Tracing water perturbation using NO_3~- , doc, particles size determination, and bacteria: A method development for karst aquifer water quality hazard assessment

摘要

Karst systems, as well as springs, are vulnerable to water perturbation brought by infiltration. In this research, sources of water perturbations were examined. The first objective is to provide a method that can determine the origin of the water flowing in the karst outlet. The second objective is to identify the associated water quality hazards caused by the infiltration source. The method relies on these parameters: turbidity, DOC, NO_3~-, particle size, and bacteria (E. coli, enterococcus and total coliforms). As the method was applied during flood events, measurement of the water flow is also needed to have a basic knowledge on the hydrodynamic of the water resource. The proposed method is based on a high resolution monitoring of physico chemical parameters of the water flowing during flood events. Using this proposed method. (1) the origin of the water can be identified, (2) the type and nature of water perturbation can be described, and (3) the type of water perturbation that accompanies contaminants such as the one with anthropogenic source (e.g. NO_3~-) and bacterial nature can be determined. In identifying the water origin, this proposed method employed NO_3~- and DOC data normalization. Values are projected in the NO_3~-_norm = f(DOC_norm) reference frame. These are aligned to the slope. Depending on the obtained slope (α), water origin can be disclosed. If α ＞ 1, the increase of concentration of DOC weighs more, characterizing water from surface runoff. Whereas, if α ＜ 1, the consideration is more on the increase of NO_3 concentration, characterizing water from unsaturated zone. However if a cannot be calculated because there is no evident slope, this characterizes the water already present in the system. Water originating from the surface runoff is prone to inorganic and bacterial contamination adsorbed by the particles. Identifying the type of water perturbation needing water treatment is important in managing the water resource. Hence, the evolution through time of NO_3~- and DOC with the particle size distribution, anthropogenic nature type of contaminant (i.e. in this study NO_3~- ). and presence or absence of bacteria were examined. This method was applied in the springs of the Toulon, an important drinking water source of the city of Perigueux in France. This site was chosen considering the following factors: (1) its karst nature being vulnerable to infiltrations, having fractures and sinkholes; (2) its land use being influenced by the anthropogenic activities such as agriculture; and (3) its observed pronounced turbidity incidence. The first flood events of two hydrological cycles were assessed. Three water origins of the spring water and the respective water quality hazards were identified: (ⅰ) water from saturated zone with minerals, (ⅱ) water from unsaturated zone with nitrate, and (ⅲ) water from surface runoff with the presence of bacteria. The second and third types of water perturbation gave evidence that the Toulon springs can be contaminated. Hence, in terms of resource management, the information obtained can be used as a basis in forecasting and planning the management actions or water quality treatments needed.