Natural and man-made sources of ionizing radiation are present in the environment in which man lives; there is continuous bombardment of man and his environment by these ionizing radiations (Eisenbud and Gesell, 1997). This is as a result of the presence of radionuclides in all activities undertaken by man. Naturally occurring radionuclide are present in the food we eat, the air we breathe and the water we drink and have resulted in health hazard among the general public. Some of these exposures are relatively constant and uniform to all individuals throughout the world (Aregunjo et al., 2004).
Jibiri et al. (1999) reported that an increase in the background ionizing radiation from numerous sources has various health side-effects on the populace. Also, photographs, exposing the body parts to radiation in the course of treatment of cancer and digital mammography can lead to high exposures (Andrew and Len, 1993). Radionuclides from both natural and artificial sources can come into contact with water though several ways. They may be deposited from air; they may also be released to the water from the ground through erosion, seepage or human activities such as mining and drilling (Aregunjo et al., 2004). Cothern and Lappenbush (1990) measured uranium in ground and surface water in the USA and reported that many of the community drinking water supplies exceeded a uranium concentration of 370 and 185 Bq L1 limits by USAEPA and WHO respectively. Bawaja et al. (1987) surveyed the lake Ontario and reported that uranium and 226Ra levels were in an average of 0.7x106 and 1.2 m Bq L1, respectively while in lake Michigan, Nelson and Metta (1983) measured the levels of 232Th and obtained an average value of 28x106 Bq L1. Avwiri et al. (2005) surveyed the radionuclide concentration of soil, sediments and water in Aba River, Nigeria and reported that the results obtained for the observed radionuclide isotopes were below international standard and concluded that the river has no radiological burden on the populace.
The geographical/geological formation of an area determines to some extent
the radionuclide present in water (Baratha, 1990). In order to give an approximate
idea of the amount the radionuclide in water, the gross alpha and gross beta
activities are measured, this gives the degree of contamination (Semko and Parekh,
2000). Gross alpha is more of a concern than gross beta for natural radioactivity
in water as it refers to the radioactivity of Th, U, Ra as well as Rn and daughter
isotopes. For anthropogenic radioactivity, gross alpha may pertain to screening
for transuranics in wastes, while gross beta to screening for fission products
in accidental reactor releases (Anonymous, 1997a; Semko and Parekh, 2000). CEC-FAO
and WHO in 1990 recommended a standard practical screening level of radioactivity
in drinking water of 100 Bq L1 for alpha and 1000 Bq L1
for beta (Cothern and Lappensbush, 1990). Onoja (2004) surveyed the gross alpha
and gross beta radioactivity in well water in Zaria and reported an average
alpha activity of 6.35±0.45 and 75.34±1.53 Bq L1 for
beta activity. Juliet (2006) also measured the gross alpha and beta radioactivity
in River Kaduna and reported a mean value of 0.117±0.002 and 0.439±0.006
Bq L1 for alpha and beta activity, respectively. The overall results
in the two separate research work show that both alpha and beta activities are
below the WHO practical screening level of radioactivity in drinking water.
Okpare Creek in Nigeria lies within latitude 05°, 27 N and 05°, 33 N and longitude 005°, 53 E and 006°, 04 E of the Niger Delta, the Creek runs North-West to South-East. It has its sources at both Umuaja in Umutu and River Niger and empty into Atlantic Ocean at Forcados (Agbalagba, 2007). Its geology is consistent with the general geology of the Niger-Delta complex. The creek plays host to various farming activities, commercial/residential refuse dumps and industrial effluent discharges from rubber processing factories and asphalt plants at various point along the Creek, thus the Creek is classified into three zones (farming Zone (A), commercial/Residential Zone (B) and industrial Zone (C)) for the purpose of this research work to know the radionuclide distribution pattern and the area of high activity concentration.
The awareness of the potential degradation of the water body and its environments by the activities of the farmers, residence/commercial activities and the effluents discharge from these companies is on the increase and there have bevarious claims and counterclaims of degradation of the Okpare creek by the host communities. This study therefore, determines the level of radionuclide concentrations (gross alpha and beta activity) in the Creek and ascertains the radiological health side-effects on the populace and the environment. These results will therefore form a base-line data of radionuclide activity for the Okpare creek.
MATERIALS AND METHODS
Sample collection and preparation: Twenty water samples were collected at 20 sampling spots using the stratified random sampling method and sample preparation procedures followed as reported earlier (Onoja, 2004). The samples were collected along the course of the river to reflect the economic and industrial activities in the river. The samples were analyzed at the Center for Energy Research and Training, Ahmadu Bello University, Zaria, Nigeria.
The background measurements, sample efficiency and the plateau test were carried out using standard methods (ASTM, 1995).
Counting: The counting equipment, a Eurysis System-eight-channel-gas-filled
proportional counter is automated. The procedure involves entering the present
time, number of cycles and the counting (operational) voltages. Also the counter
characteristics (channel efficiency and background count rate), volume of sample
used and sample efficiency were entered. The sample efficiency was calculated
||The mass of residue in the planchette got from sample preparation
|0.1 A (mg)
||The expected mass of the residue in the planchette
Gross alpha counting: For gross alpha counting the high voltage was
set at 1600 volts and samples were counted for 3 cycles of 3600 sec per cycle.
The results were displayed as raw counts, count rate (count/min) activity and
standard deviation. The data were acquired for alpha only mode and the alpha
count rate as well as alpha activity were calculated using the formula (ASTM,
Where, (α) unit coefficient is the multiplication coefficient making it possible to obtain the results expressed in the units used by the operator, eff is efficiency and Bgd is the background detection limit.
