INTRODUCTION
More than 90% of public-private water supplies in Yola area come from groundwater
(Ishiaku and Ezeigbo, 2000). The high dependence on
groundwater means that there is an urgent need to maintain existing production
wells or boreholes.
A good borehole is one that lets water flow into it easily. In other words,
it is an efficient access hole which is determined by pumping the hole at different
rates and measuring the drawdown. It is thus normal for the efficiency of a
borehole to deteriorate with time and there are many varied reasons for this
(Amah et al., 2008).
Natural, non-pathogenic bacteria occur in all aquifers and unfortunately can
grow as sticky, thick slimes in the pores of a gravel pack and/or in the cracks
in the rocks, that let water flow into the hole (Smith, 2005).
A similar process takes place in the presence of some minerals in the water.
Thus physic-chemical changes take place when water flows from the aquifer into
the hole (Todd, 1995). Unstable minerals such as calcite
(lime-scale) and iron and manganese can precipitate and form encrustations and
corrosion in the gaps used by the water to enter the hole which is sometimes
also encouraged by the growth of bacteria (Appelo and Postma,
2007). But unfortunately many borehole owners and drillers in Yola area
are neither conversant with these problems nor affect a regular maintenance
of their boreholes.
Previous published works in the study area assessed groundwater quality with
emphasis on domestic and agricultural purposes (Ishiaku
and Ezeigbo, 2000; Obiefuna and Orazulike, 2010a-c;
Obiefuna and Orazulike, 2011) with only one (Ishiaku
and Ezeigbo, 2000) highlighting the effects of corrosion and encrustation
on boreholes in the study area. Related study in other parts of the world (Shomar
et al., 2008; Sundaramanickam et al.,
2008; Daghrah, 2009; Hilles
and Al-Najar, 2011; Alslaibi et al., 2011).
In this study, an attempt was made at identifying the presence of inorganic
substances tagged Corrosion-Encrustation Index Parameters (CEIP)
known to be responsible for corrosion and encrustation in the water industry.
These parameters act as a marker for the measurement of the degree of corrosion
and encrustation in water wells and pipeline systems (Amah
et al., 2008).
MATERIALS AND METHODS
The study area falls within latitudes 9°11N and 9°24N and longitudes 12°20E and 12°34E and lies about 50 km south of the Hawal Massifs. It is bounded to the east by the Republic of Cameroun and to the west by Ngurore town. The northern boundary is demarcated by Gokra town and the southern boundary by the Mandarare town and occupies approximately 431 km2 of the land surface (Fig. 1).
The study area is underlain by the upper member of the Bima Sandstone (B3)
which is a cretaceous sedimentary unit of the Yola Arm of the Upper Benue Trough
(Carter et al., 1963; Abubakar
et al., 2006). The surface geologic units of the study area are the
fine-medium grained sandstone to the north and south and the coarse grained
sandstone to the northeast. The depth to the bedrock varies from 30 m to more
than 45 m (Obiefuna and Orazulike, 2010a). Stratigraphically,
the Bima Sandstone consist of alternating layers of poorly to moderately consolidated
fine to coarse grained sandstones, clay-shales, siltstone and mudstone with
an average thickness of more than 250 m as seen from their outcrops in the field.
Two aquifer systems namely the upper unconfined alluvial aquifer and the lower
semi-confined to confined aquifer and the lower semi-confined to confined aquifer
were identified based on hydrolithologic analyses, geological reconnaissance
and hydrogeological mapping. The upper alluvial aquifer system occurs at a dept
range of 20 to 80 m with an average thickness of about 39 m. The mean hydraulic
conductivity is 2.54 m2 day-1 with corresponding mean
transmissivity value of 237 m2 day-1 which indicate moderate
to good aquifer. The lower semi-confined to confined aquifer system occur at
a depth range of 80 to 250 m with an average thickness of about 14.52 m. The
mean hydraulic conductivity is 2.54 m2 day-1 with corresponding
mean transmissivity value of 103.51 m2 day-1 which indicate
moderate to good aquifer (Obiefuna and Orazulike, 2010a).
To the northeast a localized recharge area occurred to the west whereas to
the southwest it is northeastwards towards the Benue River (Obiefuna
and Orazulike, 2010a). They discharge naturally at points or areas where
the aquifer with its underlying relatively impermeable alluvial units such as
clay-shales and mudstone intercepts the ground surface in river or stream valleys.
