Nitrogen pollution sources in coastal groundwater discharge at an urbanized tropical coast

Fresh submarine groundwater discharge (FSGD) can be a substantial source of nitrogen (N) to tropical coastal waters. Fertilizers and sewage are generally regarded as major sources. Here, we resolve coastal nitrogen fluxes and sources in southern Kerala, India. The region has a high coastal population density, seasonally intensive tourism, and an industrialized hinterland agriculture.

Nitrate concentrations in post-monsoon coastal spring discharge was 23.7 mg/l. Nitrate leakage to groundwater from agriculture is limited in warm climates due to high denitrification associated with high temperatures. Using slim models, we estimated leakage from the main fertilized land uses of the hinterland region: Paddy fields (56±13 kg-N/ha), rubber plantations (7±2 kg-N/ha) and home gardens (14±4 kg-N/ha). These loads cannot explain the observed nitrate concentrations in coastal springs. Nitrate stable isotope (d¹⁵N and d¹⁸O) signatures imply that manure or sewage is the main N source. While the local catchment population was stable during the last decade (ca. 40,000), tourism increased from 16,000 visits in 2005 to 208,000 visits in 2017. Tourism increased ammonium stored in pit latrines by 70 %.

This rapid change seems widespread in India and Southeast Asia. We suggest that urban growth without proper sanitary facilities was the largest coastal nitrate pollution source as reflected at the investigated site, while fertilization effect was marginal due to large atmospheric losses of N.