Mapping land use suitability for development of recharge wells in the Ciliwung watershed, Indonesia

Jakarta is located in a delta that serves as an estuary for 14 rivers (Samsuhadi 2009). In hydrogeological terms, the Ciliwung Watershed is the most influential catchment area in the region (Hendrayanto 2008), passing through two provinces: Jakarta and West Java. The Ciliwung Watershed has experienced rapid population growth and urban sprawl from Jakarta to the surrounding areas of Depok, Bogor, Tangerang, and Bekasi, and one notable negative impact of this expansion in the Jabodetabek (Jakarta, Bogor, Depok, Tangerang, Bekasi) region is the lack of clean water infrastructure. As the piped water supply is limited, a significant proportion of the population uses groundwater for both domestic and industrial purposes. A water supply of guaranteed quality and quantity is one of the essential elements for continued economic prosperity (Yu et al. 2014), and uncontrolled groundwater exploration without appropriate conservation measures can causes a serious decline in the availability and quality of water, as well as a declining groundwater level (Braadbaart & Braadbart 1997).

In managing groundwater effectively, it is important to consider any technology or approach that may contribute to conservation (Riastika 2011). Common methods include the construction of dams, artificial lakes, stabilization ponds, and recharge wells. Of these, the construction of recharge wells is an attractive option, as it is a simple and relatively low-cost technology that can be applied to almost all aquifer systems to store and increase groundwater levels, improve water quality, and reduce runoff, and does not require large-scale land acquisition (Ravichandran et al. 2011).

Many studies have shown that recharge wells can significantly enhance the field capacity of soil, allowing more effective water infiltration. Sunjoto (2011) and Patel et al. (2011) designed recharge wells and conducted simulations to estimate the amount of recharged water, and both concluded that there is a connection between the geometric factor of the recharge well and its ability to recharge. Based on soil sample tests showing that soil types differ in their permeability value, the design of recharge wells must take account of soil type. Bhalerao & Kelkar (2013) reported that the construction of recharge wells can produce satisfactory results if properly planned, basing project location on aquifer suitability, hydrometeorology, and hydrology characteristics. Selection of an appropriate recharge well depends on topographic conditions, geology and soil characteristics, quality and quantity of water recharged and economic factors. This aligns with Sarup et al. (2011), who stated that the location of the recharge well should consider geological conditions, land use, land cover, water body, and other basic information. In summary, the technical and physical conditions of the site will significantly affect the effectiveness of recharge wells (Gale et al. 2002).

In response to the serious threat of declining groundwater levels in the Jabodetabek area, the governor of Jakarta issued Governor Regulation No. 20/2013 concerning recharge wells. The regulation strongly recommends optimized development of recharge wells to capture, store, and increase groundwater and to minimize rainfall-runoff, so reducing flood volume and area of inundation, as well as providing an additional water supply in the dry season. Since then, recharge wells have gradually been developed in Jakarta and surrounding areas. According to data from the Industry and Energy Agency of Jakarta, about 5,672 recharge wells had been developed in Jakarta by 2014. However, most of the Jakarta area is characterized by clay soil, which may be unsuitable for recharge wells, and the effectiveness of these recharge wells in terms of land suitability remains unknown.

To plan the construction of recharge wells in a large heterogeneous area like the Ciliwung Watershed, the scenario must include location criteria in the design of recharge wells, along with the optimum volume of water to be recharged. However, existing construction of recharge wells in the Jakarta area has not taken account of these issues, as the main consideration was groundwater threshold. The aim of the present study was to evaluate land suitability for recharge wells development in the Ciliwung Watershed, as identification of suitable locations is crucial in designing these wells. The results of this study can provide technical recommendations and a point of reference for local government in managing and conserving groundwater through the development of recharge wells in their own area.

The main objective of this study was to identify appropriate locations for the siting of recharge wells in the Ciliwung Watershed by compiling and weighting GIS data in the form of thematic maps. The essential data include soil hydrology group, groundwater level, slope, average rainfall, and land use. The land suitability map produced by overlaying several maps using GIS can be used to propose suitable locations and design for recharge wells.

The land suitability map required the compilation of 6 main thematic maps: technical geology, soil type, soil hydrology group, groundwater level, slope, and average rainfall intensity. To determine the ideal location of recharge wells, ArcGIS was used to compile and analyse those data.

The value of weigthing score calculated from Equation (1) was then converted into land suitability classification as presented in Table 2.

