Precipitation pattern changes impact in the groundwater quality at aluviões do Tejo aquifer

Abstract

In order to assess the potential impact that climate changes might have in the future groundwater chemistry, the objective of this study was to evaluate how has the past decade 2000-2009 climatic variations affected the groundwater quality of Aluviões do Tejo shallow aquifer. 2000-2009 decade was chosen due to the existence of two drought hydrologic years, in 2004/05 and 2007/08, as well as one wet year, in 2000/01. The main pressure considered was the precipitation changes with the consequent variation in the groundwater recharge. Considering that the studied area is mainly an agriculture area, it is expected that the changes in the precipitation, and therefore in the natural recharge, will impact in the nutrients (mainly nitrate) leaching to the groundwater. Therefore, nitrate concentrations were chosen as a good tracer to assess the quality status, electrical conductivity to give a broader idea of the changes in the water mineralization, and chloride as a conservative tracer that can be used for comparison with other elements. It is expected that, if recharge values are below the average and the same amount of fertilizer is applied, nutrient’s concentration in the soil will increase. However, groundwater is not immediately affected due to the nutrients travel time until the saturated zone. The following recharge episodes are likely to push further down nutrients and therefore increase groundwater nutrients concentration. During the dry years, the groundwater can even show a decrease in nutrients explainable by the small recharge in comparison to the groundwater flow that brings less contaminated water from other parts of the aquifer not affected by agriculture. The main conclusion that can be drawn from this brief analysis is that during dry periods the contaminants introduced in the soil, originated from agriculture practices or others, are temporarily retained in the soil horizon until the next wet period leaches them to the saturated zone. The time lag between the introduction of contaminants and the increase in groundwater concentrations depends on the groundwater level and the permeability of the geological formations, and several other aspects included in a classic vulnerability assessment. In this case-study, response measures are not considered a concern, if only the expected changes in precipitation pattern are considered as the driver. In this case-study, the only concern that can be highlighted is the temporary increase in nitrate concentration in groundwater that may endanger/jeopardise the achievement of Groundwater Good Chemical Status in this Nitrate Vulnerable Zone.