SMHI RH No 5, September 1992 MODELLING GROUNDWATER RESPONSE TO ACIDIFICATION Edited by Per Sanden and Per Warfvinge an REPORTS HYDROLOGY No 5, September 1992 MODELLING GROUNDWATER RESPONSE TO ACIDIFICATION REPORTFROM THE SWEDISH INTEGRATED GROUNDWATER ACIDIFICA TION PROJECT Edited by: Per Sanden Per Warfvinge Swedish Meteorological and Department of Chemical Engineering, Hydrological Institute Lund Institute of Technology Printed in Sweden, SMHI Norrköping 1992 Issuing Agency Report number SMHI RH No. 5 S-601 76 Norrköping Report date Sweden September 1992 Author (s) Per Sanden and Per W arfvinge (Editors) Title (and Subtitle) Modelling groundwater response to acidification Abstract The observed acidification of lakes and forest soils in Sweden has raised concem over the future of groundwater resources. Many of the processes which effect groundwater quality occur above the water table in the unsaturated soil. Models of the hydrology and chemistry in this zone were developed and the results of that work are presented in this report. The overall objective of this project was to obtain a better understanding of the chemical and hydrological processes involved in the acidification of soil and groundwater in order to estimate future changes due to changes in acid deposition in relation to other acidifying processes. This work concluded of this report is that the groundwater in Sweden is vulnerable to acid deposition and that drastic reductions in emissions are necessary to protect this water. The model development provides a set of tools which are generally applicable to the hydrochemistry of acidification. They demand limited calibration and the requirements for input data and parameter values can usually be met. This makes it possible to use the models for assessment of groundwater acidification. The spatial and climatic variabilities of parameters and driving variables, however, are large. The uncertainties introduced by this variability must be considered when the results are used for decision making. Key words Acidification, groundwater, hydrochemical models Supplementary notes Number of pages Language 202 English ISSN and title ISSN 0283-1104 SMHI Reports Hydrology Report available from: SMHI S-601 76 Norrköping Sweden PREFACE This report presents the modelling subprojects of the Swedish Integrated Groundwater Acidification Project. The project was initiated and mainly financed by the Swedish Environmental Protection Agency. Ulf von Brömssen and Ulla Bertils have been the officers from the agency, while the scientific coordinator has been Gert Knutsson (Dept. of Land and Water Resources, Royal Institute of Technology). The project was divided into a number of subprojects, covering field work at different research areas, analytical quality control and method development, and hydrological and hydrochemical modelling. The research areas were Masbyn and Risfallet in central Sweden (Responsible: Lars Lundin, Dept. of Forest Soil, Swedish University of Agricultural Science), a hillslope at Fårahall in the southwestem Sweden (Responsible: Gunnar Jacks, Dept. of Land and Water Resources, Royal Institute of Technology), and the Stubbetorp research basin in southeastem Sweden (Responsible: Sten Bergström, Swedish Meteorological and Hydro­ logical Institute). Some springs in the Loftsdalen area was also studied in the early stages of this project (Resonsible: Gert Knutsson, Dept. of Land and Water Resources, Royal Insitute of Technology). A separate subproject dealt with intercalibration, analytical quality control and development of a aluminum speciation method (Responsible: Folke Ingman and Lars Göran Danielsson, Dept. of Analytical Chemistry, Royal Institute of Technology). The modelling was divided into two subprojects. One of these was responsible for the development of the hydrological model (Sten Bergström, Swedish Meteorological and Hydrological Institute), the other for the hydrochemical model (Harald Sverdrup, Dept. of Chemical Engineering II, Lund Institute of Technology). The affiliation of the authors of this report are listed below: Sten Bergström, Göran Lindström, Allan Rodhe Marie Gardelin and Per Sanden Dept. of Hydrology Swedish Meteorological and Västra Ågatan 24 Hydrological Institute S-753 09 Uppsala S-601 76 Norrköping Sweden Sweden Harald Sverdrup and Per W arfvinge Richard F. Wright Dept. of Chemical Engineering II Norwegian Institute for Water Research Lund Institute of Technology P.O. Box 69 Korsvoll P.O. Box 124 N-0808 Oslo 8 S-221 00 Lund Norway Sweden Bengt Espeby and Lena Maxe Dept. of Land and Water Resources Royal Institute of Technology S-100 44 Stockholm Sweden The authors would like to thank all the persons who help us with field sampling, laboratory analysis, drawing, typing, etc. We also thank all the researchers who supplied us with data. 1 INTRODUCTION . 