopic
properties of water is related to its location in the
hydrosphere (Langmuir, 1997). Chemical species of
elements or group of elements have a direct bearing on
their environmental chemistry. Speciation levels of these
elements reflect their mobility, bioavailability to living
organisms as well as their potential toxicity.
The variations in chemical constituents of
groundwater are believed to be indicators of the series of
simple and highly complex geochemical processes within
porous sub-surface units. These, resultant from reactive
transport of fluids within the sub-surface, produce a vast
array of chemical species existing in different phases.
The ability to detect and quantify the reactive
chemical phases has in recent times, been enhanced
through computer based programs for hydrogeochemical
modeling. These models provide an insight into the
general features of natural phenomenon, rather than
specific details. Thus, it probes the confidence of
employing hydrogeochemical models for prediction of
future evolution of groundwater systems.
This article attempts to determine the distribution of
mobile chemical phases of potentially toxic elements,
through modeling, for groundwaters within the crystalline
basement of the Oban Massif.
Description of study area: The study area, Oban
crystalline basement complex, lies between Longitudes
8º00! and 8º55! E and Latitudes 5º00! and 5º45!N covering
an area of about 8,740 km2 (Edet et al., 1998), Fig. 1. The
area lies at an average height of about 150 m a.m.s.l,
rising gradually from the south northwards and falls away
towards the Cross River to the North. There is a
Corresponding Auther: A.S. Ekwere, Department of Geology, University of Calabar, Calabar, Nigeria
207
Res. J. Environ. Earth Sci., 4(3): 207-214, 2012
Fig. 1: Geologic map of the study area (Oban massif): Modified from Ekwueme (2003)
Table 1: Some aquifer parameters across the Oban massif
Location
Hydraulic conductivity
parameter
Static water
level (m)(m2/day)
Abbiati
5.80"
13.42"
Akamkpa
4.00‡ 6.0
13.15"
Awi1
0.50‡
13.18†
Mbarakom
9.50‡
10.14†
Mfamosing
8.53†
Oban
4.70‡
12.46"
Uyanga
3.89" 0.65‡
†: Edet and Okereke (2005); ‡: Edet et al. (1998); : Ekwere (2010)
Yield (m3/day)
105.00‡
190.80‡
15.00‡
180.00‡
164.16‡ 129.60‡
Aquifer thickness
(m)
24.00
58.00‡
216.00‡ 55.00† 3.10‡
40.50† 45.80‡
50.40†
30.00‡
22.00
Transmissivity
(m2/day)
120.80‡
150.80
159.12†
123.42†
51.00†
132.40
Some reported aquifer parameters from the study area
are presented in Table 1. This presents data as determined
from boreholes and wells across the massif.
granodiorites, diorites, tonolites, monzonites, charnokites
and dolerites (Ekwueme, 1990). Economic minerals that
have been identified within this basement complex
include feldspars, galena, gemstones, graphite, gold,
ilmenite, kaolin, manganese, mica, quartz, rutile, tin and
uranium (CRSG, 1989). Currently mining activities within
the study area are limited to mining of aggregates for the
construction industry.
Weathered profiles, fractures and joints are prominent
features within these rock suites and they control the
movement and storage of groundwater as they are the
main aquifers within the massif. Groundwater in the study
area occurs under water table conditions in the weathered
and fractured units, with static water level ranging from
0.00-7.00m across the massif (Ekwere, 2010).
Rates and levels of recharge to porous aquiferous
media in the study area, suffer impedance due to the top
lateritic cover characteristic of the area, (Petters et al.,
1989. This is attributed to the high clay contents of these
lateritic top soils, hence their low permeability.
METHODOLOGY
A total of fifty six groundwater samples were
collected from twenty eight locations during the dry
season (February, 2009) and the wet season (June, 2009).
These samples straddled the various geologic units within
the study area. Two samples were collected from each
location in 75Cl polyethylene bottles. The sample bottles
were soaked in 10% HNO3 for 24 h and rinsed several
times with de-ionized water prior to use. At the sampling
locations, the bottles were thoroughly rinsed with aliquots
of the sampled waters, prior to collection. One sample
from each location was preserved by acidifying to pH Ca2
with 0.5 mL of concentrated HNO3 acid for trace metals
analysis.
