Concentration of Nitrates in Spanish Carbonate Aquifers: Relation with Soil TOC and Land Use

The main objective of this study is to perform an initial diagnosis of the presence of nitrates in carbonate-rock aquifers in the Iberian Peninsula and identify the likely sources. Geostatistical methods were employed for TOC (Total Organic Carbon) mapping the studied area. The geostatistical method of spatial estimation, known as kriging, takes into account spatial correlation between experimental data through the variogram function. Highest TOC values are located in the main ranges of the Iberian Peninsula associated to forests although the highest values on nitrates are located in the agricultural areas. Nitrate concentration in groundwater ranges from less than 1 mg/L to more than 500 mg/L. Almost 4000 samples show more than 50 mg/L of nitrates, more than 1500 samples range from 37.5 to 50 mg/L, more than 1300 samples fluctuate from 30 to 37.5 mg/L and, finally, 10,400 samples show nitrate concentrations below 30 mg/L. Farming activities have a considerable impact on the state of the environment. Results of TOC geostatistical estimation have been optimal because estimation errors have been negligible. The generalized linear model procedure has shown a highly significant negative correlation between nitrates and TOC. The major conclusion of this study is that the scarcity of TOC is related to the content of nitrates in groundwater. In this sense, certain agricultural areas of Spain showing high concentrations in the occurrence of nitrates have low TOC values due to the fact that carbonate-rock aquifers are highly vulnerable to contamination. Additionally, the results of this approach indicate that most significant aquifers of this type in Spain show a good quality status (< 50 mg/L of nitrate concentration).