Buffer zones between wastewater treatment plants and receiving water bodies have recently gained interest in France. These Planted Discharge Areas (PDAs) receive treated wastewater, and may have various designs aiming to mimic “natural” kinds of wetlands. Research is needed to assess the treatment efficiency of such systems: a comprehensive study is carried out to understand the fate of water, conventional pollutants (suspended solids, organic carbon, ammonium, and phosphates), micro pollutants that are refractory to up-stream biological treatment, and pathogens. Special attention must be paid to understand the fate of the infiltrated treated wastewater in the field where PDAs are built, in order to ensure their long-term operation (system scale) and to protect the underground water bodies (site scale). To address these issues, the authors propose a comprehensive strategy combining successive stages using either geological or hydrological methods. These techniques provide the following prominent information for a proper design of PDAs: (1) the number and the location of the different soil layers; (2) the infiltration capacity of each layers; (3) the water table (height of the groundwater). Furthermore, the seasonal variation (if any) of the land characteristics are also determined. The proposed strategy successfully determined the fate of the infiltrated treated wastewater on the land considered before the implementation of the semi-industrial scale PDAs in Bordeaux. Besides, methods used for long-term PDA efficiency assessment are presented.

Although groundwater nitrogen pollution has been widely studied, the control of hydrogeological conditions on behavior of nitrogen compounds has been poorly understood. In this study, multiple stable isotopes (N/C/H/O), spectral characteristics of DOM coupled with water chemistry were used to reveal the sources and fate of nitrate and ammonium in three subareas with different hydrogeological conditions in the Central Yangtze River Basin. We identified three contrasting patterns of nitrogen sources and fate in groundwater controlled by different aquifer features. In a reducing porous aquifer mainly composed of carbonate minerals overlain by a thick low-permeability layer, the NH₄–N concentration is high (mean 4.12 mg/L) but with quite low NO₃–N concentration (mean 0.28 mg/L). The high ammonium is mainly from intense degradation of organic matter (OM), while denitrification at a higher rate results in nitrate removal. Feammox may be favored owing to abundant humics acting as the electron shuttle. In a weakly reducing to oxidizing porous aquifer mainly composed of aluminosilicate minerals overlain by a varying thickness of low-permeability layer, high ammonium occurs in a weakly reducing condition and is affected by both anthropogenic input and OM degradation, while high nitrate occurs in a more oxidizing condition and could be mainly from soil nitrogen, manure or sewage. Feammox may be also favored due to more acidic environment formed by weathering of aluminosilicate minerals, fluctuating redox condition and low abundance of labile organic carbon, while denitrification occurs at a slower rate coupled with concurrent re-oxidation of nitrite to nitrate. In an oxidizing porous - fissured aquifer system overlain by a thin low-permeability layer, the concentrations of ammonium and nitrate are both low, possibly due to strong hydrodynamic and flushing condition, although slightly higher concentration of nitrate exhibit similar sources and fate with the weakly reducing to oxidizing porous aquifer mentioned above.

The monitoring of a spring and seven piezometers in the 3 km2 Brévilles agricultural catchment (France) over five and a half years revealed considerable spatial and temporal variability in the concentrations of atrazine and its metabolite deethylatrazine (both systematically quantified at the outlet spring): maximum 0.97 and 2.72 μg L-1, mean 0.19 and 0.59 μg L-1, respectively. Isoproturon, the pesticide applied in the greatest amount, was detected in only 10 of the 133 samples. These observations can only partly be explained by land use and intrinsic pesticide properties. Geochemical measurements and tritium dating showed the importance of the stratification of the sandy saturated zone and the buffer function of the unsaturated limestone. Principal component analysis on 39 monthly data series of atrazine, deethylatrazine, nitrate, chloride and piezometric levels revealed a temporal structuring of the data possibly reflecting the existence within the aquifer of two different reservoirs with time-variable contributions. We present an integrated approach combining geochemistry and hydrogeology that leads to a better understanding of the spatial and temporal fluctuations of the pesticide concentrations in groundwater of a pilot agricultural catchment.