Groundwater Management in Coastal Area of Batang Regency to Anticipate Sea Water Intrusion

Of the water sources that exist on earth, almost all of human’s daily water needs such as drinking, bathing and washing are sourced from groundwater. Uncontrolled extraction of groundwater can cause various impacts on the environment, such as lowering of the groundwater level and seawater intrusion. Efforts to manage groundwater need to be carried out so that its availability is always maintained. The initial step of this effort is to formulate a management model as a basis for policy determination. For this reason, in this study a dynamic modeling simulation was carried out using dinamic program. The results of the research show that the priority for groundwater management efforts in the coastal area of Batang Regency is to reduce the use of groundwater for the domestic sector which can be done through saving groundwater use or by increasing the capacity of the local water company. Besides that, management through artificial recharge by making infiltration wells or biopore holes is also effective in maintaining the availability of groundwater in this area.


Introduction
The pattern of management of water resources is based on the principle of balance between utilization and conservation efforts.According to the Law of the Republic of Indonesia Number I7 of 2019 concerning Water Resources what is meant by management of water resources is an effort to plan, implement, monitor, and evaluate the implementation of conservation of water resources, utilization of water resources, and control of the destructive power of water.
Groundwater management is more difficult than surface water, because it is in the ground.In addition, if it has already been damaged, recovery efforts are also more difficult than surface water [1,2,3].For this reason, efforts to manage groundwater must be carried out as early as possible in line with the rate of utilization.
At present, integrated and sustainable management of water resources has become a major issue for the world community.In efforts to manage water resources, the assessment of groundwater potential becomes an important matter.Groundwater management is inseparable from overall water management [4,5].Good groundwater management will also have a good impact on water management as a whole.
Integrated groundwater management is defined as integration of activities within the management area, integration between sectors and integration between disciplines.Integration of management must 1313 (2024) 012031 IOP Publishing doi:10.1088/1755-1315/1313/1/012031 2 also be reflected starting from the preparation of basic concepts, policies, planning, implementation in the field and monitoring of results.
Groundwater management can also be seen as a system.Rainwater that falls on the ground surface, and seeps into the aquifer layer can be seen as system input.The flow of groundwater and chemical reactions in aquifers can be seen as processes, while the discharge of groundwater either naturally as springs or because it is taken by humans can be seen as system output [6].
Groundwater management will cover various efforts to obtain, distribute, utilize, regulate, purify and drainage water.For this reason, efforts to manage groundwater must be carried out starting from the source or catchment area to the area where it is excreted.
The coast of Batang Regency is an alluvial plain formed by fluvial processes and marine processes (Figure 1).Within this coastal aquifer there are lenses of marine clay deposits, in which salty fossil water is trapped in these lenses since these formations were formed during sea flooding.This is indicated by the presence of brackish groundwater in this region.Because of its strategic location, this coastal area is growing rapidly as indicated by the existence of the Steam Power Plant and the construction of the Batang Industrial Area.Likewise, there is a plan by the Batang Regency Government to build office and hotel superblocks, all of which will require water.Base on this description, groundwater management in the coastal area of Batang Regency is a necessity that must be realized immediately.With good management, optimal hydrological conditions will be realized, namely the production of water in accordance with the needs and requirements.One of the things that can be done in formulating groundwater management is to create models and conduct model simulations, as a basis for policy determination [7,8,9,10].

Calculation of Groundwater Availability and Water Requirement
Availability of groundwater in the study area was obtained based on analysis per sub-district which is located in the coastal area.Based on this analysis, groundwater availability in each sub-district is determined by multiplying the aquifer thickness, specific yield (Sy), and the area of each sub-district.The yield of safe groundwater for utilization is the product of the multiplication of the area, specific yield and groundwater fluctuations [11].Water requirement are calculated based on water requirement for domestic, hotel and livestock requirement [12].This is done because these sectors utilizes groundwater, while the agricultural sector utilizes river water.