Gross beta counting: The high voltage for gross beta counting was set
at 1700 volts and sample were counted for 3 cycles of 3600 sec per cycle in
beta only mode. The count rate and the activity were calculated using the formula
Alpha-activity: The alpha activity is expressed as activity concentration
C in Becquerel per liter (Bq L1). The activity concentration C is
calculated using formula (Anonymous, 1997b).
Where, Rb is observed sample count rate (s1), as is the specific activity of the alpha standard, V is the volume of the sample evaporated in liter and m is the mass in mg of the residue from volume V and the factor 1.02 is included to correct for 20 mL of nitric acid added per liter as a stabilizer.
Beta-activity: The gross beta activity is expressed as activity concentration
C in Bq L1 calculated
Where, represent the specific activity of 40K in KCl, all other terms have their usual meaning.
The standard deviation (σ), associated with the activity of the sample
collected for background is calculated (Anonymous, 1997)
Other materials/equipment used in this research work are the Geographical Positioning System (GPS) for sampling precise geographical positioning and a watch for timing.
RESULTS AND DISCUSSION
Table 1 and 2 show the preliminary results
of the detector characterization while Table 3 shows the major
result of the analysis. The results of the average efficiencies of the different
channels of the detector in alpha only mode is 35.32±0.05 while that
of beta in beta only mode is 53.58±0.11, thus the average efficiency
of beta is greater than the average efficiency of alpha with the alpha to beta
efficiency ratio approximately 4/5.
||Channel efficiencies for different modes of measurement (Nov.
|Where, r is the radius of the plenchette
|| Limit of detection of proportional counter per channel
The limit of detection gives the lowest
activity that could be detected by the detector.
The results show that the limit of detection for alpha particles ranges from
0.0044 Bq to 0.0115 Bq with an average value of 0.0080±0.001 Bq while
the limit for detection of beta ranges from 0.0193 Bq to 0.0867 Bq with an average
value of 0.0529±0.002 Bq. The overall result of the preliminary investigation
revealed low background, good plateau and low detection (Table
Table 3 shows the values of the alpha and beta activity concentration
in the three zones: viz., farming zone (A), residential/commercial zone (B)
and industrial zone (C). The alpha activity concentration have an average activity
concentration of 1.003±0.097 Bq L1 in the farming zone (A),
in the Residential/Commercial zone (B), the activity concentrations have an
average activity concentration of 4.261±0.109 Bq L1. In the
industrial zone (C), the average activity concentration of 10.296±0.489
Bq L1 was obtained. Comparing these values with that obtained by
Juliet (2006) in River Kaduna show that Okpare Creek mean alpha activity is
5.07±0.168 Bq L1 higher than that obtained at River Kaduna.
||Alpha/Beta activity concentrations of the water samples analyzed
The mean alpha activity concentration (5.187±0.170 Bq L1)
of the Creek is 5.033 Bq L1 higher than the 0.154 Bq L1 obtained in Mills Creek Australia and 1.587 Bq L1 higher than the
3.600 Bq L1 reported in Woronora River Down stream also in Australia
(Onoja, 2004). The beta activity concentrations in zone (A) have an average activity concentration
of 0.129±0.10 Bq L1. In the residential/commercial zone (B),
the average activity concentration is 0.523±0.003 Bq L1.
while in the industrial zone (C), its average value activity concentration is
0.793±0.010 Bq L1. The results of the beta activity obtained
is 0.043±0.014 Bq L1 higher than the value reported in River
Kaduna (Juliet, 2006). The mean beta activity concentration of the Okpare Creek
is 0.296 Bq L1 higher than the 0.186 Bq L1 recorded
at Mills Creek; Australia and is 0.427 Bq L1 higher than the values
recorded at Causeway river in Australia (Onoja, 2004). These high values of
the alpha activity and beta activity could be attributed to the activities in
the course of the river as the geology does not reveal the existence of radionuclide.
|| Alpha/Beta activity in the sample
||Comparison of average Alpha activity in the Zones with WHO
limit for drinking water WHO limit for drinking water
Examination of the graph shows a general low distribution of beta activity
in the three zones (Fig. 1). The alpha activity shows about
9% elevation over beta activity in the farming zone, about 27% elevation over
beta activity in Residential/Commercial zone and about 35% elevation in the
Industrial zone. The comparative low activities in the farming zone may be as
a result of good agricultural practice i.e., low usage of fertilizer. The relatively
high beta and alpha activities concentration recorded in the industrial zone
may be attributed to the effluents discharge into the river by the rubber processing
factory, the two asphalt plants and the oil spillage point at spots UGH14, UGH15,
UGH16 and UGH20, respectively. The results of the values obtained are far below
the 100 Bq L1 for alpha and 100 Bq L1 for beta WHO recommended
practical screening levels of radioactivity in drinking water and therefore
the activities in the river may not have increased the radionuclide concentration
of the water and will not have health side-effects (Fig. 2
||Comparison of average beta activity concentration in the Zones
with WHO limit for drinking water
Although, the results obtained are below the international standard recommended permissible limits for drinking waters, a regular programme of environmental audit and monitoring is hereby recommended.
The survey of gross alpha and gross beta radionuclide activity in Okpare Creek has been studied. The gross alpha and beta activity concentration along the Okpare Creek differ in quantity from location to location. This is explained by the heterogeneity of radionuclide deposits, water transportation and precipitation by organic metabolism and effluent discharge. The overall low value of radionuclide observed may be due to the present of non nuclear facilities in the studied area and the low level of radionuclide bearing rock and soil formations that constitute the geology of the area.
The measured alpha and beta activity are higher than those reported in other parts of the country. However, the values obtained are far below the WHO recommended maximum permissible limit and may not pose any serious health side-effects to the public users of the creek.