The influence of groundwater use on land subsidence in Bandung City

Bandung is a developing city with a high need for groundwater for various needs. Many research studies say groundwater usage is one of the causes of land subsidence but the results mostly just take one side either land subsidence or groundwater. If land subsidence can be estimated through the value of groundwater use, then this can help prevent calamity. Therefore, a correlation between groundwater usage and land subsidence is sought to see how much groundwater usage influences land subsidence. Land subsidence research was conducted using the Differential Interferometric Synthetic Aperture Radar (DInSAR) system with the SNAP application. The data used is the use of groundwater from 2017 to 2022 and Sentinel 1-A imagery of six images for annual research and sixty-six images for monthly research. The yield of the average settlement each year from January 2017 to May 2022 is approximately 5cm with a linear regression coefficient of determination of 21% while the groundwater use regression with subsidence per month is not linear but wavy. From this conclusion, another research can be developed regarding other factors causing land subsidence besides the factor of groundwater use which can then be seen how much influence these factors have on land subsidence that occurs.


Introduction
Today, clean water problems still occur in Indonesia, especially during the dry season.Even though water is a key component that is very often used in everyday life.This can be used as a sign that the water level is getting lower every year.The decrease in the annual water level can be caused by various things, but the biggest cause is the continuous use of groundwater [1].
Bandung is a metropolitan city that continues to experience development in various aspects, both in terms of social and economic development.Therefore, the use of water in Bandung is also experiencing an ever-increasing increase.Groundwater is still the source of water for most people in the city of Bandung.The use of groundwater is a natural product that must be monitored for its use and balanced with recharge to maintain its availability in the long term.The use of groundwater which continues to increase causes groundwater to gradually decrease.Groundwater infiltration has also decreased because the green land which is useful for infiltration has naturally changed land functions.This results in various other problems and one of them is the subsidence of the land surface each year [2].
The significant development of the city of Bandung every year has increased the various basic needs of the community, one of which is a place to live.Increasing population growth has created a need for 2 more residential land.Much of the land which was originally a groundwater recharge area has turned into residential land for tourist attractions [3].With the increasing use of groundwater, recharge becomes a determining factor for underground water balance.With recharge, groundwater can be conserved, and the land subsidence can be reduced.If recharge is not balanced with groundwater extraction, there will be changes that can result in unwanted things such as lowering of the groundwater level and subsidence of the soil surface due to the large amount of water lost underground.Based on the background explained, questions arise such as how much groundwater is used in the city of Bandung, how much land subsidence in the city of Bandung, and how much influence the use of groundwater has on land subsidence.From this question, restrictions were made, namely that only data in the city of Bandung and only the influence of groundwater use were seen so that we could get the value of the data on groundwater use in the city of Bandung, land subsidence in the city of Bandung, as well as the value of the influence of groundwater use on land subsidence.

Groundwater
According to experts, the definition of groundwater among them is as follows [4]: 1. Soemarto in 1989, Groundwater is water that occupies cavities in geological layers.The layer of soil that is located below the soil surface is called the saturated zone, then there is the unsaturated zone that is above the saturated zone to the ground surface, with its cavities filled with air and water.2. Fetter in 1994, groundwater is water that is stored in saturated zones so that it moves to various layers and soil rocks in the earth until the water comes out as springs or collects in lakes, ponds, rivers, and seas.We often know the upper limit of the water saturation zone as the groundwater table (water table ).Water stored in the saturated zone is called groundwater, which then moves some of the groundwater flow through the rocks and layers of soil on the earth until the water comes out as springs, or seeps into ponds, lakes, rivers, and seas.Rocks below the surface of the land that can store and drain water properly are called aquifer layers.In this layer, groundwater is usually taken by humans for use in everyday life.If grouped according to depth, aquifers can be called shallow aquifers (less than 50 m) and deep aquifers (more than 50 m).A free aquifer is a condition where water can move freely.A semi-confined aquifer is when water can only move in one direction.Meanwhile, a confined aquifer occurs when water cannot move or can only stay in one layer of soil.

Land Subsidence
Land subsidence is a process of land subsidence based on a certain datum (geodetic frame of reference) where there are various causative variables [5].There are many causes for land subsidence which can be categorized into two, namely due to geological factors and due to human treatment.In the category of human treatment, there is one thing that causes land subsidence.This is groundwater extraction.When more groundwater is taken than the incoming groundwater, a decrease in the groundwater level (MAT) will occur.The water that previously filled the voids in the soil is lost resulting in a void in the soil.This void in the ground causes a subsidence in the land surface above it.Land subsidence has typical characteristics or initial symptoms which can be seen in Table 1.
Table 1.Early signs of land subsidence [6] Types of Natural Disasters Vulnerable Area Initial Symptoms Land subsidence Plateu karst areas (limestone plateaus) and areas with high groundwater exploitation.
The emergence of differences in the height of the land from year to year.The emergence of holes and/or cracks on the surface of the ground, walls, walls, cracked floors.The measurement of land subsidence itself has started to be conducted in several ways and methods because this problem is starting to become a spotlight for large urban areas that are experiencing large subsidence.The simplest way to measure subsidence is to stick stakes at several points to observe changes, but this method is considered inefficient.Another way is by way of GPS (Global Position System).In this case, the GPS-receiving satellite will be placed at several points and then the coordinates will be observed periodically.The subsidence value at each point will be obtained by observing changes in the height of the ellipsoid at that point periodically [7].Apart from GPS, there is another method, namely the InSAR (Interferometric Synthetic Aperture Radar) method.InSAR is one of the techniques in geodesy science that makes it possible to calculate the magnitude of topographic changes.This method uses long-distance satellite imagery that uses a combination of values per pixel from two or more radar data [8].In this study, the method used is the DInSAR (Differential Interferometric Synthetic Aperture Radar) method, which is a branch of InSAR.Differential Interferometric Synthetic Aperture Radar (DInSAR) is a sideways radar imaging method by utilizes the phase differences of two or more SAR images with different acquisitions in their processing to obtain deformation.Deformation itself is a change in the shape, position, and dimensions of a material or a change in the position (movement) of a material either in absolute or relative terms.To calculate the deformation model in the DInSAR method the phase difference equation in the interferogram can be explained as topographical phase plus deformation phase plus atmospheric phase plus orbital phase equal to different phase [9].
The image data used is Sentinel-1 imagery, the first satellite series as part of the Copernicus program of seven satellite missions to be launched.Copernicus itself is a follow-up program from Global Monitoring for Environment and Security, abbreviated as GMES.GMES is an earth observation program established by the European Commission or EC and the European Space Agency or ESA.Sentinel-1 product modes can be seen in Figure 1.

