In the past, the hydrogeology of the sands, sandstones and gravels of the Alentejo Litoral were studied only in areas where they occur as one of the hydrostratigraphic units of the Sines and Bacia de Alvalade multilayer aquifer systems, in which they play an important role. We present the hydrogeologic characterisation of these rocks in the coastal area extending from the proximity of Morgavel Dam in the north, to about 5km south of the Cabo Sardão. Porto Covo, Almograve, Cabo Sardão and other smaller towns, are located in this area characterised by the existence of both traditional and intensive agriculture and seasonal tourism, which are the main economic activities in the studied area. As far the authors know, the hydrogeologic studies described in this communication are the first contribution in this field in this extensive area of about 196 km2.
In stratigraphic terms the studied lithologies correspond to formations in the Tertiary to Quaternary transition (Plio-Pleistocene) and are deposited at the top of the widespread carboniferous flysh (schists and greywac) formations present on the west coast of the Alentejo Region. As the Paleozoic metamorphic rocks present considerably lower permeability and porosity than the observed values for these parameters in the Plio-Pleistocene sediments, the recharge occurring in the shallow detritic phreatic aquifer cannot be absorbed by the underlying hard rocks. Therefore most of the regional flow generated from recharge at the top of the system is discharged in the sub-aerial and horizontal contact between the detritic shallow aquifer and the carboniferous rocks. These boundary conditions are responsible for the existence of several springs along this contact and, together with the hydraulic connection between the stream network in this area, define the general flow pattern from East to West. The estimated long term average recharge in the studied area is in the order of 26 hm3 (corresponding to 20% of precipitation).
The groundwater flow system was studied through traditional large diameter wells and newly drilled piezometers. Piezometric in situ measurements were complemented with the continuous monitoring of electrical conductivity, depth and temperature (using CTD probes). Hydrochemistry was characterised by in situ measurements of physio-chemical variables (pH and temperature) and water samples for laboratory analysis of major ions and other chemical species. The first contribution to characterise the chemical composition of groundwater in the studied area shows that most of the sampled waters are circum-neutral with very low mineralisation (TDS < 1000 mg/L) and are mostly of sodium-chloride type. Moreover, the results of the hydrochemical characterisation of the studied area shows that the respective typology is clearly distinguishable from groundwater in neighbour formations.