Temporal variability of nitrate concentration in a schist aquifer and transfer to surface waters

• Published in: Applied geochemistry 2001-05, Vol.16 (6), p.583-596
• Main Authors: Pauwels, Hélène, Lachassagne, Patrick, Bordenave, Paul, Foucher, Jean-Claude, Martelat, Anne
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Environmental Sciences; Exact sciences and technology; Hydrogeology; Hydrology. Hydrogeology; pyrite
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/S0883-2927(00)00062-7

Nitrate concentrations monitored for 2.5 a in the stream water and groundwater of a small catchment, 86.5% of which is devoted to intensive agriculture, show temporal variations with a maximum during winter (as much as 200 mg l −1 in groundwater and 100 mg l −1 in stream water) and a minimum at the end of summer/beginning of autumn. Variations were also observed in the stream water and shallow groundwater after rainfall. The processes involved to explain these variations, determined mainly from NO 3 − Cl −, SO 4 2−, piezometric and streamflow data, are: (a) variability of the relative contributions to stream water and shallow groundwater by upward fluxes of deeper groundwater which, as demonstrated previously, is denitrified mainly as a result of reaction with pyrite. (b) Denitrification of shallow groundwater during summer with organic matter acting as the electron donor. (c) Dilution by rain water. Nitrate concentrations in both stream water and shallow groundwater depend on the amount of precipitation, with an increased contribution from deep denitrified groundwater during dry periods. The temporal variations in NO 3 − concentration observed several metres below the water table are related to the preferential and rapid movement of NO 3 −-polluted water through fractures and large fissures, which has been estimated at 1 m day −1. Nitrate pollution in the catchment, because of the interaction with pyrite, also increases the net chemical weathering rate to values exceeding the world average.