Assessing the long-term effectiveness of nitrogen management for groundwater protection in the agricultural crop production sector in Wallonia, Belgium

Verstraeten, Elise; Alonso, Alice; Collier, Louise; Vanclooster, Marnik

doi:https://doi.org/10.5194/hess-2024-173

Preprints

https://doi.org/10.5194/hess-2024-173

Preprints

25 Jun 2024

| 25 Jun 2024

Status: this preprint is currently under review for the journal HESS.

Assessing the long-term effectiveness of nitrogen management for groundwater protection in the agricultural crop production sector in Wallonia, Belgium

Elise Verstraeten, Alice Alonso, Louise Collier, and Marnik Vanclooster

Abstract. Current nitrogen management programs within the agricultural crop production sector aim at optimizing crop productivity while minimizing environmental externalities, in particular groundwater contamination with nitrates. However, the effectiveness of these programs has been varied, with many studies indicating mixed or minimal results. Understanding the drivers of nitrate concentration in groundwater and its change is crucial for evaluating nitrogen regulations and guiding policy and management in the agricultural sector.

In this context, our study focused on assessing the effectiveness of the sustainable nitrogen management program for agriculture in Wallonia (PGDA), Belgium, on groundwater protection against nitrate contamination. We analysed nitrate concentration time series over the period 2002–2020 from 36 locations across four groundwater bodies within the Walloon nitrate vulnerable zones, situated in the agricultural belt. To capture the extent and dynamics of nitrate pollution, we developed and applied six indicators, providing a detailed view of both the current state and temporal trends of nitrate levels. Additionally, we computed spatially-explicit variables for each monitoring point to describe potential nitrate sources and their migration potential towards groundwater, and we examined their explanatory power in relation to the six nitrate pollution indicators.

Our findings indicate a modest overall improvement in average nitrate concentrations post-PGDA implementation. However, a closer examination at the individual site level reveals encouraging trends, with some locations showing pronounced decreased nitrate levels and with a decline in the average rate of change in nitrate concentration in 2020 indicating a slowdown in the rate of increase (or an acceleration in the rate of decrease) compared to 2002. Our results also underscore a complex array of factors influencing nitrate pollution and trends, with land use patterns and aquifer characteristics identified as key determinants. The study suggests that the absence of desired changes in certain areas could be attributed to a time lag between the introduction of regulatory measures and the observable impact on groundwater quality. This research highlights the intricate relationship between environmental regulation, land use, and groundwater quality, emphasizing the need for continued monitoring and adaptive management strategies to effectively address nitrate pollution in groundwater.

Received: 06 Jun 2024 – Discussion started: 25 Jun 2024

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

Download & links

Elise Verstraeten, Alice Alonso, Louise Collier, and Marnik Vanclooster

Status: open (until 20 Aug 2024)

Post a comment Subscribe to comment alert

RC1:
'Comment on hess-2024-173', Anonymous Referee #1, 28 Jun 2024 reply
1. General remarks:
The study described in this manuscript is important in the context of the need to assess the effects of actions taken to reduce nitrate pollution of groundwater and, consequently, rivers.
This requires a good methodology for determining the temporal trend of nitrate concentrations in groundwater and assessing the reversal of this trend. The study addresses this problem, and the adopted methodology for examining trend reversal seems to be appropriate. However, the methodological aspect is not properly highlighted in the manuscript. I would suggest increasing the emphasis and proportional share of the description of methodological issues of trend reversal analysis (Lowess and CP methods) throughout the text.
The second research goal, i.e. "identify the factors controlling the nitrate concentration and temporal trend", seems to be less important for the development of science (it is 2024). Moreover, in the case of this study, it is not universal, but local significance. Therefore, I would suggest moving parts of the manuscript regarding factors controlling the nitrate concentration and temporal trend to the Supplementary Material.
Instead, I would suggest going deeper into the details of the methodology description. Especially, the choice of the moment of trend reversal, number and “locations” of change points, the length of the period of examining a given trend, etc. These are universal issues, interesting for the reader involved in the analysis of water quality monitoring results.
2. Detailed remarks:
Text and Table 1 - indicator I4 - should be described rather as "slope between 20...- 20.." instead "slope in 2002". Similarly, correction should be in column “Usage and interpretation". Similarly in respect to indicator I5, as well as I6. After all, the trend lasts for some time, not 1 year. The text should precisely explain how the period (length?) of examining the upward trend was assumed (calculated?), e.g. 2001-2003, instead of e.g. 1996-2002. Or vice versa - because it is visible differently in Fig. 3 and Fig. 4. Similarly, in the case of the time period for analysing the downward trend - whether it is 2019-2021 or for example 2010-2021 (or maybe even for example 2002-2020 ?). What are the ranges analysed in this study and why? After all, it directly affects the final result of assessing the effectiveness of measures to reduce nitrate pollution. Maybe it is even the only important factor on which everything depends. This should be described in detail.