Dynamic Modeling
The groundwater management model begins by determining the parameters that affect groundwater availability, namely the rate of utilization and rate of recharge.The rate of utilization includes the use of water for domestic requirement, livestock and hotels, while the rate of recharge includes water recharge from rainfall.After determining the parameters that influence groundwater conditions in the research area, a management model is determined Simulations can be carried out for a variety of different situations with the help of the dinamic program [13,14,15,16].

Determination of Water Requirement
In this study, the water requirement that is taken into account is the demand for water sourced from groundwater, namely domestic, livestock and hotels.The requirement for water for domestic use is determined according to the size of the population and the amount of water requirement per capita per day.According to Mangku Sitepoe (1997) in [17], the requirement for water in big cities is generally > 150 liters/capita/day, in medium-sized cities it is 80-150 liters/capita/day, district cities are 60-80 liters/capita/day and village ranges from 30-60 liters/capita/day.Based on this criterion, because the research area is a regency that is developing towards a big city, the water requirement is determined to be 150 liters/capita/day.
The water requirement for livestock is calculated based on the number of livestock and water consumption per per day, where the types of livestock that are calculated for water requirement are cows, buffaloes, horses (large livestock), goats, sheep, pigs (small livestock) and poultry.The standard water requirement for large livestock is 40 liters/day, small livestock is 5 liters/day and poultry is 0.6 liters/day [18].

The Result of Modeling
The calculation results show that the availability of groundwater in the districts located in the coastal area is 648,960,234 m 3 /year, with a safe yield of 127,702,334 m 3 /year.Because the main source of groundwater recharge is from rainfall, the amount of rainfall is also calculated.Based on an analysis of annual rainfall data from six rain stations in Batang Regency, the average annual rainfall in Batang Regency is 3,818 mm/year.With an area of the district located in a coastal area of 334.33 ha, the amount of rainwater on the Batang coast is 1,276,477,628 m 3 /year.
Rainwater that falls in the study area does not completely infiltrate into the ground as groundwater recharge.Some of the rainfall will be lost into the atmosphere as evapotranspiration, some will become runoff and flow directly into the sea.Based on land use data and soil texture, the runoff coefficient is determined to be 64.8%.Furthermore, with an annual average temperature of 25.5 o C, the results of calculations using the Thornthwaite method show that the potential evapotranspiration on the Batang coast is 121.5 mm/month.With an area of 334.33 ha, the volume of evapotranspiration in the study area is 476,903,881 m 3 /year Referring to Batang Regency in Figures in 2023, the population of 6 districts in the coastal area of Batang Regency is 387,455 person.The number of large livestock (cows, buffaloes and horses) is 4,097 animal, small livestock (goats, pigs) 42,760 animal and poultry 7,759,588 animal.For the hotel sector, there are 10 hotels with 426 beds.Assuming 50% occupancy, in this calculation the number of guests taken into account is 213 person.
Based on these parameters, a model for groundwater management in the study area is formulated as shown in Figure 2. From this model, the availability and demand for groundwater and its projections can be determined as shown in Figure 3. Looking at Figure 3, it can be seen that the greatest water requirement in the Batang Coastal area is for domestic demand, followed by hotels, livestock and industry.Currently, water demand for domestic purposes reaches 21,213,161 m 3 /year, while for hotels 11,661 m 3 /year, large livestock 59,816 m 3 /year, small livestock 78,037 m 3 /year and poultry 1,699,349 m 3 /year.In 2050, the requirement for domestic water is 34,904,882 m 3 /year or an increase of more than 50% of the current water demand.The growth of hotels is relatively constant, as is the number of industries and their livestock, so that the requirement for water is also relatively constant.
The high requirement for water has an impact on the availability of groundwater.Currently the availability of groundwater is 648,960,234 m 3 /year and the water supply from local water company reaches 7,116,813 m 3 /year, while the total water demand reaches 23,062,026 m 3 /year.As a result, groundwater reserves are only 605,431,335 m 3 /year and will continue to decrease until they reach 591,739,614 m 3 /year in 2050 (Figure 4).In 2083 the water requirement will reach 64,624,124 m 3 /year or exceed 50% of the safe yield of groundwater utilization which is 127,702,334 m 3 /year, so that it will start to be included in the critical criteria.In fact, in 2124 the water demand on the Batang coast will already exceed the safe yield..

