Characteristics of hydrogeological aquifer in potential lime stone by analysis pumping test in Campurdarat District, Tulungagung Regency, Indonesia

Extraction of clean water from dug wells and drilled wells is a source of water that is obtained locally without buying, for example, clean water from the Drinking Water Company (PAM). Surface water and groundwater are generally used frequently, even though the utilization of groundwater is not optimal compared to surface water. One way to determine the potential of groundwater contained by aquifers or aquifer discharge is by pumping test wells. In this test, water is pumped to the surface through a test well dug into the designated aquifer. The purpose of this study is to determine the characteristics of hydrogeological aquifer of limestone by the pumping test analysis approach using the Theis and Cooper - Jacob methods in Campurdarat District, Tulungagung Regency. Pumping tests have been carried out on two observation and production wells with the results of the Theis method showing a transmissivity (T) of 73,193 m2/day, storage capacity (S) of 1,080×10-3 and hydraulic conductivity (K) of 1,220 m/day while Jacob shows a transmissivity (T) value of 71,682 m2/day, storage capacity (S) of 0.4512 and hydraulic conductivity (K) of 4,667 m/day. The Theis method was chosen because it has a relative error smaller than Jacob method, namely 29.77%.


Introduction
Groundwater is one of the sources of water needed for human, animal, and plant life.In recent years, groundwater use and extraction can be done by drilling shallow and deep wells and pumping the groundwater [1].
Taking clean water from dug wells and drilled wells is a source of water obtained locally without buying, for example, the clean water network from a drinking water company (PAM).Surface water and groundwater are generally often used, even though the utilization of groundwater is not optimal compared to surface water [2].
One way to determine the potential for groundwater contained by an aquifer or aquifer discharge is by pumping a test well.In this test, water is pumped to the surface through a test well dug to the intended aquifer.After being pumped because groundwater discharge is coming out of the aquifer, the initial groundwater level will certainly fall.The groundwater level subsidence is then monitored and recorded in monitoring wells placed at a certain distance around the test wells.Due to the complexity of the work and the equipment used, the pump test is time consuming and expensive, even if the confidence level in the results is high.
The parameters of aquifers such as hydraulic conductivity and transmissivity, are for the management and determination of groundwater [2].Aquifer parameters belonging to hydraulic conductivity, transmissivity and storage capacity are commonly used in groundwater models [3], [4], [5].
Over the past two decades, significant progress has been made in computer modeling groundwater and contaminant transport problems.The reliability of the model results depends on the knowledge of the hydraulic characteristics of the aquifer system.One of the most suitable methods to measure hydraulic properties because it gives results that usually show a larger area than the results of a single measurement is the pump test [6].The pump test aims to ensure that the proposed test site and associated equipment produce acceptable results and reduce uncertainty in data collection and analysis [6].
Aquifer parameters determination is an important basis for assessing groundwater resources, quantitative analysis, development and conservation, and scientific management.Case studies show that these methods are fast and accurate.The results of boundary determination are then adjusted to be of great value for scientific research and the preparation of soil engineering [7].
This study aims to evaluate the general hydrogeological conditions of limestone.Pumping tests have been carried out for two observation and production wells in Pojok Village, Campurdarat District, Tulungagung Regency.There are several methods to measure hydrogeological characteristics, namely the Theis curve method, the Cooper and Jacob method, the Chow method, and the Theim's method.But in this research, only 2 (two) methods were discussed, namely the Theis and Jacob methods [8], because groundwater flow to the pumping well is an unsteady-state condition when drawdown changes with time (rate of change of drawdown is not zero) [9].There is no source of recharge to the aquifer.That is, all the pumped water comes from the aquifer storage.The analysis of the hydraulic properties was carried out in the wells located in the study site.The basis for setting up a hydrogeolical model is the evaluation of primary data from pumping test.Based on the Earth Movement Vulnerability Zone Map, Tulungagung Regency, the research location in Campurdarat District is a very low soil movement vulnerability zone, areas that have a very low level of vulnerability to land movement.In this zone, there are rarely or rarely ground movements, both old and new ground motions, except in small areas on the riverbanks.This zone is a flat to sloping area with a less than 15% slope.The slope is not formed by sedimentary soil movement, heap material, or plastic clay or expands.

Materials and Methods
Based on the geological map of the Tulungagung quadrangle (Figure 2), the study location is located between two formations, namely alluvium (cobble, pebble, sand, silt, clay, and mud) and the Mandalika formation (volcanic breccia, lava, and tuff, with intercalation of sandstone and siltstone).

