Diurnal Fluctuations in Hydrology

Transcription

1 Effects of Photosynthesis and Groundwater Exchange on Diurnal Fluctuations of Water Quality in a Pre-Alpine River Masaki Hayashi 1, Tobias Vogt 2, Lars Mächler 2, Mario Schirmer 2 1 Department of Geoscience, University of Calgary, Canada 2 Swiss Federal Institute of Aquatic Science and Technology 0.04 Diurnal Fluctuations in Hydrology Daily cycles of hydrological variables. Usually related to solar radiation or air temperature. discharge (m 3 /s) /19 7/ 7/21 7/22 7/23 7/24 7/25 0 Muir et al. (11, Hydrol. Process. 25: 2954)

2 Base flow of a small stream near Calgary Interaction between hydrological and biological processes Eco-hydrology 0.15 flow (m 3 /s) Riparian plants start pumping Transpiration stops 0.1 9/ 9/21 9/22 9/23 9/24 9/25 9/26 9/27 11 Diurnal fluctuations of stream chemistry Dilution by snowmelt inputs Increase in nutrients by daily discharge of waste water Enrichment of dissolved species by evaporation Changes due to photosynthesis and respiration by aquatic organisms Daytime CO 2 + H 2 O + photons carbohydrate + O 2 Removal of 1 mol CO 2 from water and release of 1 mol O 2. Nighttime carbohydrate + O 2 CO 2 + H 2 O + energy Removal of 1 mol O 2 from water and release of 1 mol CO 2. Odum (1956. Limnology & Oceanography, 1: 2)

3 dissolved O 2 (mg/l) 15 Carbonate equilibrium and H 2 O + CO 2 (g) H 2 CO 3 (aq) H + + HCO 3 - Removal of CO 2 pushes the equilibrium to the left. H + is removed increase in Addition of CO 2 lowers. Example: Thur River, Switzerland DO 7 4/22 4/23 4/24 4/25 4/26 4/27 4/2 0:00 0:00.5 dissolved O 2 (mg/l) Carbonate equilibrium and response H 2 O + CO 2 (g) H 2 CO 3 (aq) H + + HCO 3 - Removal of CO 2 pushes the equilibrium to the left. H + is removed increase in Addition of CO 2 lowers. 15 Example: Thur River, Switzerland DO 7 4/22 4/23 4/24 4/25 4/26 4/27 4/2.5

4 discharge (m 3 s -1 ) Thur River data at Andelfingen Discharge > 150 m 3 s -1 causes major scouring (Uehlinger, 06, Freshwater Biol. 51: 93). 150 m 3 s -1 DO Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 15 5 DO (mg L -1 ) Thur River Study in Switzerland Observation: In addition to and DO, electrical conductivity shows diurnal fluctuations. Question: Why? Methods: Field observation, laboratory experiments, geochemical modeling. Answer: Combination of biogeochemical processes involving algae, and groundwater-surface water interaction.

5 Thur River Catchment (1700 km 2 ) Elevation range: m Niederneunforn Thur Valley Aquifer Thur Catchment Carbonate Rocks in headwater regions

6 River Corridor and Alluvial Aquifer Municipal water supplies use bank-filtrated groundwater. Thur River, Switzerland GW flow N2 N3 N m weather station Electrical conductivity (EC) in river has diurnal fluctuation. Trace the propagation of diurnal waves. Determine travel time of groundwater. What causes EC fluctuation? Vogt et al. (. Adv. Water Resour. 33: 1296) EC ( S/cm)540

7 water sample intake Field Instrumentation Niederneunforn Site meteorological station stream reach with EC/DO/ sensors Intensive Monitoring in September radiation (W m -2 ) radiation air temp. water temp DO temp. ( o C) DO (mg L -1 )

8 Intensive Monitoring in September EC ( S/cm) alkalinity (meq L -1 ) EC alkalinity 3.6 Ca DO Ca (mmol L -1 ) DO (mg L -1 ) EC ( S/cm) Intensive Monitoring in September alkalinity (meq L -1 ) EC alkalinity 3.6 Ca HCO 3- + Ca 2+ CaCO 3 + 2H + DO Ca (mmol L -1 ) DO (mg L -1 )

