Natural baseline quality of Madrid Tertiary Detrital Aquifer groundwater (Spain): a basis for aquifer management

Abstract

The Madrid Tertiary Detrital Aquifer is one of the largest and most important aquifers of Spain. This paper assesses the most relevant controls on the natural baseline quality and the dominant chemical processes within the aquifer. The hydrochemistry of the groundwater is variable despite the relative uniformity of the detrital sediments. The natural baseline is expressed as a range of values that are controlled by lithological and hydrological factors; spatial variations of groundwater chemistry are related to changes in rock type, water-rock interaction and the residence time of groundwater. The fundamental chemical processes within the Arkosic aquifer are hydrolysis of silicates, dissolution of carbonates, dissolution of evaporites (only in the vicinity of the transitional facies), ion exchange, neoformation of clays, precipitation of silica as cement, and precipitation of carbonates due to increasing temperature along the downward pathways. Some chemical and physico-chemical parameters like pH, dissolved oxygen, and hardness, and several elements like calcium, sodium, magnesium, silica, and arsenic show an evolutionary trend according to groundwater flow path. A gradual increase in arsenic concentration from recharge areas to discharge areas is observed; it is the main natural water constituent that deteriorates the quality of the fresh Madrid groundwater as a drinking water supply. The occasionally elevated arsenic concentrations originate from natural sources. The concentration and mobility of arsenic seems to be controlled by pH-dependent anion exchange processes resulting from the evolution to Na-HCO3 water.