9 2 DATA BASE (M. Gardelin and P. Warfvinge) . 13 2.1 STUBBETORP . 14 2.2 MASBYN . 17 2.3 RISFALLET . 19 2.4 VELEN . 21 2.5 TRYSILELVA . 23 2.6 DEPOSITION DATA . 24 2.7 UPTAKE DATA . 27 2. 7 .1 Nitrogen uptake . 27 2. 7 .2 Base cation uptake . 27 3 HYDROLOGICAL MODELLING ..................... 31 3.1 MODEL STRUCTURE (G. Lindström and M. Gardelin) ......... 33 3.2 SNOW MODELLING (S. Bergström and M. Gardelin) .......... 37 3.2.1 Results ........................................ 37 3.2.2 Discussion and conclusion .......................... 37 3.3 SOIL MOISTURE (M.Gardelin) .......................... 40 3.3.1 Parameter estimation . 40 3.3.2 V elen . .. 40 3.3.3 Mas byn ........................................ 41 3.3.4 Risfallet . 41 3.3.5 Discussion and conclusions ......................... 47 3.4 TRANSIT TIMES (G. Lindström and A. Rodhe) ............... 48 3.4.1 180 as a tracer .................... • • • • • • • • • • • · • · · · 48 3.4.2 Parameter estimation ............................. 49 3.4.3 Results ........................................ 50 3.4.4 Discussion and conclusions ......................... 55 3.5 SOIL TEMPERATURES (P. Sanden) ...................... 59 3.5.1 Model structure ................................. 59 3.5.2 Results ..................................... .. 60 3.5.3 Discussion and conclusions ......................... 62 3.6 GROUNDWATER LEVELS (S. Bergström and P. Sanden) ....... 63 3.6.1 Results ........................................ 63 3.6.2 Discussion and conclusions . 64 3.7 COMPARISON WITH THE SOIL MODEL (B. Espeby and P. Sanden) 66 3.7.1 Model structure . 66 3. 7 .2 Parameter estimation ............................. 68 3. 7 .3 Results ........................................ 69 3.7.4 Discussion and conclusions ......................... 70 3.8 LONG TERM HYDROLOGICAL SIMULATIONS (M. Gardelin) .. 73 4 HYDROCHEMICAL MODELLING (P. Warfvinge and H. Sverdrup) 79 4.1 MODEL STRUCTURE . 79 4.2 BASIC PRINCIPLES OF THE MODEL . 82 5 4.2.1 Mass balances . 85 4.2.2 Soil solution equilibrium sub-model . 86 4.2.3 Weathering rate model . 89 4.2.4 Nitrogen reactions . 92 4.2.5 Base cation uptake . 93 4.2.6 The cation exchange reaction . 95 4.3 IMPLEMENTATION AND USE . 98 4.3.1 PROFILE . 98 4.3.2 SAFE . 100 4.3.3 Numerical methods . 100 4.4 MODEL TEST . 101 4.4.1 PROFILE . 101 4.4.2 Weathering submodel . 104 4.4.3 SAFE ........ : . 114 5 SENSITIVITY ANALYSIS (P. Warfvinge and P. Sanden) . 119 5.1 HYDROLOGICAL MODEL . 120 5.1.1 Range of variability of conditions . 120 5.1.2 Results . 121 5.1.3 Discussion and conclusion . 123 5.2 HYDROCHEMICAL MODEL . 124 5.2.1 Sensitivity to hydrological input . 124 5.2.2 Sensitivity to time resolution . 136 5.2.3 Sensitivity to chemical model parameters . 138 6 SCENARIOS FOR ACIDIFICATION OF GROUNDWATER (P. Warfvinge and H. Sverdrup) . 147 6.1 STUBBETORP . 147 6.2 MASBYN . 150 6.3 RISFALLET . 153 6.4 CONCLUSIONS . 155 7 SCENARIO MODELLING WITH THE MAGIC MODEL (P. Sanden, L. Maxe and R.F. Wright) . 157 7.1 MODEL STRUCTURE . 157 7.2 PARAMETER ESTIMATION . 158 7.3 RESULT . 163 7.4 DISCUSSION AND CONCLUSIONS . 167 8 CRITICAL LOADS (H. Sverdrup and P. Warfvinge) . 171 8.1 METHODS . 172 8.1.1 Principles of calculation . 172 8.1.2 Calculation procedure . 173 8.1.3 Minerology . 17 5 8.1.4 Additional input data . 177 8.2 RESULTS . 178 8.2.1 Mineralogy and weathering . 178 6 8.2.2 Critical load maps . 181 8.2.3 Steady state chemistry maps . 183 8.3 CONCLUSIONS . 186 9 CONCLUSIONS . 187 10 REFERENCES . 191 7 l - I 1 INTRODUCTION The observed acidification of lakes and forest soils in Sweden has raised concem over the future of groundwater resources. In Sweden rnany households rely on groundwater from shallow aquifers for their water supply, either at their permanent residence or at their surnrner houses. Studies in the late 1980'ies revealed increased groundwater acidification and an understanding of the interaction between acidification and hydrol­ ogy. This work ended with the recognition that there isa risk for groundwater acidifica­ tion (Bertils et al., 1989). The Swedish Integrated Groundwater Acidification Project was therefore initiated by the Swedish Environrnental Protection Agency to provide informa­ tion conceming soil and groundwater acidification as a basis for policy decisions. The overall objective of the project is to obtain a better understanding of the chernical and hydrological processes involved in the acidification of soil and groundwater to be able to estirnate future changes due to changes in acid deposition in relation to other acidifying processes. This objective leads to the following tasks: • Deterrnine the quantitative characteristics of those aquifers that are vulnerable to acid deposition,