Trend Analysis of Nitrate Concentration in Rivers in Southern France

Fayçal Bouraoui,Anna Malagó

doi:10.3390/w12123374

Abstract

Excessive nutrients in rivers, lakes and aquifers are still threatening environmental health in Europe. Stringent regulations have led to progress in water quality, however hotspots with high nitrate concentrations still exist in Europe and understanding the impact of management on the nitrate concentrations and trends in these critical areas is still challenging. In this paper, we use the Exploration and Graphics for RivEr Trends (EGRET) statistical tool to eliminate the impact of flow variation, both short- and long-term, on nitrate concentration. We apply this tool to the south of France where water quality and quantity monitoring data is readily available. We compare the Mann–Kendall non-parametric approach to estimate trend and a methodology commonly used by Member States of the European Union when they report their progress in implementing the Nitrates Directive (referred to MSD approach hereafter). We showed that using the latter approach for the period 2008–2015 and the Mann–Kendall test leads to similar results in percentage of stations exhibiting trends, however with a significant disagreement on the stations exhibiting these trends. We further showed that when using flow-weighted nitrate concentrations instead of the simple mean nitrate concentration, the MSD approach results in a significant underestimation of the stations with an increasing trend. We also demonstrated that most of nitrate concentration time series are characterized by a bell-shaped curve with an increase of concentration from 1990 to mid-2000 and then a significant decreasing trend due to the implementation of management measures from mid-2000 to 2017. Most of the significant decreasing nitrate concentration trends are localized in Nitrate Vulnerable Zones that correspond to areas where strict nutrient management is required, highlighting the efficiency of the policy in place.

Highlights

Excessive nutrients in rivers and aquifers are still an environmental issue in Europe [1].The European Environmental Agency (EEA) reports diffuse pollution as the second largest significant pressure on surface water resources after hydromorphological alteration, with agriculture the dominant source of diffuse pollution [1]
When using the mean annual concentration obtained from the raw data, about 61% of the stations exhibited no significant trend, while 26%
Using the mean annual concentration obtained from the flow-weighted and flow-normalized concentrations reduced the number of non-significant trends to

Summary

Introduction

The European Environmental Agency (EEA) reports diffuse pollution as the second largest significant pressure on surface water resources after hydromorphological alteration, with agriculture the dominant source of diffuse pollution [1]. The Directive aims at reducing pollution from nitrate coming from agriculture and preventing any further pollution. To achieve these objectives, Member States (MS) are required to identify waters affected by pollution or potentially affected if no action is taken, where pollution refers to waters where nitrate concentration is larger than 50 mg/L or could be so if no measures are taken, and water bodies affected by eutrophication or that could be so if no measures are taken.

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