20
S. P. Kayastha and K. Pradhanang
Water samples from tube wells were collected after few minutes of pumping and
collected in 500 mL bottles of polyethylene [14]. The water sample from each site
was taken in a high-density polypropylene, pre-cleaned bottle and then closed with
a cap. Before sampling, sample bottles were prepared by soaking with 10% nitric
acid for 24 h and then thoroughly cleaned with double distilled water. After then,
all the bottles were dried in an oven at 120 °C for 6 h. All the water samples were
filtered through 0.45 µm cellulose acetate membrane (Millipore Corporation) and
water sample was acidified with ultra-purified nitric acid to pH < 2, immediately
after collection. Water samples were collected then kept at 4 °C and brought to
the laboratory for chemical analysis. The samples were digested with high purity
HNO 3 (Merck) within a week of sample collection for the analysis of arsenic, iron
and manganese [2]. Milli-Q water having a resistivity of 18.2 M/cm was used
for preparing all reagent solutions. Arsenic was analyzed by hydride generation
atomic absorption spectrometry (A Perkin-Elmer Model 300, UK). For accuracy,
three replications of each analysis were carried, and mean values were used for
comparisons. For each batch of the digestion set, analytical reagent blanks were
prepared and then analyzed for the same elements of the samples.
2.3 Statistical Analysis
Pearson correlation analysis was used to test the relationship between arsenic, iron
and manganese concentrations. Statistical tests were performed using significant at
p-value < 0.05. All the statistical analysis were conducted using the SAS/PC 9.3
(SAS Institute Inc., USA).
3 Results and Discussion
3.1 Distribution of Arsenic in Tube Wells
As concentration in the tube wells/well waters ranged from 0.127 µg/L to 75.7 µg/L.
The mean iron concentration in tube wells/wells was found to range from 0.34 to
8.5 mg/L, with the maximum concentration (8.5 mg/L) at Kalaiya Sub-metropolitan.
Concentration of Manganese varied from 0.20 to 1.72 mg/L. The WHO has set a
drinking water guideline concentration for arsenic of 10 µg/L, and Nepal Interim
Standard has adopted the limit of 50 µg/L. Out of total thirty-six tube wells/wells,
about 50% exceeded permissible values of WHO guideline (10 ppb) but 16.6% tube
wells/wells (Table 1) exceeded permissible concentrations of Nepal Standards of
arsenic (50 ppb). The higher values of As in some of the sites of the study area are
attributed to the groundwater with several geochemical processes, such as reductions
of Fe oxyhydroxides and dissolution of carbonate [5].
S. P. Kayastha and K. Pradhanang
Water samples from tube wells were collected after few minutes of pumping and
collected in 500 mL bottles of polyethylene [14]. The water sample from each site
was taken in a high-density polypropylene, pre-cleaned bottle and then closed with
a cap. Before sampling, sample bottles were prepared by soaking with 10% nitric
acid for 24 h and then thoroughly cleaned with double distilled water. After then,
all the bottles were dried in an oven at 120 °C for 6 h. All the water samples were
filtered through 0.45 µm cellulose acetate membrane (Millipore Corporation) and
water sample was acidified with ultra-purified nitric acid to pH < 2, immediately
after collection. Water samples were collected then kept at 4 °C and brought to
the laboratory for chemical analysis. The samples were digested with high purity
HNO 3 (Merck) within a week of sample collection for the analysis of arsenic, iron
and manganese [2]. Milli-Q water having a resistivity of 18.2 M/cm was used
for preparing all reagent solutions. Arsenic was analyzed by hydride generation
atomic absorption spectrometry (A Perkin-Elmer Model 300, UK). For accuracy,
three replications of each analysis were carried, and mean values were used for
comparisons. For each batch of the digestion set, analytical reagent blanks were
prepared and then analyzed for the same elements of the samples.
2.3 Statistical Analysis
Pearson correlation analysis was used to test the relationship between arsenic, iron
and manganese concentrations. Statistical tests were performed using significant at
p-value < 0.05. All the statistical analysis were conducted using the SAS/PC 9.3
(SAS Institute Inc., USA).
3 Results and Discussion
3.1 Distribution of Arsenic in Tube Wells
As concentration in the tube wells/well waters ranged from 0.127 µg/L to 75.7 µg/L.
The mean iron concentration in tube wells/wells was found to range from 0.34 to
8.5 mg/L, with the maximum concentration (8.5 mg/L) at Kalaiya Sub-metropolitan.
Concentration of Manganese varied from 0.20 to 1.72 mg/L. The WHO has set a
drinking water guideline concentration for arsenic of 10 µg/L, and Nepal Interim
Standard has adopted the limit of 50 µg/L. Out of total thirty-six tube wells/wells,
about 50% exceeded permissible values of WHO guideline (10 ppb) but 16.6% tube
wells/wells (Table 1) exceeded permissible concentrations of Nepal Standards of
arsenic (50 ppb). The higher values of As in some of the sites of the study area are
attributed to the groundwater with several geochemical processes, such as reductions
of Fe oxyhydroxides and dissolution of carbonate [5].