Ionic concentrations in surface waters of an agricultural catchment during storm events

Agricultural pollutants are generally carried to water courses by storm flows and infiltration waters. Because of this, their higher impact on water quality usually takes place after big rain events. This behavior emphasizes the importance of water quality studies carried out in agricultural catchments involving storm events. The results shown in this report were obtained from analyses of electric conductivity and concentrations of Ca(++), Na(+), K(+), Cl(-), NH4(+), and NO3(-) in samples collected during four storm events in a small agricultural catchment receiving frequent cattle and pig slurry applications. All the analysis were carried out with a multiparametric probe equipped with ion-selective electrodes. In general, higher ion concentrations were observed in the first stages of storm events and were followed, in most cases, by concentration decreases, sometimes registered before the stream reached its maximum flow rate. The most probable explanation for this behavior is related to the arrival of runoff water carrying materials proceeding from slurry applications to the stream. Dragging of these materials is promoted by the formation of a surface crust on the top soil of the fallow land that represents approximately half of the area being studied. Later on, continuous flow increase promotes a dilution effect that leads to a fall in the ion concentrations. The low base-flow index of the catchment (0.47) have an influence too on this behavior by making most of ions are transported by runoff. The only ion that showed strong differences when compared with the general behavior is Na(+), whose concentrations seem to be mostly governed by dilution effects. In this case, measurements taken during high flows showed concentrations similar to, or even lower than those obtained from base flows. N - NO3(-) y N - NH4(+) discharges were calculated for each event and stated as percentages of total annual discharge. These results were compared with percentages of water discharge to asses the quantitative importance of N losses in storm events related to total losses. N - NO3(-) losses were higher in 3 of the 4 selected events, reaching in one of them 11.3% of total N - NO3(-) losses. N - NH4(+) losses showed the opposite behavior. The aim of this work is getting progress in the knowledge of agricultural pollutants transport dynamics. Advances in this knowledge are of capital importance to minimize the impact of agricultural practices in water bodies.