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Approach to estimate the nitrate degradation capacity of deep porouse aquifer at two waterworks in the Lower Rhine Valley

Doreen Mäurer1, Frank Wisotzky2
1 Tauw GmbH, soil and groundwater
2 Ruhr-University Bochum, Hydrology department

O 12.2 in Grundwasserqualität

29.05.2014, 12:40-13:00, H19, NW II

 

Groundwater quality is a concern in aquifers which are affected by agricultural areas worldwide. Decades ago huge amounts of nitrogenous fertiliser are applied in agricultural used areas, like in the Lower Rhine Valley. Significant quantities of nitrate ended up in surface water and aquifers which were used for public water supply. In the upper quaternary aquifers the concentrations of nitrate often exceed the threshold value of 50mg/l from the Dinking Water Ordinance. Therefore the groundwater extraction is frequently carried out from deeper tertiary aquifers, because there are lower nitrate concentrations. But this change in extraction depth is mostly only a medium-term solution. With a time lag the nitrate can reach the extraction wells of deep aquifers. This depends on the local hydrogeological/geochemical conditions of the aquifer.

The chair of Applied Geology at the Ruhr-University Bochum carried out hydraulic/hydrochemistry investigations in two waterworks with different field/laboratory experiments.

The aim of these investigations were to

·        assess the natural potential of the aquifer to reduce nitrate

·        estimate the risk of increasing nitrate concentration in deep wells.

The special approach relied on the assessment of the complex hydrogeological processes in deep aquifers in respect to lithotrophic/organotrophic denitrification. For evaluation of the hydraulic situation/hydrogeochemistry and especially to compensate the low density of observation wells in deep aquifers new wells (one multi-level with 13 different levels) were installed up to a depth of 182m. Soil samples from two wells were used to carry out batch tests (S4) / recycle column experiments (CE). The CE are representative for closed systems, where relevant parameters of solid material (sedimentary bound sulphur SPyrit, total sulphur Sges, organic/anorganic carbon Corg/Canorg) / of water (nitrate, sulfate, hydrocarbonate, iron, manganese) were quantified.

Results of the soil material showed that the content of SPyrit, Sges, Corg, Canorg are representative for the tertiary aquifer and nearly unchanged or increased with the depth. Comparatively high contents of Canorg attest the existing potential of neutralisation of protons (H+), which will be delivered during the lithotroph denitrification. The results of the CE were used for the process orientated modelling with the computer code PhreeqC. Additionally analytical methods (e.g. calculation by using a representative unit of 1m³) were used to estimate the nitrate degradation capacity of porouse media.

The final risk assessment for the two waterworks showed that there is enought sedimentary bound material in the tertiary sediments to reduce existing nitrate effectivly. Between 40-71% of the sedimentary SPyrit / 2-58% of the sedimentary Corg, which were built in the CE, contributed to sucessful nitrate degradation. So that for at least for the next one hundred years a risk of nitrat concentration in deep wells can not to be expected.



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Letzte Änderung 02.11.2013