Assessment of Groundwater Recharge and Groundwater Quality in The Upstream of Tondano Watershed, North Sulawesi, Indonesia

Raw water need increases along with the development of a region. In terms of quantity and quality, surface water tends to decrease, which causes an increase in groundwater exploitation. For groundwater in the Tondano watershed to be sustainable and meet the required quality standards, it is necessary to know about groundwater recharge and groundwater quality. This study’s objectives are to calculate groundwater recharge using a simplified groundwater balance and to evaluate groundwater quality using physico-chemical parameters. The method used in this study is to estimate groundwater recharge by calculating groundwater balance using data on annual rainfall, annual evapotranspiration, and annual runoff. Meanwhile, for groundwater quality, Physico-chemical properties were measured, including pH, TDS, EC, and temperature, at 21 measurement points, including springs, dug wells, and drilled wells. The results indicate that groundwater recharge calculation shows a positive value of 318.34 mm/year. The measurement of Physico-chemical parameters in the field suggests that the pH value is in the range of 6.1-7.9, the TDS range is 50-200 mg/L, the EC range is 110-400 μs/cm, and the temperature range is 23.4-35.6 °C. The analysis using World Health Organization standards and Ministry of Health standards show that the groundwater quality is suitable for use.


Introduction
Water is a limited natural resource essential for human existence and health [1].The need for water each year is increasing along with population growth and the development of a region.However, the increase in demand is not matched by limited availability, and the quality tends to decrease due to reduced catchment areas, massive development without regard to the environment, and excessive use without regard to the sustainability of water sources [2], especially those originating from surface water.So this has resulted in people switching to using groundwater to meet their needs for better quality and more accessible obtain.
The Tondano watershed is one of the priority watersheds that acts as the center of economic development in North Sulawesi Province [3].It has a river length of 39.9 km and a watershed area of 544.75 km 2 [4].Administratively, it is located in four districts in North Sulawesi Province, namely Minahasa Regency, North Minahasa Regency, Manado City, and Tomohon City.Cities in the Tondano watershed area are developing rapidly, thereby increasing the need for water for domestic use, especially groundwater.Therefore, it is necessary to study water recharge and water quality so that groundwater in the Tondano watershed remains sustainable and meets the requirements for use.In order to maximize their use and prevent any adverse effects, groundwater resources should be assessed and managed [5].Changes in groundwater quality are primarily brought on by interactions between rocks and water, as well as oxidation-reduction reactions that take place as water percolates through aquifers.In addition to these mechanisms, groundwater flow in aquifers from recharge to discharge areas transports toxic and nontoxic contaminants, water-born pathogens, and other water-quality degrading criteria [6].Physical and chemical characteristics strongly impacted by geological formations and human activity determine how groundwater quality fluctuates in a particular region [7].
Watersheds are natural ecosystems bounded by ridges [8].Rainfall in the region will eventually pour into the sea after passing through small and larger rivers.In the watershed area, two areas are known: the upstream area and the downstream area.These two areas are interconnected and influence each other in one watershed unit.The watershed roles include catchment, storage, and distribution of water.This study aims to predict annual groundwater recharge based on simple groundwater balance calculations using annual precipitation, annual evapotranspiration, and annual runoff.This study also assessed groundwater quality based on the Physico-chemical parameters to make groundwater suitable for domestic use.Many assessments of groundwater quality using Psyco-chemistry parameters have been reported [6,7,[9][10][11].
This study was conducted upstream of the Tondano watershed, as shown in Figure 1.Geographically is located at a longitude of 124º51'58.13"E-125º1'44.66"Eand a latitude of 1º27'58.99"N-1º16'56.03"N.Geological conditions based on regional geological maps are consist of lacustrine and fluviatile deposits (Qs), young volcanic rock (Qv), tondano tuff (Qtv) and volcanic rock (Tmv) (Effendi & Bawono, 1997).Hydrogeological conditions based on regional hydrogeological maps, there are three types of groundwater presence: aquifer in which flow is intergranular, the aquifer in which flow both through fissures and interstices, and aquifer (fissures or porous) of poor productivity and region without exploitable groundwater [13].

Groundwater Recharge Calculation
Simple groundwater balance calculations estimate groundwater recharge in the study area, as shown in Equation 1 [14].The data needed in the calculation includes annual precipitation data and mean annual temperature data from 2012-2021.Annual precipitation data were obtained from the three stations closest to the study area: ARR Tikala-Sawangan, MRG Tikala-Kaleosan, and ARR Tikala-Rumengkor, as shown in Figure 2. In contrast, the annual mean temperature data were obtained from the Tondano-Peleloan climatology station.The average annual precipitation in the study area was calculated using annual precipitation data from the three rainfall stations processed using the Thiessen polygon method.The Turc method is then used to determine the actual evapotranspiration using the mean annual temperature and average annual precipitation data.To calculate the actual evapotranspiration using data on mean annual temperatures, Turc presented the empirical method in Equation 2 [15].
Where: ETr actual evapotranspiration (mm) P annual precipitation (mm) Tm mean annual temperature (°C), and L thermal indicator, as determined by Equation 3.
The empirical runoff equation from the Department of Agriculture of India (1990) was used to determine surface runoff.Equation 4shows the format of the runoff estimation based on this method [14].

