Nitrates as indicators of aquifer interconnection. Application to the Campo de Dalías (SE - Spain)

Relatively high concentrations of NO3 usually have their origin in processes of organic pollution and excessive use of inorganic fertilizers. In the case of agricultural areas, these activities may generate great quantities of nitrates. This fact has enabled us to use the nitrate ion to characterize an exploited aquifer unit of a very complex aquifer system, as is the case for the Campo de Dalias aquifer system (SE Spain). The shallow boreholes, which draw water from intensively cultivated superficial formations, yield waters with a high NO3 content. When the boreholes are deeper and penetrate low-permeability formations in the superficial layers, the waters contain little NO3; similar to what happens in areas of recharge where agricultural activities are absent. In addition, using longitudinal sections, it is possible to demonstrate the possible existence of connections between superficial and deep aquifers using NO3 as a tracer. Similarly one can locate those sectors responsible for contamination caused by the poorly lined boreholes themselves, when they perforate more than one aquifer in a multiple aquifer formation.