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Pattern and direction of groundwater flow and distribution of physical-chemical properties of groundwater in Randublatung basin

The people in the Randublatung basin (Grobogan, Blora, and Bojonegoro Regencies) using groundwater for daily needs and agriculture activity. As the initial step of basin-based groundwater management, it is necessary to understand the groundwater potential in this area: pattern and direction of groundwater flow and groundwater physical-chemical properties (pH, temperature, total dissolved solids, and electrical conductivity). This research aims to analyze the pattern and directions of groundwater flow, the physical-chemical properties, and the correlation between the two. This research method is field measurement of 45 different spots of dug wells in the Randublatung basin. Our results indicate that the pattern and direction of groundwater flow in the Randublatung basin are heading to Bengawan Solo River and then following the river’s flow. The groundwater physical-chemical properties measured: pH value is 6.8 on average, the temperature is 28.9 °C on average, TDS concentration is 409 mg/L average, and electrical conductivity rate is 843 μS/cm average. There is no significant correlation between groundwater flow with pH value and groundwater temperature. However, groundwater TDS concentration and electrical conductivity rate in the Randublatung basin increase as groundwater flows to the Bengawan Solo River, which is affected by the minerals of aquifer rocks (alluvial deposits).


Introduction
Groundwater is one of the water resources that are relatively easy to get and has a good quality. People using groundwater for daily needs and agriculture activity, for example, people of the Randublatung basin. The Randublatung basin is located in Grobogan, Blora, and Bojonegoro Regencies, with 206 km 2 of area. Potential groundwater capacity in the Randublatung basin for the unconfined aquifer is 23.000.000 m 3 /year and 9.000.000 m 3 /year in a confined aquifer [1]. The total population of Grobogan Regency in 2019 is 1.351.429 people, and the population growth rate is 0,56% [2]. Meanwhile, in the same year, the population of Blora Regency is 925.642 people, and the population growth rate is 1,02% [3]. Other than that, Bojonegoro Regency had 1.331.077 people in 2019 [4]. Population growth rate indicates an increase in daily activities and agriculture activities so that the use of groundwater will also increase. Excessive groundwater use can cause various problems, such as groundwater level changes and land subsidence [5]. Basin-based groundwater management is necessary so that the Randublatung basin does not negatively impact excessive groundwater use. In basin-based groundwater management, the pattern and direction of groundwater flow and the physical-chemical properties of groundwater are included in the technical component, namely the evaluation of the potential of groundwater resources [6]. Therefore, it is essential to know the groundwater flow system and the distribution of physical-chemical properties of groundwater in the Randublatung basin as the first step of basin-based groundwater management.
In this research, we analyze the pattern and direction of groundwater flow and distribution of physical-chemical properties of groundwater using Geographic Information System (GIS). This method has been applied in many similar research, such as on Pekanbaru Regency, Indonesia in 2015 [7], Harare, Zimbabwe in 2019 [8], Kupang Regency, Indonesia in 2020 [9], Coimbatore, India in 2021 [10], Sistan and Baluchestan Province, Iran in 2021 [11]. This research aims to analyze the pattern and direction of groundwater flow and the distribution of physical-chemical properties of groundwater in the Randublatung basin. In addition, to analyze the relationship between pattern and direction of groundwater flow and the physical-chemical properties of groundwater. This research is expected to be an initial description of groundwater in the Randublatung

Study area
The study area of this research is in Randublatung Basin. The basin's location is in Grobogan Regency, Blora Regency, and Bojonegoro Regency, as seen in Figure 1. [12] The location of Randublatung Basin physiologically is in the Depression Zone of Randublatung, which is between two hills. On the north side, there is Rembang Hills and on the south side is Kendeng Hills. So that from topography aspect, the elevation on the north side and the south side is from +100 m to +200 m. While the elevation in the Randublatung basin is about below +50 m to +100 m.  The geology of the Randublatung basin is shown in Figure 2. The formation of rocks from older to youngest in the Randublatung basin is Bulu Formation (Tmb), Wonocolo Formation (Tmw), Ledok Formation (Tml), Mundu Formation (Tpm), Lidah Formation (QTl), and Alluvial Deposits (Qa) [13]. The formation of rocks shows that there are marl and limestone on the bottom side of this basin and gradually turns into clay and sandstone on the upper side. Hydrogeological, formation of rocks that can become an aquifer is Lidah Formation (QTl) and Alluvial Deposits (Qa). Lidah Formation (QTl) consists of clay and sandy in some places. Meanwhile, Alluvial Deposits (Qa) consists of sandstone and gravel. So, there are two types of aquifers in the Randublatung basin, confined aquifer and unconfined aquifer [1]. Data from the water table is used for the pattern and direction of groundwater flow analysis. The physical-chemical properties of groundwater are directly measured on the field. Water table and physical-chemical properties were measured in 45 dug wells in the Randublatung basin. The dug wells that were measured are dug wells that were not exposed to rainwater directly.  [14] The water table is needed to determine the direction of water flow. The direction of water flow can be obtained by measuring the water table from neighboring wells in meters above sea level (masl). Water table measurement can be performed only in an open well, such as dug wells.