Field measurements included temperature, electrical
conductivity, total dissolved solids, pH and Eh were
208
Res. J. Environ. Earth Sci., 4(3): 207-214, 2012
Table 2: Results of analysis for ground water (dry season)
Sample
code
Temp. pH
EC
TDS Fe
Mn
Ni
Cr
GWB 1
26
5.9
46.3 29.68 0.012
0.024 0.001
0.001
GWB 2
28
5.86 38.6 24.74 0.056
0.274 0.006
0.001
GWB 3
27
5.92 42.2 27.18 0.009
0.147 0.002
0.006
GWB 4
27
5.84 34.6 22.18 0.134
0.071 0.002
0.002
GWB 5
26
5.14 326
209.0 0.011
0.047 0.009
0.007
GWB 6
29
5.08 250
160.3 0.065
0.368 0.001
0.001
GWB 7
27
5.51 67.0 42.95 0.027
0.062 0.001
0.001
GWB 8
29
6.02 72.4 46.41 0.007
0.318 0.002
0.009
GWB 9
32
6.34 69.2 44.36 0.016
0.061 0.001
0.001
GWB 10
28
5.40 77.4 49.62 0.002
0.043 0.001
0.001
GWB 11
29
5.87 50.6 32.44 0.003
0.066 0.001
0.005
GWB 12
26
5.72 83.0 53.21 0.170
0.050 0.001
0.001
GWW 1
26
6.34 56.2 36.03 0.009
0.008 0.015
0.005
GWW 2
26
6.46 326
20.90 0.003
0.428 0.001
0.004
GWW 3
28
6.06 46.4 29.74 0.334
0.013 0.051
0.021
GWW 4
27
5.82 56.8 36.41 0.006
0.088 0.003
0.007
GWW 5
27
5.97 454
291.0 0.127
0.374 0.001
0.004
GWW 6
26
6.04 474
303.9 0.007
0.425 0.005
0.009
GWW 7
28
5.88 586
375.6 0.059
0.443 0.002
0.002
GWW 8
27
5.56 616
394.9 0.001
0.146 0.001
0.008
GWW 9
27
5.62 622
398.7 0.019
0.161 0.039
0.009
GWW 10 28
6.18 482
309.0 0.005
0.086 0.001
0.006
GWW 11 28
5.92 612
392.3 0.163
0.151 0.003
0.014
GWW 12 26
6.79 48.8 31.28 0.001
0.102 0.001
0.001
GWW 13 27
5.12 52.8 33.85 0.013
0.033 0.012
0.006
GWW 14 28
5.71 24.8 15.9 0.082
0.049 0.001
0.001
GWW 15 27
6.86 52.0 33.33 0.113
0.122 0.008
0.001
Mean
27.4 5.82 157.3 127.6 3.980
37.41 3.380
2.670
*: All values in ppm, except EC and temperature in :s/cm and ºC, respectively
Table 3: Results of analysis for ground water (dry season)
Sample
code
Temp. pH
EC
TDS Fe
Mn
GWB 1
30.4 6.96 160
110
1.472
0.048
GWB 2
28
6.74 76
120
1.513
0.197
GWB 3
28
7.02 94
140
0.916
0.102
GWB 4
27
6.27 60
40
1.436
0.053
GWB 5
29
6.32 370
570
0.872
0.023
GWB 6
28
6.29 180
120
0.655
0.089
GWB 7
28
7.12 60
40
1.759
0.039
GWB 8
26
7.86 240
370
0.104
0.224
GWB 9
28
8.65 320
190
0.137
0.056
GWB 10
28.5 7.29 430
280
0.176
0.097
GWB 11
27
7.85 180
280
0.120
0.082
GWB 12
28
7.89 120
80
0.201
0.114
GWB 13
28
6.90 310
210
0.046
0.005
GWW 1
28.6 7.69 90
60
2.421
0.068
GWW 2
28
7.06 190
130
0.012
0.082
GWW 3
27
6.55 240
370
4.018
0.128
GWW 4
29.4 6.45 100
70
0.731
0.104
GWW 5
30
6.76 410
290
9.115
0.007
GWW 6
28
6.82 430
570
2.264
0.119
GWW 7
29
6.64 520
780
2.784
0.201
GWW 8
28
5.88 240
170
0.004
0.014
GWW 9
27
5.94 376
540
0.028
0.017
GWW 10 28
6.70 200
140
0.003
0.093
GWW 11 27
6.48 578
890
0.107
0.127
GWW 12 28
7.06 80
120
0.001
0.086
GWW 13 28.5 7.14 120
180
0.009
0.028
GWW 14 27
7.62 580
470
9.800
0.015
GWW 15 27.5 6.98 310
480
8.760
0.094