In text, no referral to Fig. 3, and there is no information on the basis of which data the time series was created.

lines 148-149 - "The window length..." - no explanation of the reason for this approach. Why eight years?

lines 152-152 - Helsel & Hirsch, 2002 - no in References.

In text, no referral to Fig. 4, and there is no information on what data this time series is based on.

Fig. 5 - no explanation of how to understand the meaning of the descriptions next to the "more desirable / less desirable" arrows in the context of histogram results.

Fig. 6 - no explanation of how to understand the meaning of the descriptions next to the arrows "desirable effect / undesirable effect" in Kendall rank correlation.

lines 404-406 - if precipitation is a variable in this research, it should be called "precipitation" or "rainfall", instead of "recharge". "Recharge" means that evapotranspiration is already included in this value. By the way, in Table 2 is "Rainfall", not "Recharge" - ?.

References seems to be incomplete –

regarding the base materials, for example:
Hirsch et al. 1991 - Selection of methods for the detection and estimation of trends in water quality.
Esterby, 1996 - Review of methods for the detection and estimation of trends with emphasis on water quality applications.
Grath et al. 2001 - The EU Water Framework Directive: statistical aspects of the identification of groundwater pollution trends, and aggregation of monitoring results
Craig & Daly, 2010 - Methodology for establishing groundwater threshold values and the assessment of chemical and quantitative status of groundwater, including an assessment of pollution trends and trend reversal.
In relation to relatively new articles, for example:
Frollini et al., 2021 - Groundwater quality trend and trend reversal assessment in the European Water Framework Directive context: an example with nitrates in Italy.
Comments on at least these indicated publications would be needed.
3. Technical/editorial remarks
lines 165-169 - incorrect paragraph formatting (font, line spacing, etc.)

line 460 - no data about the publisher/institution and publication place.

Reply
Citation: https://doi.org/10.5194/hess-2024-173-RC1

Elise Verstraeten, Alice Alonso, Louise Collier, and Marnik Vanclooster

Viewed

Total article views: 99 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	BibTeX	EndNote
73	20	6	99	2	2

HTML: 73
PDF: 20
XML: 6
Total: 99
BibTeX: 2
EndNote: 2

Views and downloads (calculated since 25 Jun 2024)

Month	HTML	PDF	XML	Total
Jun 2024	73	20	6	99

Cumulative views and downloads (calculated since 25 Jun 2024)

Month	HTML	PDF	XML	Total
Jun 2024	73	20	6	99

Viewed (geographical distribution)

Total article views: 90 (including HTML, PDF, and XML) Thereof 90 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 28 Jun 2024

Short summary

This study evaluates the effectiveness of Wallonia's sustainable nitrogen management program against groundwater nitrate contamination, 20 years post-implementation. We analysed nitrate concentration time series from 36 locations, finding a modest overall improvement and variability across sites. Results reveal interrelated controlling factors, with land use and aquifer characteristics being key. Lack of improvement may be due to a time lag before the impact of regulatory measures is observable.


Total:	0
HTML:	0
PDF:	0
XML:	0