Prediction of iron and manganese release during riverbank filtration

Transcription

1 Prediction of iron and manganese release during riverbank filtration T. Grischek, S. Paufler 1

2 Outline Introduction Study area How much iron could be reduced? Conclusions 2

3 Processes during RBF Hiscock & Grischek

4 RBF and iron (common expectation) River Groundwater portion % Bank filtrate portion % Oxygen depletion, Denitrification, Fe(II) release as result of degradation of DOC 4

5 RBF site Torgau, Germany Monitoring cross-section I 42 vertical wells Distance to river: about 300 m Monitoring cross-section II Depth of filters: m bgl Abstraction rates: 150 m³/h/well Sandhu (2009) 5

6 Mean iron concentration 1993 Hochschule für Technik und Wirtschaft Dresden 0 MP 7/1: 20 MP 7/3: 18 Deich River Elbe Elbe 0,5 old branch Alte Elbe m a.s.l. 0,2 Up to 100 mg/l Spremberger Schichten 41 0,09 MP production Well Brunnen borehole 33 Well 33 0,06 MP ,7 9 5,7 26 0,4 MP 33 0, ,8 14, Around 30 mg/l Cross-section II 19 Meßprofil Torgau-Ost II 6

7 Decreasing iron concentration Torgau-Ost: Monitoring cross-section II β(fe tot ) in mg/l Well 33-50% in about 20 years MP 33 MP 43 MP 53 Well 33 MP 53 Date 7

8 DOC removal in cross-section II Cross-section II MP 6 near well 33 sediment fraction < 2 mm Fe mg/kg Sample Depth (m bgl) Fe (mg/kg) Mn (mg/kg) DOC in mg/l, DOC in mg/l, No change in DOC conc. = No change in Fe(II) conc.? 8

9 How much iron could be released? 9

10 Column experiment for extraction of total iron >70% of total iron conc. extracted Fe tot 3550 mg/kg (Fe) in mg/kg Time Zeit in in ddays Fe tot 2470 mg/kg Hydroxylamine hydrochloride solution after Brand (1989) A1 A2 B1 B2 10

11 Well 2 Hochschule für Technik und Wirtschaft Dresden Hypothetical iron release along the flow path 150 m³/h River V = 800,000 m³ ρ = kg/m³ Fe tot = 3,000 mg/kg 4,000,000 kg Fe pool in aquifer Well 1 Well 2 Well 3 Q well = 0.7*10 6 m³/a (50% bf) Fe(II) well = 20 mg/l River Fe removal = 14,000 kg/a > 250 years potential release of iron 11

12 Potential RBF site Embaba, Egypt 20m Potential RBF well 1,600,000 kg Fe pool in aquifer Q well = 1.2*10 6 m³/a (90% bf) Fe removal = 3,600 kg/a > 440 years potential release of iron Nile river Fraction < 2 mm Nile riverbed 11,000 mg/kg TOC 37,600 mg/kg Fe 780 mg/kg Mn Aquifer sediment 1,100 mg/kg TOC 11,100 mg/kg Fe 130 mg/kg Mn Groundwater: Fe > 3 mg/l Nile water: DOC < 5 mg/l, less degradable 12

13 RBF and iron (revised figure) Hochschule für Technik und Wirtschaft Dresden River Groundwater portion % Anoxic conditions; low redox Generally substantial Fe(II) Low variation in water quality Bank filtrate portion % Oxygen depletion, Denitrification, Fe(II) Dissolution/desorption of Fe(II) maximum permissible value for Fe in drinking water (0.2 mg/l) is usually exceeded at RBF sites Sandhu (2009) 13

14 Conclusions Fe(II) is a subject of concern for most RBF sites Check Fe(II) in land-side groundwater Fe(II) concentration can also decrease during RBF It takes decades to achieve steady-state conditions Prediction based on BDOC/TOC and Fe tot in sediment 14

15 Acknowledgements BMBF FHproUnt2012: Optimization of bank filtration and subsurface removal of iron and manganese (03FH042PX2). ESF grant (no ) Water company FWV Elbaue-Ostharz GmbH Contact Thomas Grischek (Mail: Sebastian Paufler (Mail: Faculty of Civil Engineering Division of Water Sciences 15

16 Mass balance approach DOM = (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) DOM O CO NO 3- + HPO H 2 O + 18 H + DOM + 94,4 NO ,4 H CO ,2 N 2 + HPO ,2 H 2 O NH O 2 2 H + + H 2 O + NO 3 - von Gunten et al Observed removal: 2.9 mg/l DOC and 0.1 mg/l NH 4 + Oxidation requires only about 3.5 mg/l O 2 Along flowpath 10.4 mg/l O 2 and 22 mg/l NO 3- consumed 16

17 Decreasing Mn concentration Mean manganese concentration Dresden Niederpoyritz -70% in about 20 years 17