Impact of more than thirty years of intense exploitation on groundwater flow system and chemistry of a carbonate karst aquifer in a semi-arid region. The case study of Sierra de Humilladero (Málaga province, S Spain).

In water-stressed areas such in S Spain, non-continuous outcrops of carbonate rocks hold limited groundwater volumes of good quality that very often constitute the only source of water for urban supply. Uncontrolled pumping in these important aquifers results in most cases in the substantial modification of the groundwater flow distribution, leading to water quality impairment and associated aquatic ecosystems degradation. The small carbonate massif of Sierra de Humilladero has been selected to address the impact of intensive pumping during more than thirty years. Using public databases, reports and recent field-data, a spatial-temporal piezometric study together with a tentative End Member Mixing Analysis (EMMA) were performed to quantify and timely assess the input source contribution from surrounding aquifer formations to statistically support the hypothesis launched by precedent authors. Results show that intensive groundwater exploitation has provoked a sustained lowering of piezometric levels (3-4m/year), up to 93 meters since 1998, along with a dramatical groundwater impairment, promoted by hydraulic gradient inversion effect with surrounding permeable geological formations. After comparing piezometric data and computed end-member mixing fractions, three stages in groundwater quality evolution have been differentiated: stage 1 (1992-2010), characterised by the influence of the chemical signature of Miocene waters (1-9% mixing ratio) in the composition of sampled groundwater, coinciding with the period of maximum rate of piezometric lowering (> 5 meters/year); stage 2 (2011-2016), denoted by the progressive mixing with evaporite groundwater from Triassic substratum (up to 20% in 2016) at depth (380-360 m a.s.l.), as the overexplotation effect on piezometric levels kept decreasing but at a is less intense than in stage 1, and; stage 3 (2018-today), in which pumping rate was reduced and chemical types are not clearly defined (random mixing fractions).

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