Biogeochemical processes in infiltration basins and their impact on the recharging effluent, the soil aquifer treatment (SAT) system of the Shafdan plant, Israel
Résumé
Soil aquifer treatment (SAT) systems utilize infiltration basins for recharging pre-treated effluent into the aquifer for water quality improvement. The present research focuses on the diurnal and seasonal variations of oxygen, carbon and nitrogen species in the infiltration basins and in the uppermost vadose zone for determining the impact of different environmental conditions on the composition of the infiltrating effluent. The study was conducted in one of the infiltration basins of the Dan Region Sewage Reclamation Project (Shafdan), Israel. Solar radiation and aeration were recognized to be the two main factors controlling the chemical composition of the effluent within the basin and the uppermost vadose zone. The effluent entering the basin show initial enrichment with dissolved oxygen (DO) due to gas exchange caused by "injection'' through the inlet fountain and turbulence during basin filling. The DO, pH and dissolved inorganic carbon (DIC) fluctuate diurnally as a result of intense in-basin daylight photosynthesis and nighttime respiration. DO and pH increase during day and decrease during night and the DIC shows an opposite behavior. The daylight net primary production was similar to 240 mu mol L-1 d(-1) and the diurnal respiration rate was similar to 430 mu mol L-1 d(-1) (assuming that the nighttime respiration rate is constant over a whole diurnal cycle). The gross primary production during daylight was similar to 450 mu mol L-1 d(-1). Dissolved organic carbon (DOC) and inorganic nitrogen species (NH4+ + NO3) varied significantly in the uppermost vadose zone, however, neither the vadose zone nor the basin showed diurnal fluctuations. It appears that chemical variations between three sampling campaigns conducted in different seasons stemmed mainly from the efficiency of basin aeration that resulted from changes in basin's operation regime and local environmental conditions. The results of this study lead to the following basin management recommendations: (1) keeping short flooding and drying cycles and allowing complete basin desiccation before the next flooding; and (2) flooding the basin during daytime, starting at dawn to maximize oxygen production and minimize infiltration during nighttime. Following these routines would result in higher redox potential of the infiltrating effluent.