Figure 2. Geology Map of Study
The pumping test aims to analyze groundwater discharge.The aim is not only to determine the ability of the drilled well to produce groundwater but also to determine the passability of the watercarrying layer (aquifer) [2].The pumping test is pumping water from a well with a certain discharge, observing the decrease in the water level during pumping and observing the recovery of the water level after the pump is turned off according to a certain time interval.The pumping test can be divided into two: for testing wells with a step drawdown test and aquifers with a long period test.Data regarding aquifer characteristics is a factor that must be considered in studying groundwater, especially to determine the capacity of groundwater that can be stored in the soil layers and the capacity that can be used.
According to [10], transmissivity values range from less than 12.4 m 2 /day to more than 12.400 m 2 /day.If an aquifer has a transmissivity coefficient value of less than 12.4 m 2 /day, then the aquifer is only sufficient for fulfillment that requires a small groundwater discharge.However, if the value is 12.4 m 2 /day or more, then the groundwater discharge is sufficient for industrial needs, urban activities, and irrigation.The pumping method is a very simple procedure.Selected observation wells have been pumped into the aquifer system, and groundwater subsidence is calculated in pumped test wells or in neighboring pumped wells at regular intervals.When the pumping test was stopped, the groundwater level in the observation well increased [7].
The best method used to evaluate the aquifer parameters is the pump test method.This test involves approaching the water of the water and its rates and characteristics of the water and the water well and/or one or more [11].Researchers have proposed several methods and estimations of groundwater parameters in recent periods to analyze pumping test data [12].A test is a process for introducing or withdrawing water from a well and calculating the aquifer's response (change in pressure) at the same test or in a nearby observation well.All aquifer properties (mainly hydraulic conductivity, transmissivity, and specific storage) are used for estimation based on the hydraulic response model.
Binary pumps and gear pumps can be installed for shallow wells where only small drains are required, manual pumps, and flow rates up to 500 m 3 / day.The suction height must not exceed 7 m for efficient and continuous service.When larger discharges from shallow wells are required, centrifugal pumps are generally used.The assembly can be mounted with a horizontal or vertical axis.The horizontal design is efficient, easy to install and maintain, and is usually connected directly to an electric motor.Due to the low suction lift, pumps are often positioned a short distance above the water level in large diameter wells [13].
The quantity of groundwater in one area is closely related to the system and the characteristics of the aquifer of its constituent materials, including the type capacity and transitivity [14].Aquifer parameters for determining groundwater potential include namely, K-value (hydraulic conductivity), T-value (transmittance), S-value (storage coefficient), and r-value (pumping effect distance) [15].To obtain the value of the aquifer parameter, a pumping test of the aquifer from resident wells for a short period of time is carried out, taking into account the hydrological conditions of the region.Groundwater responds with short-term withdrawal, for example, by pumping from 1 to 4 hours [16], [17].This pumping test is a method that can be widely used to determine the technical characteristics of groundwater [18].The method used to analyze aquifer parameter calculations is the Cooper-Jacob calculation approach for groundwater pumping or subsidence data, and Theis calculation is for data on the increase or recovery of groundwater level after pumping [17], [13].
The storage coefficient (S) (without unit) is the volume of water the aquifer can release or store per unit area in a unit variation of the aquifer level/aquifer level.The value of S has no units, and this price can be determined by the type of aquifer and the amount of groundwater contained in an area.Although the transmissivity (T) is the penetration of water in the soil through an aquifer of the same thickness, the speed K (hydraulic conductivity) increases by h (thickness of the aquifer).
The analysis was conducted using 2 (two) methods, namely Theis and Cooper-Jacob.The difference significant between both is Theis method's assumption that the aquifer is confined (it is bounded on the top and bottom by confining layers).The aquifer is compressible, and water is released instantaneously from the aquifer storage with a decline in the head due to pumping.While Cooper-Jacob method has unconfined aquifer [9].

Theis Method
The hydraulic properties were analyzed in wells located at the research site, knowing the characteristics of the aquifer and the pumping test analysis approach using the Theis method.The extent of the reduction of the water table, multiplied by the accumulation coefficient and then added together for the entire area of influence, will be equal to the discharge.Since water must be obtained from reduced storage in the aquifer, the groundwater level will continue to decline as long as the aquifer is functioning effectively and indefinitely.Therefore, in theory, there is no constant flow.However, the extent of groundwater level reduction will continue to decrease as the area of influence widens [19].
Basically, the Theis curve method utilizes the developed ideal groundwater reduction curve [20].Graphically, this curve is superposed manually on field subsidence data.Once deemed appropriate, the required flow components are then determined.In general, the stages of the analysis of the Theis curve method are as follows [19] : 1. Prepare the Theis curves from the Appendix and if you can, plot the curves on translucent paper to make working with step 3 easier 2. Plot the groundwater level reduction data on log-log paper with t/r 2 as the x-axis and s as the yaxis.We call this the data curve.Remember that the scale of the two curves must be exactly the same.
3. Perform a superposition of the Theis curve and the data curve.The trick is to place the Theis curve above the data curve.Change the position of the Theis curve so that the curve line coincides with as many points on the data curve as possible.The x and y axes of both curves must remain parallel when changing positions.4. Once the best position has been found, determine one of the intersection points anywhere along the meeting position or at all points on both curves.From that point, determine the value , , s and W(u). 5. Determine the transmissivity value and the storage coefficient (S) with the equation : The equation for groundwater subsidence and storage coefficient, according to Theis is [19]: where: s (r,t) = a decrease in the distance r (m) from the well at time t (second) is pumped Q = pumped well discharge (m 3 /sec) T = transmissivity of aquifer (m 2 /sec) S = storage coefficient/storativity (dimensionless) t = time since pumping started (seconds) r = the radius of the observation well is measured against the pumped well (m) The value of W(u) can be obtained using the normal type curve created by designing the value of W(u) vs u, or the reverse curve that can be created by drawing the value of W(u) vs 1/u, in a register graph with W(u) as the vertical axis shown by Figure 3