Groundwater Management by Reducing Population Growth
Taking into account the calculation results of groundwater utilization, it is known that most of the water utilization on the coast of Batang Regency is for the population requirement or for domestic requirement.Likewise, the rate of increase in water use is also in line with the rate of population growth.Based on this phenomenon, the simulation of reducing water use will emphasize water use in this domestic sector.
There are two ways that can be done to reduce the amount of water used for domestic needs.The first way is to reduce the population growth rate (for example with the family planning program or population migration), while the second way is to reduce water use or conserve water.
In an effort to reduce water use by reducing the population growth rate, a reduction of 25% was made, so that if the previous population growth rate was 1.79%, it was changed to 1.75%.The simulation results are shown in Figure 5. Looking at Figure 5, it is known that by reducing the population growth rate, there is no phenomenon of a significant decrease in water demand.Total water demand in 2050 will reach 36,323,081 m 3 /year.Entering the critical criteria will occur backwards a year to 2084.

Groundwater Management by Water Saving
Efforts to manage groundwater by saving water are carried out by reducing the amount of water used per person per day by 25%, so that previously the amount of water used was 150 l/person/day to 112.5 l/person/day.The simulation results are shown in Figure 6.Looking at Figure 6, it is known that by saving water, there is a phenomenon of a very significant decrease in water demand.The population's water requirement in 2050 will only reach 28,027,526 m 3 /year.Critical conditions for new groundwater utilization will occur in 2099 and for more than 100 years ahead there will be no excess of safe yield.

Groundwater Management byArtificial Recharge
One way to maintain groundwater recharge is to reduce the amount of rain that becomes runoff.The way to reduce runoff is to make infiltration wells, biopores and ponds.
For this reason, in this simulation it is assumed that there will be an additional 10% infiltration of rainwater into groundwater.The 10% value used as the basis for adding groundwater absorption is the suitability of the simulation and the expected reality.

Groundwater Management by Increasing Capacity of Local Water Company
Efforts to manage groundwater can also be carried out by increasing the capacity of local water company whose water sources are not groundwater.For this reason, this management is carried out by increasing local water capacity by 25%.The simulation results are shown in Figure 8.

Groundwater Management by Reducing Population Growth, Water Saving, Artificial Recharge and Increasing Capacity of Local Water Company
In this alternative, the parameter of population growth rate, the amount of water used for domestic requirement is reduced by 25%, addition of groundwater infiltration 10% and increasing local water company capacity 25%.The simulation results show that there is a very significant decrease in water demand followed by a steady decrease in groundwater recharge, so that the amount of stored groundwater will only decrease to 639,014,443 m 3 /year in 2050 (Figure 9).For the next 100 years there will be no critical conditions in terms of utilization groundwater let alone exceeding safe yields.

Conclusion
From the simulation results, it can be seen that the priority for groundwater management in the Coastal Area of Batang Regency is to reduce groundwater utilization for the domestic sector, which can be done through savings in groundwater usage or by increasing capacity of local water company.Besides that, management through artificial recharge by making infiltration wells or biopore holes is also effective in maintaining the availability of groundwater in this area.

Figure 2 .Figure 3 .
Figure 2. Groundwater Management Model in the Coastal of Batang Regency

Figure 4 .
Figure 4. Reduced groundwater storage due to increased water demand

Figure 6 .
Figure 6.Groundwater management by water saving

Figure 7 .
Figure 7. Groundwater management by artificial recharge

Figure 8 .
Figure 8. Groundwater management by increasing capacity of local water company

Figure 9 .
Figure 9. Groundwater management by Reducing Population Growth, Water Saving, Artificial Recharge and Increasing Capacity of Local Water Company