|
Fig. 1: |
Map of the study area showing some well locations and sampling
points |
Groundwater samples were collected from twenty-five locations within the study area. The water samples from boreholes in which pumps are already installed, were collected after about two hours of pumping and the screen interval of the well represents the average sample depth. The samples were bailed, using a stainless steel bailer, from a depth of two meters below the water table which more or less indicates the sample depth. The samples were collected in 1000 mL plastic bottles and field filtration was carried out through filter papers (0.45 μm) to remove suspended solids. They were then carefully sealed, labeled and stored in a refrigerator at 4°C prior to analyses within 24 h of collection. Electrical Conductivity (EC), hydrogen ion concentration (pH), Temperature (°C), Dissolved Oxygen (DO) were determined in the field. The temperature and electrical conductivity were measured with the aid of HACH spectrophotometer equipment model No. DR/2400 temperature and conductivity meter whereas pH and DO were determined with HACH Instruments Incorporated USA spectrophotometer equipment model No DR/2400 pH meter and oxygen meter, respectively. Chemical analyses of other ions were performed in the laboratory employing standard methods, Atomic Absorption Spectrophotometer for cations and conventional titration for anions (by titrimetric method, using the Titro Process Dosimat 665 equipment). Ions were converted from milligram per litre to milliequivalent per litre and anions balanced against cations as a control check of the reliability of the analyses results.
Statistical analysis: Data analyses to check for statistical errors
(a reflection of random fluctuations in the analytical procedure) was carried
out. It was done (from electrical balance EB calculations since the sum of positive
and negative charges in the water should be equal) and expressed mathematically
as follows (Appelo and Postma, 2007):
where, cations and anions are expressed as meq L-1 and inserted with their charge sign. The sums are taken over the cations (Na+, K+, Mg2+ and Ca2+) and the anions (Cl¯, HCO3¯, SO42¯ and NO3¯) and were found to be within the acceptable range of ±5%.
RESULTS AND DISCUSSION
The results of the chemical analyses are presented in Table 1 whereas Table 2 indicates the range and percentage of Corrosion and Encrustation Index Parameters (CEIP) in sampled borehole water.
Corrosion parameters: The following corrosion index parameters (pH,
DO, HCO3, TDS, CI, H2S and Temperature) were evaluated and used to
assess the corrosion potentials of sampled borehole waters. Table
1 indicate a pH values varying from 6.5 to 7.8 with a mean value of 7.14
revealing 40% acidic water suggesting corrosion and 60% alkaline water suggesting
encrustation. These values however fall within the WHO (2006)
recommended range of 6.5 to 8.5.
The bicarbonate values ranged from 50 to 207 mg L-1 with 96% of
the sampled water having bicarbonate value above 50 mg L-1 (Johnson,
1975) indicating corrosive groundwater. The dissolved oxygen varies from
0.02 to 2.80 mg L-1 with 12% of the sampled water having dissolved
oxygen value above 2.00 mg L-1. It thus suggests corrosive groundwater.
The relatively high concentrations of HCO3 and DO in the sampled
water tend to lower the pH of the water, increase its acidity and cause corrosion
of metallic objects used in borehole construction. Groundwater in the study
area is generally shallow and thus contains relatively high concentrations of
dissolved oxygen in places (Schwartz and Zhang, 2003).
Furthermore high concentration of bicarbonate in the sampled water may be due
to increase in dissolved carbondioxide generated in the soil zone when water
comes in contact with calcite and limestone as well as from fossil fuel combustion
released into the atmosphere.
Table 1: |
Physicochemical characteristics of groundwater samples for
yola area on the bases of Corrosion-Encrustation Index Parameters (CEIP) |
 |
The shallow groundwater will subsequently become rich in carconic acid. In
addition, the interaction between the liberated carbondioxide and the atmospheric
precipitation results in the formation of carbonic acid as follows (Schwartz
and Zhang, 2003):
Thus this antecedent acidic rain water subsequently infiltrates into the groundwater
system to lower the pH of the groundwater and increase acidity. Hence about
4% of the sampled groundwater has hydrogen sulphide concentration above 2 mg
L-1. However, the relatively low level of chloride (<160 mg L-1)
and TDS values (<122 mg L-1) are generally less than 500 mg L-1
for chloride and less than 1000 mg L-1 for TDS based on Johnson(1975)
suggest that these parameters contributes negligibly to corrosion of boreholes
in the study area.
The higher the temperature of groundwater the more aggressive the environment
and consequently the tendency towards corrosion of borehole installation as
found in boreholes drilled in the Chad Basin with surface temperatures of not
less than 28°C (Amah et al., 2008).
Furthermore, temperatures ranging from 25 to 27.5°C recorded for boreholes in Calabar area have been suggested as one of the causes of corrosion of wells in the area. Accordingly the relatively high acidity (or low pH) and bicarbonate (HCO3) and temperature (Fig. 2a) are likely one of the contributing factors to corrosion of some boreholes in the study area (Table 2).
