Investigating the Sources of Nitrogen Contamination in the Shallow Aquifer of Jakarta using a Newly Developed Distributed River-Aquifer Flow and Transport Model

Abstract:

Recent observations in the shallow aquifer of Jakarta show a rise in nitrate (NO₃^-) levels. Groundwater is extensively used in the city to compensate for the limited public water supply network and therefore the risk to public health from a rise in NO₃^- concentration is high. NO₃^- has been identified as a cofactor for methemoglobinemia in infants, a disease which can lead to death in extreme cases.

 The NO₃^- levels detected are still below regulatory limits for drinking purposes but strategies are necessary to contain the growing problem. To this end, the main sources and pathways of inorganic compounds containing nitrogen (N) – i.e. nitrate, nitrite (NO₂^-) and ammonium (NH₄⁺) - were investigated. We combined 3 years of field measurements in the Ciliwung River, the major river flowing through Jakarta, with a distributed river-aquifer interaction model to characterize the N-cycle in both systems and quantify the contribution of river infiltration in the overall groundwater N budget. The computed infiltration fluxes were compared to estimates of leaks from poorly maintained septic tanks, which are extensively used in the city, to identify the main source of groundwater contamination.

 Observations show a strong and interdependent spatial and seasonal variability in the levels of NO₃^-, NO₂^- and NH₄⁺ in the river, which is caused by changes in nitrification/denitrification rates due to variations in dissolved oxygen concentrations. Simulation results suggest that such dynamics in the river cause river to aquifer contamination patterns to likewise change over space and time, which leads to heterogeneous vulnerability distributions. The estimated contribution of river-N infiltration to the observed NO₃^- groundwater levels is small if compared to that originating from all leaking septic tanks inside Jakarta. However, in the vicinity of the Ciliwung, river to groundwater N-loading can play an important role in the local NO₃^- groundwater levels because it is highly concentrated.