Cooper -Jacob Method
This method is commonly known as the Jacob Method.Similar to Theis method, Jacob's method is also used for unsteady flows.Jacob's method is a derivation of Theis' formula.However, it is more consistent and less expensive because only one observation well is required, and the aquifer is an unconfined aquifer.The assumptions that must be met are the same as Theis Method, but the value of U has a limit smaller than 0.01, so the value of r is less than the value of t is large.From the derivation of the Theis method, Jacob derives the equation for the subsidence of the groundwater table S to be [19].
or can be written in logarithm Hydraulic conductivity is a unit of velocity of the ability of rock layers to allow water to pass [13].Hydraulic conductivity is influenced by physical properties, namely porosity, grain size, grain arrangement, grain shape and their distribution.The hydraulic conductivity values of several types of rock can be seen in Table 1.

Results and Discussion
To determine the characteristics of hydrogeological aquifer, the pumping test is the primary method.These characteristics include the coefficient of water permeability or the coefficient of transmissivity (T), the coefficient of hydraulic conductivity (K), and the coefficient of water reserve (S).These values can be determined through a constant discharge continuous pumping test (long period test).
According to the survey results, the test pumping location is located in Pojok village, Campurdarat District, Tulungagung Regency, with one production well and two observation wells (Figure 5), and the coordinates can be tabled as in Table 2 below.The main objective of hydrogeological research is the measurement of the hydraulic conductivity of the aquifer.The distribution of hydraulic characteristics of permeable mediation can be an important   From the results of Table 3 above, the Theis method was chosen to analyze the hydrogeological characteristics of the aquifer in the study area because the K value is close to limestone material (Table 1 Limestone Kh = 0.94 m/day) and has a relative error smaller than Jacob method namely 29.77%.

Conclusions
The characteristics of hydrogeological aquifer of limestone based on pumping tests on shallow groundwater in Pojok village, Campurdarat district, Tulungagung regency using Theis and Cooper -Jacob methods.It can be concluded that the aquifer strength parameters of Theis method show that the average transmissivity (T ) is 73.193 m 2 /day, the storage capacity (S) is 1.080x10 -3 and the average hydraulic conductivity (K) is 1.220 m/day while Cooper-Jacob shows that the transmissivity (T) is 71.682 m 2 / day, the storage capacity (S) is 0.4512 and the hydraulic conductivity (K) is 4.667 m/day.The Theis method was chosen to analyze the hydrogeological characteristics of the aquifer in the study area because it has a smaller relative error than the Jacob method of 29.77%.The characteristics of hydrogeological aquifer with the parameters (mainly hydraulic conductivity, transmissivity, and specific storage) are used for estimation based on the hydrogeological model.

Figure 1 .
Figure 1.Map of Tulungagung Regency and Study Locations below.

Figure 3 .
Figure 3. Graph of Relationship Between 1/u and W(u)

Figure 5 .
Figure 5. Map of the Planned Pumping Test Location

Figure 7 .K = 4 .
Figure 7. Graph Experiment of Cooper -Jacob Methods Based on Figure 7 above, for the graph of the relationship between t and s, the Cooper -Jacob method is taken if t 1 = 1 minute with the equation s 1 = 0.0322 x + 2.586, we get s 1 = 2.6182 m.Meanwhile, if t 2 = 11 minutes with this equation, we get s 2 = 2.9402 m so that the groundwater level decreases per cycle log s = 0.3220 m.Aquifer transmissivity (T) Q = 246.320m 3 /day  = 2.30′ 2∆ T = 280.021m 2 /days Hidraulic Conductivity (K) B = 60 m ′ =   K = 4.6670 m/days = 5.4x10 -5 m/second.

Table 2 .
Coordinate Location of Pumping Test

Table 3 .
Recapitulation Characteristics of Hydrogeological Aquifer