Groundwater quality overview towards sustainability in dryland oases

Código

I-EBHE0009

Autores

ATHMANE KHETTOUCH, Mohammed Hssaisoune, Yassine Ait Brahim, Yassine Ez-zaouy, Thomas Hermans, Lhoussaine Bouchaou

Tema

WG 1.11: Water Quality Under Global Changes

Resumo

The most important oases in southern Morocco are situated within the Drâa River Basin (DRB), considered one of the ten driest watersheds worldwide. The limitation and scarcity of surface water resources exert high pressure on groundwater resource. In the last decades, climate change, population growth, and recent increases in cash crop production aggravate the problem of water salinization, putting human well-being at high risk. This study aims to assess groundwater quality in the DRB using hydrochemical characteristics from various survey sources conducted over the DRB. The concentration at which constituents become objectionable for drinking purposes was derived from both the World Health Organization (WHO) and context-specific guideline values as a crucial aspect for consumer acceptability. Hydrochemical characteristics of water were simplified into a single comprehensible value using the Water Quality Index (WQI) and Heavy Metal Pollution Index (HPI) to assess overall water quality. Water samples were collected from various sites, including boreholes, wells, Khetteras, and springs, designated to meet the water needs of the population across different aquifers in key valleys and oases. Additionally, the suitability of groundwater for irrigation purposes was assessed using sodium absorption ratio (SAR), Residual Sodium Carbonate (RSC), and the permeability index (PI). The results show that more freshwaters (EC<1500 µs/cm2), belonging to the good quality range (WQI<100), were observed in the upstream oases and valleys, whereas high saline waters reaching half the level of seawater deteriorate water quality in the last hyper-arid oases. Insufficient water flushing through the root zone is a consequence of the gravity-fed irrigation mode adopted and is responsible for the high content of sodium and chloride, which emerge as the predominant ions along the flow path. Groundwater quality exceeds the guideline limits recommended by the WHO and the MSV for most elements due to their high mineralization and reaches alarming levels within the current context of overexploitation and high evaporation rates. Moreover, water degradation affects its suitability for agricultural use, with 30% of groundwater likely impacting crop water availability and 13% exhibiting severe reduction in infiltration rate. Nevertheless, sodium hazard was not associated with CaCO3 and MgCO3 precipitation (low PI and RSC). In some areas, nitrate contents imply anthropogenic contamination, whereas the concentrations of heavy metals were more associated with the geological setting than anthropogenic activities. As perspectives, water-demanding watermelons, alfalfa and henna should be substituted with alternative crops consuming less water and tolerating high salinity. Expanding the sewer system, particularly in rural areas, is required to reduce nitrate content in shallow aquifers contaminated by septic tanks.