9 Carbonate equilibrium and CO 2 removal H 2 O + CO 2 (g) H 2 CO 3 (aq) H + + HCO 3 - K 2 = [H + ][CO 3 2- ] [HCO 3- ] HCO 3 - H + + CO 3 2- [CO 3 2- ] = K 2 [HCO 3- ] [H + ] [ ] : chemical activity molar concentration K 2 : dissociation constant CO 2 removal decrease in [H + ] and [HCO 3- ] But, =.5 [H + ] -.5 [HCO 3- ] -3 CO 2 removal shift [CO 3 2- ] increase Calcium and carbonate reaction Ca 2+ + CO 2-3 CaCO 3 (calcite) K s = [Ca 2+ ][CO 2-3 ] K s : solubility constant Increase in [CO 2-3 ] precipitation of calcite mineral Gravels from Thur River bed

10 9.5 Carbonate equilibrium diagram H 2 O + CO 2 (g) H + + HCO - 3 CO 2 partial pressure alkalinity P CO2 400ppm in the atmosphere alkalinity (meq/l) 9.5 Carbonate equilibrium diagram H 2 O + CO 2 (g) H + + HCO - 3 CO 2 partial pressure alkalinity P CO2 400ppm in the atmosphere alkalinity (meq/l)

11 9.5 Carbonate equilibrium diagram H 2 O + CO 2 (g) H + + HCO - 3 CO 2 partial pressure alkalinity 9/23 16: 9/23 : P CO2 400ppm in the atmosphere Thur River Sample P CO2 = 250 to 1500 ppm alkalinity (meq/l) Always supersaturated. Hysteresis complex process EC ( S cm -1 ) P CO2 (ppm) Geochemical calculation using PHREEQC Saturation Index = [Ca2+ ][CO 3 2- ] K s 500 P CO2 400 ppm calcite S.I. 0 EC 0 calcite saturation index

12 Calcite precipitation experiment Outdoor experiment failure! Sample collection Controlled light chamber Light chamber experiment Gravels were submerged in Thur River water. Exposed to light for hours, followed by darkness. Photon intensity = 1 E s -1 m W m -2. Two beakers were subjected to the same condition. Calcite-coated gravel Calcite beaker Live Cladophora sp. on gravel Periphyton beaker

13 500 Results: Calcite beaker light on 9.0 EC ( S cm -1 ) 450 P CO2 = 4 ppm Calcite SI = EC :00 12:00 1:00 0:00 6:00 No change in EC Calcite precipitation is blocked. Why? Super-saturation and kinetic inhibition Dissolved organic carbon is known to inhibit calcium precipitation even at SI of 13 to (Lebrón & Suárez, 199). Other ions can inhibit precipitation; magnesium (Berner, 1975), phosphate (House, 197). Periphytons can generate a thin (< 1 mm) diffusive boundary layer that has much lower P CO2 and higher than bulk water (House et al. 199; Tobias and Böhlke, 11). Biological process is required for precipitation.

14 EC ( S cm -1 ) Results: Periphyton beaker light on P CO2 = 170 ppm Calcite SI = EC.4 P CO2 = 70 ppm Calcite SI = :00 12:00 1:00 0:00 6:00 EC does not recover after photosynthesis stops. What causes the EC increase? Back to field data. Downstream propagation of chemical signals Niederneunforn to Andelfingen Niederneunforn Andelfingen Thur Valley Aquifer Average discharge = 12 m 3 /s Channel length km Flow velocity m/s Travel time 4-5 hr

15 DO (mg L -1 ) Downstream propagation of chemical signals 15 :00 16: Niederneunforn Andelfingen Photosynthesis is locally controlled. EC ( S cm -1 ) :30 17:00 21:00 EC recovery involves largerscale processes. 4 Alluvial aquifer upstream of Niederneunforn Input of carbonate rich groundwater Niederneunforn Andelfingen Thur Valley Aquifer Mostly bedrock channel below Niederneunforn Little input of carbonate rich groundwater Recovery of EC is due to groundwater input.

16 Conclusion Conceptual Model day night photosyn. O 2 CO 2 micro environ. CSI precipitn. calcite EC respiration O 2 CO 2 CSI GW input EC Hayashi et al. (12, J. Hydrol : 93) Implications Similar observations are commonly made in karst springs around the world (Dandurand et al. 192; Spiro & Pentecost, 1991; Finlay, 03; Liu et al. 0). In USA, 40 % of stream gauging stations had the water saturated with respect to calcite (Tobias and Böhlke, 11). Carbonate precipitation with photosynthesis may have major influence on watershed-scale geochemical cycle in many rivers.