GWW 16 27
7.64 110
80
0.027
0.046
Mean
28.0 6.75 247.38 233.54 1.710
0.081
Ni
0.018
0.036
0.004
0.003
0.019
0.014
0.005
0.012
0.013
0.015
0.006
0.007
0.041
0.009
0.016
0.024
0.012
0.014
0.019
0.010
0.008
0.044
0.005
0.009
0.012
0.009
0.061
0.010
0.003
0.016
Cr
0.008
0.019
0.007
0.009
0.016
0.012
0.027
0.034
0.019
0.014
0.020
0.003
0.006
0.006
0.004
0.008
0.003
0.011
0.007
0.004
0.012
0.009
0.005
0.017
0.011
0.021
0.036
0.003
0.007
0.012
Cd
0.001
0.001
0.002
0.002
0.001
0.001
0.002
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.003
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.058
Pb
0.001
0.001
0.001
0.001
0.001
0.001
0.004
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.008
0.003
0.001
0.001
0.001
0.001
0.003
0.001
0.001
0.001
0.001
0.001
0.001
0.154
Na
3.662
4.339
1.989
2.562
4.597
4.603
4.401
4.237
2.459
3.745
2.957
9.000
4.124
4.382
2.939
3.006
4.367
4.240
4.951
4.335
4.274
4.553
4.408
4.380
3.915
1.163
3.708
0.006
Ca
12.26
35.20
30.15
17.02
62.06
30.12
30.11
122.1
30.11
20.09
15.19
13.40
118.4
104.4
7.273
22.26
9.002
454.13
22.04
15.02
135.2
22.05
18.05
21.15
15.07
15.13
13.15
0.005
Mg
0.559
5.168
3.604
1.016
4.993
5.198
0.847
5.337
0.424
2.063
1.789
4.900
3.498
5.285
1.369
2.394
5.283
5.586
5.150
4.371
5.281
4.424
4.668
4.007
1.789
0.447
1.761
0.001
K
1.563
3.455
1.915
1.065
3.284
2.999
1.449
3.211
0.821
0.809
0.912
2.080
2.174
4.463
2.997
0.858
5.922
5.172
1.847
4.266
4.953
4.548
4.941
2.262
2.038
1.645
0.542
0.002
HCO3G
30.5
97.6
48.8
30.5
61.02
30.5
73.2
128.1
42.8
18.3
18.4
12.25
73.2
122.3
30.5
24.7
85.4
18.5
18.3
36.6
48.8
36.8
54.7
30.5
24.6
24.4
54.9
47.27
SO42G
2.94
30.77
9.091
1.092
113.6
61.54
564.1
136.4
13.64
5.88
29.41
1.22
40.91
45.47
68.18
29.41
0.572
47.06
43.59
50.07
1.282
91.18
117.9
32.35
27.05
5.128
12.83
58.62
NO3G
1.607
5.892
0.003
0.893
8.214
35.18
47.51
10.89
0.005
11.25
3.571
21.20
6.255
45.89
0.535
8.036
4.643
23.04
40.01
19.82
3.143
15.71
19.46
40.89
4.825
0.009
1.786
14.08
ClG
5.998
5.998
2.496
3.499
14.49
33.96
56.98
19.49
4.998
9.997
3.994
21.56
10.52
32.99
2.056
8.996
0.801
92.97
22.99
21.93
87.47
23.49
17.99
17.05
8.520
3.999
1.022
19.86
Cd
0.005
0.003
0.001
0.003
0.007
0.005
0.002
0.015
0.009
0.001
0.012
0.001
0.001
0.001
0.001
0.007
0.001
0.002
0.001
0.001
0.001
0.002
0.001
0.001
0.001
0.001
0.018
0.001
0.001
0.014
Pb
0.001
0.001
0.001
0.001
0.004
0.001
0.001
0.009
0.017
0.007
0.002
0.001
0.001
0.001
0.003
0.012
0.001
0.001
0.001
0.001
0.001
0.003
0.001
0.001
0.001
0.015
0.021
0.001
0.001
0.004
Ca
7.46
26.05
12.80
8.01
18.12
11.27
7.06
42.84
8.12
14.06
8.06
30.04
60.11
8.03
16.03
6.04
18.13
22.07
67.25
28.06
11.36
97.14
15.15
13.02
16.08
18.94
10.21
7.66
14.22
21.50
Mg
1.01
3.67
0.37
0.27
2.86
1.83
0.56
1.96
0.4
0.49
2.04
2.58
1.12
0.32
2.69
0.74
1.29
