Introduction
Increasing population and anthropogenic activities have brought about many environmental problems globally. One of such problems is land subsidence described as the gradual differential settling or sudden sinking of the ground surface due to the movement of ground materials. Land subsidence is generally caused by human activities, alterations to the earth’s surface and underground geologic processes. Specific causes include underground mining of solid minerals, and the collapse of such mines; withdrawal of groundwater and petroleum; dewatering or drainage of organic soils; sink holes, wetting of dry low density soil; and, natural sediment compaction. Land subsidence has received researchers׳ attention in different parts of the world. In this research, land subsidence problems resulted from the overuse of ground water in Khoy is investigated.
Materials and Methods
The study area is Khoy basin located in the north of West Azerbaijan Province. latitude and longitude of the area include 44° 47´ 37״ to 44° 55´ 08״ E and 38° 35´10״ to 38° 37´ 24״ N. The study utilized water extraction data obtained to model the subsidence susceptibility surface of the study area. Total quantity of water extracted in the study area was calculated by combining extraction figures that including the annual discharge for all the hand pump boreholes. Since the data required for the study have spatial components that can change over time. GIS was proved useful in the storage, integration and display of the data. The study used water discharge figures to model the susceptibility of the study to land subsidence in a GIS environment. The map produced showeds that the very high and high subsidence susceptibility zones are found in the surrounding areas of Khoy. Using a GIS for the modeling enabled a data base to be created which could be updated as new facts would emerge. The data on water extraction was linked to an existing data base of the base map of the study area.
Results and Discussion
Data on subsidence susceptibility index of the study was used to produce subsidence surface map which was divided into 4 subsidence zones using a reclass tool of arc map’s spatial analyst extension. Figures 11 and 12 show the spatial distribution of the subsidence, while Table 3 shows the areal distribution of the zones., As seen in the table very high zone covers 10%, while the low zone covers 28%. The analysis using water extraction is a good estimate of land subsidence; therefore, it serves as a good tool for policy makers to monitor environmental hazards. Areas of special interests could also be monitored. For example, the map shows that areas of high and very high zones are found in the established Firuragh area of Khoy. The study demonstrated the use of GIS to visualize the subsidence of the study area. This environment allows for the ease of data editing, integration, analysis, and storage. The resultant product will help to identify, and locate sensitive areas that can be impacted by subsidence so that emergency managers can customize disaster relief efforts. Using a GIS for the modeling enabled a data base to be created so that it could be updated as new facts emerge. It is recommended that a subsidence unit should be established within Khoy so that the rate land subsidence could be monitored, especially in the vulnerable areas identified in the study.
Conclusions
On the one hand over-exploitation of groundwater resources leading to low capacity levels, but on the other hand it results in land subsidence in some parts of Iran. Due to the easy access and simply tapping of underground water resources, the use of these resources has increased substantially in recent years, and unfortunately in some areas the exploitation of groundwater resources is more than their store. The main criteria for zoning include the presence or absence of fissures, the extent of water-table drop, the kind of deposits, and the original depth of water table. In general, four zones were separated and mapped, including very high, high, medium. The map produced shows that the very high and high subsidence susceptibility zones are found in the Firuragh area. From the map it can be seen that very high zone covers 10%, while the high zone covers 29%. According to the results, the water level of the Firuragh Plain had dropped about 5/04 meters over the last 12 years, whereas, the annual average drop in water level in the Firuragh plains is about 42 cm. Thus, the drop in water level has caused land subsidence in the plain.
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