High bicarbonate concentrations in groundwater arising from the presence of
unstable minerals such as calcite in aquifer zone can result in CaCO3
clogs and reduce well performance. Calcite deposition is often observed in wells
where methane is present which is subsequently oxidized to CO2 by
microbial oxidation. These usually take the form of a calcified slime which
looks like white spots in rock or casing surfaces. About 5% loss in specific
capacity of wells arising from high concentrations of dissolved oxygen, total
alkalinity and low pH has been reported in New York by the United States Geological
Survey (Johnson, 1975).
Encrustation parameters: The degree of encrustation taking place in
the well screen openings and water bearing formations can be evaluated employing
the following encrustation index parameters such as Iron (Fe), Manganese (Mn),
hydrogen iron concentration (pH) and Total Carbonate Hardness (TCH). The concentrations
of Iron (Fe) and Manganese (Mn) in the sampled water ranged from 0 to 1.80 mg
L-1 and 0 to 0 mg L-1, respectively. These results indicate
that 16 and 0% of the 25 boreholes sampled have Fe and Mn contents greater than
0.3 mg L-1 for Fe and 0.1 mg L-1 for Mn respective requirements
for the occurrence of chemical encrustation on well screen and aquifer to take
place (Table 2). The results thus indicate that only few samples
will stain laundry and plumbing fixtures but encrustation from precipitation
of iron and manganese compounds on well screens, pipes and aquifers are quite
uncommon. However, the high concentration of iron in few of the samples in the
study area is due to corrosion of well structure and piping materials in contact
with acidic groundwater, especially if the borehole was shut down for some time
or not flushed out before pumping (Amah et al., 2008).
Encrustation may also occur when suspension of fine particles of clay and silt
are carried unto the screen probably due to improper development of the borehole
or wrong choice of screen slot size or screening a portion of the hole containing
an abnormal amount of these materials (Amah et al.,
2008).
The carbonate hardness recorded ranged from 10 to 140 mg L-1 with
a mean value of 47.79 mg L-1 with only 4% of the sampled groundwater
exceeding 100 mg L-1 revealing low risk level in most of the sampled
groundwater (Johnson, 1975; Amah et
al., 2008). Furthermore, according to Durfor and
Becker (1964) classification 76% of the sampled groundwater are soft whereas
24% are moderately hard to hard indicating minimal contribution to encrustation.
Thus encrustation of groundwater wells in few of the wells is mainly due to
high total iron, high pH (alkalinity) and high TCH contents of the water (Fig.
2b).
|
Fig. 2 (a-b): |
Plot of histogram of percent (a) Corrosion Index Parameters
(CIP) and (b) Encrustation Index Parameters (EIP) against water quality
parameters |
Oxidation of organics in leachates found in the study area (which is high in
organic carbon) raises the pH of groundwater and precipitates CaCO3
especially if carbonate is near saturation. These cause CaCO3 clogging
which reduces well performance as mineral build up in the intake zone reduces
effective hydraulic conductivity of the system.
Prevention of borehole deterioration: This study indicate that the study
area generally has low corrosion and encrustation risk and borehole failures
may be largely due to poor pump selection, use of inferior materials as riser
pipes and poor pump installation and operation. These lead to burning of pumps
and/or collapsing and falling into boreholes (Ishiaku and
Ezeigbo, 2000).
Borehole corrosion and encrustation risk observed in few locations could be minimized if appropriate materials are used in well construction and completion by borehole designers and drillers. These include the use of corrosion resistant casings and well screens made of non-ferrous metal alloys (stainless steel) as well as PVC pipes and coating of borehole installation materials with galvanized zinc and tar.
Encrustation due to high concentration of iron in the sampled groundwater may be controlled through oxidation of soluble ferrous ions to insoluble ferric ion. The precipitates formed as a result is subsequently removed by energetic flocculation, filtration and chlorination. Furthermore, conduction of pumping test prior to pump selection and installation and use of good quality materials as riser pipes is strongly recommended. In addition regular maintenance of boreholes involving air lifting, jetting, acid treatment and chlorination is recommended.
CONCLUSION
The assessment and interpretation of CEIP indicate low risk level to corrosion and encrustation for majority of boreholes in the study area. These suggest that high rate of borehole failures in the area could be due to poor pump selection and operation, use of inferior materials as riser pipes and lack of regular maintenance of boreholes. Hence, conduction of pumping test prior to pump selection and installation and regular maintenance of boreholes involving air lifting, jetting, acid treatment and chlorination is recommended.
ACKNOWLEDGMENTS
The study described in this report is based on data generated for a doctorate degree dissertation by the first author Gabriel Obiefuna under the supervision of Prof. D.M. Orazulike. The authors will like to thank the Adamawa State Water Board Yola for their assistance during field work and to the Federal University of Technology Yola for giving the first author study fellowship to pursue a doctorate degree programme at the A.T.B.U Bauchi.
Finally I will like to thank Mr. Valentine for typing the manuscript and Ibrahim Ahmed for drafting the figures.