3.42
2.92
3.06
1.64
2.78
2.07
1.99
0.99
1.21
0.35
0.61
0.72
1.59
Na
4.65
5.71
2.42
2.07
4.79
5.6
4.93
5.09
2.3
2.96
3.01
4.97
1.76
2.45
6.7
2.72
4.06
6.97
5.87
6.76
4.32
4.45
5.66
5.69
5.02
3.72
2.65
4.02
4.41
4.35
K
1.01
4.69
2.11
0.19
4.04
0.96
1.35
1.62
0.74
0.6
1.32
3.86
0.74
0.99
29.81
1.21
32.18
30.16
28.72
32.04
3.26
3.62
9.79
10.13
3.26
2.56
2.58
1.02
32.18
8.51
Cl
128.5
256.4
56.74
27.99
98.2
149.9
232.9
187.5
123.6
596.6
126.4
50.62
103.1
103.5
75.48
26.74
105.5
130.9
180.2
194.6
104.5
126.8
48.49
33.46
28.05
22.14
308.7
27.29
37.99
127.35
HCOG3
72.14
97.55
52.14
24.16
58.74
18.25
36.47
94.56
67.1
213.5
186.2
54.89
127.9
30.41
42.63
12.62
48.64
66.98
24.22
12.14
24.34
21.92
36.49
48.64
28.2
36.48
384.3
32.14
30.48
68.42
SO42G
36.25
24.99
15.34
175.2
98.2
18.24
27.29
145.7
839.6
294.1
188.5
36.37
31.84
13.64
145.5
70.21
45.5
120.5
82.4
98.6
84.09
57.4
59.09
86.21
18.16
42.1
471.7
21.22
138.6
120.23
NO3G
0.69
0.95
0.09
0.18
1.25
2.57
0.26
10.04
8.27
4.14
2.14
1.8
1.94
0.48
2.88
0.23
0.53
3.18
11.72
7.27
1.18
0.34
1.04
1.01
8.49
2.13
1.98
0.63
0.73
2.69
carried out using standard field equipment (PHT-027
multi-parameter water quality probe). Prior to
measurement of pH, the electrode was calibrated using pH
6.88 and 4.01 buffer solutions at a similar temperature to
the water samples. The same meter and an ionode ORP
electrode were used to measure Eh.
Chemical analyses were carried out for the major ion
concentrations of water samples using the standard
procedures recommended by APHA (1995). Trace metal
contents were also determined by Atomic Absorption
Spectrometry (AAS).
RESULTS AND DISCUSSION
Concentrations of major and trace chemical species
are presented in Table 2 and 3. The analytical results
209
Res. J. Environ. Earth Sci., 4(3): 207-214, 2012
respectively in the dry season. In the wet season ranges
were 0.3-3.7mg/L for Mg and 0.2-32.18 mg/L for K.
Potassium exh ibited the lowest concentration levels
relative to other cations. This is common in natural waters
due to its tendency to be fixed by clay minerals and
precipitate in the formation of secondary minerals
(Matheis, 1982). However K+ recorded the highest mean
concentration of 13.97mg/L next to calcium (23.09 mg/L)
in the wet season.
The major anions and nutrient analyzed were
bicarbonate (HCO32G), chloride (ClG), sulphate (SO42G) and
nitrate (NO3G). Sulphate and bicarbonate were the
dominant anions within the dry season with mean
concentration values of 61 and 48 mg/l, respectively. In
the wet season chloride was most dominant with an
average concentration value of 130.5 mg/L. Sulphate had
a mean value of 123 mg/L and bicarbonate lesser at
70 mg/L. Nitrate was least with mean values of 14 and
2.7 mg/L in the dry and wet seasons respectively.
