N and P behaviour in alluvial aquifers and in the soil solution of their catchment areas: How land use and the physical environment contribute to diffuse pollution.

The role of land use and the physical environment in N and P pollution of alluvial aquifers was analysed at three levels of information: (1) aquifer (N and P in groundwater), (2) soil transect (potentially leachable N and P in the soil solution) and (3) aquifer's catchment area. The study was carried out in the Oja and Tirón alluvial aquifers and their catchment areas (northern Spain). Nitrate was the dominant N form, both in groundwater and the soil solution of aquifers' catchment areas. Orthophosphate and organic-P were the codominant P forms in the aquifers. Orthophosphate was the main form in the soil solution. During the period 2005-2017 no significant decrease in nitrate pollution was observed, suggesting the need to review current Nitrate Vulnerable Zone (NVZ) designations. Since nitrate is highly mobile, it tended to accumulate in stagnation zones at the lower reaches of the aquifers. P did not accumulate in the same zones due to its low solubility. Principal component analyses (PCAs) of the aquifers, soil transects and aquifers' catchment areas revealed that the observation scale influences the environmental factors that can be detected as intervening in groundwater pollution. At the aquifer scale, links were found between nitrates and land use, topographic, hydrogeological and climatic factors. The protective effect of natural areas against nitrate pollution was noteworthy, while agriculture was associated with pollution. At the soil transect scale, an altitudinal gradient governed soil particle size distribution and land use, separating mountain forest soils from agricultural soils. The negative relationship between clay contents vs. nitrate and orthophosphate in the soil solution pointed to a regulatory role of clay. At the catchment scale, the size and physical characteristics of the catchments and land use distribution determined macronutrient availability in the soil solution and, in turn, N and P groundwater distribution.