Determination of the denitrification capacity of unconsolidated rock aquifers using 15N tracer experiments at groundwater monitoring wells - development of a new method to assess actual and future denitrification in aquifers Dissertation zur Erlangung des Doktorgrades der Fakultät für Forstwissenschaften und Waldökologie der Georg-August-Universität Göttingen vorgelegt von Diplom Geologe Wolfram Eschenbach geboren in Burg Göttingen, 2014 1. Gutachter: Prof. Dr. Heinz Flessa 2. Gutachter: Prof. Dr. Jürgen Böttcher Tag der mündlichen Prüfung: 28.01.2014 "Ich bin mir jedenfalls bewusst, dass ich keine Weisheit besitze, weder groß noch klein." Sokrates Table of contents Figures ...................................................................................................................................... V Tables ....................................................................................................................................... VI Danksagung ........................................................................................................................... VII Abstract ................................................................................................................................... IX Kurzfassung ............................................................................................................................ XI Preface and Outline ............................................................................................................. XIII 1 General Introduction .......................................................................................................... 1 1.1 Nitrate pollution of groundwater ......................................................................................... 1 1.2 The nitrogen cycle ............................................................................................................. 3 1.3 Denitrification in groundwater ............................................................................................ 6 1.4 Objectives of this thesis...................................................................................................... 9 2 Online measurement of denitrification rates in aquifer samples by an approach coupling an automated sampling and calibration unit to a membrane inlet mass spectrometry system ......................................................................................................... 11 Abstract .................................................................................................................................11 2.1 Introduction......................................................................................................................12 2.2 Experimental ....................................................................................................................15 2.2.1 Study site ....................................................................................................................15 2.2.2 Set-up .........................................................................................................................16 2.2.2.1 Incubation of aquifer mesocosms and sampling procedures......................................16 2.2.3 On- and offline measurement and analysis of dissolved gases .........................................17 2.2.4 Automated online analysis using ASCU-MIMS .............................................................18 2.2.4.1 Principle of online dissolved gas analysis with MIMS .............................................19 2.2.4.2 Sampling procedure and analysis............................................................................19 2.2.5 Manual offline approach with GC-IRMS .......................................................................21 − 2− 2.2.5.1 Sampling and offline analysis of NO3 , SO4 and denitrification gases .....................22 2.3 Calculations......................................................................................................................23 15 − 2.3.1 N abundance of denitrified NO3 ................................................................................23 2.3.2 Mathematical approaches to determine the contribution of labelled denitrification gases in a mixture of N2 and N2O gases ..........................................................................................23 2.3.2.1 Method I − Mulvaney ............................................................................................24 2.3.2.2 Method II − isotope pairing method........................................................................25 2.3.2.3 Method III − Spott and Stange ...............................................................................26 2.3.3 Precision and limit of detection .....................................................................................26 I 2.3.4 Estimating potential bias from in situ degassing .............................................................27 2.4 Results and Discussion ......................................................................................................28 2.4.1 Comparison between ASCU-MIMS and GC-IRMS gas Analysis ....................................28 2.4.2 Comparison of three different mathematical 15N approaches ...........................................30 2.4.3 Denitrification rates and influence of the experimental set-up on the measured time pattern of denitrified (N2+N2O) .............................................................................................36 2.4.4 Comparison of ASCU-MIMS with previous methods .....................................................40 2.5 Conclusions......................................................................................................................41 3 Predicting the denitrification capacity of sandy aquifers from shorter-term incubation experiments and sediment properties .......................................................... 43 Abstract .................................................................................................................................43 3.1 Introduction......................................................................................................................44 3.2 Materials and methods ......................................................................................................47 3.2.1 Study sites ...................................................................................................................47 3.2.2 Sampling procedures ....................................................................................................48 3.2.3 Laboratory incubations.................................................................................................48 3.2.3.1 Standard treatment ................................................................................................48 3.2.3.2 Intensive treatment ................................................................................................49 3.2.4 Analytical techniques ...................................................................................................50 3.2.5 Calculated parameters ..................................................................................................52 3.2.6 Statistical analysis and modelling..................................................................................53 3.2.7 Basic assumption and methodical limitations of the presented approach ..........................54 3.3 Results .............................................................................................................................55 3.3.1 Incubations and independent variables: grouping of aquifer material ...............................55 3.3.2 Time course of denitrification products, denitrification rates and cumulative denitrification at the end of incubations .................................................................................55 3.3.3 Sediment parameters ....................................................................................................57 3.3.4 The stock of reactive compounds and its availability for denitrification during incubation............................................................................................................................59 3.3.4.1 Standard treatment ................................................................................................59 3.3.4.2 Intensive treatment ................................................................................................61 3.3.5 Relationship between the cumulative denitrification and sediment parameters .................61 3.3.6 Regression models to predict Dcum(365).........................................................................63 3.3.6.1 Predicting Dcu m(365) from intial denitrification rates ...............................................63 3.3.6.2 Predicting Dcu m(365) from sediment parameters ......................................................64 3.3.7 Predicting the stock of reduced compounds (SRC) from Dcum(365) and estimation of the minimal lifetime of denitrification (emLoD) ..........................................................................66 3.4 Discussion ........................................................................................................................68 3.4.1 Groundwater redox state and sample origin ...................................................................68 3.4.2 Predicting Dcu m(365) from initial denitrification rates and