Concentrations of trace metals (Fe, Mn, Ni, Cr, Cd
and Pb) are also presented in Table 2 and 3. Ranges of
values of the selected trace metals exhibit a general trend
of increase from the dry to wet season. Iron (Fe)
concentration in groundwater ranged from 0.001 to 9.80
mg/L for both sampling seasons. Known primary sources
of iron are silicates and aluminosilicates (olivine,
pyroxene, amphibole mineral groups and the mica
biotite), which characterize metamorphic and igneous
rocks (Deer et al., 1992). Pyrite (FeS2) and magnetite
(Fe2O4) are other common minor minerals. Iron (Fe2+) is
largely mobilized and redistributed during the chemical
weathering of igneous and metamorphic rocks. Relatively
show the pH ranged from 5.08-6.86 in the dry season. The
waters tended to be less acidic to alkaline in the wet
season with pH range of 5.88-8.65. These differences in
pH are related to variation in soil CO2 and bicarbonate
concentration. The Electrical Conductivity (EC) which
expresses ionic strength of solution, varied between
20.9 to 622/cm in the dry season and 60-800/cm in the
wet season. The water in the massif exhibited average
values of Total Dissolved Solids (TDS) concentration as
121ppm in the dry season and 265ppm in the wet season.
Assessment shows that that EC and TDS concentration
values are lower in the dry season compared to the wet
season.
The major cations analyzed included calcium,
magnesium, sodium and potassium. The concentrations
exhibited the relationship, Ca2+ > Na+ > Mg2+ > K+.
Calcium (Ca2+) was the most dominant cation accounting
for about 78 and 61% of total cation in the dry and wet
seasons respectively. Concentration of Ca2+ ranged from
7.27-135.2 mg/L in the dry season and 6.04-97.14 mg/L
in the wet season. This dominance of Ca2+ may reflect the
process of chemical weathering of silicates and the
common occurrence of calcium carbonate (Langmuir,
1997). Water-rock reactions of mineral phases such as
plagioclase within the basement can be adjudged from
this.
Sodium (Na+) was next in dominance with
concentration ranges of 1.2-9.0 and 1.8-7.0 mg/L in the
dry and wet seasons respectively. The mean values were
3.98 and 4.35 mg/L for the dry wet seasons respectively.
The other cations, magnesium (Mg2+) and potassium
(K+) ranged between 0.4-5.4 and 0.5-5.9 mg/L
Table 4: Correlation coefficient of parameters from the dry season
Temp.
EC
TDS
pH
Na
Temp.
1.000
EC
0.011
1.000
TDS
0.011
1.000
1.000
pH
- 0.085
- 0.335
- 0.335
1.000
Na
- 0.136
0.416
0.416
- 0.385
1.000
K
- 0.080
0.735
0.735
- 0.321
0.514
Ca
- 0.080
0.190
0.190
- 0.098
0.257
Mg
- 0.098
0.642
0.642
- 0.306
0.695
Cl- 0.146
0.547
0.547
- 0.293
0.420
HCO3
0.029
0.012
0.012
0.022
0.199
SO4²
- 0.026
0.014
0.014
- 0.199
0.119
NO 3
- 0.108
0.307
0.307
- 0.197
0.570
Table 5: Correlation coefficient of parameters from the wet season
Temp.
EC
TDS
pH
Na
Temp.
1.000
EC
- 0.315
1.000
TDS
- 0.292
0.844
1.000
pH
- 0.209
- 0.062
- 0.251
1.000
Na
0.471
- 0.376
- 0.096
- 0.251
1.000
K
0.364
- 0.087
0.027
- 0.136
0.612
Ca
0.038
0.000
0.158
- 0.200
0.302
Mg
0.269
- 0.182
0.029
- 0.236
0.785
ClG
- 0.114
0.385
0.221
0.064
- 0.203
HCO3
- 0.450
0.704
0.407
0.115
- 0.579
0.395
0.687
0.335
0.329
- 0.625
So42 −
NO3G
- 0.137
0.280
0.218
0.352
0.014
K
Ca
1.000
0.428
0.800
0.552
0.454
0.020
0.352
1.000
0.512
0.533
0.625
0.069
0.187
K
Ca
Mg
1.000
0.532
- 0.036
- 0.236
- 0.296
0.186
1.000
- 0.103
- 0.387
- 0.448
0.111
1.000
0.221
0.537
- 0.145
- 0.298
- 0.270
0.126
210
Mg
1.000
0.503
0.438
- 0.061
0.493
ClG
HCO3G
SO4²G
NOG3
1.000
0.100
0.314
0.513
1.000
0.285
0.214
1.000
0.475
1.000
ClG
HCO3G
SO42G
NO3G
1.000
0.391
0.273
0.163
1.000
0.778
0.128
1.000
0.397
1.000
Res. J. Environ. Earth Sci., 4(3): 207-214, 2012
higher values in the wet season are attributed to leaching
of iron (Fe) from soil material into the water table as
water level increases alongside increase in precipitation.
