Streamwater Chemistry

Transcription

1 Streamwater Chemistry 1) Dissolved major ions 2) Suspended and dissolved organic matter 3) Dissolved nutrients and biological transformations 4) Dissolved gases 5) ph

2 1) Dissolved major ions TDS (Total dissolved solids) = sum of all dissolved major ions. Operationally defined Whatever passes through 2 micrometer filter TDS of ~100 mg/l is world average [= 0.1 grams per 1000 grams water = 0.1 ppt, 100 ppm] TDS of <500 mg/l is drinking water standard for U.S. [0.5 ppt] Lower Colorado: TDS > 800 mg/l [0.8 ppt] Levels above 1000 mg/l can cause biological harm How to measure TDS? Evaporation Conductivity electrical conductance of water due to dissolved ions.

3 Components of TDS Salinity all anions and cations (effectively same as TDS in freshwater) Hardness = sum of Ca 2+ and Mg 2+ In U.S. used as synonym for alkalinity FW < 0.5 ppt Ocean ave. = 35 ppt Alkalinity Quantity of compounds that shift ph > 7. HCO 3, CO 3 2, OH units: mg/l (ppm) CaCO 3, or meq/l Alkalinity often used as surrogate for stream fertility production of crustaceans (gammarids, crayfish) and fish is often higher with high Ca 2+ concentrations; aquatic insect production less sensitive. Fig. 4.6 shows salmonid production across an alkalinity gradient. low alkalinity limits production variation at high alkalinity What other environmental factors correlated with high alkalinity?

4 Sources of TDS? Why is streamwater much more concentrated than rainwater? [FIG. 4.1] * * Contact with minerals in soil Weathering of rock (water is weak acid) carbonate rocks (limestone)» high in Ca 2+, Mg 2+, HCO 3 igneous or noncarbonate rocks» granite, slate, sand (SiO 2 ) Groundwater inputs (long contact with soil/rock)

5 Biological Effects Fig. 4.7 shows effects of road salt on water salinity (Cl ) along ruralurban gradient. natural [Cl ] Note these concentrations are just for Cl ; total TDS would be much higher.

6 2) Suspended and dissolved organic matter Seston suspended particulate matter, including plankton, organic detritus, and inorganic material. important food for collector filterers NOTE: See Lecture 10 slide on DOM! Dissolved Organic Material (DOM) material < 0.45 micrometers, includes leachates from living organisms and soils, and decaying detritus important food for bacteria and other microbes and micrometazoans important export from thawing permafrost

7 3) Dissolved nutrients (N,P,C,Si) N P Humans export N and P to oceans globally Fig. 13.3: N and P vary with land use type Forest to agriculture gradient Human inputs NO 3 in fertilizer PO 4 in animal wastes NO 3 in acid rainwater

8 Mineral nutrients in streams (mainly N and P) are available for biological use IF they can be assimilated Retention vs. Biology Nutrient spiraling is a way of describing the affinity of stream organisms for a particular nutrient What factors might be important for this? Nutrient Spiraling

9 Export of Nutrients from Streams is a major source of pollution Exporting streams (i.e. ag drainages) moves excessive amounts of N and P to the gulf of Mexico The dead zone is caused by the boombust cycle of marine algae being subsidized by this nutrient export Decaying algae are decomposed by microbes on the ocean floor and the concentration of O2 decreases why?

10 4) dissolved gases: O 2, CO 2, CH 4, N 2 O O 2, CO 2, CH 4, N 2 O, etc. Factors controlling concentration Solubility Temperature [TABLE 4.1] Atmospheric pressure (altitude) For example, at 5 C» sea level, 100% O2 saturation is 12.5 mg/l» 3200 m altitude, 100% saturation is 8.6 mg/l What biological processes regulate O 2 and CO 2 concentrations? Aerobic respiration Photosynthesis Concentration in atmosphere at sea level O 2 = 21% CO 2 = 0.03% CH 4 = % N 2 O = % (ratio of atmospheric O 2 to CO 2 = 700) Supersaturation occurs when water has oxygen concentration exceeding 100% of solubility turbulent mixing biological activity photosynthesis How would concentration of O 2 and CO 2 be different in groundwater? How would organic pollution affect the dissolved oxygen in a stream?

11 Biologically derived Gasses (CH 4, N 2 O) Insoluble gasses produced as byproducts of microbial activity using dissolved nutrients as terminal electron acceptors Production of these gasses governed by redox potential within sediments or adjacent riparian zones The N cycle in particular is complex with respect to redox sensitivity

12 5. What is ph and what controls it? What is it? Negative log 10 of [H + ] If [H + ] = mol/l, ph = Why is it important? Life tolerance (~4.5 to ~9.5) ph of various liquids, rain, and lakes Natural gradients in ph Fig. 4.8: Sampling across streams in acidic regions of southern England microarthropods macroarthropods Main point? Fewer species adapted to low ph.