Data (mostly secondary) on required soil characteristics were obtained from relevant agencies that included the Geological Agency of Indonesia, the Ministry of Agriculture, and the Research Center for Soil and Agro-climate. These data included technical geology maps, soil type maps, soil hydrology group maps, groundwater level maps, and land use maps. Soil permeability was measured directly (primary data), based on soil samples collected from 4 sampling points distributed upstream to downstream, including the road side of Jogjogan at Cisarua Bogor; Pajajaran Regency at Bogor, the Universitas Indonesia Campus at Depok, and Sukapura Sub-District in North Jakarta (see Figure 1). For representative sampling of existing soil permeability conditions in the Jakarta area, the final sampling point was outside the Ciliwung Watershed. To determine soil permeability, these samples were then tested in the soil mechanics laboratory of the Faculty of Engineering at Universitas Indonesia.

The evaluation of land suitability involved overlaying the existing map of recharge wells and the land suitability map, using ArcGIS 10.1. The main target of the evaluation was to establish what percentage of existing recharge wells in the Jakarta area were on suitable land. The existing map of recharge wells was redrawn by processing (digitizing) the coordinates of recharge wells, based on the 2014 data provided by the Industrial and Energy Agency of Jakarta.

A simple design was developed to estimate the dimension of recharge wells that might be implemented in the Ciliwung Watershed. The main data required for the design related to the volume of water from excessive rainfall-runoff over the given area. These data were obtained from rainfall analysis and calculation of recharge well dimensions.

The designed rainfall intensity for the rainfall-runoff analysis was calculated on the basis of maximum daily rainfall data from eight rainfall station as shown in Table 3. The rainfall data were then analyzed using the extreme Log-Pearson Type III distribution.

As a model area for proposing the recharge wells design over the Ciliwung Watershed, a specific location was selected. The regions around Bojong Gede and Cibinong represented the middle area of Ciliwung Watershed was employed. The area was categorized as qualified land and has a composite runoff-coefficient of about 0.75.

The average value of soil permeability from each sampling point based on laboratory test is presented in Table 4.

Table 4 shows that the middle area of Ciliwung Watershed has the lowest soil permeability. Meanwhile, the downstream area of Ciliwung Watershed has higher soil permeability than the middle area because it has less clay content of soil. The soil permeability test results in upstream area of Ciliwung Watershed shows the highest permeability values.

Because of Bojong Gede and Cibinong located in between of Soil Hydrology B and C, and occupied similar land utilization, the value of soil permeability is assumed of the average soil permeability in the middle area of Ciliwung Watershed (represented by Padjajaran Regency and Universitas Indonesia) which is around 0.481 × 10⁻⁴ cm/s.

Based on Figure 3, four classes of land suitability can be identified in the Jakarta region: highly unqualified land, not qualified land, less qualified land, and qualified land. The highly unqualified land is located mainly in the northern part of Jakarta, accounting for less than 1% of existing recharge wells. A further 8.9% of existing recharge wells are found in the unqualified land in the districts of Cempaka Putih, Johar Baru, and Pulo Gadung. About 42.3% and 48% recharge wells are developed in less qualified land and enough to qualify land, respectively. It is clear, then, that more than 80% of existing recharge wells in Jakarta are constructed on land of moderate suitability (enough to qualify land and less qualified land) while the remaining wells are equally distributed across highly unqualified and unqualified land.

A simple calculation involving Equations (2)–(6) illustrates the preferred design of recharge wells to be constructed in the selected areas of the Ciliwung Watershed. Both individual and communal rectangular recharge wells were simulated, and the results are presented in Table 5.

The designed dimensions for rectangular recharge wells (width and length) are 1 meter for single wells and 2 meters for communal. The estimated heights of recharge wells (singular and communal) are 2.1 meters and 3.2 meters, respectively. These heights are still below the maximum recommended by the Indonesian National Standard (2002) for recharge well construction.

In the Ciliwung Watershed, some areas of the Jakarta region do not meet the qualifying criteria for construction of recharge wells. According to the land suitability map, these areas have shallow groundwater levels and are mainly categorized as group C soil hydrology and highly unqualified land. Any qualifying land in the Jakarta region is distributed around the Pasar Minggu, Ciracas, Pasar Rebo, and Jagakarsa areas, as shown in Figure 3. According to Figure 2, the optimal sites for recharge well construction are in Cisarua (around Ciawi and Citeko), Bogor.

Proposed materials and components for designated recharge wells (adopted from the Department of Settlement and Regional Infrastructure 2002) are presented in Table 6.

A land suitability map for the construction of recharge wells was developed by overlaying several thematic maps, using ArcGIS 10.1. The results show that 80% of existing recharge wells in the Jakarta region (part of the Ciliwung Watershed) are constructed on enough to qualify land and less qualified land; the remaining 10% of existing wells are constructed on highly unqualified and unqualified land. The land suitability map offers a guide for future construction of more effective recharge wells. It is suggested that the construction of recharge wells in the Ciliwung Watershed should be concentrated in Cisarua, Bogor. The recommended type of well (singular and communal) is rectangular. Further study is urgently required to evaluate the effectiveness of recharge wells in the field.