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Combining stable isotopes with contamination indicators: A method for improved investigation of nitrate sources and dynamics in aquifers with mixed nitrogen inputs.

Minet, Eddy P.; Goodhue, R.; Meier-Augenstein, Wolfram; Kalin, R.M.; Fenton, O.; Richards, K.G.; Coxon, Catherine E.

Authors

Eddy P. Minet

R. Goodhue

Wolfram Meier-Augenstein

R.M. Kalin

O. Fenton

K.G. Richards

Catherine E. Coxon



Abstract

Excessive nitrate (NO3-) concentration in groundwater raises health and environmental issues, which must be addressed by all European Union (EU) member states under the Nitrates Directive and the Water Framework Directive. The identification of NO3- sources is critical to efficiently control or reverse NO3- contamination that affects many aquifers. In that respect, the use of stable isotope ratios 15N/14N and 18O/16O in NO3- (expressed as o15N-NO3- and o18O-NO3- respectively) has long shown its value. However, limitations exist in complex environments where multiple nitrogen (N) sources coexist. This two-year study explores a method for improved NO3- source investigation in a shallow, unconfined aquifer with mixed N inputs and a long established NO3- problem. In this tillage-dominated area of free-draining soil and subsoil, suspected NO3- sources were diffuse applications of artificial fertiliser and organic point sources (septic tanks and farmyards). Bearing in mind that artificial diffuse sources were ubiquitous, groundwater samples were first classified according to a combination of two indicators relevant of point source contamination: presence/absence of organic point sources (i.e. septic tank and/or farmyard) near sampling wells and exceedance/non-exceedance of a contamination threshold value for sodium (Na+) in groundwater. This classification identified three contamination groups: agricultural diffuse source but no point source (D+P-), agricultural diffuse and point source (D+P+) and agricultural diffuse but point source occurrence ambiguous (D+P±). Thereafter o15N-NO3- and o18O-NO3- data were superimposed on the classification. As o15N-NO3- was plotted against o18O-NO3-, comparisons were made between the different contamination groups. Overall, both o variables were significantly and positively correlated (p < 0.0001, rs = 0.599, slope of 0.5), which was indicative of denitrification. An inspection of the contamination groups revealed that denitrification did not occur in the absence of point source contamination (group D+P-). In fact, strong significant denitrification lines occurred only in the D+P+ and D+P± groups (p < 0.0001, rs > 0.6, 0.53 < slope < 0.76), i.e. where point source contamination was characterised or suspected. These lines originated from the 2-6% range for o15N-NO3-, which suggests that i) NO3- contamination was dominated by an agricultural diffuse N source (most likely the large organic matter pool that has incorporated 15N-depleted nitrogen from artificial fertiliser in agricultural soils and whose nitrification is stimulated by ploughing and fertilisation) rather than point sources and ii) denitrification was possibly favoured by high dissolved organic content (DOC) from point sources. Combining contamination indicators and a large stable isotope dataset collected over a large study area could therefore improve our understanding of the NO3- contamination processes in groundwater for better land use management. We hypothesise that in future research, additional contamination indicators (e.g. pharmaceutical molecules) could also be combined to disentangle NO3- contamination from animal and human wastes.

Journal Article Type Article
Publication Date Nov 1, 2017
Journal Water research
Print ISSN 0043-1354
Electronic ISSN 1879-2448
Publisher Elsevier
Peer Reviewed Peer Reviewed
Volume 124
Pages 85-96
Institution Citation MINET, E.P., GOODHUE, R., MEIER-AUGENSTEIN, W., KALIN, R.M., FENTON, O., RICHARDS, K.G. and COXON, C.E. 2017. Combining stable isotopes with contamination indicators: A method for improved investigation of nitrate sources and dynamics in aquifers with mixed nitrogen inputs. Water research [online], 124, pages 85-96. Available from: https://doi.org/10.1016/j.watres.2017.07.041
DOI https://doi.org/10.1016/j.watres.2017.07.041
Keywords Nitrate; Groundwater; Stable isotope; Contamination indicator; Diffuse source pollution; Point source pollution

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