The chemical status of groundwater and pollution risk in the Azores archipelago (Portugal)

Groundwater quality in the Azores archipelago (Portugal) is a major issue for the definition of water policies, as about 98 % of the water supply is supported by aquifers. Therefore, this issue was addressed in the Azores River Basin District Plan made in order to comply with EU Water Framework Directive (WFD), using an approach developed in order to link land use pattern and groundwater chemistry. Monitored groundwater corresponds to cold waters with a pH range between 5.95 and 8.44. A median value of conductivity (188.8 μS/cm) suggests that in general the samples are low mineralized, and are mainly in the range from Na–HCO3 to Na–Cl type waters. Average content of nitrate ranges between 0.02 and 66.2 mg NO3/L, and an evaluation of chemical status of groundwater with respect to NO3 content was made in groundwater bodies in which it was possible to compute arithmetic mean values and the respective confidence limit at 95 %. From the 21 groundwater bodies in which this computation was made possible, only in São Jorge Island a single value exceeds the maximum admissible limit of 45 mg NO3/L. Therefore, only in this case a groundwater body is in poor chemical status due to non-point pollution sources. Results are consistent with the set of constructed potential pollution risk maps, except in São Jorge where the area occupied by moderate to very-high-risk areas is higher comparing to other islands. Risk maps were constructed using vulnerability to pollution determined according to the DRASTIC methods, and overlying this index value with land use and net N-pollution load layers. Results show that generally pollution risk is very low to moderate.