Extensive usage of vital freshwater is highly undesirable in the current era of climate change and population growth. However, the 6 millionm3/y of brackish seepage that occurs in the deep Horstermeer Polder, located in between the cities of Amsterdam and Hilversumin the Netherlands, is mitigated by using up to 207 millionm3/y of freshwater. This is very unsustainable, especially since the
pressure on the freshwater reserves in the Netherlands is rising. Therefore, this research focuses on evaluating a new mitigation measure of the brackish
upconing below the polder: the extraction of brackish water. In March 2021 AD, a pilot well has been installed in the polder, and its effects were assessed in this research. To enable the evaluation, an extensive analysis of all available measurements of the geology, hydraulic head, and chloride distribution in the area was performed, and a 3D regional variable-density groundwater model using MODFLOW 6 was built. The model result has a reasonable good agreement with these measurements, since at 70% of the measurement locations, the simulated hydraulic head deviates less than 30 cm, and at 80% of them, the simulated chloride concentration deviates less than 200 mg/l. Moreover, compared to previous research, the representation of the brackish upconing was significantly improved. The effects of the pilot well were calculated using five different operational settings. The results indicated that a shallow well, starting below -50 m NAP, reduces the chloride load in the polder more than a deep well (-100 m NAP). However, even in the best scenario, i.e. an active shallow well with a constant pumping rate of 100 m3/h, the reduction of the chloride load is limited to 5.5%. In perspective, these results suggested that at least 24 of these wells are needed to lower the chloride concentration in the discharge water out of the polder to 250 mg/l. On the contrary, the shallow wells cause a considerable average drawdown of 4 cm at surface level in a radius of 50 meters around the well, which might result in damage to wooden pile foundations in the northeast of the polder.

It has been determined that the plastic waste load in the Petanu river (Bali, Indonesia) can be as high as 2015.5 kg/day in the beginning of the rainy season. To restore the ecosystem and protect human livelihood this load should be reduced drastically. The enormous pollution rate is largely due to the massive amounts of mismanaged (plastic) waste at household level. This is the part of the waste that is either burned or dumped by the households, rather than collected or brought to a recycling facility. Extensive mismanagement of waste is an indication of an inadequate functioning waste management system. In Bali, and this watershed specifically, the existing system is decentralised and its waste management strategies rely heavily upon public participation. As a result, in some areas households have limited options regarding waste handling due to a lack of a collection system and other waste services. At the same time, households need to pay for those services. In order to improve the current waste management, it is therefore important to understand the trade-off made by the households between the environmental impact of plastic waste and cost of the waste management system.

In this report, this trade-off is studied by creating a real-world, coupled economic-environmentalmodel, of the Petanu’s watershed in the Gianyar regency. The model consists of two parts: 1) a production possibility frontier (PPF) and 2) a utility curve. The PPF is an arc curve that visualises the relation between the plastic waste load originating from households and the average impact on the monthly purchasing power of a household. The purchasing power is impacted since users of the waste management system need to pay for the services. The utility curve, on its turn, visualises a households trade-off between plastic waste load and impact on monthly purchasing power. Coupling these curves gives insights in how well the current waste management system satisfies the preference of the households regarding cost versus pollution rate. If the current waste management matches perfectly with these preferences, the point of tangency of both curves would be the location on the curve representing the current plasticwaste load and impact on the purchasing power. If the point of tangency has a lower plastic waste load and hence higher impact on purchasing power, households are willing to invest more in their waste management to improve the environmental quality of the Petanu river.

To gain insight in the trade-offs people make regarding four disposal methods (Self-Service, Pick-Up, Burn and Dump), cost and time, a stated choice experiment was set up. This is done by means of a questionnaire in which respondents were asked to make choices between hypothetical choice situations. The choice sets consist of different combinations of the disposal methods with varying attribute levels for cost and time. The survey has been conducted under 300 respondents from six different villages, located in the watershed of the Petanu river. With the help of data analysing program Biogeme, an open source Python package, the stated choice data has been transformed into the utility curve. Besides a general utility curve for the whole population living in the watershed of the Petanu river, also utility curves have been established for certain segments of the population, based on age, educational level and currently used disposal method. Hence, the questionnaire contained also questions about socio-demographic characteristics, the currently used disposal method and the corresponding cost. The latter two are not only used for the segmented utility curve but also for the creation of the PPF. The PPF is built-up by defining five different scenarios, i.e. the current situation, three scenarios with an improved waste management system and a scenario without a waste management system. The scenarios with an improved system have an increasedwaste collection rate or additional locationswhere recyclables can be handed in, in exchange for money. For all scenarios, the corresponding average cost per household and the plastic waste load in the Petanu river have been calculated. The quadratic best fit to these data points gives the PPF.

By combining the PPF with the general utility curve it was found that an average household in the watershed of the Petanu is willing to pay more for the waste management system than they are currently doing in order to decrease the plastic waste load in the Petanu river. It was even found that households want to achieve very low plastic waste load values and are willing to invest 47,400 IDR per month to reduce their contribution to plastic waste load by 100 grams per day. An exact equilibrium point is however not found as the PPF entails to many uncertainties at very low plastic waste load values. Nevertheless, the model gives a good indication and it is therefore recommended to increase the current collection rate of household waste. Furthermore, it is recommended to invest in TPS3R facilities in which waste is sorted and send to recycling facilities. Moreover, the combination of the PPF with the segmented utility curves gave some very interesting
insides. First of all, the younger generation (· 31 years old) has a higher willingness to pay for the reduction of plastic waste load, than the older generation. This is likely the result of the awareness programs on waste management at schools. Hence, it is recommended to expand this educational program. Secondly, educational level is positively correlated with willingness to pay. Remarkably, income level and gender did not have a significant relation with willingness to pay. Lastly, time is a significant determinant for the choice of disposal method, which corresponds to a certain plastic waste load. Therefore, it is recommended that waste management at household level should be as time-efficient as possible. This means the collection should be as much as possible be done at the doorstep of the houses and recycling bins should be located close-by.

All in all, it has been demonstrated that the households in the watershed of the Petanu river are very willing to pay for the waste management services, however, current waste management options are too limited. It is now the task of the governmental institutions and community leaders to enable collection of waste for every household in every village.