Measuring water table
Measuring the water table can be done by using measurement tapes. Figure 3 shows how to obtain the depth of the water table in masl. The first step is measuring water level depth from the edge of the well casing (AC). Then, measure the height of the well casing (AB). Depth of water table (BC) can be calculated from the subtraction of AC and AB. To obtain water table in masl, elevation data is needed. Subtraction between the elevation of ground level and depth of water table will produce water table in masl. AB is the height of the well casing. BC was obtained by subtraction of AC and AB [14] .

Measurement of groundwater physical-chemical properties
The physical and chemical properties of groundwater can indicate the quality of groundwater [15]. The physical-chemical properties of groundwater in this research are pH, temperature, total dissolved solids (TDS), and electric conductivity. Value of pH shows groundwater's composition. Low pH values (below 4) indicate acid water that can dissolve metals in contact with water. Water with pH 7 means neutral water. High values of pH (above 8) indicate alkaline water. The temperature data of groundwater reflects its hydrological condition [14]. If the groundwater temperature is close to local surface temperature, the groundwater belongs to the shallow active water cycle. TDS in the groundwater is from mineral substances in aquifer rocks [15]. Groundwater in the shallow area generally has less than 1.000 mg/L TDS concentration [16]. Electric conductivity in groundwater represents the total amount of salts dissolved in the water [14]. It means that with the larger conductance, the water is more mineralized [15]. Measurement of groundwater physical-chemical properties is directly done on the field using Hannameter. The temperature was measured in Celsius (oC), TDS in mg/L, and electric conductivity in µS/cm.  [14] Groundwater flow patterns consist of equipotential lines. As seen in Figure 4, the direction of groundwater flow will be obtained through the equipotential lines. Groundwater will flow from high equipotential to low equipotential. Mapping of the equipotential line is using GIS. Water table masl from 45 dug wells plotted according to the coordinates to create a raster of the water table. Then, to create the equipotential lines, convert the raster map to contour shape using a contour tool. The contour represents the equipotential line of groundwater flow. The direction of groundwater flow then can be determined from the groundwater equipotential map.  [14] .

Analysis of groundwater physical-chemical properties distribution
Groundwater physical-chemical properties data from 45 dug wells were processed using GIS to map groundwater physical-chemical properties. One map for each parameter. So, there will be a map of pH value distribution, a map of groundwater temperature distribution, a map of TDS distribution, and a map of electric conductivity distribution.

Analysis of the relationship between the pattern and direction of groundwater flow with the physicalchemical properties of groundwater
The method to analyze the relationship between the pattern and direction of groundwater flow and groundwater's physical-chemical properties is by overlaying the maps. Map of groundwater flow pattern overlaid on each map of the distribution of physical-chemical properties of groundwater. Then, the relationship between the groundwater flow and the physical-chemical properties value of groundwater can be seen through the overlay map.

Results and Discussions
Results of water table masl and physical-chemical properties measurement are shown in table 1. Water table masl varies between 21 masl to 94.6 masl. The water table measurement result indicates that the topography of the Randublatung basin affects the water table level. On the west side of the basin, the elevation is around +100 m, and the water table is about 80 masl to 94.6 masl. While on the east side of the basin, the elevation is around +50 m, and the water table is around 30 masl to 21 masl. The pattern and direction of groundwater flow are shown in Figure 5. The direction of groundwater flow in the Randublatung basin is heading to Bengawan Solo River and then following the flow of Bengawan Solo River.    The map of physical-chemical distribution is shown in Figure 6. The pH value of groundwater in the Randublatung basin varies between 6.1 and 7.6. Figure 6(a) shows that the highest pH value is located in the middle of the Randublatung basin. The groundwater temperature measurement result is shown in Figure 6(b) explains that the groundwater temperature in the Randublatung basin is between 27.3 o C and 33.5 o C. The highest temperature is located on the east side of the Randublatung basin, with the sampling condition relatively at the same time, namely at noon. Figure 6(c) shows that the concentration of TDS in the groundwater of the Randublatung basin varies between 50 mg/L and 710 mg/L. The lowest TDS concentration is located on the northwest side of the basin. In comparison, the highest is on the northeast side of the basin. While in Figure 6(d) shows that the electric conductivity rate in the groundwater of the Randublatung basin is between 120 µS/cm to 1430 µS/cm. The distribution map of groundwater electric conductivity that quite similar to the distribution of TDS. Groundwater with the lowest electric conductivity value is located on the northwest side of the basin, and the highest is on the northeast side of the basin. Our data show no clear indication that the physical-chemical properties of the groundwater follow the flow direction, as shown in Figure 7. Figure 7(a) shows the map overlay between groundwater flow and distribution of pH value. As already mentioned, the groundwater flow is heading to Bengawan Solo River. However, on the west side of the Bengawan Solo River, groundwater has a high pH value and groundwater with a low pH value. It can be said that there is no significant correlation between groundwater flow and groundwater pH value.
The relationship between groundwater flow and groundwater temperature in the Randublatung basin is shown in Figure 7(b). The temperature value is increasing on the east side of the basin. However, on