Groundwater investigation in the Dumoga area, north Sulawesi province, Indonesia

Dumoga is one of the areas in North Sulawesi Province, Indonesia, designated as an agriculture and food crop area. Groundwater is one of the sources of raw water and irrigation in this area. Therefore, it needs detailed information about the quantity and quality to support groundwater exploitation and management for sustainable use of groundwater resources. This study investigates the groundwater resource potential and quality, which has not been previously studied. The research methods used geological observation, springs mapping, geoelectrical survey, measurement of groundwater level, and physicochemical, including temperature, pH, total dissolved solids (TDS), and electrical conductivity (EC). The result shows that the Dumoga area has a high potential for groundwater resources, with sand and clayey sand as the main aquifers. Groundwater flows from around the research area to Dumoga River in the middle. Even though it has a high potential for groundwater resources, not all groundwater is recommended for irrigation water because of the quality, especially from the southwestern and eastern Dumoga areas.


Introduction
In the spatial planning of the North Sulawesi Province for 2014-2034, Dumoga is designated as an agricultural area for food crops [1].Many residents rely on groundwater as their primary water source, which is also utilized to support irrigation.The central government constructed two major irrigation systems [2] and 61 groundwater wells.As groundwater is used for irrigation, it is crucial to assess the quality and quantity of this water resource.Irrigation water must meet certain standards or quality criteria to ensure it does not harm plants and their yields [3,4].Several key parameters in water need to be determined, including Electrical Conductivity (EC), Total Dissolved Solids (TDS), pH, and temperature.Electrical Conductivity (EC) indirectly measures the salt concentration in water, which is essential for assessing water suitability for crop cultivation and monitoring nutrient solution concentration [3].High EC in nutrient solutions is favorable for plants, but EC levels exceeding the threshold can damage crops [5].Total Dissolved Solids (TDS) are the concentration of dissolved 1311 (2024) 012028 IOP Publishing doi:10.1088/1755-1315/1311/1/012028 2 minerals in water, commonly comprising carbonate, bicarbonate, chloride, sulfate, nitrate, magnesium, sodium, calcium, and potassium [6].pH indicates the concentration of hydrogen ions in water.Extremely acidic water or alkaline can cause corrosion issues and potential maintenance difficulties [7].Temperature is critical to determine its effects on chemical reaction rates, gas solubility reduction, taste and odor enhancement, and other factors [7].Apart from the parameters EC, TDS, pH, and temperature, it is also essential to understand the geological conditions of the groundwater-bearing formations.The availability of groundwater is significantly influenced by hydrogeological properties, such as rock types (lithology) [8,9].This study investigates the groundwater resource potential and quality, which has not been previously studied.

Materials and Methods
To estimate the lithology types at the research area, a field check and observation were conducted and combined with resistivity measurements at 7 points geoelectric place (GL-1, GL-2, GL-3, GL-4, GL-5, GL-6, GL-7), as shown in Figure 1.The resistivity measurements allow us to infer the lithology types because the electrical properties of a medium are influenced by the constituent rocks or mineral compositions, water content, rock homogeneity, permeability, texture, temperature, and geological age [10].Different rock types exhibit varying resistivity values, as shown in Table 1 [11,12,13,14,15,16,17,18].However, it is important to note that in the soil, resistivity is more controlled by the water content rather than the mineral composition and depends on the quantity, type, and temperature of pore water in the soil [13].Resistivity measurements were carried out using the Wenner method due to its reasonably good horizontal and vertical resolution and sufficient depth of penetration [19].In the Wenner configuration, the distance between the current and potential electrodes is the same [19,20].The results of the resistivity measurements are then interpreted for the type of lithology.The lithology type is then interpreted into an aquifer, aquitard, aquifuge, and aquiclude layers.An aquifer is a layer or rock formation that can store and transmit water quite well, such as sand or gravel.Aquitard is a layer or rock formation that can store water but has limited water flow, such as sandy clay.Aquiclude is a layer or rock formation that can store water but is very difficult to transmit water in significant quantities, such as clay.An Aquifuge is a body or rock layer that cannot store or transmit water, such as dense igneous rock [21].
In addition to conducting geoelectric measurements at the research site, depth to the water table (GWL) measurements, as well as measurements of Electrical Conductivity (EC), Total Dissolved Solids (TDS), pH, and Temperature, were carried out at 24 dug wells (DW), 11 drill wells (W), and 11 natural springs consisting of 7 springs (SP) and four hot springs (HP), as shown in Figure 1.