13 Biological effects of excessive [H + ] Loss of body Na + and failure to acquire Ca 2+ Damage to respiratory surfaces (fish gills, mayfly gills) and egg development Leaching of toxic Aluminum and other metals from soils into streams (Fig. 4.9) Alteration of substrate availability for microbes shift in primary forms of metabolism Examples (from text): Invertebrates: species composition changes along ph gradient in Swiss streams Fish: Brook trout decline while blacknose dace and sculpin can be eliminated by low ph in northeastern US

14 Sources of Acidity Natural Acidification Poorly buffered soils (noncalcareous soils) [see equations] Humic substances (dissolved organic material from wetlands, etc.) Anthropogenic acidification Addition of NO 3, SO 4 2 in acid rain What makes streams (and lakes) vulnerable to acidification? Figure 16. Regions of sensi0vity to acid rain in the United States. Also shown are the isopleths of the ph of precipita0on; for example, all of the eastern U.S. currently has an average ph of rainfall between 4 5, where "neutral" rainwater has a ph of ~6.

15 What is the ph of distilled water? ph = 7 = log 10 [10 7 ] #1: H 2 O H + + OH Why is ph of pure rain only 5.6? #2: H 2 O + CO 2 H 2 CO 3 HCO 3 + H + Carbonic acid forms and dissociates into weak acid. How much more acidic than neutral? [10 7 / = ] = 25 Why is acid rain << ph 5.6? Strong acids disassociate in water H 2 SO 4 2H + + SO 4 2 HNO 3 H + + NO 3 But most streams are not acidic!

16 What makes water acidic? addition of H + what are sources? Acids (e.g., carbonic acid (Eqn #2)) Addition of CO 2 from atmosphere, groundwater #2: H 2 O + CO 2 H 2 CO 3 HCO 3 + H + What makes water more alkaline (ph > 7)? addition of OH what are sources? Reactions of water with bicarbonate (HCO 3 ) and carbonate (CO 2 3 ) ions #4: HCO 3 + H 2 O H 2 CO 3 + OH #5: CO H 2 O HCO 3 + OH Where does carbonate and bicarbonate come from??? from watershed!! (groundwater contact with limestone, (CaCO 3 ) a source of HCO 3 )

17 Only 3 forms of dissolved inorganic C their relative proportions depends on ph. CO 2 free carbon dioxide HCO 3 bicarbonate CO 3 2 carbonate Most streams in ph range of 6.59, and HCO 3 dominates. If H + added to stream, neutralized by OH formed from reaction of water with HCO 3 (Eqn #3) or with CO 3 2 (Eqn #4) and ph does not change much. Adding enough H + can use up OH provided by CO 3 2 or HCO 3 and lower ph, eventually producing dissolved CO 2. #2: H 2 O + CO 2 H 2 CO 3 HCO 3 + H +

18 Bicarbonate Buffering System è Streams with high alkalinity (HCO 3 or CO 3 2 ) can hold a lot of H + without notable change in ph. #3: H 2 O + CO 2 H 2 CO 3 HCO 3 + H + CO H + How does Acid Rain + Stream Water = no change in ph? OH neutralizes H + and more OH forms immediately from reaction of CO 3 2 or HCO 3 with water! ph will not change until supply of CO 3 2 or HCO 3 is exhausted most streams Why are some streams more susceptible? Limestone geology (CaCO 3 is source of HCO 3 ) More acidic rainfall (humans)

19 A carbonate(d) twist (Eqn #6) Carbonic acid (from rain) reacts with limestone in soil: H 2 CO 3 + CaCO 3 Ca HCO 3 Calcium ion reacts with abundant HCO 3 in stream to form Calcium bicarbonate: #6: Ca HCO 3 Ca(HCO 3 ) 2 CaCO 3 + H 2 O + CO 2 CaCO 3 can precipitate out of stream water under what conditions? Removing CO 2 drives the equation to the right. How can CO 2 be removed??

20 In hard waters: CO 2 removed in two ways: 1) Biological activity Shoreline algal photosynthesis (mostly lakes) Chara (skunk weed) Removes CO 2 and becomes encrusted with CaCO 3 What happens at night? #6: Ca HCO 3 Ca(HCO 3 ) 2 CaCO 3 + H 2 O + CO 2

21 2) Physicalchemical processes where excess dissolved CO 2 vented e.g., Travertine terraces (e.g., Mammoth Hot Springs, Yellowstone) Supersaturated CO 2 in subterranean water degasses upon contact with atmosphere #6: Ca HCO 3 Ca(HCO 3 ) 2 CaCO 3 + H 2 O + CO 2 What would happen in stream below a dam if water rich in calcium bicarbonate were released from the hypolimnion of a deep reservoir during summer stratification? Hint: Deep reservoir water is supersaturated with CO 2

22 Synopsis: CO 2 dissolves into surface water to equilibrium HCO 3 and CO 3 2 enter through surface/ground water Controls on ph? 1) buffering reactions of carbonic acid 2) amount of carbonate and bicarbonate derived from rock weathering (produces OH ) 3) buffering reactions also influenced by salinity, temperature, but we re not concerned with that here Bicarbonate Buffering System deceptively simple Wetzel: in alkaline, hard water lakes, often twice the content of Ca 2+ and HCO 3 found than predicted on the basis of chemical equilibria.

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