Site Conceptual Model for a Complex Coastal Tropical Alluvial Aquifer Interbeded with Volcanic Ashes

A Site Conceptual Model (SCM) was developed for the alluvial aquifer where the city of Pedernales is located, in the Ecuadorian Pacific Coast (Manabi Province). This aquifer had been used mainly for domestic water supply since 3 or 4 decades ago (seventies); in the last decade (from 1998), a lot of new pumping wells had been installed, not only for domestic use but also for industry (mainly shrimp) and touristic developments.

The aquifer is conformed by non consolidated irregular non continuous layers of sands, silts and clays, interbeded with fine volcanic ashes. The mean aquifer thickness is 25 m. and regional groundwater flow gradient follow the terrain morphology which is mainly towards the west and south west, from the hills to the coast, locally controlled by buried channels, the Tachina River and pumping wells with flow discharge ranging between 0.1 L/s and 1.5 L/s (1.6 gpm to 23.8 gpm). Because these complex geology, the aquifer behaves mainly confined and semiconfined, with very low direct rain infiltration; the main recharge in the study area is lateral and comes from the upper hills.

Since the population and the industry had been growing, and due to poor sewage and storms system drainages, also by the intensive use of the aquifer, its contamination occurred mainly from sewage and a hydrocarbon release.

Due to the lack of historical data, and in order to assess the hydrogeology and groundwater flow and to get a more reliable SCM and corrective actions, an aggressive campaign consisting on soil sampling using a Direct Push Technology in 27 sites was carried out as well the drilling and Installation of 40 groundwater monitoring wells; also a Geographical Information system and Numerical Modeling were used as an aid for the SCM.

Finally a more reliable SCM was established, a detailed hydrocarbon plume was delineated, showing natural attenuation. Also the main source of hydrocarbon release was controlled, and although the most of the pumping wells have no risk for domestic water use, several well head treatment systems were installed in order to provide water with better quality conditions.