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Studying arsenic concentration in drinking water using GIS

Jahanzaib Sohail Rana
GIS Expert
Environment Protection Agency
Lahore, Pakistan
[email protected]

Abstract
Arsenic is a poisonous substance, which is released both from certain human activities and naturally from the Earth’s crust. Humans may be exposed to arsenic mainly through food and water, particularly in certain areas where the groundwater is in contact with arsenic-containing minerals. In Pakistan, drinking water supplies are generally obtained from surface water sources (such as rivers, canals or lakes) or the underground aquifers. The reliance on groundwater as opposed to surface water sources for drinking of over pumping has also caused the deterioration of groundwater quality.

A vast concentration of various quality parameters occurs in groundwater at different places and WHO guidelines are often exceeded. A survey conducted by the Punjab Environment Protection Department and UNICEF during 2000-04 shows high concentration of arsenic in drinking water samples collected from various sites in Lahore. Arsenic mapping requires a broad zonal understanding of the whole dynamics to achieve a strategic mapping and remediation. This project shows the arsenic surface of the entire region on the basis of sample points and shows regions in Lahore where arsenic exceeds the permissible WHO limit and where it is in permissible limit. This helps in future prediction.

Approximately 70 percent of drinking water supplies in Pakistan come from aquifers. This reliance at present is growing. However, groundwater in Pakistan is being contaminated by raw sewage irrigation and land disposal of industrial effluents and through the use of deep soakage pits and heavy application of fertilizers and pesticides. The intrusion of saline water into the fresh water zone is a result.

The present research paper was aimed at mapping the arsenic presence, its depth and concentration level by using the interpolation technique. The absolute location of study area is Lahore, North Punjab (lat 31.545397, lon 74.310887 Pakistan). This research paper produced the surface of arsenic in the year 2007 for this purpose. Geographical information system and satellite image processing was used to identify, locate, map and analyse the existing data for modeling the hazard zones in the district.

Introduction
Groundwater is one of the most important sources of drinking water and is not present in abundance in nature. The contamination of groundwater with arsenic is one of the serious problems encountered in developing countries. Thus there is a rising need to map the level of arsenic concentration, trend of arsenic flow and the temporal changes that occur in concentration level. The present study was an effort to map arsenic concentration at a district level that might lead to a broader understanding of its regional presence and significance. India and Bangladesh has been reported to suffer a lot from arsenic contamination problems. Under the World Health Organization (WHO) standards, the permissible level of arsenic in drinking water for Pakistan is 0.05 mg per liter (50ppb) which is said to be safe.

The geochemistry of arsenic had been recently been reviewed by Thornton (1996). The main constituent of around 200 mineral species is arsenic. Out of these, 60% are arsenates, 20% sulphides and sulphosalts and the remaining 20% includes arsenides, oxides and silicates. From the observations in the Cordoba it was concluded (Astolfi et al., 1981) that the regular intake of drinking water containing more than 0.1 mg l-1 of arsenic leads to clearly recognisable signs of arsenic toxicity and ultimately in some cases to skin cancer.

The solution to such problem lies in understanding the intricate relation between the various socio-economic factors associated with it. It has been found that arsenic consumed may be quickly excreted from the body through methylation (often termed as detoxification) in the body, through mostly urine. But this methylation reaction needs methyl donors coming from nutritional methionine-rich food sources like green vegetation and meat. Thus economic conditions and demographic status are also a key to such solution. The geology of the area, land use and irrigation and drainage pattern attributes play significant role in finding a plausible solution to such problems.

GIS acts as an excellent tool to unify data from various sources and integrate them into a single environment to analyse the relationship amongst them. The satellite images help in identifying the various land use patterns and may provide a clue in identification of patterns and source with respect to its geological setup. Thus geometrics can act as a decision support tool to analyse the various data sources for mapping the risk zones of the area.

The present project was mooted by EPD Punjab in collaboration of JICA, which provides the equipment to measure the arsenic in collected samples. The latest satellite images were used to detect the various land use and land cover of the district to verify the changing pattern of surface water presence of the area. This imagery also helps in identifying illegal human activities which can cause the presence of arsenic. An attempt has been made to model the affected zone map using GIS.


Table 1.1: Chemical nature of arsenic at a glance

Study Area
The study area is Lahore.

The map shows the sample point of arsenic at a tubewell location.

Methodology
The EPA laboratory staff and the concerned inspector conducted physical surveys (areas of their jurisdiction) to take the sampling points with GPS. GPS device was used to mark the point with latitude and longitude on the map and assign the sample number. The samples were collected in polystyrene bottles with capacities ranging from 0.5 to 1.5 L. The bottles were thoroughly washed with water, before taking samples. Nitric acid was used as preservative in sampling bottles for arsenic and the pH was brought down. Before starting analysis of drinking water, the instrument was calibrated with freshly prepared standard solutions. The standard solutions or sample solutions were diluted with equal amount of TISAB before measurement and each standard solution was mixed. The total volume was sufficient to immerse the electrode and measure the fluoride concentration. The calibration was verified by running standard solution of known concentration as sample.

Total ion strength adjustment buffer preparation. 1 molar solution of sodium chloride, 0.75 M solution of sodium acetate and 0.25 M acetic acid solutions were prepared with distilled water. Equal amounts of all three solutions were mixed to get buffer (TSAB).

Equal amounts of TISAB and samples were mixed and brought to room temperature. The total volume was sufficient to immerse the electrode and measure the fluoride concentration.

Arsenic testing. The data attached with each sample point are shown here. This is the attributed data against sample location with spatial. The map was geo-referenced with UTM projection and WGS-84 datum.

This map shows the range of tube well installed in depth.

Result and discussion
Total sample points collected from the study area are 339. Only 129 sample were within the permissible limit 50ppb< and 220 were in the 50ppb<. By using these 339 sample points was generated the surface of arsenic in Lahore showing that the most of the areas in the city are affected with arsenic. For validating the surface of Arsenic, the specific area or where the value shows a very abrupt change is again sample tested. The rainy season attenuated arsenic contamination in the area. Arsenic concentration decreased with increase in the depth of water sampling. Hence deep well pumping could be employed as a sustainable source of arsenic-free water.

In this map, areas shown by blue circle sample points are within the permissible limits and the brown triangle are those exceeding the permissible limit. The surface with yellow colour is good indicator but the gradual darker colours show the concentration of arsenic. This surface shows arsenic level in ground water in the whole area. Now we take the next step to identify the indicators which is responsible of arsenic concentration.

Acknowledgment
I want to thank Allah who made it possible for me to complete this research paper. I also express my gratitude to the members of Environment Protection Agency of Punjab and my elder brother Shahzad Faheem Rana who has assisted me at each step of my life.

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