A reverse scenario for the dry season is related to
precipitation of iron compounds with decrease or
recession of water levels and amount of rainfall. These are
related to the prevailing pH-Eh (redox potential) of the
waters (Hem, 1986).
Manganese (Mn) concentrations ranged from 0.0050.443 mg/L. The mean values were 0.154 and 0.081mg/L
for the dry and wet seasons. Concentration levels of Ni,
Cr, Cd and Pb were also recorded. Mean values during
the dry and wet seasons respectively for these elements
are; Ni (0.001, 0.016), Cr (0.005, 0.012), Cd (0.001,
0.004), and Pb (0.002, 0.004) with all values in mg/L
Trace metals have been known to be deposited by
precipitation in particulate form, associated with washout
of windblown dust or aerosols (Nriagu, 1996).
Higher levels of trace metals are noticeable on the
western flank of the study site, characterized by higher
human population density. This trend could be adjudged
to biodegradation of organic wastes with release of metal
loads eventually leached into sub-surface waters. The
western sector of the study area also has a history of small
scale mining activities. Studies have shown however that
metal concentrations in groundwater are diminished by
influx of unpolluted waters and by removal of metals by
adsorption and precipitation processes (Rosner, 1998;
Paulson, 1999; Berger et al., 2000). These series of
processes could be adjudged from the concentration levels
of metals which are within permissible limits of WHO
(2001) standard for potable water.
(Mg)-HCO3, Ca(Mg)-Cl and Na-K-Cl in the dry season.
In the wet season the controlling species are Na(K)-HCO3
and Na(K)-SO4. Na and Cl exhibit a very subtle
coefficient of 0.42 in the dry season.
A rather significant but negative correlation exist
between Na+ and SO42G. This could be from glauberite,
possibly associated with evaporites from the sedimentary
terrain in close proximity. Chloride and sulphate increase
from the dry to the wet seasons. Sulphate behaves as a
mobile ion in relatively oxidizing groundwater
environment except in conditions of oversaturation of
gypsum or anhydrite (Michard et al., 1996). This mobile
SO42G could enhance CEC, leading to dissociation of NaCl waters from crystalline basement or other sources
(Pearson et al., 1989).
Hydro chemical modeling: The modeling package
PHREEQC (Parkhurst, 1995) and VISUAL-MINTEQ
were used to calculate distributions of aqueous species
and mineral saturation indices using the measured ground
and surface water compositions. The employment of
VISUAL-MINTEQ as a complimentary modeling tool
was due to its more extensive database of mineral and
aqueous species for interpretation purposes, in
comparison to PHREEQC.
Mineral equilibrium calculations for groundwater are
useful in predicting the presence of reactive minerals in
groundwater system and estimating mineral activity;
(Taylor et al., 1996; Deutsch, 1997; Zhu et al., 2008).The
groundwater or solution is considered to be in equilibrium
with regards to a particular mineral if the saturation index
(SI) = 0. It is considered to be undersaturated if SI<1 and
oversaturated if the SI>1.
Correlation matrices: Pearson correlation matrix is
normally used to find relationships between two or more
variables. Variables showing r>0.7 are considered to be
strongly correlated, whereas r>0.5-0.7 shows moderate
correlation at a significance level (p) of <0.05 (Table 4
and 5). Correlation of samples from the dry season shows
that there exist, a significant positive correlation between
the following parameters; electrical conductivity (EC),
TDS with potassium (K). A moderate positive correlation,
exist between Ca-Mg, HCO3, Cl; Mg-EC, TDS, Na and
Na-K, NO3 .
Correlation analysis of wet season samples reveals a
significant positive correlation for EC with TDS and
HCO3 G, Mg HCO3G. Moderate positive correlation exist
between; EC-SO4, Mg-Ca, K and negative for Na-HCO3,
SO4.
