Procedimiento para evaluar cargas internas de nutrientes en cuerpos de agua

Abstract

On the basis of nutrient lixiviation experiments and variations in redox potential in water bodies, a procedure is proposed to evaluate internal nutrient loads. This procedure was evaluated for a water body of 1800 ha with a maximum depth of 38 m and an average depth between 11 and 22 m, depending on the variations in storage volume. Combined sediment samples from deep, intermediate and shallow areas of the water body were suspended in water with magnetic agitation and supplies of air or N2 (g) for control of redox potential. The suspensions were monitored, measuring dissolved minerals and nutrients. Carbon dioxide produced by mineralization of organic matter in the sediments was quantified by determining variations in electrical conductivities due to the precipitation of carbonate in alkaline solutions. The lixiviation of nutrients per unit mass of sediments was calculated, considering that this occurs in the upper 0.15 m of sediments with a density of 170 kg/m3. The nutrient loads per unit area were plotted as a function of redox potential and tendency lines were adjusted. Based on redox potentials for the water body as published in the scientific literature, areas of the water body were calculated that represent specific redox potential intervals. Applying the tendency lines, the internal phosphorous and nitrogen loads were determined, detecting more lixiviation of nitrogen and phosphorus from sediment samples originating from deep and intermediate areas than from the shallower areas of the water body. While nitrogen release and CO2 production increased with increasing redox potential, lixiviation of phosphorous and iron increased with decreasing redox potential as a result of reductive dissolution of iron-containing minerals and associated phosphorus. Lixiviated phosphorous represented approximately 1% of total phosphorous in the sediments. The results suggest that internal phosphorus load mainly occur from June to October, during stratification of the water body, while the internal load of nitrogen occurs from December to April. Accumulated internal loads of nitrogen and phosphorus were 1,153.5 and 3.7 t/yr, respectively. These results were compared with the nutrient balance for the water body of 13.7 t P/yr which is 370% the estimated internal phosphorus load and 914 t N/yr or 79% of the estimated internal nitrogen load, resulting in an overestimation of the nitrogen load and an underestimation of the phosphorus load by considering changes in redox potential published in the literature. Recommendations were formulated to optimize the proposed procedure.