Conceptual model of hydrogeology condition around Tambakboyo artificial lake in Sleman Regency, Yogyakarta

. Groundwater nowadays has become one of the public necessities. Tambakboyo Artificial Lake in Sleman Regency, Special Region of Yogyakarta Province mainly functions as a recharge lake for the local groundwater. The location of this lake at the economic and educational center leads to an increase in groundwater use, affecting the quality and quantity of groundwater. The groundwater flows through hydrogeological conditions around Tambakboyo Artificial Lake, part of the Sleman-Yogyakarta Groundwater Basin. This study aims to know how groundwater system works by developing the hydrogeology conceptual model of the Tambakboyo Artificial Lake. Hydrogeological direct observation in the field consists of measuring groundwater level, geoelectrical survey, slug test, and secondary data of bore log and climate data. All those data were analyzed and interpreted to develop a 2-dimensional conceptual model of the hydrogeological condition. The analysis shows that the study area is urban with a recharge of approximately 331.80 mm/year. The hydrostratigraphy around Tambakboyo Artificial Lake is commonly formed by the gravelly sand, silty sand, and breccia layers, with sand and silty sand acting as one aquifer system. The clay layer was found as an aquifer base at a depth ranging between 68 and 120 meters. The thickness of the aquifer decreases from north to south with the permeability value (K) around 4•10 -5 – 2•10 -4 m•s −1 . Based on the relationship between surface water and groundwater, Tambakboyo Artificial Lake can act as a recharge lake for its main purpose.


Introduction
Artificial lake nowadays has become an infrastructure which functions as a groundwater conservation media.Tambakboyo Artificial Lake in Sleman Regency, Special Region of Yogyakarta Province mainly functions as a recharge lake for the local groundwater.However, groundwater quality has been tremendously threatened by various activities due to rapid urbanization in last two decades.For instance, most citizens in Yogyakarta City are using on-site sanitation systems that may harm the groundwater because of the excessive wastewater seepage or leakage [1].Groundwater use in this region is increasing rapidly, especially in the middle of the area (Depok District, Ngemplak District, and Ngakglik District) which is at the center economic and educational developing area, and due to the limited surface water supply, increasing population density, high producers of agricultural commodities in the Sleman Regency, so that these three areas become the research area.

Background
Groundwater over-pumping can cause negative impacts such as decreasing groundwater level and the quality.A groundwater modeling may help to manage the above problems.Groundwater modeling can describe complex hydrogeological subsurface conditions [2].Most groundwater modeling research has used the MODFLOW model, which can simulate groundwater flows over a wide range and in various natural systems [3][4][5].Conceptual model becomes the basic of groundwater modeling.A hydrogeological conceptual model is a simple form of actual conditions or an idealization of existing conditions of a particular area and how groundwater flow systems work [6,7].The hydrogeological conceptual model consists of: identifying the model boundaries, defining the hydrostratigraphy system, defining the aquifer characteristics, and defining the groundwater flow system [3,4].Therefore,a hydrogeological conceptual model in Tambakboyo Artificial lake is needed before developing the numerical groundwater modeling and becoming this research's objective.
The geological conditions in Sleman Regency are dominated by Sediment of Mount Merapi.Geological formations are divided into volcanic deposits, sediments, and intrusive rocks, with volcanic deposits representing more than 90% of the area.In this zone the rocks consist of volcanic deposits, tuff, breccia ash, agglomerates and inseparable molten lava (Qmi) [8].
The research area is in the Merapi Aquifer System, The thickness of the Merapi Aquifer System varies greatly, in general the thickness increases towards the south.In the Yogyakarta Graben area, namely in the Ngaglik area, the thickness of the Merapi Aquifer System reaches 80 meters, in the Bedog and Karangayam areas around 140 meters and in the Yogyakarta city area it reaches 150 meters.This thickness decreases again outside the Yogyakarta Graben, which is around 45 meters south of Yogyakarta City.In the Bantul Graben area, around Bantul City, the thickness of the Merapi Aqiufer System has increased again to 125 meters [9].Vertically, the Merapi Aquifer System can be divided into two main aquifers, the upper aquifer, the lower aquifer and the basic aquifer [10].

Methods
The method of this research is collecting primary data and secondary data.Geological direct observation, measuring the boundary condition of the research area and measuring groundwater level from 105 dugwells were conducted as a primary data.Dug well observation point can be seen at Figure 2. Collecting boreholes data, geoelectrical survey, pumping test, digital map, geology regional map and climatological data from Balai Besar Wilayah Sungai Serayu-Opak were collected as secondary data.All those data were analyzed and interpreted to develop a 2-dimensional conceptual model of the hydrogeological condition.All the research methods shown in Figure 3.  Analyzing geological data is a fundamental point in this method, because we have to know the natural conditions of the water pathway.Parts of the geological analysis are geological mapping, regional geological map analysis, analysis of log drilling results, research area boundary conditions data, and analysis of geoelectric results.
After we know the condition and character of the geological parameters, we proceed to hydrological analysis to know the characteristics of the surface water and the groundwater.To determine hydrological conditions we need climatological data, annual rainfall data, annual temperature data, runoff, depth of ground water level, and recharge values of groundwater.
The result of the analysis is a conceptual model of hydrogeological condition,that can be used as a tool to know the recharge ability of Tambakboyo Artificial Lake.