The major exchangeable ions Ca-Mg and Na-K
correlate positively with correlation coefficients found to
be within same order across sampling seasons. It can
therefore be said that the concurrent increase or decrease
in the cations is the result mainly of ion exchange effects.
The recognizable chemical species association controlling
the groundwater types from correlation results are; Ca
Predicted aqueous species: Metals can exist in water as
many aqueous species and the type(s) of species can
effect mineral solubility, adsorption/desorption behavior
and possibly bioavailability. The predicted predominant
aqueous species from hydrochemical modeling are given
below for the metals measured and detected in
groundwater samples.
Iron: Iron existed in many aqueous species which
included; Fe2+, FeO(OH), Fe2O3, FeHCO3, FeSO4(aq),
FeCO3, FeOH+(aq), FeCl+, FeHSO4+, FeCl3, Fe3(OH)45+ and
Fe(OH)3G. Fe2+ was the dominant specie constituting
about 94%. Other species FeHCO3 and FeSO4(aq),
constituted (<6%) and (<2%), respectively. According to
Reimann and Caritat (1998) Fe is toxic to humans in
drinking water at levels of >200mg/L. For the Eh-pH
systems [(Fe-O-H), (Fe-C-O-H), (Fe-Si-O-H), (Fe-O-HSi), (Fe-C-Si-O-H); Brookins (1988)], under the
prevailing Eh-pH condition, Fe2+ is immobile. Its mobility
will further be hindered in the area by precipitation as Fe
oxides (hematite, magnetite), oxyhydroxides (goethite)
and other co-precipitating metal phases (Edet et al.,
2004).
211
Res. J. Environ. Earth Sci., 4(3): 207-214, 2012
Nickel showed a near equal concentration of the various
aqueous species in most samples. Speciation of cadmium
appeared to be dependent on pH for the various samples
with no predominance of a particular species, but contents
of Cd, its carbonate and chloride were common in most
samples. Chromium with minimal concentrations in most
samples, exhibited a predominance predicted to be Cr2+.
Free metal species are the most bio-available and
toxic form of trace elements that exist in natural water
(Apte et al., 1995). The presences of considered metals as
free ions are relatively mobile under prevailing Eh-pH
conditions. Their low concentrations in addition to the
presence of limiting mineral phases (carbonates and
sulphates) reduce the risk of their contamination to
groundwater.
Manganese: The various species of manganese were,
Mn2+, MnHCO3, MnSO4, MnCO3(aq), MnCl, Mn(OH)42G,
Mn2(OH)3, MnCl2, Mn(NO3)2(aq) and Mn3+. Mn is nontoxic and the dominant phase was the ionic Mn2+
accounting for over 88% in the groundwaters. Lesser to
this was MnHCO3 constituting <6%. Under the present
pH- Eh of the groundwater, Mn2+ is immobile posing no
problem in the area.
Nickel: The Ni species in the groundwater were, Ni2+,
Ni(OH)2(aq), Ni(OH)3G, Ni(SO4)22G, NiCl, NiCl2(aq),
NiCO3(aq), NiHCO3G and NiOH, NiSO4(aq). Nickel showed
a near equal concentration of the various aqueous species
in the groundwaters. Ni2+ compounds are non-toxic
(Reimann and Caritat, 1998) and its immobility under the
present pH-Eh condition, Ni does not pose any threat to
the groundwaters in the area.
Mineral saturations states and control on groundwater
composition: Mineral saturation states were calculated as
part of the output from the modeling programs. They are
useful for indicting what minerals might be dissolving or
precipitating into or from groundwater, or controlling the
groundwater composition. The calculations are based on
an equilibrium model, so the results are only an
indication, as kinetic factors may inhibit approach to
equilibrium.
The minerals which exhibited oversaturation (SI>1)
were; goethite, hausmanite, hematite, jarosite-k,
manganite, pyrochroite, pyrolusite and ferrihydrite,
Fe(OH) . These species can be said to be supersaturated
as they exhibited saturation of up to 10 orders of
magnitude or more (i.e., SI>10 +). These were basically
oxides, hydroxides and hydrated oxides of Fe and Mn,
(Ekwere, 2010).
