Influence of groundwater exploitation on the ecological status of streams in a Mediterranean system (Selva Basin, NE Spain)

Abstract

The ecological status of streams depends on an equilibrium between hydrological processes and biological dynamics. Water discharge is the main requisite for a wealthy riparian habitat. Nevertheless, human practices severely affect water availability through stream water derivation and groundwater withdrawal. In this sense, impacts upon aquifer water storage and its effects on base flow generation have a significant effect on stream biology. Consequently, biological indicators will point out poor conditions resulting from such human impacts. In this paper, the effects of groundwater exploitation on stream discharge and surface water quality are evaluated and compared to biological indicators in a Mediterranean catchment. The stream-aquifer relationship is investigated by considering the hydrological context of each river reach, including human pressures, and the hydraulic head in the contiguous alluvial aquifer, where it exists. These data allow us to differentiate distinct types of reaches that are defined according to a "Stream-Aquifer relationship and chemical Pressure" (SAP) classification, which is used later on to standardize the different hydrochemical and biological features of the sampling points. Stream water and groundwater hydrochemistry are compared to depict the hydraulic behavior of the sampled watercourses during wet and dry periods. Specific elements are used as tracers of groundwater inputs, wastewater influence, or even stagnant conditions during the dry season. This dataset defines a framework to interpret the biological status of each reach based on the Iberian biological monitoring working party indicator (IBMWP). Affinities between hydrological and hydrochemical conditions with biological indicator values allow the causal effects of groundwater exploitation on stream ecology to be defined. The use of multivariate principal component analysis shows that the dataset variance is distributed according to the SAP classification, and that variable grouping is in agreement with the observed hydrological processes and their effects on biological indicator values. This work provides evidence of the importance of groundwater dynamics on biological indicators in a human-modified environment. When using the SAP classification, biological indicators acquire a broader meaning as they reveal the status of biological processes and the causal references. Such information is relevant for water management assessment within the context of the European Water Framework Directive, as it emphasizes the control of groundwater exploitation as a key parameter in the preservation of stream ecological status and the achievement of the objectives of the directive