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

Relatively high concentrations of NO3 usually have their origin in processe s of organic pollution and excessive use of inorganic fertilizers. In the c ase of agricultural, areas, these activities may generate great quantities of nitrates. This fact has enabled us to use the nitrate ion to characteriz e an exploited aquifer unit of a very complex aquifer system, as is the cas e 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 penetrat e low-permeability formations in the superficial layers, the waters contain little NO3; similar to what happens in areas of recharge where agricultura l activities are absent. In addition, using longitudinal sections, it is po ssible to demonstrate the possible existence of connections between superfi cial and deep aquifers using NO3 as a tracer. Similarly one can locate thos e sectors responsible for contamination caused by the poorly lined borehole s themselves, when they perforate more than one aquifer in a multiple aquif er formation.