Surface water-groundwater interaction and source of nitrate in municipal water supply aquifer, San Jose, Costa Rica

Deterioration of groundwater quality by anthropogenic input of nitrate is a significant issue in the Central Valley of Costa Rica. Nearly 50% of the Central Valley municipal water supply is derived from volcanic aquifers. Nitrate concentrations in some of these aquifers have been steadily increasing in recent years, reaching levels above or near the WHO drinking water standard. Potential sources of nitrate are commercial fertilizer application by coffee plantations and untreated domestic sewage. Groundwater-surface water interactions in the system are complex. Rivers are alternately effluent and influent depending on the geology and seasons. Groundwater is recharged by direct soil infiltration and downward leakage of rivers polluted by domestic sewage. Precipitation, river and groundwater samples were collected over a 10-month period from the Mancarrón River watershed for geochemical and isotope analyses. Groundwater recharge areas were inferred from geological and isotopic information. Weak correlation between nitrate and chloride concentrations in two of the wells suggests the local groundwater is impacted by nitrate from commercial fertilizers through direct soil infiltration. Correlation between nitrate and chloride in a spring suggests the nitrate source is sewage.