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Acidification, Buffering, and Salt Effects in the Unsaturated Zone of a Sandy Aquifer, Klosterhede, Denmark
Author(s) -
Hansen Bent Kjær,
Postma Dieke
Publication year - 1995
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/95wr02217
Subject(s) - weathering , saturation (graph theory) , acid rain , groundwater , acid neutralizing capacity , gibbsite , aquifer , leaching (pedology) , kaolinite , vadose zone , soil acidification , dissolved silica , deposition (geology) , hydrology (agriculture) , geology , environmental chemistry , dissolution , environmental science , soil water , soil science , mineralogy , acid deposition , chemistry , soil ph , geochemistry , sediment , geomorphology , mathematics , geotechnical engineering , organic chemistry , combinatorics
Acidification of groundwater in a noncalcareous sandy aquifer at Klosterhede, Denmark, is the result of acid rain deposition. In the 4‐ to 5‐m‐thick unsaturated zone the pH ranges from 4.2 to 4.9 with Al concentrations of up to 0.8 mmol L −1 . The groundwater at the top of the saturated zone still has a pH below 5. Deposition of sea salt affects the solute profiles, and its importance varies both spatially from the forest margin to the inner part of the forest and temporally through seasonal variations in infiltration and dry deposition. As a result, pulses of high solute concentrations travel downward through the unsaturated zone. The cation exchange capacity (CEC) of the sediments ranges between 0.2 and 1 meq 100 g −1 , and in the acidified zone, base saturation is around 17%. The pore waters are close to equilibrium with gibbsite, supersaturated for kaolinite, and strongly undersaturated for other silicateminerals. Mass balance calculations on increases in dissolved silica over depth suggest that the buffering effect of silicate weathering is small. Buffering processes and solute transport were modeled with the code PHREEQM. Simulation of pre–acid rain weathering indicates that this process operates on a timescale of thousands of years, yielding minimum pH values near 5.2 and a base saturation of greater than 70%. The present leaching of Al 3+ rich acid water from the soil yields acidification rates of 7 and 10 cm yr −1 for weathering of a naturally weathered and a pristine profile, respectively. Simulation of infiltration of sea‐salt pulses indicates that the cation distribution quickly becomes attenuated by the exchanger composition. However, due to coupling of gibbsite equilibrium with ion exchange processes, downward traveling pulses with high solute concentrations will cause pH variations throughout the unsaturated zone by precipitation and dissolution of gibbsite. Accordingly, the general acidification pattern at Klosterhede is overprinted by salts effects in a complicated fashion.