Wednesday, July 14, 2010

Monitoring Arsenic

Monitoring arsenic in ground water should be planned nationally. The following illustrates the procedures that need to be taken to monitor arsenic.

1. Random testing of tube wells throughout the country to determine the extent of the problem.

2. Subsequently, blanket testing of all wells in selected districts should follow, to identify each and every contaminated well.

3. Blanket testing programmes could also include other activities that are essential for additional monitoring and management operations, such as the :
-location of each well using a Geographic Information System (GIS)
-diagnosis of arsenicosis patients in the district surveyed
-introduction of various water treatment measures in that district.

Blanket testing system

Geographic Information System (GIS)

These large scale monitoring methods have help to detect the amount of arsenic in the area where further actions can be taken .

Tests for arsenic found in water can either be done:

-In a laboratory, where the water samples are delivered. (laboratory assays)
-using field testing kits for in-situ monitoring. (field assays)

Current field assays available....

Testing of drinking water by field kit of arsenic

Compared to laboratory assays, field assays are generally inexpensive and can produce a large number of screening results in a short time. Although they may be less sensitive compared to laboratory assays, it is acceptable if our purpose of sampling is merely for sample screening or site surveys.

The following table shows a summary of the current field assays for monitoring arsenic

Current laboratory assays available....

Laboratory assays are generally required to measure arsenic levels down to parts per billion (ppb) levels, or in other words as μg/L for water or μg/kg for solids. Laboratory analysis normally requires us to conduct a pretreatment, either by acidic extraction or acidic oxidation digestion of the environmental sample. Pretreatment involves the transfer all of the arsenic in the sample into an arsenic acid solution, which is then measured by laboratory assays, such as Atomic Fluorescence Spectroscopy (AFS), Graphite Furnace Atomic Absorption (GFAA), Hydride Generation Atomic Absorption Spectroscopy (HGAAS), Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS).

Compared to field assays, these instruments tend to be more bulky, expensive to operate and maintain and require fully equipped laboratories for us to maintain and operate. However, these instruments have the advantage of higher sensitivity, which allows us to acquire more accurate results.

The table below shows the major assays for laboratory arsenic analysis.

The table below shows a comparison of both the arsenic detection methods

When monitoring arsenic, especially in water samples, it is important that we know the current requirements for measuring the pollutant. The current maximum contaminant level (MCL) for all forms of arsenic in groundwater is 50 μg/L (50 ppb). This is set by EPA based on a Public Health Service standard. According to the WHO guideline, the MCL for arsenic is 10ppb.
For our blog, we will only discuss some of the field and laboratory instruments mentioned above, which can be found in the next few segments.

All in all, the decision on which type of monitoring method is more suitable depends on the resources available within each country and the purpose of our sampling.

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