Controlling factors and driving mechanism of nitrate contamination in groundwater system of Bandung Basin, Indonesia
ABSTRACT CONTROLLING FACTORS AND DRIVING MECHANISM OF NITRATE CONTAMINATION IN GROUNDWATER SYSTEM OF BANDUNG BASIN, INDONESIA By Ahmad Taufiq Student ID: 32013501 (Doctoral Program in Geological Engineering) Number of population, industry, and economic activities in Indonesia are growing rapidly and the impacts on natural environments raise awareness about water quality issue over the country. Bandung Basin, one of the most rapidly growing urban areas in Indonesia, was assessed for NO3- contamination in groundwater systems, and its controlling factors and driving mechanisms were investigated with the aim to demonstrate novelty on the use of combination of parameters of stable isotope ratios in nitrate (?15N and ?18O in NO3-), groundwater age (using CFC-12 age tracer), and socioeconomic parameters (land-use, population, and economic database). Groundwater NO3- concentrations at present time did not exceed WHO limit for all the analyzed samples (3.00 mg/L in average with maximum value of 20.69 mg/L, n = 102). Dual stable isotopic analysis together with CFC-12 groundwater age determination suggest that anthropogenic activities are the major causes for increasing NO3- concentrations in the groundwater. Those activities are industry and domestic for urban areas, and farming for rural areas. In general shallow unconfined aquifer is more vulnerable to NO3- contamination compared with deep confined aquifer because denitrification occurs in deep anoxic aquifer and this led to attenuation of NO3- pollution as groundwater flows. However, it seems likely at groundwater depression cones in urban areas more concentrated waters are transported from shallow aquifer into deep aquifer system through downward vertical fluxes due to excessive pumping. Result of principal component analysis (PCA) on NO3- concentrations with socioeconomic parameters indicated that industrial and population growths are the main driving mechanisms related to groundwater NO3- contamination. In this study, the NO3- concentration is tested as a mixing tracer in estimation of mixing ratios in the depression areas with different magnitudes of groundwater drawdown. To demonstrate the use of NO3- concentration as the mixing tracer, the estimation results of mixing ratio are compared by using two methods: (1) the mixing ratio by using all parameters (the total mixing ratio), and (2) the mixing ratio by using only NO3- concentration (the NO3- mixing ratio). There is a positive correlation among the total mixing ratios, CFC-12 concentration, and modeled vertical flux. The comparison of calculating the total mixing ratio and the NO3- mixing ratio showed insignificant correlation which means NO3- concentration is ineffective as the mixing tracer in deep groundwater. The results illustrate how quality of urban groundwater has deteriorated. This approach is useful to understand how the NO3- contaminant behaves in large basin aquifer system under urban environments. This study can propose the importance of the use of hydrochemistry data in combination with groundwater residence time and socioeconomic parameters. The proposed approach might be applicable in other developing regions too because increasing populations may be associated with increasing nitrogen loadings. Information from our study should be important as a base for protection and preservation of groundwater quality.