Impact of groundwater resource management on coastal aquifers and adjacent wetlands in south Portugal

Ongoing research shows that groundwater resource management in the Eastern Algarve, south Portugal, has greatly affected the quality of coastal aquifers and adjacent wetlands over time. Three important periods of groundwater management can be distinguished. Until 1999, groundwater was the only source for public supply and agriculture, annually consuming near 100 million m3. In the groundwater basin of the Ria Formosa lagoon, an internationally recognized wetland, 65% of mean annual groundwater recharge was consumed by agriculture. Despite heavy pumping, phenomena of groundwater overexploitation, such as decreasing water levels or seawater intrusion, were only observed during long periods of drought. Coastal groundwater discharge was certainly limited, which intensified the recycling of groundwater induced by irrigation and return flow of local groundwater. This led to groundwater salinization in the upper aquifers, more severe in the sand than in limestone aquifers, due to higher residence times in the former. Nitrate contamination, triggered by excess fertilization and leakage from septic tanks, increased dramatically along with groundwater salinization, owing to its conservative behaviour, with NO3 concentrations exceeding 300 mg/l. In the lagoon winter blooms of green ulvoid algae were observed in the intertidal zone. These algae form thick mats on the mud flats and have little access to ocean water, so that temporary increases in nutrient-loaded groundwater outflow may play an important
After 1999, public water supply switched to surface water supplied by large reservoirs. Many municipal wells were abandoned, resulting in an increase of groundwater discharge and local freshening phenomena. Meanwhile, agriculture, by far the largest consumer, continued to rely solely on groundwater.
After 2001, a large area was connected to an irrigation district supplied by surface water. This resulted in a sharp rise of the water table, followed by an almost complete attenuation of interseasonal oscillations. Coastal groundwater discharge in the area has greatly increased, due to the deactivation of a large number of wells and the additional recharge from excess irrigation with surface water. Groundwater freshening has occurred in the upper aquifer, mainly as a result of dilution. Nitrate concentrations have dropped, but when subtracting the dilution effect, it can be seen that nitrogen inputs into the soil continue to rise. The algal blooms observed in recent years may be related to the increased nutrient transport towards the lagoon. A complete cessation of groundwater withdrawal for irrigation in the area would lead to significant and long-lasting coastal water and nutrient discharge. This could result in denser populations of algal blooms, with potentially serious impacts on biodiversity and tourism development.