Figure 1. Research location
The depth to the groundwater level (GWL) measurements, analyzed along with the ground surface elevation at each sampling location, can generate GWL contours, which reveal patterns and directions of groundwater flow.The TDS values can describe the water types for each sample [8], as shown in Table 2.
Table 2. Water Types based on TDS [8] Water Type TDS (mg/L) Pure Water 0 -1,000 Brackish Water 1,000 -10,000 Saline Water 10,000 -100,000 Brine Water > 100,000 Water quality parameters can also be categorized into several classes, where Class I represents water suitable for raw drinking water.Class II refers to water suitable for recreational facilities, freshwater fish cultivation, livestock, and irrigation.Class III includes water suitable for freshwater fish cultivation, livestock, and irrigation.Class IV is water suitable for irrigation (Table 3) [22].Determination of the groundwater safety level for irrigation purposes can also be based on specific parameters, including EC, TDS, and pH values, as shown in Table 4 [23].

Results and Discussion
Based on the field observations of the surface lithology in the research area, it is predominantly composed of alluvium (sand and clayey sand).In addition to alluvium, there are breccia and sandstone lithologies from the central to the western parts of the research area, while towards the eastern side, there are tuff and limestone lithologies with some andesitic lava, as seen in Figure 2. The sub-surface lithology was identified based on the geolectrical survey.The resistivity values range from 0.03 Ωm to 25,000 Ωm (Figure 3).The resistivity values of clay ranges 0 Ωm -10 Ωm, sandy clay (10.01 Ωm -20 Ωm), clayey sand (20.01 Ωm -30 Ωm), sand (30.01 Ωm -100 Ωm), breccia at GL-1, GL-2, GL-5, GL-6, GL7 (>100 Ωm) and limestone at GL-3, GL-7 (>100 Ωm).Based on the type of lithology from the geoelectric interpretation results, it is indicated that sand and clayey sand are the lithology that functions as an aquifer.Those aquifers have a resistivity range from 20 to 100 for GL-1, GL-2, GL-3, GL-4, GL-5, GL-6, and GL-7, are shown in Figure 4.The resistivity value for the aquifer at this research location is almost the same as the results of research by Sedana et al. at Manado, where the aquifer resistivity value is below 123 Ωm [14].The field observation and groundwater level (GWL) elevation analysis show that the groundwater flow in the research area flows from the surrounding areas toward the center, where the Dumoga River is located.The groundwater level depth ranged from 0.2 to 10 m below the surface (Figure 5).The groundwater level depth is not much different from other research on alluvium lithology carried out by Nurfaika and Kasim in Limboto, where the groundwater level depth ranges from 0.3 m to 7.45 m [24].The data measurement of EC, TDS, pH, and temperature is shown in Table 5.A TDS range ranged from 33 mg/L in DW17 to 2.905 mg/L in HP04, and EC ranges from 68 μs/cm to 5.800 μs/cm.The pH range was from 5.88 in HP01 to 9.43 in W09, while for temperature, the range ranged from 26.1°C in SP01 to 73.5°C in HP01.
According to Table 3 and Table 4, the analysis of groundwater quality classifications and groundwater safety levels for irrigation, eight samples are not recommended for water quality classifications and not recommended or slight to moderate at groundwater safety levels for irrigation categories, as shown in Table 5.

Conclusions
This research is important to support the government program for maintaining food crops area in Dumoga due to the need for water sources.This research aims to determine the potential of groundwater in both quantity and quality through field surveys, geoelectrical measurements, and physicochemical measurements of groundwater.The Dumoga area is dominated by sand and clayey sand, with breccia and sandstone found in the western and limestone, lapilli tuff, and andesitic lava in the eastern research area.Groundwater in the research area flows from the surrounding regions towards the center of the Dumoga River.The primary aquifer is sand and clayey sand, with groundwater depths around 0.2 m to 10 m from the surface.Most hot spring water is unsuitable for raw water and irrigation sources due to their temperature, pH, and EC being more than standard.Some groundwater in the southwestern and eastern Dumoga area also did not recommend irrigation due to their quality.The results of groundwater research in the Dumoga area show that this area has good groundwater potential.However, several locations have groundwater quality that does not meet the irrigation and raw water requirements.It should be avoided to use groundwater with bad quality for irrigation in order to minimize crop failure.The groundwater resources in Dumoga can support government programs that maintain this area for food crops area.

Figure 2 .Figure 3 Figure 4 . 7 Figure 5 .
Figure 2. Surface lithology of the research area It was found that there are several hot springs in the Dumoga area.The hot spring with the highest EC value, reaching 5,800 μs/cm, is located at HP04 in West Dumoga District and associated with breccia lithology.This hot spring is estimated to be mixed with formation water, causing its EC value.On the other hand, the hot spring with the highest temperature, measuring 73.5°C, is found at HP01 in East Dumoga District.In general, groundwater with high EC, TDS, and pH values is located in the eastern part of the research area.Specifically, high EC and TDS values are observed in the village of Doloduo in the East Dumoga District, particularly in the dug wells with high temperatures, as shown inFigures  6,7,8,and 9.

Table 5 .
Result of Ground Water Measurement