Hydrogeology Conceptual Model of Meninting Dam and Its Surrounding Area, West Lombok Regency, West Nusa Tenggara, Indonesia

The Meninting Dam is one of the dams recently built in West Lombok, Indonesia. Hydrogeologically, this dam was built on the Mataram-Selong Groundwater Basin, which is known to be a highly productive aquifer. However, in this area, no extensive research in terms of hydrogeological conditions has been conducted. Therefore, the objective of this research is to present a conceptual model of the hydrogeology of the Meninting Dam and its surrounding area. This study was carried out by collecting primary and secondary data on the geological, hydrological, and hydrogeological conditions in the research area. Results show that there are two layers of unconfined aquifer, one layer of aquitard, and one layer of confined aquifer in this area. The unconfined aquifer is built by layers made up of pebbles, sand, clay, gravel, and boulders, but also a layer of pumiceous tuff with a thickness up to 40 m and a hydraulic conductivity value between 10-1 mm/s and 5x10-2mm/s. The confined aquifer is composed of pumiceous tuff and has a thickness range of up to 100 m and a K value of about 5 x 10-3 mm/s. Between unconfined and confined aquifers, there is a thick layer of aquitard made up of laharic breccia.


Introduction
Freshwater scarcity is a major problem in the world today, and is expected to worsen as population growth and economic development drive demand [1].The Meninting Dam, which serves as the study site, is one of the dams recently constructed in West Lombok, Indonesia, and is located in the village of Bukit Tinggi, Gunungsari Sub-district, West Lombok Regency, West Nusa Tenggara [2], (Figure 1).It only began construction in 2019 and is part of the National Strategic Project (NSP) to support food and water security, particularly in Eastern Indonesia [3].The dam has an area of 53.6 hectares of inundation reservoir and dams the Meninting River [4].With the construction site above the Mataram-Selong Groundwater Basin [5], which is known to be an aquifer with high productivity [6], there is no in-depth research related to the hydrogeological condition of the Meninting Dam area.Therefore, the objective IOP Publishing doi:10.1088/1755-1315/1291/1/012020 2 of this research is to come up with an idea of how the hydrogeology of the Meninting Dam and the area around it works, through a conceptual model.
A model is a simplification of the natural world's complexity [7].A model is a representation of reality that seeks to explain the behavior of various aspects of reality and is never as complex as the actual system represented [8].A conceptual model is typically created for a site-specific hydrogeological setting, but it can also be created for a generic geologic setting [7].

Research Methodology
Primary and secondary data on the geological, hydrological, and hydrogeological conditions in the research area were gathered for this study.Primary data collection, such as geological field observations and rocks sampling, is done on-site.While secondary data in the form of rainfall data, temperature, borehole-drill logs, and geoelectrical survey reports were also collected from local authorities and evaluated.

Analysis of Geology and Hydrogeology
From March to April 2022, this study was conducted in the study area.It ranges in elevation from 38 to 425 meters above sea level and includes the quarter and tertier volcanic zones.Regionally, the study area is within the Kalibabak Formation (TQb), that is composed of breccia and lava as the basement., the Lekopiko Formation (Qvl), which consists of pumice tuff, lava breccia, and lava, and the Alluvium Formation (Qa), which consists of pebble, granule, sand, clay, and coral fragments [9] (Figure 2).
According to the Hydrogeological Map of Lombok Island and the Western Part of Sumbawa Island [6], the study area is covered in three aquifer classifications.The first type is highly productive aquifer with widely varying transmissivity, a wide range of depths to the water table, and wells yielding more than 5 l/sec in general.The second is a moderately productive aquifer with widely varying transmissivity, great depth to the water table, and wells yielding less than 5 l/sec.The third type is locally productive aquifer with largely varying transmissivity, generally no groundwater exploitation by drilling due to great depth to the groundwater table, locally small spring can be captured (Figure 3).

Analysis of Hydrological Data
The hydrological data analyzed is rainfall data obtained through CHIRPS 2015 (Climate Hazards Infrared Precipitation with Stations) over a 20-year period (2002-2021).
The estimation of groundwater recharge rate are important aspect of the data input into the groundwater model.As a result, the rate of recharge in this study is calculated using a simple water balance formula [10]: And runoff is calculated using the following formula [11]: Where, Ro : Runoff (mm/year) P : Precipitation (mm/year) Tm : Temperature (°C) A : Area (km2)

Geolectrical Data Evaluation
The geoelectric method is the most commonly used method for groundwater exploration [12]- [19].The geoelectric method, which is based on the concept of subsurface determination, provides data on the structure, composition, and subsurface conditions [20].Aside from that, it can be used to determine the location and distribution of aquifers [14].
The planned drilled well of PT.Air Minum Giri Menang-SPL Penimbung, Penimbung Village, Gunungsari Sub-district, and West Lombok Regency is subjected to geoelectrical estimation [21].Geoelectrical measurements were taken at four different locations around the planned location of the drilled wells, namely: Based on geoelectrical investigations, as shown in Figure 4, indicate that the study area contains two aquifers: a shallow aquifer at a depth of 5-50 m and a deep aquifer at a depth of 66-170 m.

Borhole Data Evaluation
Logbor data comes from the Meninting Geology Final Report [22], which is used to interpret subsurface geological conditions.The logbor data used is the logbor point across the axle of the Meninting dam, with geological conditions ranging from young to old and consisting of topsoil, river sediment, the terrace sediment, volcanic breccia, and andesite in descending order.This data is comparable to geoelectrical measurement data collected at PT Air Minum Giri Menang, Penimbung Village.

Results and Discussion
The goal of this study was to improve basic knowledge of hydrogeologic features in order to better understand the surface and shallow groundwater relationships observed in a portion of the Upper Mataram-Selong Basin.Based on the review and analysis of the primary and secondary data described in the previous chapter, the hydrogeological conceptual model of the study area is presented in Figure 5.Of course, many obstacles and problems arose during the implementation of this research, which became the limitations of this research.The data obtained is still very limited, but it is hoped that it will be supplemented with more and more accurate data in the future, such as data on wells or drilling in the study area's upstream area and monitoring wells that do not yet exist.

Figure 5 .
Figure 5. Hydrogeological Conceptual Model of Study AreaIn the model, the hydraulic conductivity of the aquifers determines how easily groundwater moves within the aquifer systems.
According to the result and discussions, the following conclusions can be drawn from this research: a.The study area comprised of a two-layer unconfined aquifer, one layer of aquitard, and one layer of a confined aquifer.The unconfined aquifer is composed of layers of pebbles, sand, clay, gravel, and boulders, as well as a layer of pumiceous tuff with a thickness of up to 40 m and hydraulic conductivity values ranging from 10 -1 mm/s to 5x10 -2 mm/s.The confined aquifer is made of pumiceous tuff and has thicknesses of up to 100 m as well as a K value of about 5 x 10 -3 mm/s.There is a thick layer of aquitard made up of laharic breccia with a K value of about 5x10 -4 mm/s between an unconfined and confined aquifer.b.The annual precipitation in the study area ranges between 1706 and 2943 mm.The evapotranspiration rate was approximately 1329-1507 mm/year.The annual runoff values ranged from 79 to 139 mm.Groundwater recharge rates in the study area range from 148 to 575 mm/year.

Table 1 .
The coordinate point of geolectrical measurements