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Change in nitrate concentration in groundwater; background report to the nitrate compliance checking level study. : Results of measurements at common and special national monitoring wells.

Het verloop van de nitraatconcentratie van het grondwater; achtergrondrapport Nitraatdieptemeetnet : Resultaten van metingen bij LMG- en N-putten

Synopsis

Permanent wells on agricultural fields can be used in a monitoring network for monitoring nitrate trends with depth in the upper five meters of groundwater. For monitoring nitrate at greater depths, wells outside the fields provide sufficiently reliable measurements. With such a nitrate monitoring network, it will likely be possible to demonstrate the occurrence or absence of nitrate breakdown (denitrification) as well as the potential adverse secondary effects of denitrification on groundwater quality. The precise quantification of denitrification and its side effects requires a specialised research approach and is not feasible within the framework of a regular monitoring network. This is the outcome of a study carried out by the RIVM by order of the Ministry of Infrastructure and Environment (I&M) and the Ministry of Economic Affairs, Agriculture and Innovation (EL&I). These findings have an important bearing on the structure of a monitoring network for the determination of the trend of nitrate in groundwater with depth (nitrate profile monitoring network). They can also be used by these ministries to facilitate an informed decision on whether such a network should be put in place. The results of a nitrate profile monitoring network are relevant in the discussion with the European Commission pertaining to the Netherlands minerals policy. Nitrate concentration shows no trend with depth within the upper five meters of groundwater under agricultural fields on deeply drained sandy soils, where the highest nitrate concentrations are measured. Nitrate concentration decreases with depth under agricultural fields on other types of sandy soils. The results of this study reveal that a lower nitrate concentration at greater depth (more than five meters) is related to both old levels of fertilisation and to denitrification in the deeper soil layers. As such, this study confirms the findings and conclusions of previous compliance checking level studies.
 

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