Physico-chemical Properties
Groundwater Physico-chemical properties were measured on groundwater samples from twenty-one groundwater emergence points consisting of eight springs, eight dug wells, and five drilled wells, as shown in Figure 3  Assessment of the samples was carried out by comparing the values of the physics-chemical parameter measurements with the standard values, as shown in Table 1.The interpretation of these values is an initial assessment of the condition of groundwater quality before conducting further research to decide whether groundwater is suitable for domestic use.

Results and Discussion
Based on collecting secondary data and direct measurements in the field, which are then processed, groundwater recharge estimates and assessments of water quality in the upstream Tondano watershed are obtained.

Groundwater Recharge
The average annual precipitation value for the study area was calculated using the precipitation data from the three rain stations using the Thiessen polygon method, as shown in Figure 4.The maximum precipitation was in 2017 at 3090.72 mm, while the lowest was in 2019 at 1106.18 mm.For ten years, the average annual precipitation was 2028.91 mm/year.

Figure 4. Annual precipitation
The annual mean temperature was obtained from the Tondano-Peleloan climatology station.Figure 5 shows the ten-year mean temperature values from 2012-2021.The lowest mean temperature value of 25.76 °C occurred in 2013, while the highest mean temperature value of 26.98 °C occurred in 2016.The average value of the annual mean temperature to be used in calculating groundwater recharge is 26.64 °C.Data on average annual precipitation and annual mean temperature are calculated using the Turc method to obtain an actual evapotranspiration value of 1424.92 mm.Then, using the runoff equation from the Department of Agriculture of India (1990), a surface runoff value of 285,65 mm was obtained.The estimated groundwater recharge in the study area is 318,35 mm.Based on these results, the estimated value of groundwater recharge from the calculation results shows a positive value.The groundwater recharge value is 15.69% of the average annual precipitation.

Groundwater Quality
Physico-chemical parameters measurement of groundwater in the field was carried out using a Hannamemeter at 21 points of emergence of groundwater, as presented in Table 2.The data on the physico-chemical properties of groundwater were then compared with the standards for drinking water quality (WHO standard) and Permenkes Number 492/2010 regarding drinking water quality requirements based on the findings of the field measurements.Total Dissolve Solid (Figure 6), Electrical Conductivity (Figure 7), pH (Figure 8), and temperature are among the variables evaluated (Figure 9).  Figure 7 shows that the EC values of twenty-one groundwater samples meet the WHO standard, but only one sample, MA 1, does not meet them.However, in another study, the sample was still classified as good with moderately saline water and having varying ionic concentrations [20].The minimum EC value of groundwater samples is 110 µs/cm, and the maximum is 420 µs/cm, with an average of 222.86 µs/cm.8 shows that the pH values of thirteen groundwater samples meet both WHO standards and the Permenkes, but eight samples do not.In another explanation, the WHO assigns a minimum pH value of 5 [17], so the eight groundwater samples can still be said to meet the standards.The minimum pH value of groundwater samples is 6.1, and the maximum is 7.9, with an average of 6.57.The Permenkes standard for temperature is ±3 °C from the average air temperature in the area.The average value of the mean annual temperature in the study area is 26.64 °C, so the standard temperature is in the range of 23.64 °C -29.64°C.Figure 9 shows that the temperature values of seventeen groundwater samples meet the Permenkes standard, but four samples do not.If using the WHO standard, which is in the value range of 15°C -34°C, only one sample does not meet the standard, namely the MA 8 sample, because this sample comes from a hot spring.The minimum temperature value of groundwater samples is 23.4 °C, and the maximum is 35.6, with an average of 28.48 °C.

Conclusion
This study provides an overview of groundwater conditions upstream of the Tondano watershed.Based on the results of groundwater recharge calculations, the estimated recharge in the study area shows a positive value of 318,35 mm.The groundwater recharge value is 15.69% of the average annual precipitation.In terms of the evaluation of groundwater quality, it generally complies with WHO and Permenkes requirements, making it suitable for domestic usage.The groundwater must undergo a series of further evaluations before it can be declared fit for drinking.

Figure 1 .
Figure 1.Study area map

Figure 2 .
Figure 2. Rainfall station location  =  −   −  (1) Where: U Groundwater Recharge (mm) P Annual Precipitation (mm) ETr Actual Evapotranspiration (mm) Ro Surface Runoff (mm) . Measurements were carried out using a Hannameter directly in the field as soon as possible after taking the sample to get the ideal result.Temperature, pH, Total Dissolve Solid (TDS), and Electrical Conductivity (EC) were measured by Physico-chemical parameters.Data on the Physicochemical characteristics of groundwater gathered in the field can be utilized for data verification in the laboratory and to provide a preliminary assessment of the groundwater quality[16].The measurement results of Physico-chemical parameters are compared with the Guidelines for drinking water quality (WHO standard) and Permenkes Number 492/2010 concerning drinking water quality requirements.

Figure
Figure8shows that the pH values of thirteen groundwater samples meet both WHO standards and the Permenkes, but eight samples do not.In another explanation, the WHO assigns a minimum pH value of 5 [17], so the eight groundwater samples can still be said to meet the standards.The minimum pH value of groundwater samples is 6.1, and the maximum is 7.9, with an average of 6.57.

Table 1 .
Groundwater quality requirements for drinking water

Table 2 .
Groundwater Physico-chemical Properties in the Study Area