Rainfall
To input recharge to the conceptual model, average rainfall is required.To get that, data are collected for five years period of time from 2017 to 2021 [11].By computing, the average monthly rainfall and annual rainfall, the average annual rainfall is 2064 mm/year.

Evapotranspiration
The actual evapotranspiration is also vital to be estimated.In this case, Turc, widely used empirical method [12], is used to determine the value of ETa as function of annual precipitation and mean temperature.Turc's equation is shown as the following: Where: : real evapotranspiration (mm/year) : rainfall (mm/year)   : annual mean temperature (°C) By inputting the average annual rainfall of 2064 mm and annual mean temperature of Sleman Regency is 25,70 °C, the result shows that the value of ETa is approximately 1382 mm/year.

Runoff
A lot of methods have been developed to estimate runoff.Sharma's method [13] can be applicable in Yogyakarta groundwater basin due to the fact that Yogyakarta province is considered to be rural area and the method is highly recommended for this type of land.It is used as function of average temperature, annual rainfall, area of watershed.The equation of runoff calculation is given as the following:  = From the data obtained from BPS Sleman Regency, average temperature is 25,70 °C, 2064 mm of annual rainfall, and 5,89 km 2 of area of watershed.Therefore, the value of net runoff without considering the impervious cover effect is approximately 606 mm/year.

Groundwater recharge in research area
In urbanized area, the natural groundwater recharge can be inhibited as impervious cover enhances runoff and limits infiltration.However, urban development introduces new sources of recharge: leakage from water and wastewater distribution and collection systems.The population in the Yogyakarta City is served by sewage system, it is believed that wastewater evacuated from on-site sanitation, latrines, soakaways and main leakage from the water supply become the major source of groundwater recharge [14].The leakage from wastewater system is described in the Table 1.All the components of urban recharge in research area can be summarized in Table 2. From calculation in Table 2, the value of net urban recharge to groundwater divided by the research area, can be estimated as approximately 255mm/year.Base on general concept of water balance with consideration of natural and urban recharge (amount of water in is equal to amount of water out), the equation of groundwater recharge calculation is derived as: Where: P : Rainfall (mm/year) Eta : Actual evapotranspiration (mm/year) Rou : Runoff which consider the effect of impervious cover (mm/year) Uu : Net Urban recharge (mm/year) U : Groundwater recharge (mm/year) By computing all parameters above, the groundwater recharge with consideration of natural and urban recharge results as 331.80 mm/year.

Pumping test
An accurate hydraulic conductivity is necessary in conceptual model because it has strong influence on groundwater flow.This hydraulic parameter can be determined by means of an aquifer test, particularly slug test.In this test, 6 dug wells have been pumped at the average constant rate of 1.22 liter per second.Based on Hvorslev's method which is applicable for unconfined aquifer type, the time-recovery is interpreted to yield the hydraulic parameters of the aquifer, shown in equation ( 4).The location of slug test can be seen at Figure 4. Where: K is hydraulic conductivity (m/s) r is radius of the well casing (m) R is the radius of the well screen (m) L is the length of the well screen (m) t37 is time it takes for the water level to rise or fall to 37% of the initial change(s).

Results and discussion
The data of groundwater elevation that collected from 105 dugwells (shown at Figure 5), it shows that depth to groundwater getting deeper to south directions.And the geological data that collected from 9 boreholes data (shown at Figure 6), it shows that research area are commonly formed by the gravelly sand, silty sand, and breccia layers, with sand and silty sand acting as one aquifer system.The conceptual model from north to south section shown at Figure 7 giving the illustration that Tambakboyo Artificial Lake stands above the aquifer layer that mostly formed by sand material, and the clay layer was found as an aquifer base at a depth ranging between 68 and 120 meters, and from West to East section shown at Figure 8.By calculating the hydrological parameters that consists of Precipitaion is 2064 mm/year, evapotranspiration actual 1382 mm/year, run-off 606 mm/year with the Groundwater flow direction from north to south giving the final value net urban groundwater recharge is approximately 331.80 mm/year, shown in Figure 9. Based on the relationship between surface water and groundwater, Tambakboyo Artificial Lake can act as a recharge lake for its main purpose.

Conclusion
Based on the result, the conceptual model of hydrogeological condition of Tambakboyo Articial Lake and Its sourrounding can be concluded: • The groundwater flow direction from the north to south Tambakboyo Artificial Lake, based on investigated water table data.• Tambakboyo Artificial Lake stands above the aquifer layer that mostly formed by sand material, and the clay layer was found as an aquifer base at a depth ranging between 68 and 120 meters.• The recharge in the research area is approximately 331.8 mm/year.Based on the slug test data, the hydraulic conductivity of the aquifers is 1,4•10-4 m/s and is categorized as high hydraulic productivity.

Fig. 6 .
Fig. 6.Geological map.By using Hvorslev's Method the slug test result shows that the hydraulic conductivity value of the aquifer is around 4•10 -5 to 2•10 -4 m•s −1 , with the average value is 1,4•10 -4 m•s −1 .The conceptual model from north to south section shown at Figure7giving the illustration that Tambakboyo Artificial Lake stands above the aquifer layer that mostly formed by sand material, and the clay layer was found as an aquifer base at a depth ranging between 68 and 120 meters, and from West to East section shown at Figure8.

Fig. 7 .
Fig. 7. Hydrogeological conceptual model from North to South section.

Fig. 8 .
Fig. 8. Hydrogeological conceptual model from West to East section.

Table 1 .
Estimation of the potential wastewater recharge in the research area.