Undersaturated mineral phases included the
following; anhydrite, aragonite, calcite, dolomites,
gypsum, rhodochrosite, siderite, melanterite and the vast
species from the analyzed elements. These included
carbonates, sulfates as well as chlorides. The waters were
unsaturated or less supersaturated with respect to most of
the minerals, reasons largely differences in
physicochemical conditions. Reactions of groundwater
with, CO3 and SO4 minerals are often expected to reach
equilibrium within years in the aquifer. This however
depends on the abundance of such mineral phases within
the porous media.
The modeling indicates that groundwater
compositions are controlled by a complex set of minerals
and processes. Dissolution, precipitation and cation
exchange are the dominant of the processes within
groundwater systems. Some elements tend to be
controlled by mineral solubilities and equilibrium
processes. Others invariably are controlled either by
unknown minerals or kinetic reactions. Some elements
e.g., Fe, are controlled by mineral solubilities and
equilibrium processes, though not directly important to
Lead: Species of lead (Pb) in the groundwaters included;
[Pb2+, Pb(CO3)22+, Pb(NO3)2(aq), Pb(OH)2(aq), Pb(OH)3G,
Pb(SO4)2, Pb2OH3, Pb3(OH)42+, Pb4(OH)44+, PbCl,
PbCl2(aq), PbCl3G, PbCl42+, PbCO3(aq), PbHCO3+, PbNO3+,
PbOH+, PbSO4(aq)]. The dominant dissolved specie was
Pb2+ constituting > 70% of all species. Pb(CO3)22+ and
PbCl are in the range of 2-17% and >8%, respectively.
The distributions of the species show no marked
difference for the various groundwater types. Pb2+ appears
to be mobile but however poses no problem of
contamination as Pb is low (mean 0.079 mg/L) and
PbCO3 will restrict its mobility (Edet et al., 2004).
Cadmium: The major ionic species of Cd in the
groundwaters is Cd2+. Other species were, Cd(NO3)2(aq),
Cd(CO3)22G, Cd(OH)2(aq), Cd(OH)3G, Cd(OH)42G,
Cd(SO4)22G, Cd2OH3+, CdCl+, CdCl2(aq), CdCO3(aq),
CdHCO3+, CdNO3+, CdOH+ and CdSO4(aq). Speciation of
cadmium appeared to be dependent on pH for the various
samples with no predominance of a particular species, but
mixtures of Cd and its carbonate and chloride were
common in most samples. At present Ph-Eh condition,
Cd2+ which is toxic and carcinogenic is immobile and if
present in high concentrations may be adsorbed by clay,
thus reducing its danger potential (Edet et al., 2004).
Chromium: Chromium with minimal concentrations in
most samples, exhibited a predominance predicted to be
Cr2+. The only other specie was CrOH+. This potentially
toxic element is immobile posing no threat to
groundwaters as its content was low at a mean of
0.009 mg/L.
Iron exist as many aqueous species, but the
predominant ones were invariably predicted to be siderite,
goethite, haematite, ferrihydrite and Fe. Manganese had
hausmanite, manganite, pyrochroite and pyrolusite as
predominant species. Lead in aqueous form was predicted
to be distributed in several species, but Pb2+, aqueous Pbsulphate and carbonate made up a greater percentage.
212
Res. J. Environ. Earth Sci., 4(3): 207-214, 2012
Edet, A.E., C.S. Okereke, S.C. Teme and E.O. Esu, 1998.
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water quality, they can affect the precipitation or
dissolution of other more toxic elements, e.g., Pb and Ni.
CONCLUSION
The groundwater status alternates from slightly acidic
to slightly alkaline from the dry to the wet seasons. The
concentration of potentially toxic elements resulting from
hydrochemical processes is low and may be diminished
by influx of unpolluted water and removal of metal by
adsorption and precipitation processes.
Correlation shows major exchangeable ions, Ca-Mg
and Na-K, to correlate positively, indicating cation
exchange as one of the major dynamics of the
hydrochemical status.
Speciation calculations show that Fe, Mn, Ni, Pb, Cd
and Cr are present as free ions and as series of other
species in all the analyzed groundwater samples. The
groundwater is super-saturated with respect to goethite,
hematite, ferrihydrite, jarosite-k, hausmanite, manganite,
pyrochroite and pyrolusite (basically oxides and
hydroxides of Fe and Mn). However the low
concentrations of these metals in addition to the presence
of limiting mineral phases (carbonates and sulphates)
reduce the risk of their contamination to groundwater
within the study area.
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