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

1) Dissolved major ions TDS (Total dissolved solids) = sum of all dissolved major ions [TABLE 4.2] –Regional variation TDS of 120 mg/L is world average[= 0.12 grams per 1000 grams water = 0.12 ppt] TDS of <500 mg/L is drinking water standard for U.S. Lower Colorado: TDS > 800 mg/L[0.8 ppt] How to measure TDS? –Evaporation –Conductivity – electrical conductance of water due to dissolved ions.

Salinity -- all anions and cations (effectively same as TDS in freshwater)FW < 0.5 ppt Ocean ave. = 35 ppt Hardness = sum of Ca 2+ and Mg 2+ –In U.S. used as synonym for alkalinity 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) often higher with high Ca 2+ concentrations; aquatic insect production less sensitive. Fig. 4.6 shows salmonid production across a alkalinity gradient. - low alkalinity limits production - variation at high alkalinity What other environmental factors correlated with high alkalinity?

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

Biological Effects –Fig. 4.7 shows effects of road salt on water salinity (Cl - ) along rural-urban gradient. Note these concentrations are just for Cl - ; total TDS would be much higher.

2) Dissolved nutrients (N,P,C,Si) N P 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 Human alteration of N and P export to oceans globally

3) S uspended and dissolved organic matter Seston - suspended particulate matter, including plankton, organic detritus, and inorganic material. Dissolved Organic Material (DOM) – material < 0.45 micrometers, includes leachates from living organisms and soils, and decaying detritus

4) dissolved gases (N 2, O 2, CO 2 ) and pH N 2 –source for N-fixing cyanophytes (blue-green algae) –gas bubble disease in fish where N 2 supersaturated (e.g., below large reservoirs) Analogous to “the bends” O 2, CO 2 –Factors controlling concentration Solubility temperature [TABLE 4.1] atmospheric pressure (altitude) –Supersaturation? turbulent mixing biological activity –photosynthesis Concentration in atmosphere at sea level O 2 = 21% CO 2 = 0.03%

What is pH and what controls it? 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. What is it? –Negative log 10 of [H + ] If [H + ] = 4.5 × 10 −4 mol/L, pH = Why is it important? –Life tolerance (~4.5 - ~9.5)

Biological effects of excessive [H + ] 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 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 from soils into streams (Fig. 4.9)

Sources of Acidity –Natural Acidification Poorly buffered soils (non-calcareous 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 “vulnerable” to acidification?

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

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

The form of dissolved inorganic C depends on pH. –CO 2 free carbon dioxide –HCO 3 - bicarbonate –CO 3 2- carbonate Most streams in pH range of 6.5-9, 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 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 - exhausted Why are some streams more susceptible? –Limestone geology (CaCO 3 is source of HCO 3 - ) –More acidic rainfall (humans) most streams Bicarbonate Buffering System  Streams with high alkalinity (HCO 3 - or CO 3 2- ) can “hold” much H + without notable change in pH. #3:H 2 O + CO 2 H 2 CO 3 HCO H + CO H +

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: –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?? #6: Ca HCO 3 - Ca(HCO 3 ) 2 CaCO 3 + H 2 O + CO 2

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

2) Physical-chemical processes where “excess” dissolved CO 2 vented e.g., Travertine terraces (e.g., Mammoth Hot Springs, Yellowstone) #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? Supersaturated CO 2 in subterranean water degasses upon contact with atmosphere Hint: Deep reservoir water is supersaturated with CO 2

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.”