Characterization and modeling of an alluvial aquifer in a semi-arid mountainous area (case of the Er-Rich aquifer, High Atlas, Morocco)

Arid zone climatic conditions generate floods, often brief and intense, so the corresponding alluvial aquifers recharge and discharge rapidly because they consist of relatively thin, permeable deposits. However, the Er-Rich alluvial aquifer is characterized by relatively low piezometric fluctuations even in periods of prolonged drought. Understanding the hydrodynamics of this aquifer and the different components of the groundwater budget is the preliminary step to analyze this phenomenon. The geological data show a collapse-basin limited on both sides by long mountain ridges, on which alluvial and detrital sediments have been deposited. Then, based on 40 boreholes data, completely crossing the quaternary formation, the aquifer is unconfined, and consists of sand and gravel with some clay and silt. Locally, it's including deposits of Tertiary age. The aquifer materials are commonly segregated by size into lenses and beds, which can affect the movement and availability of water. Beds and lenses of sand, gravel, or mixtures of the two yield most of the water. The deposits may be more than 20 m thick and some km wide, much of their total thickness is saturated during the flood period, and, in many places, they yield large amounts of water. Thus a mathematical groundwater model was developed, employing conceptual groundwater modelling approach. The model covers an area of 110 km(2) with one layer, which is unconfined and contains 69 rows and 210 columns with a uniformly horizontal spacing equal to 200 m. For this purpose, Groundwater Modelling Software (GMS) was used which supports the MODFLOW-2000 code. The model is calibrated in steady and transient state considering the period from 1980 to 2008. The calibrated model shows increasing porosity (5 to 25%), from old terraces to the most recent alluvium, according to the degree of weathering and the percentage of clay. It shows also the importance of the river flows in the water balance of the aquifer; its contributions participate up 67% of the inputs to the Er-Rich aquifer. This equilibrium is the result of a global regulation at the watershed, by the presence of perennial sources (ex. : Sidi Hamza) upstream of the modeled domain, capturing the deep Liassic reservoir, and by the existence of a reduced outlet downstream of the aquifer.