Summation of Biogeochemical Research of Sierra Nevada catchments

1. Summation of Biogeochemical Research of Sierra Nevada catchments Kate Samelson Kendra Morliengo-Bredlau Ben West Corey Lawrence

2. Origin of the Chemical Compositions of Some Springs and Lakes Robert M. Garrels Fred T. Mackenzie

3. Introduction • Analysis of spring water from Sierra Nevada -known analysis of water constituents - primary analysis of igneous rock minerals and soil minerals derived from them - the system is closed: little loss or gain of water or CO2 - the chemical composition of the rocks studied are representative of continental crust, and a widespread application to rock-water systems can be used.

4. primary igneous rocks + soil water high in CO22- = soil minerals + spring water

5. Weathering Relations • Parental granite rocks: quartz diorite and microcline • Dissolved content of spring water comes from attack of CO2 rich soil water on the silicates • Kaolinite is the main weathering product

6. Weathering Reaction Test 1. back-react with kaolinite to determine if original rock minerals can be formed 2. subtract cations and anions in snow water from spring water to determine the minerals derived form the rock *HCO3 is a fudge factor

7. Weathering Reaction Results 1. Reactions balance for the closed system 2. Weathering product is kaolinite 3. Original rock materials were reconstructed *Reactions depend on depth and retention. Rock material encountered will affect pH, rate and weathering products

8. If the above reaction occurs, then: [Ca2+][SiO2]8 K = ------------------ [H+]2 So, a water undersaturated with montmorillite and saturated with kaolinite should have a value < K Assume: CO2 rich water + plagioclase  kaolinite…  montmorillite

9. Evaporation Experiment Procedure Conditions 1. The water remains in equilibrium with a CO2 pressure of 10 -3.5 atm 2. Temperature remains constant at 25°C 3. Pure water (except for a little CO2) is continuously removed form the system 4. Assume that any solids formed remain in equilibrium

10. Results • Waters emerge from closed system when they have reached compositions similar to general Sierra water, but it will continue to gain and lose different constituents. • Difficult to deduce complete reactions using evaporation technique because there are reactions that occur in the presence of the parent rock and processes are asymmetric

11. Geochemical and Hydrologic Controls on the Composition of Surface Water in a High-elevation Basin, Sierra Nevada, California Mark W. Williams Aaron D. Brown John M. Melack

12. Introduction • Emerald lake watershed is a high altitude basin in the southern Sierra Nevadas • Solute Composition in lake changes due to three distinct periods: -snow pack runoff -transition period between runoff and summer flow -low flow from late summer into winter

13. Background • Traditional dogma: composition of surface water is controlled by solutes in equilibrium with bedrock weathering products. if so, then, if groundwater discharge is major source of stream flow during storm events in granite basins, chemical weathering is the major process that neutralizes incoming acidity…but the chemical weathering process may be overwhelmed and too slow at buffering in reservoirs with low retention rates

14. Objectives • Determine sources of solutes in stream flow -focus on origin of Ca+, reactive silicate and HCO3- • Determine whether stream water is in equilibrium with mineral weathering products • Investigate if acidity due to atmospheric deposition is neutralized by weathering of other processes • Investigate effects that subsurface routing has on composition of surface water

15. Site Information • Soils are strongly acidic • Bedrock: granite and granodiorite • Soils are mainly derived from weathering of the bedrock • Snowfall accounted for ~95% of the precipitation input during study year -very vulnerable to acid pulse

16. 1. All water samples were analyzed for major inorganic ions and reactive silicates (Si) 2. Buffering capacity determined with Gran titration method way to establish [ ] of HCO3- 3. Garrels and Mackenzie approach used to determine whether the streamwater content was a product of the basin/catchment 4. Cl- was used as a scaling factor to determine base cations in snowpack melt waters 5. Reservior residence time was determined using a 6LiBr tracer 6. Ca2+:Na+ ratio was used to help explain possible soil buffering process 6. Ca2+:Na+ ratio was used to help explain possible soil buffering process soil retains Ca2+ more than Na+, Na leaches out faster than Ca is cation exchange in soils is a major buffering process, the ratio of Na:Ca should be high in snowmelt runoff and low in end of melt season 7. Back reactions are difficult to calculate due to differences in retention time, depth (rock encountered) used contributions of Ca2+ to calculate products of mineral weathering contribution % change with seasonal changes in water flow …hydrologic flow paths change …relative importance of different biogeochemical processes change Methods

17. Residence Times: • 4/10/1086 – 8/30/1986  143 days Residence time: 7 – 23 days -Averages in May ’86 and ’87 had almost daily turnover in talus -second method incorporating soil saturation reported soil retention rates of minutes to hours before being available to surface flow • Groundwater discharge in low flow was ordered from months to years

18. Conclusions • Mass balance did not work: wrong weathering reactions used other processes beyond weathering of plagioclase contributed to dissolved solutes deficit of HCO3- indicates additional sources of alkalinity beyond mineral weathering • Weathering model inaccurate: model is good for low flow periods but not as well with high flow unknown and possibly synergistic effects during high flow stream water in ELW are in steady state with weathering products during low flow periods

19. Conclusionscont… • Mineral weathering does not seem to be the primary process of buffering buffering of acidic cations occurs too rapidly to be attributed to silicate weathering cation exchange seems to be the major buffering agent because their kinetic rates match what is needed to buffer during such short retention times • ELW is subject to acid pulses from low pH snow melt buffering during “high flow” comes from soils buffering during “low flow” comes from weathering buffering controls change

20. Conclusions cont… • Snow pack runoff in ELW infiltrates soils and unconsolidated materials, undergoes reactions with soil water and soil exchanges and is then discharged to stream flow • Granitic basins are sensitive to atmosphic deposition of acids due to low residence time during melt periods