How can water quality be improved when the urban waste water directive has been fulfilled? A case study of the Lot river (France)

Abstract

The Lot river, a major tributary of the downstream Garonne river, the largest river on the Northern side of the Pyrenees Mountains, was intensively studied in the 1970s. A pioneering program called "Lot Rivière Claire" provided a diagnosis of water quality at the scale of the whole watershed and proposed an ambitious program to manage nutrient pollution and eutrophication largely caused by urban wastewater releases. Later on, the implementation of European directives from 1991 to 2000 resulted in the nearly complete treatment of point sources of pollution in spite of a doubling of the basin's population. At the outlet of the Lot river, ammonium and phosphate contamination which respectively peaked to 1 mg N-NH L and 0.3 mg P-PO L in the 1980s returned to much lower levels in recent years (0.06 mg N-NH L and 0.02 mg P-PO L), a reduction by a factor 15. However, during this time, nitrate contamination has regularly increased since the 1980s, from 0.5 to 1.2 mg N-NO L in average, owing to the intensification of agriculture and livestock farming. Application of the Riverstrahler model allowed us to simulate the water quality of the Lot drainage network for the 2002-2014 period. We showed that, with respect to algal requirements, phosphorus and silica are well balanced, but nitrogen remains largely in excess over phosphorus and silica. This imbalance can be problematic for the ecological status of the water bodies. Using the model, for simulating various scenarios of watershed management, we showed that improvement of urban wastewater treatment would not result in any significant change in the river's water quality. Even though arable land occupies a rather limited fraction of the watershed area, only the adoption of better farming practices or more radical changes in the agro-food system could reverse the trend of increasing nitrate contamination.

Recent Trends (2012–2016) of N, Si, and P Export from the Nemunas River Watershed: Loads, Unbalanced Stoichiometry, and Threats for Downstream Aquatic Ecosystems

“…Mid-term (over some consecutive years) and frequent (at least monthly) monitoring of rivers that drain large watersheds allows to provide reliable datasets to analyse the temporal trends in discharge and nutrient transport [34]. This monitoring time span and frequency requirements are due to three main reasons: (1) Climatic microcycles may last for multiple years and are characterized by prolonged dry or wet phases; (2) climate, through hydrology, affects nutrient cycles; and (3) the effects of ongoing climate changes interact with those of implementation of policy directives that target nutrient reduction (e.g., land use, sewage water treatment plants, and the use of fertilizers) [36,[50][51][52]. Due to reasons (1) and (2), a one-or two-year sampling program may fall in unusually low or high discharge periods that are not representative of a climatic microcycle and therefore lead to an incorrect conclusion regarding discharge (and nutrient) trends.…”

“…Šileika et al [29] suggest that converting 20% of arable land to pasture, together with sewage treatment plant improvements, would reduce the yearly TN export to 12,000 t, which is close to the target set for Lithuania. Improvement of agricultural practices would not necessary result in lower contribution of diffuse sources to N loads as the transfer of N from arable land to surface and ground water largely depends on storage and immobilisation processes within soils [36,52]. During the Soviet Union period, the low cost and the intensive application of N fertilizers in agriculture likely caused N accumulation in soils.…”

“…Overall, long-term monitoring is also needed because of slow ecosystem response times, e.g., lag times between changes in terrestrial nutrient management and land-water nutrient transfer or riverine loads decrease and within river nutrient transformations processes. Different studies have demonstrated that substantial nutrient reductions (i.e., those related to N loads) may occur many years after the implementation of initiatives, such as the reduction in fertilizer use, due to the accumulation and slow release of N from soil [20,34,36]. A few investigations carried out in small catchments within the geographical area of the Baltic States have highlighted large variations in agricultural practices and climate and analysed their effect on N loss [37].…”

Nutrient inputs and hydrology together determine biogeochemical status of the Loire River (France): Current situation and possible future scenarios

“…We tested two contrasting agricultural scenarios (Garnier et al, 2018;Desmit et al, 2018). As a first step, we considered the same current conventional rotations, with full application of the current regulations (balanced fertilization following the agronomical recommendation with respect to current yield objectives and catch crop introduction before spring crops, Justes et al, 2012), called good agricultural practices (GAP).…”

“…These models were used for estimating the global distribution of nutrients export from large river. Fully based-process models such as SWAT (Arnold et al, 1998;Malagó et al, 2017), RiNUX (Loos et al, 2009) require calibration procedures, whereas Riverstrahler (Billen et al, 1994;Garnier et al, 2002;Garnier et al, 2018), PEGASE (Descy et al, 2011) and QUAL-NET (Minaudo et al, 2018) are more generic. When metrics are calculated for evaluating these models, NRMSE, slopes or biases are chosen.…”