Monitoring nitrate reduction: hydrogeochemistry and clogging potential in raw water wells

Abstract

The mainly agricultural input of NO3- and compliance with drinking water guideline values pose major challenges for many water suppliers. Additionally, associated changes in hydrochemistry, especially concerning products of NO3- reduction (Fe2+/3+, Mn2+/4+, Ca2+, Mg2+, SO42-, HCO3-) and subsequent reactions, can have a major influence on mineral saturation states and well yield: well productivity can be strongly reduced by mineral precipitation and silting. To evaluate hydrogeochemical evolution and clogging potential for a given well field, thorough hydrochemical and geochemical investigations are required. Therefore, time-dependent and depth-specific ion concentrations in water samples (n = 818) were analysed in a catchment area of a waterworks in western Germany. The sediments of the aquifers were extensively investigated for their geochemistry (CS, scanning electron microscope, aqua regia digestion and dithionite solution; n = 253). In addition, PhreeqC was used to model saturation indices in order to identify possible mineral precipitation in the wells. Results show a high NO3- input into deep wells screened in Tertiary sediments due to an admixture of Quaternary groundwater. Directly at the Quaternary-Tertiary boundary, chemolithotrophic NO3- reduction consuming pyrite occurs. Protons released during the process are pH-buffered by dissolving carbonate minerals. Overall, the hydrochemistry and especially the saturation indices are strongly influenced by NO3- reduction and its degradation products. A change in well yield has not yet been observed, but future clogging by ochre formation or sintering cannot be excluded.

Trace Element Mobility during Corg-Enhanced Denitrification in Two Different Aquifers

“…S sulf showed two measurements below the detection limit (0.05 g/kg) and a maximum of 5.9 g/kg in 159 m. Thus, the maximum values of S tot and S sulf were each found at this depth. The mean S sulf content was 1.2 g/kg [30]. The Fe content in the Quaternary ranged from 6.6 to 10.1 g/kg, which was considerably lower than the Tertiary mean value of 21.6 g/kg.…”

“…Nitrate-contaminated groundwater from the Quaternary flows into the Tertiary due to the production wells. Lithotrophic NO 3 − reduction has good potential in the Tertiary sands, with an average sulfide-S (S sulf ) content of 1.2 g/kg [30]. Associated is the potential of trace element-rich sulfides such as pyrite (FeS 2 ).…”

“…From numerous samples of the entire thickness, 30 samples were selected and geochemically analyzed in greater detail. Hydrochemical data were provided by the water supplier and had already been evaluated through the first geochemical analyses by Ortmeyer et al [30]. Sampling for isotopic analysis was conducted at a multilevel well at all 11 filter depths (with the deepest filter at 156 m b.g.l.…”

“…Sediments of the Quaternary could not be analyzed for S and C content because sampling was not possible immediately after drilling and the sediment was in contact with atmospheric oxygen for several days. The S sulf and C org contents of the Quaternary-Tertiary aquifer were previously presented in Ortmeyer et al [30]. For the comparison of these 2 study areas, the data were processed comparatively, in addition to other analyses.…”

“…Hydrochemical data were provided by the water supplier and were previously evaluated for the Quaternary-Tertiary aquifer in Ortmeyer et al [30]. The data are discussed comparatively in this study.…”