Materials and Methods
This study uses a quantitative correlation method to see the correlation between the use of groundwater and subsidence in the city of Bandung as a research area.The instruments needed in this study were SNAP and SNAPHU software, SLC Image Sentinel 1-A data, and groundwater use data in the city of Bandung [11].
The research began with a literature study and data collection.Land subsidence data was obtained using SNAP while groundwater use data is secondary data.Both data are processed and then checked for correlation.Then also look at the amount of land subsidence, if it is large then artificial recharge can be carried out.then from these results, conclusions can be drawn.
Meanwhile, for DInSAR processing using the SNAP application, there are several steps.There are preprocess sentinel 1-A, TOPSAR split, using two sentinel images as master and slave, back geocoding, enhanced spectral diversity, interferogram, TOPSAR deburst, top phase removal, multi look, goldstein phase filtering, snaphu export, unwrapping, snaphu import, range doppler terrain correction are carried out on the import results and the results that have been phased to displacement, then the two are combined in Stack tool step, band maths, ROI statistical analysis, and the land subsidence value for each area studied is produced [12].

Results and Discussions
Groundwater use data obtained from Energy and Mineral Resources of the City of Bandung produces data on the volume of extraction for the 2017-2022 period as well as the average depth of the groundwater table in the City of Bandung for 2021-2022 as in Figure 2.

Figure 2. Groundwater withdrawal volume
According to groundwater level data obtained, the average groundwater level in 2021 is 40.232m, while in 2022 data obtained up to June the average groundwater level is 46.979 m.From the data above it can be seen that on an annual average, the groundwater level in the city of Bandung has decreased by about six meters.When viewed from the groundwater level per month, the lowest depth was 49.02 m in December 2021 and the closest to the ground surface was 30.95 m in May 2021 so the difference in level obtained is 18.07 m, and for the average groundwater level in Bandung City in 2021 to 2022 it is 42.48 m while for the average difference each month from 2021 to 2022 it is 0.13 m which means surface conditions groundwater on average every month has increased by 0.13 m.Volume of Groundwater Withdrawal (m³) Figure 3. Graph of groundwater level and groundwater withdrawal It can be seen from Figure 3 that the groundwater table does not completely change following the water withdrawal that occurs.The two data are not completely compatible but have quite similar patterns so it can be concluded that groundwater extraction affects changes in the groundwater level that occur but not completely because of many other factors such as recharge which help reduce the decrease in the groundwater level that occurs.The results of land subsidence were obtained after processing data from Sentinel 1-A imagery using the SNAP application as in Figure 4 until Figure 9 for each year.This study used as many as twelve images for research per year.Based on the statistical results of ROIs, the amount of subsidence per district is obtained based on the minimum, maximum, average, and standard deviation values.The following is the average data from 2017 to 2022 in Table 2.
observation time from January 2017 to July 2022.To see more data, a regression is conducted for a period per month as in Table 4, and seeks for more fit regression with curve expert as in Table 5.The results of the linear regression values are still exceedingly small, so we look for a regression that matches the value of the high coefficient of determination.Truncated Fourier Series regression for 2017-2020 and Sinusoidal regression for 2021-2022 were obtained.Regression was also conducted for groundwater table data with land subsidence at one well point at the ESDM Office of West Java Province which is located at Jalan Soekarno Hatta No. 576, Sejati, Buah Batu, Bandung City as in Figure 11.

Conclusions
From the results and previous discussion, it can be concluded that Bandung City's total groundwater usage from January 2017 until May 2022 is 62,843,306 m 3, and land subsidence annually has an average of 6 cm from 2017-2021.According to the results of linear regression of the value of groundwater use and land subsidence annually, the use of groundwater affects land subsidence by 21%.So from that we can say that groundwater usage only affects 21% of land subsidence.This means land subsidence most likely occurs due to many other factors other than groundwater.
From the conclusions above, there are several suggestions that the author can give, namely the use of groundwater cannot be avoided, therefore groundwater conservation must always be improved.In addition, land subsidence also continues to occur in several areas of the city of Bandung.This should always be observed every year because many things can cause this to happen and can be fatal in the long term.Meanwhile, to reduce land subsidence that occurs due to the use of groundwater, Artificial Recharge can be conducted, and measuring the value of Land Subsidence using the DInSAR method with the SNAP application can be a solution for monitoring the amount of settlement effectively each year compared to manual measurements using stakes.

Table 4 .
Data on land subsidence and groundwater use per month for 2017-2022

Table 5 .
Determining coefficient value